NBS

PUBLICATIONS

NAri INST OF STANDARDS & TECH R.I.C.

/p ^ A1 11 00985933

/Procedures used at the National Bureau QC100 .U57 V492;1977 C.I NBS-PUB-C 1977
J

J
DEC S iStl

us^^n"^ Procedures Used at the National Bureau of Standards

/""^To Determine Selected Trace Elements in

C.-2. Biological and Botanical Materials

Edited by:

R. Mavrodineanu

Analytical Chemistry Division

Institute for Materials Research

National Bureau of Standards

Washington, D.C. 20234

Sponsored by the

U.S. Environmental Protection Agency

and

Partially supported by the

U.S. Energy Research and Development Administration

U.S. DEPARTMENT OF COMMERCE, Juanita M. Kreps, Secretary

Dr. Sidney Harman, Under Secretary

Jordan J. Baruch, Assistant Secretary for Science and Technology

V NATIONAL BUREAU OF STANDARDS, Ernest Ambler, Acting Director

Issued November 1977
Library of Congress Catalog Card Number: 77-608301

National Bureau of Standards Special Publication 492

Nat. Bur. Stand. (U.S.), Spec. Publ. 492, 295 pages (Nov. 1977)

CODEN. XNBSAV

U.S. GOVERNMENT PRINTING OFFICE

WASHINGTON: 1977

For sale by the Superintendent of Documents, U.S. Government Printing Office, Washington, D.C. 20402

(Order by SD Catalog No. C13. 10:492). Stock No. 003-003-01858-9. Price $7.50

(Add 25 percent additional for other than U.S. mailing).
FOREWORD

In the act of Congress which provides the basic authority

of the National Bureau of Standards (NBS) , an essential function of the Bureau is the "preparation and distribution of

standard samples such as those used in chemical analysis . . and "the development of methods of chemical analysis . . .".

At the Bureau these two functions are closely joined in

the Standard Reference Materials (SRM's) program. In this

program the desired characteristics of a quantity of material,

sufficiently large to make it available to a large number of

users over a reasonably long time, is determined. By using

these SRM's to calibrate instruments, to test the accuracy

of newly developed methods, or in a routine program to

monitor laboratory results, the quality of analytical data

can be assured. More than nine hundred and fifty SRM's are

available now from NBS for use in science and industry.

These SRM's cover a large variety of applications and include

inorganic and organic compounds certified for their chemical

composition (major, minor, and/or trace elements) , or for

physical properties such as, for example, their optical,

thermal, electrical, or radioactive characteristics.

A significant number of these SRM's are certified

through the measurements performed in the Analytical Chemistry

Division of the NBS Institute for Materials Research. These

measurements require that the systematic and random errors

of the procedures used to determine the particular element

or compound desired be sufficiently well known to state the

concentration of that analyte on the certificate within a

required uncertainty level. It is one of the major responsibilities of the Division to select suitable procedures from

the literature or, if necessary, to develop new or modified

procedures to insure that the necessary accuracy is obtained

during the certification process.

iii
This publication contains a detailed description of the

procedures used at NBS to determine twenty-five trace elements

in biological and botanical matrices. They were used to

determine the trace element content of spinach, orchard

leaves, pine needles, tomato leaves, bovine liver, blood

serum, etc. The dissemination of these procedures, and

their careful use, should permit other analysts to perform

measurements with an accuracy comparable to those obtained

at NBS.

This volume was prepared at the request of the U. S.

Environmental Protection Agency who also partially supported

the development of the procedures and/or SRM's. The U. S.

Energy Research and Development Administration also supported

a portion of the work described in this report. This support

is gratefully acknowledged.

Philip D. LaFleur, Chief

Analytical Chemistry Division

July 1977

iv
LIST OF CONTRIBUTORS

J. R. Baldwin, D. A. Becker, R. W. Burke,

B. I. Diamondstone , T. E. Gills, S. H. Harrison,

G. J. Lutz, E. J. Maienthal, R. Mavrodineanu

,

L. T. McClendon, T. J. Murphy, P. J. Paulsen,

T. C. Rains, H. L. Rook, and S. A. Wicks

Analytical Chemistry Division

Institute for Materials Research

National Bureau of Standards

Washington, D. C. 20234

V

TABLE OF CONTENTS

PAGE

ABSTRACT 1

INTRODUCTION by R. Mavrodineanu 2

SECTION I. Sample Preparation

T. J. Murphy 5

SECTION II. NEUTRON ACTIVATION ANALYSIS

A. Determination of Arsenic, Antimony and Copper

in Biological and Botanical Materials Using

Neutron Activation Analysis

T. E. Gills. 9

B. Determination o£ Platinum in . Biological and

Botanical Materials Using Neutron Activation

Analysis

D. A. Becker and T. E. Gills ,. . . . 12

C. Determination o£ Vanadium and Manganese in

Biological and Botanical Materials by Neutron

Activation Analysis

S. H. Harrison 15

D. Radiochemical Separation for the Determination

of Mercury and Selenium in Biological and

Botanical Materials Using Neutron Activation

Analysis

H. L. Rook, T. E. Gills and G. J. Lutz .... 20

E. Determination of Chromium in Biological and

Botanical Materials Using Neutron Activation

Analysis

L. T. McClendon 23

F. Rapid, Quantitative Separation for the

Determination of Selenium Using Neutron

Activation Analysis

H. L. Rook 26

SECTION III. SPARK SOURCE MASS SPECTROMETRIC

ISOTOPE DILUTION

Determination of Cadmium, Copper, Iron, Lead,

Mercury, Molybdenum, Nickel, Selenium, Silver,

Tellurium, Thallium, and Zinc

P. J. Paulsen 33

vii
PAGE

SECTION IV. ATOMIC ABSORPTION AND FT.AME EMISSION

SPECTROMETRY

A. Determination of Beryllium, Cadmium, Copper,

Magnesium, Manganese, Mercury, Nickel, and Zinc

by Atomic Absorption, and of Calcium, Potassium,

Rubidium, and Sodium by Flame Emission

Spectrometry, in Biological and Botanical

Materials

.

T. C. Rains 49

B. Determination of Calcium, Potassium, and Sodium

in Flour by Emission Flame Spectrometry

J. R. Baldwin and R. Mavrodineanu 66

SECTION V. MOLECULAR ABSORPTION SPECTROMETRY

Procedures for the Determination of Arsenic,

Copper, and Nickel by Molecular Absorption

Spectrometry

R. W. Burke and B. I. Diamondstone 73

SECTION VI. FLUORESCENCE SPECTROMETRY

Determination of Beryllium by Fluorescence

Spectrometry

S. A. Wicks and R. W. Burke 85

SECTION VII. POLAROGRAPHY

Analysis of Botanical Standard Reference

Materials by Cathode Ray Polarography

E. June Maienthal 91

ACKNOWLEDGEMENT 106

APPENDIX 108

viii
LIST OF TABLES

TABLE NO. PAGE

SECTION II

1. Selenium contents o£ Standard Reference

Materials, ppm 31

SECTION III

1. Trace elements (yg/g) in Coal, SRM 1632, by

spark source mass spectrometry isotope

dilution 47

2. Trace elements (yg/g) in Beef Liver, SRM 1577,

by spark source mass spectrometry isotope

dilution 47

3. Trace elements (yg/g) in Spinach, SRM 1570,

by spark source mass spectrometry isotope

dilution 48

3b. Cd and Cu in spinach (Lot 6-2), additional

sampling 48

SECTION IV

1. Determination of sodium, potassium, and calcium

(yg/g) in a wheat flour sample on an "as

received "basis 70

2. Determination of sodium, potassium, and calcium

(yg/g) in a rice flour sample on an "as

received"basis 71

SECTION V

1. Spectrophotometric determination of arsenic in

tissue samples by the arsine-silver

diethyldithiocarbamate procedure 78

2. Spectrophotometric determination of copper in

three new NBS botanical SRM's 82

SECTION VI

1. Beryllium content of three SRM's as determined

by the acetylacetone extraction 2-hydroxy-3-

naphthoic acid spectrofluorometric method. . . 89

ix
TABLE NO. PAGE

SECTION VII

1. Polarographic determination of iron (acid

soluble) in orchard leaves 98

2. Polarographic determination of aluminum (acidsoluble) in orchard leaves 99

3. Polarographic determination of iron (total)

in orchard leaves 100

4. Polarographic determination of aluminum (total)

in orchard leaves 101

5. Polarographic determination of bismuth and

lead in orchard leaves 102

6. Polarographic determination of nickel in

orchard leaves 103

LIST OF FIGURES

FIGURE NO.

SECTION II

1. Sample Combustion Tube and Mercury Trap 27

X
PROCEDURES USED AT THE NATIONAL BUREAU OF STANDARDS

TO DETERMINE SELECTED TRACE ELEMENTS IN

BIOLOGICAL AND BOTANICAL MATERIALS

R. Mavrodineanu, Editor

Abstract

This volume consists of thirteen papers describing the

analytical procedures selected at the National Bureau of

Standards (NBS) for the determination of Ag , Al , As, Be, Bi,

Ca, Cd, Cr, Cu, Fe, Hg, K, Mg, Mn, Mo, Na, Ni, Pb , Pt , Sb

,

Se, Te, Tl, V, and Zn in biological and botanical materials.

These procedures are used at the present time for the certification of various substances issued by NBS as Standard

Reference Materials, and belong to the following analytical

disciplines: sample preparation, neutron activation analysis,

spark source mass spectrometric isotope dilution, atomic

absorption and flame emission spectrometry, molecular absorption spectrometry, fluorescence spectrometry, and polarography

.

Further details on the analytical methods including sample

preparation, purity of reagents, and problems associated with

blanks are given in sixteen additional papers which are

reproduced in the Appendix to this volume.

Key Words: Analytical disciplines; analytical procedures;

atomic absorption spectrometry; biological

materials: blanks; botanical materials ; chemical

species; flame emission spectrometry; fluorescence

spectrometry; molecular absorption spectrometry;

neutron activation analysis; polarography; pure

reagents; sample preparation; spark source mass

spectrometric isotope dilution; tissues.

1
INTRODUCTION

This work consists of a collection o£ analytical procedures used in the Analytical Chemistry Division, Institute

for Materials Research, of the National Bureau of Standards

(NBS) , for the determination of trace levels of Ag , Al , As,

Be, Bi, Ca, Cd, Cr, Cu, Fe, Hg , K, Mg , Mn, Mo, Na , Ni, Pb

,

Pt, Sb, Se, Te, Tl, V, and Zn in biological and botanical

materials. These procedures were critically selected or

adapted, and often specially developed, by the scientific

staff members of the Analytical Chemistry Division to provide

measurements with the best obtainable accuracy. They were

considered to be most appropriate for the analysis and certification of various Standard Reference Materials (SRM's)

issued by NBS such as SRM 1570, Trace Elements in Spinach;

SRM 1571, Orchard Leaves; and SRM 1577, Bovine Liver.

The description of these procedures is given here with

sufficient detail to permit the analyst to use them as a

protocol for routine analyses in his laboratory.

They are assembled according to the analytical disciplines involved in the measuring process:

I- Sample Preparation

II- Neutron Activation Analysis

III- Spark Source Mass Spectrometric Isotope Dilution

IV- Atomic Absorption and Flame Emission Spectrometry

V- Molecular Absorption Spectrometry

VI- Fluorescence Spectrometry

VII- Polarography

As a general rule, and unless otherwise indicated, all the

acids and distilled water used in the measurements are of

high-purity and were prepared at NBS. A detailed description

of the methods and instrumentation used to produce these

reagents, including the analytical techniques used to test

their purity, is given in a paper reproduced as Appendix 1.

2
The important subject of blanks, a determining factor

common to all analytical measurements, is examined in Appendix 2, which also includes a detailed description of the

environmental conditions necessary to control this essential

factor, and to insure a maximum protection against contamination . An evaluation of the lyophilization method for preconcentration and drying of various analytical samples is

presented in Appendix 3.

Also included in the Appendix are reprints of thirteen

papers which provide further relevant and more detailed

information on the analytical procedures included in this

publication.

In order to describe adequately materials and experimental procedures discussed in this work, it was occasionally

necessary to identify commercial products by manufacturer's

name or label. In no instance does such identification imply

endorsement by NBS or that the particular product or equipment are necessarily the best available for that purpose.

The analytical procedures assembled in this publication

are believed to provide the best measuring capabilities

available at this time. They are, however, continuously

being revised, improved, or replaced by more accurate ones.

Hence, it is recommended that the analyst who desires

to be informed of these advancements maintain a contact with

the scientific staff of the Analytical Chemistry Division at

NBS.

January, 1977 R. Mavrodineanu

3

SECTION I

Sample Preparation

T. J. Murphy

1. Drying

All analytical results for the tissue Standard Reference

Materials should be expressed on a dry weight basis. For

non-volatile elements, drying may be performed on the sample

to be analyzed, but for volatile elements, such as mercury

and selenium, drying should be done on separate samples other

than the samples for analysis.

The only animal tissue certified to date as a Standard

Reference Material is SRM 1577, Bovine Liver. This material

was lyophilized during preparation, so f reeze-drying is the

recommended drying method.^ The material should be dried by

lyophilization for at least 24 hours using a cold trap at or

below-50 °C and at a pressure not greater than 30 Pa (0.2 mm

Hg) . Even though this material has been previously lyophilized, drying should be done on separate samples for volatile

elements such as mercury and selenium.

To date, we have worked on four botanical SRM's, spinach,

pine needles, tomato leaves, and orchard leaves. All contain

varying amounts of water. The removal of this water is not a

simple problem since additional volatiles can also be lost.

Desiccator drying over P2O5 or Mg(C10it)2 is not satisfactory as the rate of moisture loss is too slow. Oven drying

at 105 °C and vacuum oven drying at 75 °C are also unsatisfactory since both methods cause large losses of volatiles

other than water.

Oven drying for two hours at 75 °C, vacuum drying for

24 hours at room temperature, and f reeze-drying (lyophiliza-

^ An evaluation of the lyophilization procedure is given in

Appendix 3.

5
tion) for 24 hours are all acceptable methods. The loss of

drying by these different methods showed that all three

methods agreed to about 0.2 percent. For volatile elements,

drying should be performed on separate samples and results

corrected to a dry basis.

In general, the samples were dried in the following

manner: a suitable sample (usually about 2 g) was placed in

a weighing bottle (with cover) . The bottle and contents were

weighed and transferred uncovered to an oven, vacuum oven or

freeze-drier as desired. After leaving the samples for the

desired time under the desired conditions, the bottles and

contents were placed in a desiccator and covered. After

equilibration, the weighing bottles and contents were again

weighed and the moisture loss calculated. For the nonvolatile elements the samples were used directly for analyses.

For the volatile elements (Hg, Se, As, etc.) separate samples

were taken for analysis and corrected to dry weight from the

calculated moisture loss.

2. Wet Ashing

The basic wet ashing system employed in the decomposition

of tissue samples involved the use of nitric -hydrofluoricperchloric acids. The acids and water used in these dissolutions were prepared by sub-boiling distillation. The

preparation and analyses of these acids and water are described

in a paper included as Appendix 1. Covered beaker chemistry

was used for all elements except the volatile elements such

as Hg, As, Se, and Cr.

For the non-volatile elements such as Pb, Cu, Ni, and

Cd the following procedure was used.

A suitable sample (usually 1 g) was transferred to a

Teflon FEP beaker and treated with 5 ml nitric acid, 1 ml

hydrofluoric acid and 2 ml perchloric acid. The beaker was

covered with a Teflon cover and placed on a hot plate at low

6
heat. After the action of nitric acid slowed, the temperature

was raised to about 110 °C and the beaker and contents were

heated overnight. The cover was then removed and washed with

water. The solution was heated to fumes of perchloric acid

and the heating was continued until fumes of perchloric acid

were no longer noted.

After removing the beaker from the hot plate and allowing it to cool, the sides of the beaker were washed with

water and the resulting solution was again heated until fumes

of perchloric acid were no longer noted.

For the analysis of samples where perchloric acid could

not be used (Cr, for example) , sulfuric acid was substituted

and quartz instead of Teflon beakers were used. A suitable

sample (usually 1 g) was transferred to a quartz beaker and

treated with 5 ml of H2O and 10 ml HNO3. The beaker was

covered with a quartz watch glass and the solution was

digested overnight on a hot plate. Five ml of sulfuric acid

were added and the solution was evaporated to fumes of

sulfuric acid. The beaker was then covered and nitric acid

was added dropwise to the hot solution to destroy the residual

organic matter. The solution was then transferred to a

Teflon FEP beaker and any remaining siliceous material was

washed into the Teflon beaker. To remove silica, 1 ml of

hydrofluoric acid was added and the solution was evaporated

to fumes of sulfuric acid.

For the volatile elements, dissolution was effected under

reflux conditions. The system consisted of a 125 ml quartz

flask fitted with a quartz condenser having a 24/40 standard

taper joint. The condenser was filled with quartz beads.

A suitable sample (usually about 1 g) was transferred

to the flask and the sides washed with a few ml of water.

The condenser was then attached and cold water ('v-lO °C) was

circulated through the condenser jacket. Ten grams of HNO3

were then added through the top of the condenser and the

solution was allowed to digest for about four hours while
the temperature was gradually increased. Nine grains of

sulfuric acid (or perchloric acid) were then slowly added

through the top of the condenser and the temperature was

increased

.

The water was drained from the condenser jacket and the

solution was allowed to concentrate and reflux overnight.

This treatment results in a clear solution, except for undissolved siliceous matter. When possible, this was dissolved

by transferring the solution to a Teflon beaker and adding a

small amount of hydrofluoric acid.

All operations involving sample preparation were performed in Class 100 environmental conditions to reduce

environmental contamination. Whenever possible all analytical operations were carried-out in the same controlled

conditions. A thorough discussion of these conditions, indispensable to minimize sample contamination, and to insure

the production of accurate measurements, will be found in

Appendix 2.

NOTE: The symbol for milliliters used throughout this manuscript

was ml instead of mL.

8
SECTION II

NEUTRON ACTIVATION ANALYSIS

Determination of Arsenic, Antimony, Chromium, Copper,

Manganese, Mercury, Platinum, Selenium, and Vanadium

A. Determination of Arsenic, Antimony and Copper

in Biological and Botanical Materials

Using Neutron Activation Analysis

T. E. Gills

1. Objective

The object is to provide

isolating and determining ars

neutron activation analysis,

materials

.

an effective procedure for

enic, antimony and copper using

in biological and botanical

2. Principle

This procedure involved the neutron irradiation of a

biological or botanical sample and subsequent solubilization

of the radioactive material by nitric and perchloric acids.

After obtaining the irradiated material in a soluble form,

the radioactive solutions were made to 6M HClOi* and passed

through 6M HClOit equilibrated inorganic ion exchange columns

of cuprous chloride and tin dioxide. Copper and antimony

were absorbed on the cuprous chloride while arsenic was

absorbed on the tin dioxide. Sodium, the major interference

in the analysis of biological materials, was eliminated by

passing through to the eluate fraction.

3. Instrumentation

Pneumatic tube RT-3 of the NBS Reactor (NBSR) and a

Ge(Li) detector system coupled to a multichannel analyzer

were used. For further details see Appendices 4 and 5.
4.

5.

Reagents

Cu standard:

As standard:

Sb standard:

Carriers

Na carrier:

Acids

Inorganic Ion

Exchangers

NBS SRM's:

100 mg Cu metal (99.99% pure) dissolved

in high purity HNO3 and diluted to 100 ml

with high purity H2O.

150 mg AS2O3 (99.95% pure) dissolved in

high purity HCl and diluted to 100 ml

with high purity H2O.

278 mg antimony potassium tartrate

(reagent grade) dissolved in a minimum

amount of high purity HCI-HNO3 and

diluted to 100 ml with high purity H2O.

Cu, As, Sb - 1.0 mg/ml (reagent grade

oxides and chlorides)

.

1 mg/ml (reagent grade chloride)

.

HClOi,, HNO3, HF (ACS reagent grade).

Cuprous chloride, tin dioxide.^

Orchard Leaves (1571) , Bovine Liver (1577)

Sample Preparation and Analysis

Sample sizes of approximately 250 mg are placed in clean

polyethylene microcentrifuge tubes and heat sealed. Standards

consisting of solutions of copper, arsenic, antimony and

'\>250 mg of SRM 1571 Orchard Leaves are treated in the same

manner. Standards and samples are irradiated for 30 minutes

in the pneumatic tube RT-3 of the NBSR in a flux of

^^^6x10 ^ ^n* cm ^s ^. After 24 hours decay, samples and standards

are post-weighed and/or pipetted into 125 ml boiling flasks

which have been fitted with air condensers. The irradiated

materials are then dissolved in 1:3 mixtures of HClOtt- HNO3 .

^ These inorganic ion exchangers were obtained from Carlo Erba,

Division of Industrial Chemicals. Via C. Imbonati, 24,

Milano, 20159, Italy.

10
I£ silica containing material is present, 3 to 4 drops of HF

may be added. After dissolution, 10 ml of 6M HClOit is added

and the solution is passed through 3-cm columns of cuprous

chloride and tin dioxide respectively. Copper and antimony

are absorbed on the cuprous chloride column while arsenic is

absorbed on the tin dioxide column. After the columns are

washed, the contents are transferred separately into 45 mm

petri dishes and counted.

6. Procedure

Weigh the irradiated samples into 125 ml boiling flasks.

Add 2 ml of Cu, As, Sb and Na carriers. Add 15 ml of the

HClOif-HNOs mixture (1:3) and heat until the sample is dissolved. Continue heating until the volume is reduced to

'^1 ml. Allow sample to cool.

Place each inorganic exchanger in a polyethylene exchange

column; a height of 3 cm is sufficient for sample sizes of 1

gram. Mount the columns in series with the cuprous chloride . column at the top. Wash each column three times with 5 ml

of 6M HClOif . Discard wash.

Add 5 ml of 6M HClOi^ to the cooled sample and transfer

to the cuprous chloride column. Allow the sample solution

to pass through both columns.

Wash both columns three times with 5 ml portions of

6M HClOit . Discard all washings.

Transfer separately the cuprous chloride and tin dioxide

columns into 45 mm petri dishes and count each fraction.

Remarks

The use of the inorganic ion exchangers as outlined in

the above procedure provides a rapid and efficient separation for trace amounts of copper, arsenic and antimony from

the matrix radioactivities. The technique also allows the

analyst to process a series of samples simultaneously.

11
Calculation

I£ the induced activities are measured in an identical

manner, comparing counting rates in the sample and standard

will give the weight of the desired element in the sample by

the following simple relation:

Total cts/min Standard _ wt (yg) Standard

Total cts/min Sample wt (yg) Sample

Example

Sample

(yg element)

(Total cts/min Sample) (wt (yg) Standard)

Total cts/min Standard

Sample

(yg element)

(SxlO'^cpm) (10 yg)

(2. SxlO'^cpm)

= 20 yg

B. Determination of Platinum in Biological and Botanical

Materials Using Neutron Activation Analysis.

D. A. Becker and T. E. Gills

1. Objective

To provide a simple and selective separation procedure

for determining trace quantities of platinum in tissues,

sediments and waters by neutron activation analysis (NAA)

.

2. Principle

The most sensitive and useful nuclear reactions for quantitation of platinum involves isotopes of low gamma energies

which are difficult to detect in an activated tissue, sediment

or sometimes aqueous matrix. This procedure is based on the

electro-chemical theory of spontaneous deposition of an element onto a less noble element. The nuclear reaction used

is ^^^Pt(n,Y) ^^^Pt^ ^^^Au. The end product of the reaction,

12
^^^Au, has a 3.15 day half-life and y-ra-y energies of 158 keV

(37 percent abundant) and 208 keV (8 percent abundant) which

are more suitable parameters for the detection.

3. Instrumentation

Pneumatic tube RT-3 of the NBS Reactor, large volume

Ge(Li) detector coupled to a multi-channel analyzer (see

Appendices 4 and 5)

.

4. Reagents

Pt standard: NBS SRM 680 - 1 mg is dissolved in a minimum volume of high purity aqua regia (hot)

Aqua regia: I+4+I-HNO3 :HC1 rHaO

Pt carrier: 5 mg Pt/ml

Na carrier: 1 mg Na/ml

Ca carrier: 1 mg Ca/ml

Silver: metal powder

5. Sample Preparation and Analysis

Approximately 250 mg of the sample is weighed into a

clean polyethylene micro vial and heat sealed. The platinum

standard solution is treated in the same manner. Sample and

standard are placed in an irradiation "rabbit" and irradiated

for one hour in the NBS Reactor in a thermal neutron flux of

'\'6xl0 ^ ^n* cm ^'S ^. After a 24 hour decay period to reduce

short half-life matrix radioactivities, the samples are

opened and transferred to 100 ml beakers containing carriers

of elements of interest, and dissolved in aqua regia. After

dissolution and evaporation to 2 ml the samples are washed

into a 50 ml centrifuge tube with 24 ml of H2O which contains

finely divided silver powder. The mixture is shaken to

allow for spontaneous reduction of the platinum. The mixture is filtered through a glass fiber-filter pad to retain

the silver metal powder upon which the platinum had been

13
deposited. The filter pad is mounted and counted in a standard geometry. The platinum standard solution is treated in

the same manner as the sample.

6. Procedure

Transfer each irradiated material to a 100 ml beaker.

Add 1 ml of Pt, Na, and Ca carriers. Add 10 ml of aqua

regia and heat until the sample has dissolved and the volume

has been reduced to 2 ml.

Let the sample cool. Place 50 mg of finely divided

silver powder into a 50 ml round bottom centrifuge tube

(with polyethylene- lined cap) and add 24 ml of H2O.

Transfer the sample solution to the centrifuge tube and

shake for 10 minutes.

Filter the mixture through a 25 mm glass -fiber filter

pad. The silver metal powder containing the platinum is

retained on the filter pad. Discard the solution.

Place the filter pad in a 45 mm petri dish and count on

a Ge(Li) detection system.

Remarks

This procedure is rapid, quantitative and selective for

platinum. The sodium and calcium carriers are needed as

hold-back carriers for the decontamination of the Pt from

the matrix. Both photopeaks of ^ ^ ^Au (158 keV and 208 keV)

can be used individually for quantitation.

Calculation

If the induced activities are measured in an identical

manner, comparing counting rates in the sample and standard

will give the weight of the desired element in the sample by

the following simple relation:

14
Total cts/min Standard

Total cts/min Sample

wt (pg) Standard

wt (yg) Sample

2.5x10'* cpm

5x10^* cpm

1 yg

X

X

2 yg

(yg element)

C. Determination of Vanadium and Manganese in

Biological and Botanical Materials by

Neutron Activation Analysis.

S. H. Harrison

1. Objective

Determination of vanadium and manganese at yg/g levels

in biological and botanical materials using Instrumental

Neutron Activation Analysis (INAA)

.

2. Principle

Instrumental Neutron Activation Analysis is applied to

samples for vanadium and manganese determinations utilizing

the thermal neutron reactions ^^V(n,Y)^^V and ^ ^Mn (n , y) ^ ^Mn

.

Measurement of the radiation emitted and comparison to a

known standard made radioactive under the same conditions

enable quantitative determination of the vanadium and

manganese in the sample.

3. Instrumentation

For instrumental analysis of complex matrices such as

biological or botanical materials, a high resolution Ge(Li)

semiconductor detector, preferably a resolution less than

2.2 keV for the 1332 keV gamma of ^"Co is used in conjunction

with a multichannel analyzer of at least 2048 channels.

15
4. Reagents

The standard solutions are made of high purity metals

or oxides dissolved in purified reagents (see Appendix 1)

.

Distilled Water

Nitric acid, diluted 50 percent for dissolution of

standards (V)

.

Acetic acid, reagent grade, diluted 50 percent for

dissolution of standards (Mn)

.

5. Sample Preparation and Analysis

For this INAA procedure, careful sample preparation is

important. Precleaned plastic containers and instruments

are used whenever possible for sample handling, but nickel

spatulas and stainless steel knives are acceptable.

A sample size of about 300 mg of dried tissue is adequate for analyses. For irradiation, the sample is pelletized to provide a small uniform sample. This pellet is

then sealed in a small precleaned polyethylene bag, and the

sample is sealed in a second bag for double encapsulation.

Primary standards for vanadium and manganese analyses

are made by dissolving pure metals in a suitable acid

medium. Solution standards are prepared to approximate the

tissue sample sizes.

Tissues are irradiated for about five minutes in a flux

of 1x10^ ^n*cm ^s ^. After irradiation, the tissue samples

are transferred to new containers for counting.

After decay times of from 2 to 12 minutes the samples

are counted for five minutes live time ( e.g. for a matrix

such as sediment it may be necessary to wait longer for some

of the ^^Al activity to decay). Since ^^V decays with a

half-life of 3.75 minutes, it is helpful if the total

activity and decay rate of the standard approximate that of

the sample.

16
Manganese can be determined in the same count as

vanadium, however, a lower uncertainty may be obtained if a

second count is taken after short-lived isotopes have decayed

away. Thirty minutes to two hours is a reasonable decay

time for this second count.

6. Procedure

The sample- standard comparator technique is used for

quantitative determination of vanadium and manganese. The

predominante y-ray emitted by ^^V has an energy of 1434 keV.

There are no commonly observed interferences at this energy.

The predominante y-ray for ^^Mn has an energy of 847 keV.

The half -life of ^^Mn is 154.6 minutes. There is. a commonly

observed y-i^ay interference with the 847 keV peak. This

interference is caused by the 843 keV y-ray of ^^Mg, which

has a half -life of 9.46 minutes. Two choices are available

for avoiding this interference: The second most prominent

y (1811 keV) of ^^Mn may be used for quantification or one

can wait until the 9.46-minute ^^Mg has decayed. In using

the second alternative, it would be impossible to determine

V and Mn in the same count.

7. Calculation of V in Tissue

Vanadium was chosen for the calculation example because

of the special problems in measuring short-lived isotopes.

The commonly used activation analysis equation is:

A = A e

o

-Xt

(1)

where X = the decay constant,

= the decay time,

the activity of the isotope at that decay

t

A

time and

A

o

the activity of the isotope when t = o.

17
When using this equation, A is assumed to be equal to A,

the average activity of the isotope during the live time

counting interval, At^. This is a valid assumption only

At

when < 0.5 (Tj is the half-life of the isotope of interest)

In the case of vanadium, At« = 5 min. Tj = 3.75 min, and

At I

= 1.33 not less than 0.5. Therefore, for the calculation

of vanadium, it is necessary to use the following equation

XAAt

-Xti -Xt2

e -e

(2)

where ti

t2

At

the decay time at the start of the count,

the decay time at the end of the count and

the clock time counting interval.

Example

Sample wt

Standard wt

297.7 mg sample

0.4022 mg V/g soln x 0.1000 g

soln = 0.04022 mgV

A (sample)

ti (sample)

1 2 ( s amp 1 e

)

At^ (sample)

A (sample) /mg sediment

1965 cts/min

12.00 min

17.80 min

5.80 min = clock time counting

interval

,

6.601 cts/mg sediment min

A (standard)

ti (standard) =

^2 (standard)

At^ (standard) =

"K (standard) /mg V =

12,867 cts/min

12.00 min

17.77 min

5.77 min = clock time counting

interval

.

3.199x10^ cts/mg V min

18
Tj C^^V) = 3.75 min

At

> 0.5, therefore equation 2 is used to calculate A

XAAt^^ ,595

^0 = — —where X = 3.75 ^lin -Ati -At?

e -e

After substituting the time and activity parameters in

equation 2 for the sample and then the standard, the folflowing values for A^ are obtained:

A^ (sample) /mg sample = 98.82

Aq (standard) /mg V = 4.777x10^

A (sample) 98.8 2 cts/mg sed. — = = 2 . 068x10"^ mg V/mg

A^ (standard) 4.777x10^ cts/mg V

sample or cone. = 20.68 yg V/g sample.

19
D. Radiochemical Separation for the Determination of Mercury

and SElenium in Biological and Botanical Materials

Using Neutron Activation Analysis.

H. L. Rook, T. E. Gills and G. J. Lutz

1. Objective

The objective is to determine mercury and selenium at

concentration levels as low as 10 ppb using neutron activation coupled to a selective radiochemical separation

procedure.

2. Principle

The principle of the procedure is the irradiation of

an unknown sample along with standards in an intense thermal

neutron flux for a period of several hours inducing the

reaction ^ ^ ^Hg(n, y) ^ ^ ^Hg and ^ "^Se (n , y) ^ ^Se . The mercury and

selenium activities are subsequently separated from the

matrix radioactivities by combusting the sample in a stream

of oxygen and trapping the volatile elements in a liquid

nitrogen cooled trap.

3. Instrumentation Apparatus and Reagents

Instrumentation and apparatus required include: a tube

combustion furnace, a quartz tube combustion assembly, a

high resolution gamma-ray detector, a pulse height analyzer,

precipitation and filtering equipment. For further details

see Appendix 4 and 5.

Reagents required are tank oxygen, mercury oxide, concentrated nitric acid, ammonium hydroxide, liquid nitrogen,

and thioacetamide . The chemicals are of reagent grade.

4. Procedure

The samples are weighed and encapsulated in quartz

tubes for irradiation. If expected levels of mercury are

very low, encapsulation is done in a Class 100 clean

20
room.^ The quartz encapsulated samples are subsequently

packaged along with encapsulations of mercury solution standards in polyethylene rabbits and irradiated in the NBS

Reactor in a flux of 6x10^ ^n/cm^ s for approximately six hours.

After irradiation, the individual quartz ampoules are rinsed

with cone. HNO3 and water. The samples are then immersed in

liquid nitrogen for a few minutes and opened by scoring the

vial with a file and pressing at the scratch mark. The

samples are then transferred along with approximately ten

milligrams of HgO carrier into porcelain combustion boats

(1x10 cm) . The combustion boat is then inserted into the

combustion assembly. An oxygen stream of 20 cm^/min is

allowed to flow over the sample. If the sample is a biological material, it is allowed to burn freely after igniting

with a torch. After combustion, the tube is heated to about

800 °C for 20 minutes. Portions of the tube not covered by

the furnace are flamed down with a gas oxygen torch to

further distill mercury into a liquid nitrogen cooled condenser. The assembly is then allowed to cool and the distilled

mercury in the condenser is transferred with nitric acid and

water into a 4-oz polyethylene bottle. The solution containing the mercury and selenium is diluted to a selected

standard volume and shaken to homogenize the solution.

Standards can be carried through the procedure or an aliquot

can be transferred to a 4-oz bottle and diluted.

All samples and standard solutions are counted on a

Ge(Li) detector in conjunction with a multichannel pulse

height analyzer. The 68 and 77 keV peaks of ^^^Hg and the

265 keV line of ^^Se are used in the evaluation.

An improvement in sensitivity can be achieved by reducing the volume of the counting sample by precipitating the

mercury as a sulfide. Selenium coprecipitates with the HgS

.

^ See Appendix 2.

21
Bromine-82 can be removed during the combustion step

with a heated silver wool trap. Silver wool also removes

selenium so the procedure is applicable when only mercury is

to be determined. Bromine-82 can also be removed during the

precipitation step by the addition of hold-back carrier.

5. Calculation

The concentration o£ mercury and selenium is calculated

using the standard-comparator method:

Total cts/min Standard

Total cts/min Sample

Wt (yg) Standard

Wt (yg) Sample

Example

StandardCounts per min 9068

Wt (yg) 1.53

SampleCounts per min = 1831

Wt (yg) X

9068 cts/min

1831 cts/min

1. 53

X

X .31 yg

Note: For further information see Appendix 6.

22
E. Determination of Chromium in Biological and Botanical

Materials Using Neutron Activation Analysis.

L. T. McClendon

1. Objective

The objective is to provide a simple and accurate procedure for determining chromium in biological and botanical

materials using neutron activation analysis.

2. Principle

This procedure is designed for a rapid, selective determination of trace quantities of chromium in tissues, by

separating it from the matrix activity produced when these

materials have been irradiated in a nuclear reactor. Experiments have shown that a volatile form of chromium is present

in some tissues so that dissolution of the sample in a

closed system and entrapment of the vapor fumes in a HNO3 or

HCl solution is recommended.

3. Instrumentation

Pneumatic tube RT-3 or RT-4 of the NBS Reactor, Ge(Li)

detector system or NaT detector system. Further details

will be found in Appendices 4 and 5.

4. Reagents

Cr standard: 0.5000 g Cr metal (99.91 pure) dissolved in high purity HCl and diluted

to 1000 ml with high purity H2O.

Cr carrier: 1 mg Cr/ml, (CrCls'lO H2O reagent

grade) dissolved in HCl, 1.5M.

High Purity

Cone . , 6M

70-72%

101 (w/v) in 3M HzSOi^

H2O:

HNO3 :

HClOi^

Ce2 (S0^) 3 :

23
Tribenzylamine: 1% (w/v) in CHCI3

NBS SRM: Orchard Leaves (1571).

5. Sample Preparation and Analysis

Approximately 250 mg of the sample is placed in a clean

quartz vial or polyethylene tubing and sealed. The chromium

standard solution and Orchard Leaves ('^^250 mg) are treated

similarly. Samples and standards are placed in an irradiation "rabbit" and irradiated for an appropriate period of

time (1 to 6 h) in RT-3 or RT-4 of the NBS Reactor. After a

sufficient decay period is allowed to minimize some of the

matrix activity, the sample container is washed with 1:1

HNO3 to remove any exterior contamination. If Cr is >_2 yg,

the Cr concentration can be determined instrumentally without

chemical treatment. If the concentration is below 1 ppm a

radiochemical separation may be needed. Thus the sample is

transferred to a 50-ml Erlenmeyer flask for "open" or "closed"

flask dissolution. The chromium standard solution and comparative standards (Orchard Leaves, Coal, etc.) are treated

in the same manner as samples.

6. Procedures

a. Instrumental Analysis

If the sample and standards were preweighed into a container that was suitable for both irradiation and counting,

the following operation may be omitted. Weigh the sample

into a suitable counting vessel. Calibrate the detection

system for assaying the 320 keV gamma ray emitted by Cr-51

and count sample for a suitable time period.

b. Radiochemical Separation

Weigh the sample into a 50 ml boiling or distilling

flask. Add 1 ml of Cr carrier, 5 ml cone. HNO3 and 3 ml of

cone. HClOit. Place on hot plate and heat using a medium

24
temperature setting until sample has dissolved and Cr^^ has

been oxidized to Cr^^. To the hot solution add 4 ml of hot

eerie sulfate solution and heat for five minutes. Let

sample cool. Transfer the sample to a 50-ml extraction tube

equipped with a polyethylene - lined top using 15 ml of 1 . 5M

HCl. Mix well. Add 10 ml of tr ibenzylamine solution to the

extraction vessel. Agitate for five minutes. Let stand

until phases have separated, take a 5-ml aliquot of the

organic phase and count on a gamma detection system. Transfer

the trap solution to a suitable counting vessel and count if

a "closed system dissolution" was used.

Remarks

This solvent extraction procedure is selective for Cr VI

which makes it advantageous to dissolve the sample in an

oxidizing media. Ceric sulfate is added to the dissolved

sample to maintain the chromium in solution as Cr VI. If

gold, antimony, or mercury are present in large quantities,

they may also extract under the conditions described. The

gamma-ray energies of these three elements are sufficiently

different from the 320 keV gamma ray emitted by Cr-51 so

that no interference will occur.

Calculation

The amount of chromium in a sample is calculated using

the comparator technique, since the measured counting rate

using a standard measuring set-up is proportional to the

total activity:

Wt (yg/g) of Cr in Sample _ Counts/min of Cr in Sample

Wt (yg/g) of Cr in Standard Counts/min of Cr in Standard

25
Example

Cr in Orchard Leaves (O.L.)

1 yg o£ Cr standard yields 10.4x10^ cpm;

count rate of Cr in 1 g of O.L. yields 26.8x10^ cpm.

Cr in 1 g O.L.

1 yg

yg Cr in O.L.

Cr in O.L.

26.8x10^ cpm

10.4x10^ cpm

26.8x10^ cpm

10.4x10^ cpm

2.577 yg/ g sample

.

F. Rapid, Quantitative Separation for the Determination

of Selenium Using Neutron Activation Analysis.

H. L. Rook

1. Objective

The analytical procedure described in this section was

developed to permit the quantitative determination of selenium

in biological and botanical materials, at the part per

million and sub-part per million levels, using neutron

activation analysis (NAA) and with a minimum of chemical

manipulations. The physical separation method used minimizes

the errors associated with chemical separations, and, in

addition, permits the simultaneous determination of mercury.

2. Principle

The method is based on the irradiation of the analytical

sample in a nuclear reactor followed by the separation of

The material used in this section was taken from a work

published by the author in Analytical Chemistry, Vol. 44,

No. 7, p. 1276 to 1278 (1972)

.

26
selenium by thermal volatilization in an oxygen stream. The

selenium separated in this manner is transferred quantitatively to an adequate container, and the activity o£ the

sample is determined by counting with a Ge(Li) detector and

2048-channel pulse-height analyzer.

3. Procedure

Apparatus. The sample combustion train (Figure 1) was

made from an outer 19/38 standard taper Vycor^ joint which

had been extended to a total length of 30 cm. The condenser

was made from an inner 19/38 standard taper Vycor joint,

extended to 25 cm, which was inserted through a liquid

nitrogen cold trap made from a small polyurethane acid

shipping container.

FLOWMETER

GAS FLOW

VALVE

OXYGEN

INLET

SAMPLE \

^QUARTZ'

WOOL

^ JOINT

Figure 1. Sample Combustion Tube and Mercury Trap.

Reagents. Standard solutions were made by dissolving a

weighed quantity of high purity elemental selenium in concentrated high purity nitric acid and diluting to volume with

^ In order to specify procedures adequately, it has been

necessary to identify commercial materials in this report.

In no case does such identification imply recommendation

or endorsement by the National Bureau of Standards, nor

does it imply that the material identified is necessarily

the best available for the purpose.

27
distilled water. The selenium concentration used was 0.5910

mg/ml , which was dilute enough to prevent any significant

neutron self -adsorption, but large enough to negate loss due

to wall adsorption of selenium in the storage container.

Procedure. For analysis, 0.5-gram samples were encapsulated in cleaned quartz tubes. Standard solutions were

encapsulated in a like manner. Five sample tubes were

arranged in a concentric ring with a standard in the center

in a polyethylene rabbit and irradiated in pneumatic transfer

facility RT-3 of the National Bureau of Standards Reactor [1]

for two to six hours at a thermal neutron flux of approximately 6x10^ ^n* cm ^'s ^. Flux monitors were not used, as the

radial flux variation is less than ±2 percent in RT-3. The

samples were allowed to decay 5 days to minimize personnel

radiation exposures.

The quartz tubes were washed in 1:1 nitric acid, cooled

to liquid nitrogen temperature, and broken ^^^5 mm from the

top. The samples were weighed into a ceramic combustion

boat, and 2.5-mg of mercuric oxide carrier added. Since

selenium oxides are difficult to dissolve in mineral acids,

inactive selenium carrier was not used. Mercury carrier was

adequate to eliminate adsorption of selenium on the walls of

the Vycor condenser and polyethylene counting bottle through

the formation of mercury selenide. This was readily soluble

in nitric acid, enabling the dissolution procedure developed

for the mercury separation to be used. The boat was inserted

into the combustion chamber, an oxygen flow of '^30 cc/min

was passed over the sample and the sample was then ignited

with an oxygen-gas torch. The sample was allowed to burn

freely with the downstream quartz wool plug heated to red

heat to ensure complete combustion of volatile hydrocarbons

and carbon produced during the sample burn. After combustion

ceased, the ash was heated to approximately 1000 °C for five

minutes and the entire combustion tube heated down to the

28
standard taper joint to drive all volatile compounds of the

sample into the condenser.

The combustion tube was allowed to cool for five minutes,

the liquid nitrogen cold trap was removed, and the condenser

was placed joint-end down into a 4-oz polyethylene bottle.

The condenser was warmed to room temperature, and the selenium and mercury carrier washed into the bottle with 10 ml

of concentrated nitric acid and three 10-ml portions of distilled water. The total volume of solution was adjusted to

50 ml and the sample was counted using a 60-cc Ge(Li) detector

in conjunction with a 2048-channel pulse-height analyzer.

The 265 keV gamma ray of selenium-75 was used for the determination, as the 136 keV gamma ray suffers from interferences

from mercury-197m.

After irradiation, the standard was frozen in liquid

nitrogen, broken in a beaker, allowed to melt, and decanted

into a 4-oz polyethylene bottle. The broken quartz vial and

beaker were washed with six 5-ml portions of distilled water

and the final volume was adjusted to 50 ml. The specific

activity at the end of the irradiation was compared to that

of the sample for quantitative results.

4. Discussion

In this work, the method for the separation of volatile

elements, originally developed for the determination of

mercury, was studied and adapted to the selenium problem [2].

Four significant improvements over currently used procedures

have been attained: the procedure has a minimum of chemical

manipulations, thereby minimizing technique related errors;

the radio-selenium separation is quantitative, thus eliminating errors involved in the determination of chemical yields;

the procedure is based on a physical, rather than a chemical

separation allowing the same technique to be used on a wide

variety of matrices; and the determination of mercury may be

conducted simultaneously using this separation procedure.

29
The method was tested using radiotracer experiments and

excellent agreement was obtained by independent analysis

using isotope-dilution spark-source mass spectrometry. The

procedure has been subsequently used to determine the selenium concentrations of three new Standard Reference Materials

being issued by the National Bureau of Standards.

Results

The method was tested for complete recovery of selenium

by burning non- irradiated samples of biological material

with known quantities of selenium-75 nitrate solutions

added. The recovery selenium-75 was brought to 50-ml volume

and compared to the activity of a like quantity of tracer

diluted directly to 50-ml volume and counted in a standard

configuration. The results of these tests indicated a

complete recovery of selenium-75 tracer. A total of five

separations was carried out with a mean recovery of 99.1

percent and a relative standard deviation of 1.1 percent.

Using the analytical procedure developed, the selenium

concentration in three new Standard Reference Materials

(SRM) being issued by the National Bureau of Standards was

determined. These materials are:

The method described was used to determine the selenium

contents in these materials with an accuracy in the mean

results of better than ±10 percent relative at the 95 percent

confidence level. The results were in excellent agreement

with independent analyses using isotope-dilution sparksource mass spectrometry. The individual results are given

in Table I.

SRM 1630

SRM 1577

Mercury in Coal

Bovine Liver

SRM 1571 Orchard Leaves

30
Table I. Selenium contents of Standard Reference

Materials, ppm.

Results Obtained by Neutron Activation Analysis Using the

Combustion Technique

SRM 1571

Orchard Leaves

SRM 1577

Bovine Liver

SRM 1630

Mercury

in Coal

Mean

2s

0.079 1.08 2.16

0.084 1.05 2.03

0.083 1.16 2.14

0.075 1.13 2.08

0.074 1.12 2.08

0.069 1.16 2.05

0.085 1.17 2.14

0.075 1.11

0.087

0.079 1.12 2.11

±0.012 ±0.08 0.09

Results Obtained by Isotope-Dilution Spark-Source

Mass Spectrometry

0.08 ± 0.02 1.11 ± 0.04 N.D.

Not determined.

The results indicate that the described separation

affords a degree of confidence in the final results not obtainable using the nondestructive activation analysis technique.

Yet with the simple one-step combustion separation, samples

may be processed in a minimum of time (approximately 15

minutes each), maintaining the process-time advantage of

nondestructive activation analysis when large numbers of

samples must be analyzed. The technique has the added

31
advantage that the mercury content, which is often of great

interest in biological materials, may be determined simultaneously .

5. References

[1] LaFleur, P. D. and Becker, D. A., Eds., NBS (U.S.)

Tech Note 548, 10 (1970)

.

[2] Rook, H. L., Gills, T. E., and LaFleur, P. D. , Anal.

Chem. 44, 1114 (1972).

32
SECTION III

SPARK SOURCE MASS SPECTROMETRIC ISOTOPE DILUTION

Determination of Cadmium, Copper, Iron, Lead, Mercury,

Molybdenum, Nickel, Selenium, Silver, Tellurium,

Thallium, and Zinc.

P. J. Paulsen

1. Introduction

The spark source mass spectrometer (SSMS) can detect

all elements in the periodic table at detection limits down

to a few ng/g. In actual samples there will be interferences

between elements, however all elements are inherently detectable with similar sensitivity. Quantitative results

will require concentrations significantly higher than these

"detection limit" values.

Isotope dilution (ID) analysis can, in theory, be

applied to any element having two or more stable isotopes.

If one considers the various practical aspects of SSMS

analysis, approximately 50 elements are amenable to isotope

dilution SSMS analysis. Our laboratory has had occasion to

perform SSMS ID analysis on a total of 36 different elements

and a wide variety of matrices during the last 10 years.

Isotope dilution SSMS has multi-element capabilities.

As many as 28 elements have been measured simultaneously by

isotope dilution in a single sample while doing a survey

type analysis (designed for low detection limits in pure

materials, but not designed for the best precision). In a

precise quantitative analysis of a NBS SRM for certification,

up to 11 elements have been determined simultaneously from a

single sample.

Using the element copper as an example, the general

procedure for mass spectrometric isotope dilution analysis

is as follows:

33
1. A known amount of ^^Cu isotope (spike) is added to

a weighed amount o£ sample to be analyzed.

2. The ^^Cu spike is equilibrated (mixed) physically

and chemically with the natural copper in the

sample. Equilibration is usually obtained by dissolution of the sample in the appropriate acids.

3. The equilibrated copper is separated from the sample

in a form suitable for mass spectrometr ic measurements .

4. The altered ^^Cu/^^Cu isotopic ratio is measured

mass spectrometr ically.

The concentration of copper is then calculated using the

formula

where

ppm (by weight) WK

M

(A

W =

K =

R = A and B

sp sp

A and B

(BR

B R)

Spike weight in yg ; M = sample weight

in grams

Ratio of natural atomic weight to atomic

weight of spike

Measured altered isotopic ratio a/b

Abundances of isotopes a and b in spike

Abundances of isotopes a and b in natural

From this equation it is seen that it is the ratio (R) of the

two Cu isotopes that is required to calculate the copper concentration. The total amount of copper is not measured. It

is therefore not necessary to either recover 100 percent of

the copper in the sample or know recovery efficiency in order

to obtain quantitative results. Therein lies the biggest

advantage of using isotope dilution analysis.

34
Once equilibrium is obtained between the "spike" and

"natural" isotopes o£ an element, procedures for separation

can be adopted which optimize such factors as group separations, low blanks, clean separations, etc., without the

restraint of having a controlled, known recovery. Efficient

recovery methods are, of course, desirable since the sensitivity of the method is dependent on recovery.

2. General Considerations for Multi-Element SSMS Isotope

Dilution

a. Concentration Range Covered

Quantitative analysis good to about ±5 percent can be

obtained if from 1x10 ^ to 1x10 g of an element can be

loaded into the instrument for sparking. Smaller amounts

have been measured on occasion, however, the precision suffers proportionally as the concentration is lowered. For a

1-g sample this gives quantitative values at the 1 to 0.1

ppm level. Lower concentrations can be determined at this

precision if the sample size is increased, however, in any

system there is a limit to either the sample available or

the amount of the sample that can be effectively handled.

For some of the more common elements the system blank may

be the limiting factor and it may increase with the sample

size depending on its source. There is no upper concentration limit on the use of isotope dilution SSMS, however at

high concentrations there are usually other techniques of

analysis which are more cost effective and more precise than

the ±5 percent obtained with SSMS photoplate detection. Because of the value of the separated isotopes ($1 to $10/mg

+ time involved in preparation) , spikes are held to less than

1 mg. Samples with high concentrations are dissolved, aliquoted and then spiked in order to conserve spikes.

35
b. Spike Levels

A given amount of spike will cover only a limited range

of concentrations, the range will depend on the natural isotopic ratio, the purity of the separated "spike" isotope and

the precision and accuracy with which the altered isotopic

ratio can be measured. A valid analysis cannot be performed

when the measured altered isotopic ratio differs from either

the natural or "spike" ratio by only the measurement uncertainty (± percent standard deviation or ± accuracy) . With

photographic plate detection the plate uniformity limits

precisions to ±5 percent and accuracy (for ratios ^1) depends

on obtaining a good plate calibration curve.

The best accuracy with the photoplate is usually

obtained if the amount of spike added is adjusted to give a

measured isotopic ratio of 1 (R=2 if the natural ratio =1)

.

This procedure minimizes the errors associated with photoplate

calibration and also gives the maximum number of measurable

ratios for a given situation. If electronic detection SSMS

is used, the accuracy is not dependent on having R=l and a

wider concentration range can be covered with a given spike.

The effective range is still limited by the measurement

precision and the isotopic ratio of the spike and natural

element

.

c. Preconcentration

The need for obtaining physical and chemical mixing of

the "spike" and natural isotopes for a solid sample is met

by dissolving the sample in acids. This step both dilutes

the sample and adds extraneous anions. The high absolute

sensitivity of the SSMS makes it impractical to consume more

than a total of about 10 mg of material in a normal analysis.

Some form of preconcentration is therefore usually necessary

in order to reach this consumable size and hence obtain

maximum sensitivity. The ideal preconcentration procedure

36
would yield only those elements to be analyzed and no others,

This would give the best analytical sensitivity while minimizing interference problems. The presence of a large

excess of an element not being analyzed in the SSMS sample

can cause interference problems. This is especially true if

it is an unwanted cation which carries along an equivalent

anion from the acids used in dissolution - double trouble.

Isotope dilution SSMS is a practical method mainly because a number of elements can be determined simultaneously.

This means that any preconcentration method used needs to

provide group separations. In some types of samples this

can be obtained simply by excluding the major components of

the sample. A number of preconcentration procedures have

been used for SSMS isotope dilution. These include ion exchange, solvent extraction, electrodeposition, precipitation

with and without carriers, and high temperature volatilization. Electrodeposition onto high purity gold wires (photoplate detection) is an especially attractive method since it

has a very high rejection factor for organics and anions

originating from the acids used in sample dissolution.

The SSMS readily detects organic molecules. If organic

material from either the sample or the reagents used in preconcentration is present in the sample loaded into the SSMS,

interferences can result.

d. Interferences

As in all analytical methods it is the identification

of interferences that enables the proper action to be taken

which avoids errors in analysis. There are three general

types of interferences in SSMS ID analysis.

1. Isobars - adjacent Z elements with isotopes

having the same nominal masses: Interferences of this type

can always be identified and it occurs with only a few elements. In some cases proper selection of the isotopes

measured may minimize the magnitude of the interference,
and/or allow correction for the interference. Chemical preconcentration procedures could also be used to remove the

undesired element. In any case, this type of interference

could not go undetected.

2. Multicharged ions of high mass falling at the

same nominal mass as the singly charged ion of the desired

element: The SSMS disperses ions according to their mass to

charge ratio (m/e). The energy available for ionization in

the spark is so high that all elements can be multiply ionized. This can result in interferences when two elements of

different masses and different charge states have the same

nominal mass to charge ratio. The relative intensities of

the +1, +2, +3, etc., ions of an element follows a predictable pattern (intensity drops off by a factor of ^^^3 for each

additional charge) . This type of interference is therefore

readily recognized by a visual examination of the photoplate.

Again, in many instances, interferences can be avoided or

corrected for by measuring the proper isotopes of both elements. In some cases the interference can be further minimized or avoided if a multiply charged ion of the desired

element can be measured. Again this type of interference

would not go undetected.

3. Interference by compounds of the same nominal

mass as the element being analyzed: Hydrocarbons fit this

classification. The resolution of any SSMS using photoplate

detection is adequate to resolve and identify the presence

of a hydrocarbon line with the same nominal mass as an

element line. Quantitative measurements can be made on the

resolved element lines providing the hydrocarbon line is not

too intense. In this case the sample preconcentration

method could be modified to lower hydrocarbons or in some

situations (explained later) the interference can be avoided

by measuring a multiply charged ion of the element.

38
Other types o£ compound lines will usually not have a

large enough mass difference from an element of the same nominal mass to be resolved into separate lines. The major difficulty with this type of interference is that the intensity of

compound interferences cannot be as easily predicted from other

information on the photographic plate. However, since the

SSMS can detect all elements there is good information on what

elements are available as starting material to form the compounds. The stability of a compound in the form of a positive

ion (the species being detected) under the conditions that

exist in the spark source is very small. It is only the extremely high sensitivity of the SSMS combined with the presence

of elements at high concentrations that allows the detection

of such ions. Under most conditions these interferences occur

only for elements being determined in the low ppm range.

When checking for potential interferences a list of

possible compounds can be obtained by following these general rules

1. Consider only the major elements (as detected by

SSMS)

.

2. All samples, regardless of source, have adequate

0, H, (and possibly carbon) available for forming

compounds

.

3. Anions of the acid used in sample dissolution can

appear in all possible forms, i.e. , ClOit gives Cl^,

+ + + CIO , CIO2 , CIO3, plus "cation" combinations thereof.

4. Group la, lb, 2a, and 2b elements have a tendency

to form dimers , trimers , etc., and these can also

pick up oxygen and hydrogen, or anion fragments.

5. The compounds that cause interferences need to exist

as ions for less than 1 ys; during their lifetime

(as a detected ion) no collisions occur with other

ions, molecules, or any surface. The combinations

that will occur therefore may appear to be strange

when viewed from a classical chemical background.

39
The presence of compounds can be detected by their

generation of abnormal isotopic ratios and of lines which

do not correspond to any element lines. If the compound has

an element with more than one isotope it may allow corrections to be made. Abnormal lines that can be identified

give good clues to the possibility of a related compound.

For instance, if a sample (with high NaC10i+ content) shows

lines which are identified as NaClot then the spectrum

should also be checked for NaClot, NaClO"^, Clot, Clot, etc.

One approach to this type of interference problem is to

measure the isotopic ratios for all elements being analyzed

in an unspiked sample. If the ratios found are the natural

ratios of the elements it can be assumed that no interferences exist. This approach requires good control over the

sample preconcentration process.

There is a simple method of avoiding errors of analysis

from compound interference that will apply to most but not

all elements. The method is to measure the isotopic ratios

for elements on a +2, +3, or +4 ion instead of the +1 ion.

The stability of a compound with a +2 or +3 charge on it is

so low that such species cannot be detected. The +2 and +3

ions of an element on the other hand are only a factor of

^^^3 and '^^lO times less intense than the +1 ion.

There will be a loss in sensitivity in such an analysis

and the method cannot be used much below 1 yg. The precision of isotope ratio measurements using the photoplate

is the same for all charge states (limited only by plate

uniformity) so the precision does not suffer. There is one

major qualification that prevents use of multiply charged

ions for all elements. Both measured isotopes of the multiply charged ion of the element must fall at fractional mass/

charge (m/e) values if that charge state is to be usable.

This is necessary since the unit m/e positions are very

likely to have either a +1 element line or possibly a

40
compound line. This is especially true at the lower m/

e

region of the plate where most of these types of measurements would be made.

Interference by compound lines is usually only significant in the analysis of low ppm concentration elements.

The interference can be minimized by keeping elements not

being analyzed out of the sample, and interference can be

eliminated if a multiply charged ion can be measured. The

major problem is being able to spot the existence of this

type of interference.

3. Analysis of Organic Materials Using SSMS Isotope

Dilution

Most of the SSMS ID analysis at NBS has been for the

certification of Standard Reference Materials (SRM's). The

procedures adopted have emphasized accuracy and precision

over other factors such as speed and turn around time.

Since a wide range of analytical techniques are applied to a

SRM certification, the SSMS ID analysis stressed only those

elements where it would be most effective. In general, most

elements above 100 to 500 ppm would be analyzed by other

methods to a better precision than the ±5 percent obtained

by SSMS photoplate detection. More elements could have

been analyzed by SSMS ID than were reported if it had been

necessary

.

The organic materials (not all SRM's) that have been

analyzed by SSMS ID are: orchard leaves, beef liver, coal,

tomato leaves, pine needles, spinach, tuna fish, urine,

wheat, rice, fuel oil, and river bottom extract. The elements determined for NBS organic SRM certification are: Pb

,

Tl, Hg, Te, Cd, Ag, Mo, Se, Zn, Cu, Ni, and Fe. A total of

35 elements have been analyzed for SRM certification if

other types of SRM's are included.

41
The chemical preconcentration methods used in the

analysis of the organic materials include: electrodepos i

tion onto high purity gold wires, ion exchange, vaporization

of volatile elements, and co-precipitation.

Our analysis of coal, beef liver, and spinach SRM's

will be used as examples for SSMS ID analysis.

A detailed description of the instrumentation and techniques used to perform the actual analysis by SSMS ID will

be found in the four papers reproduced in the Appendices 7

through 10. Although the chemical species discussed in

those papers are contained in a metallic matrix, the same

analytical procedures are applicable to organic matrices.

4. Preliminary Analysis

The preliminary analysis serves two functions in a SRM

analysis. First, several group separation techniques can be

tested to see which system recovers the greatest number of

elements of interest. Second, the concentration levels of

the different elements needs to be determined in order to

adjust the spike level for each element for optimum precision

and accuracy in the actual analysis. If no information is

available on the sample, it is spiked with 100 ppm, 10 ppm,

and 1 ppm of a master spike solution containing "spikes" of

28 elements. This range of spike levels can be obtained by

changing both sample and spike amounts, but it is best to

keep the spike above 1 yg. Analysis of these three samples

would give reasonably accurate results over a concentration

range from '^^500 ppm down to '^^O.l ppm.

If an element not in the spike is detected, its concentration can be estimated by using one of 28 spiked elements

as an internal standard. In all of the SSMS ID analyses

performed to date the preconcentration procedure has been

selected to give the maximum number of desired elements from

a single sample processing. Two independent procedures (two

SSMS samples) would have doubled both the chemical and the

42
SSMS workload but would not have doubled the number of

elements determined.

What has been just referred to as a preliminary analysis for our NBS SRM SSMS ID analysis would, in fact, provide

adequate accuracy and precision for "final" analysis of many

types of analyses. Such a system would be much more efficient than the one we feel obligated to use on NBS SRM's.

5. Procedure Used for Analysis of Coal, Beef Liver, and

Spinach Spiking

Weighed samples of the organic material were spiked

with proper amounts of each element's "spike" in order to

obtain an equilibrated ratio (R) of 1. Silver has a natural

ratio of 1, it is spiked to give a ratio of 2.

The amounts of spike required were computed using the

results of the preliminary analysis. The spike elements can

be added one element at a time, however, our preferred

method is to mix all the spikes together in the proper proportion and then make just one addition. In either case,

spike calibration samples are prepared at the same time

using the same pipet setting and spike solutions as used on

the samples. Known amounts of the pure natural elements are

also added to these spike calibration samples. The natural

solutions are obtained by quantitative dilutions of a solution prepared from a known amount of the pure natural

element. SSMS analysis of the spike calibration sample

gives an accurate determination of the amount of spike

actually added to the samples.

"Blanks" are also spiked at the same time as the samples

using only 5 to 10 percent as much spike as was used on the

sample (but not less than 0.1 yg) . This level of spiking

can safely measure the blanks down to less than 1 percent of

the value being determined in the analysis. The blank samples

are then treated in an identical manner as the real samples.

43
Spike Equilibration

Equilibration is obtained by the acid dissolution of

the samples. These samples were dissolved by a stepwise

treatment with HNO 3 , HClOi^ , and HF (no HF for beef liver).

The amounts of reagents required were determined during the

preliminary analysis.

A detailed discussion of the procedures used for the

dissolution of samples contained in an organic matrix is

given in Section I, "Sample Preparation" by T . J. Murphy of

this work. The analyst should refer to this section when

such matrices are under consideration.

6. Preconcentration

All three samples were separated-preconcentrated by

electrodeposition onto high purity gold wire cathodes. This

technique is used often since it has a number of advantages.

The method is simple and uses no reagents beyond those

needed for sample dissolution. This results in low analysis

blanks. The gold wires are loaded directly into the SSMS

for analysis and give high elemental sensitivity with photoplate detection. Electrodeposition gives good rejection of

the alkalies and alkaline earths (high in these samples)

which are seldom analyzed by SSMS ID. The elements are

recovered in a reduced form and therefore no extraneous

anions or cations are added to the sample being sparked.

Following dissolution (equilibration) the samples were

evaporated to dryness and redissolved in 10 ml of distilled

water. They were plated for three hours or more at voltages

from 2.5 to 3.0 V. The elements recovered depended on both

the plating voltage and the acid content of the plating

solution. Zinc recovery is very sensitive to the presence

of small amounts of acid in the plating solution. Unless

the sample is dried carefully, zinc will not plate. The

six-nines pure gold wire used for electrodeposition is about

44
1/2 mm diameter. The wire is plated only on the tip for a

distance of about 2 mm.

7, Sample Sparking

Two cathode wires are plated during deposition. These

are loaded so that their 2 mm of plating overlap and that

the sparking will occur along this 2 mm length. Two photoplates are exposed using 1/2 of the plated surface for each.

A graded series of 20 exposures are made on each plate

starting from IxlO"^^ coulombs and going up to 1x10 ^

coulombs. The electrodes are moved so as to spark new sample on each exposure. The gold wire becomes warm while

making the 10 ^ exposures on the first plate. This outgasses

the sample appreciably and the second plate exposed has significantly lower hydrocarbon content (interference) . Some

elements (Cd, Se) may also be volatilized and show lower

intensities on the second plate.

8. Photoplate Analysis

The plates are carefully examined for any of the types

of interferences discussed previously. Based on this information, appropriate lines and charge states are selected for

each element for the densitometric measurement of the

altered isotopic ratio. Even though electrodeposition gives

the "cleanest" possible spectrum, significant amounts of the

sample's major cations and anions (from acid) are retained

in the plated sample by occlusion, adsorption and absorption.

The procedures used on these samples result in perchlorate

being the major anion in the final solutions. Perchlorate

fragments showed up in all three organic SRM's. The SSMS

spectrum show lines for Cl^, ClO^, CIO2, and Clot. The

ClO^ gives lines at m/ e 51 and 53, these would interfere

with SSMS ID analysis of V and Cr respectively if they had

been determined in any of the samples (they were not) . The

45
Clot gives lines at m/e of 67 and 69 (two lines are due to

the fact that CI has two isotopes) . Zinc is spiked with

^^Zn and therefore Clot can interfere with the analysis of

Zn. Fortunately, the amount of Clot interference can be

determined by measuring the line at 69. Also the measurement

of the ^^Zn/^'*Zn ratio can be made at Zn^^ (if enough Zn is

recovered to give a measurable +3 zinc line) where zinc is

free from Clot interference.

Even if interferences are not detected, as many different charge states of an element are measured as is

practical. This gives the maximum number of ratios (we stop

at ~20/element) and improves the accuracy of the analysis.

(A multiply charged ion is used only if both isotopes fall

off unit mass and have no other type of interference) . The

+1 data for an element is calculated separately to determine

if it gives significantly different results than that from

the multiply charged ions. If it is different, it is assumed

to be caused by an undetected interference and the +1 data

are discarded.

Results

The results of the analysis of coal, beef liver and

spinach are given in tables 1, 2, and 3.

46
Table 1. Trace elements (yg/g) in Coal, SRM 1632, by spark

source mass spectrometry isotope dilution.

Pb

Bottle 358

1 22.6

2 31.6

Bottle 292

1 27.1

2

3 30.8

4 28.7

Average 2 8

Ag Cd Se

0.041 0.15 3.02

0.065 0.14 2.88

0.057 0.17 3.02

0.22

0.12 -- 3.03

0.07^ 0.17 3.0

Zn Cu Tl

32 17.7 0.642

36 20.1 0.648

18.4

41 17.6 0.603

36 19.3 0.532

44 17.2 0.532

38 18 0.59

3. Recommended value < 0.1.

Table 2. Trace elements (yg/g) in Beef Liver, SRM 1577, by

spark source mass spectrometry isotope dilution.

Pb Cd Ag Mo Se Zn Cu Tl

1 0.41 0 . 32 0.058 3.25 1.13 126 181 0.048

1 0.41 0. 29 1.12 128 183 0.046

2 0.38 0. 26 0.060 3.08 1.12 131 184 0.050

2 0.33 0.27 0.081 1.07 182 0.051

3 0.36 0. 27 0. 054 3.34 1.12 130 184 0.047

3 0. 33 0.27 0.077 3. 24 1.12 130 197 0.050

4 0.33 0. 28 0.061 3. 24 1.10 121 183 0.045

6 0.34 0. 26 0.067 1.12 120 181 0.043

•age 0. 36 0. 28 0.06 3. 23 1.11 126 184 0.047

47
Table 3. Trace elements Cyg/g) in Spinach, SRM 1570, by

spark source mass spectrometry isotope dilution.

Sample No. Lot No. Pb Tl Cd Cu Zn

304 6--2 1 .15 0 .028 2 .11 12. 18 49. 0

305-1 13--2 1 .09 0 .037 1 . 54 11. 13 50. 0

305-2 13--2 1 .10

a

1 . 53 11. 25 50. 2

306 9--1 1 .15 0 .029 1 .50 11. 36 48. 7

Average 1 .12 0 .031 1 .67 11. 5 49. 5

Insufficient data obtained for sample 305-2.

Table 3b. Cd and Cu in spinach (Lot 6-2) , additional

sampling

.

Element yg/g

Cd 2.32^

Cu 12.54

Original sample depleted; additional material of the same

lot was analyzed.

The close agreement for the two independent runs on Lot

6-2 for Cd and Cu indicates that the Cd is inhomogeneous

in the sample and should not be certified.

48
SECTION IV

ATOMIC ABSORPTION AND FLAME EMISSION SPECTROMETRY

A. Determination of Beryllium, Cadmium, Copper, Magnesium,

Manganese, Mercury, Nickel, and Zinc by Atomic Absorption,

and of Calcium, Potassium, Rubidium, and Sodium by Flame

Emission Spectrometry in Biological and Botanical Materials.

T. C. Rains

1. Objective

The procedures described in this section were developed

and applied to the quantitative determination of traces of

beryllium, cadmium, copper, magnesium, manganese, mercury,

nickel, and zinc, using atomic absorption spectrometry

(AAS) , and calcium, potassium, rubidium and sodium, using

flame emission spectrometry (FES) , in a variety of biological

and botanical materials.

The uncertainty of these methods are 2 to 10 percent,

and improvements in the accuracy of the measurements can be

attained if a number of precautions are taken. Instrumental

parameters must be optimized for each element; standard

solutions should be prepared to contain the same chemical

species as are present in the unknown sample. Also, the

optimum precision and accuracy are attained by AES and FES

when the technique of bracketing is used [1-4],

2. Principle

The absorption of radiant energy by ground state atoms

in the gaseous state is the basis of AAS. When radiation of

proper wavelength passes through a vapor containing ground

state atoms, some of this radiation will be absorbed by excitation of particular atoms (3)

.

The magnitude of the absorption signal is directly

related to the number of these ground state atoms found in

49
the optical path. Ground state atoms necessary for the

measurements are produced from the sample material, by

vaporization of the material introduced in an adequate

excitation source followed by dissociation of the molecular

species into neutral atoms. Normally these steps are carried

out by a nebulizer associated with a flame or by a nonflame

excitation device [5,6].

In flame emission spectrometry a combustion flame is

used to provide the energy necessary to atomize and to excite the atoms introduced into the flame. The solution containing the element of concern is converted into small

droplets by a nebulization process. The small droplets

enter the flame where they are dried, vaporized, and dissociated. The dissociated species are then excited to emit

characteristic radiation having an intensity proportional to

the number of excited particles. The emission spectra

produced by various chemical species excited in combustion

flames is relatively simple, and the discrete monochromatic

radiations can be easily separated by a monochromator and

measured by a photoelectric device with a minimum of radiation

interferences [1]. In certain circumstances, the concomitant

or flame gases can produce atomic lines or overlapping band

structures which may interfere with the measurements. To

overcome these difficulties in FES, it is recommended that

repetitive optical scanning in the derivative mode be used.

This technique was described by Snelleman et al . [7].

Further discussions on the principles of AAS and FES will be

found in references 1 to 6.

3. Procedure

1. Instrumentation

A number of commercial instruments are available to

perform measurements by AAS and FES, and it is assumed that

the analyst is familiar with his particular instrument. It

50
is not possible in this section to give detailed instructions

concerning specific instrumental parameters to assure stability, linearity, adequate flame conditions, etc. for all

the available commercial instruments. For AAS, the Perkin

Elmer Models 403 and 603 were used in this work.^ For FES,

the instrumentation described by Snellman et al . (7) was

used.

2. Reagents

All standard stock solutions were prepared from high

purity metals or salts^ as described in reference 2,

Chapter 13 reproduced in Appendix 14. All other solutions

were prepared from ACS reagent grade chemicals. All acids

were NBS purified [8]

.

3. Sample Preparation and Analysis

It is difficult to cover the many problems associated

with sampling the variety of plant and animal tissues. The

sample, of course, must be representative, and it must be

protected from contamination and deterioration prior to

analysis [9, see also Appendix 2] . It must be determined

at the start if the analysis is to be reported on the "as

received" basis or after drying to a constant weight by one

of several methods [1,9].

^ In order to describe adequately the materials and experimental procedures, it was occasionally necessary to

identify commercial products by manufacturer's name or

label. In no instance does such identification imply

endorsement by the National Bureau of Standards, nor does

it imply that the particular product or equipment is

necessarily the best available for that purpose.

^ Editor's Note - The weights given in this reference were

calculated on the assumption that the compounds are 100

percent pure.

51
a. Sample Preparation and Analysis for Na , K, Rb

,

and Ca by Flame Emission Spectrometry.

The following procedure is applicable for the dissolution of a variety of tissues such as NBS Orchard Leaves,

SRM 1571, and Bovine Liver, SRM 1577, for the determination

of the elements mentioned above:

Transfer a 2 to 5-g sample of tissue to a 100 ml

Teflon beaker and add 25 ml of dilute nitric acid

1:1. Digest on a hot plate for one hour, and cool

and rinse the sides of the beaker with water. Add

10 ml of HNO3 and 5 ml of HF and evaporate to

approximately 10 ml. If the sample contains black

particles of unreacted carbon, repeat the addition

of 10 ml of HNO3. Add 10 ml of HClOi, , heat to

strong fumes of perchloric acid, cool and wash the

sides of the beaker and continue the digestion

until the sample volume is reduced to approximately

2 ml . In some analyses where the concentration of

the analyte is less than 1 yg/g, separation and

preconcentrat ion are required, therefore, the

volume of samples should be evaporated to near

dryness to remove excess acid. Transfer the

solution to a 100-ml volumetric flask and dilute

to volume with water.

Proceed with the FES determination as described

for each individual element.

4, Determination of Sodium by FES

Prepare a standard stock solution of sodium from

NBS SRM 919, Sodium Chloride, as described in

reference [2] (see Appendix 14) . Prepare a series

of standard working solutions from the standard

stock solution containing 1.0, 2.0, 3.0, 4.0 and

5.0 yg Na/ml in 0 . IN HCIO4 . Transfer duplicate

52
test portions of the sample to 100-ml volumetric

flasks. The aliquot taken will depend upon the

concentration of sodium in the sample. The final

concentration should be 2.0 to 3.0 yg Na/ml in

the final volume of 100 ml.

To one volumetric flask, add 100 yg of Na and

dilute the solution to volume. Label this

sample as spiked with 1.0 yg Na/ml. Start the

FES instrument [7], set the wavelength control

to 589.0 nm, and, install an oxy-hydrogen burner

(total consumption) in the instrument. Set the

tank regulators and flow rates for the oxygen and

hydrogen as recommended by the burner manufacturer.

Light the oxy-hydrogen burner and allow the burner

and the spectrometer to stabilize for 15 minutes

while aspirating water. Nebulize the 5.0 yg

Na/ml or higher standard into the flame and adjust

the amplifier gain until the emission signal is

reading 90 percent transmission or 90 divisions

on the strip chart recorder. Nebulize water

after each sample or standard and record the

percent transmission. Nebulize the 1.0, 2.0, 3.0,

4.0 and 5.0 yg Na/ml into the flame and record

the percent transmissions. Repeat the measurements until the percent transmissions of the

standards do not deviate by more than ±0.5

percent. Nebulize the unknown solutions and

record the percent transmissions. Repeat the

nebulization of water between all samples.

Nebulize a standard solution after each 3 to

5 unknown solutions. (Note: If the FES instrument drifts, the standard solutions should

be repeated more often.) Determine the net

percent transmission for the standards and

53
unknowns by substracting the background and

water readings from each standard and unknown.

Calculate the concentration of the unknown

sample as follows:

Calculation

Set up a calibration curve using a least square

fit on a hand calculator. (Note: If a hand

calculator or computer is not available, plot on

rectilinear graph paper the percent transmission

corrected for the background, as the ordinate,

and the concentration of the standards, expressed

in micrograms of analyte per milliliter, as the

abscissa. The calibration curve should pass

through the origin) . Determine the concentration of the unknowns from the calibration curve.

If the recovery of the standard addition is not

100 percent, correct the concentration by the

following equation:^

where: x = yg/ml of analyte in the unknown as

determined from the calibration curve.

s = amount of standard added, yg/ml in

final volume.

y = yg/ml of analyte found in unknown

with standard added as determined

from the calibration curve.

C = concentration yg/ml.

^ The calculation procedure used here is also applied to the

succeeding determinations described in this section.

54
Calculate the analyte concentration o£ the original

sample as follows:

let W = weight of solid sample, g,

Vs = volume of liquid in which W is dissolved,

ml

,

V = test portion of liquid sample or

aliquot of Vs , ml,

V2 • • • ^f

Z = dilution factor = a,12a^ • • • a„n

where: = volume to which V is first diluted, ml,

a^ = aliquot of V^, ml,

V2 = volume to which a^ is diluted, ml,

a^ = aliquot of , ml, etc. to V and a , I ^ L n n

V£ = final volume on which measurement is made,

ml

.

C = analyte concentration of V^, yg/ml; then

analyte in the original sample,

c-z-v

yg/g = v»w

^

5. Determination of Potassium by FES

Prepare a standard stock solution of potassium as

described in reference [2], and prepare a series of

standard working solutions from the standard stock

solution containing 1.0, 2.0, 3.0, 4.0 and 5.0

yg K/ml in 0 . IN HCIO1+ . Proceed as directed in

paragraph 4 (page 52) except set the wavelength

control to 766.5 nm.

Calculations

Proceed as directed in Calculations, page 54.

55
6. Determination o£ Rubidium by FES

Prepare a standard stock solution of rubidium as

described in reference [2] , and prepare a series

of standard working solutions from the standard

stock solution containing 0.1, 0.2, 0.3, 0.4, and

0.5 yg Rb/ml in 0 . IN HClOit which also contains

100 yg K/ml . Transfer duplicate test portions of

the sample to 10-ml volumetric flasks. (Note:

Select an aliquot of the sample that will provide

a final concentration of 0.2 yg Rb/ml.) To one

volumetric flask, add 1.0 yg of Rb and dilute the

solution to volume. Start the FES instrument [7]

,

set the wavelength control to 780.0 nm and proceed

as directed in item 4 (page 52)

.

Calculation

Proceed as directed in Calculations, page 54.

7. Determination of Calcium by FES

Prepare a standard stock solution of calcium as

described in reference [2] using NBS SRM 915, Calcium

Carbonate. Prepare a series of working solutions of

calcium containing each 1000 yg K/ml in 0.1 N HCIO1+

.

The concentration of the standards will depend upon

the type of tissue to be analyzed. (Note: For

NBS-Bovine Liver, standard solutions containing 0.2,

0.4, 0.6, 0.8 and 1.0 yg Ca/ml were prepared. For

NBS-Orchard Leaves, standard solutions containing

2.0, 4.0, 0.6, 0.8 and 10.0 yg Ca/ml were prepared.)

Select an aliquot of the unknown and spike with an

appropriate concentration of calcium as described

in item 4 (page 52). (Note: A solution of K

must be added to the unknown to provide a final

concentration of 1000 yg K/ml.) Start the FES

56
instrument [7] , set the wavelength control to

422.7 nm and install a premix laminar flow burner

with a 5-cm slot burner head. Set the tank regulator and flow rates for air, nitrous oxide, and

acetylene as recommended by the manufacturer and

light the air-acetylene flame. Switch to nitrous

oxide-acetylene, while observing all safety precautions recommended by the manufacturer for

nitrous oxide-acetylene flame. Proceed as

directed in paragraph 4 (page 52)

.

Calculations

Proceed as directed in Calculations, page 54.

b. Sample Preparation and Analysis for Be, Cd,

Cu, Mg, Ni, Mn, and Zn by Atomic Absorption Spectrometry.

The sample preparation procedure described in paragraph

3, page 51, is used for the dissolution of a variety of

tissues such as orchard leaves (SRM 1571) , and bovine liver

(SRM 1577) for the determination by AAS of the elements

mentioned above.

8. Determination of Beryllium by Nonflame AAS

Prepare a standard stock solution of beryllium as

described in reference [2] , and prepare a series of

standard working solutions from the standard stock

solution containing 0.005, 0.010, 0.015, 0.020 and

0.025 yg Be/ml in 0 . IN HNO3. If the unknown solutions contain more than 10 yg Ca/ml, add an equal

amount of calcium to the beryllium standard working

solutions. Transfer duplicate test portions of

sample to 100 ml Teflon beakers, evaporate to near

dryness. Add 4 ml of 0 . 2N HNO3, and transfer to

10-ml volumetric flasks. Select an aliquot that

will provide a final concentration of Be in the range

57
o£ 0.010 to 0.015 yg Be/ml. To one volumetric flask,

add 0.05 yg of Be and dilute the unknown solution

to volume. Start the instrument (Perkin-Elmer Model

603 equipped with the HGA-2100 graphite furnace)

as described in the instruction manual [12]. Turn

on the beryllium hollow cathode lamp, adjust the

lamp current to 14 mA, and set the wavelength to

234.9 nm using a slit width of 0.7 nm. Turn on the

D2 arc lamp, and allow the hollow cathode and D2

arc lamps to warm for 15 minutes, then balance the

two beams. Set the HGA-2100 furnace at the following

instrumental conditions: dry for 30 seconds at 100 °C;

char for 40 seconds at 700 °C; atomize for 7 seconds

at 2800 °C. Set the instrumental condition as follows:

peak height integration time for 5 seconds; scale expansion for 1.0. Using an Eppendorf pipette, transfer

0.005 to 0.025 yg Be/ml, establish a calibration curve

and repeat the calibration curve until the absorbances

are within ±5 percent. Determine the net absorbances

of the unknown and standard solutions. (Note: to

increase the precision of the analysis, analyze each

sample in triplicate.)

Calculation

Proceed as directed in Calculations, page 54, except

plot absorbance instead of transmission versus

concentration

.

9. Determination of Cadmium by Nonflame AAS

Prepare a standard stock solution of cadmium as

described in reference [2] and prepare a series of

standard working solutions containing 0.002, 0.004,

0.006, 0.008 and 0.010 yg Cd/ml in 0 . IN HNO3 . Proceed as directed in paragraph 8 (page 57) except

for the following instrumental conditions: for the

58
Perkin Elmer 603 instrument set the cadmium hollow

cathode lamp at 8 mA and wavelength at 228.8 nm;

set the HGA-2100 furnace: dry for 30 seconds at

100 °C; char for 40 seconds at 250 °C; atomize

for 7 seconds at 2100 °C.

Calculations

Proceed as directed in Calculations, page 54, except

plot absorbance instead of transmission versus

concentration.

10. Determination of Copper by AAS

Prepare a standard stock solution of copper as described in reference [2] , and prepare a series of

working solutions of 0.5, 1.0, 2.0, 3.0 and 4.0

yg Cu/ml in 0 . IN HCIO., that contain 500 yg Na/ml

.

(Note: For plant material, potassium should be substituted for sodium.) Transfer duplicate test

portions of the sample to 25-ml volumetric flasks.

Select an aliquot of the sample that will provide

a final concentration of 1.0 to 2.0 yg Cu/ml. To

one volumetric flask add 25 yg of Cu and dilute

the unknown solution to volume. Attach a premixed

air-acetylene burner having a three slot burner

head to the Perkin-Elmer 603 AAS unit as described

in the instruction manual [12] , and turn on the

unit containing a copper hollow cathode lamp. Set

the hollow cathode lamp current at 15 mA, set wavelength to 324.7 nm, and use a slit width of 0.2 nm.

Turn on the D2 arc lamp, allow the lamp to warm for

10 minutes, and balance the two beams. Set the instrumental conditions as follows: mode-concentration,

integration time for 5 seconds, scale expansion 2.0.

Turn on the air-acetylene flame and allow the burner

to warm for 15 minutes while nebulizing water. Con59
struct a calibration curve by nebulizing the series

of analyte standards and measuring their absorbance.

Always nebulize water before and after taking a

measurement of the standards or unknown solutions.

Obtain a net absorbance by subtracting any reading

obtained for the water from the readings of the

standard solutions. Repeat the calibration curve

until the absorbances are within ±2 percent. Measure

the absorbance of the unknown including a reagent

blank. (Note: To increase the precision and accuracy

of the analysis, measure a low standard, unknown, and

then a higher standard.) Obtain the net absorbance

by substracting any reagent blank and/or readings

obtained for the water from the readings of the

standard solution.

Calculations

Proceed as directed in Calculations, page 54, except

plot absorbance instead of percent transmission

versus concentration.

11. Determination of Magnesium by AAS

Prepare a standard stock solution of magnesium as

described in reference [2] , and prepare a series

of working solutions of 0.1, 0.2, 0.3, 0.4, and

0.5 yg Mg/ml in 0 . IN HCIO^^ that contains 0.1 percent lanthanum solution. (Note: Magnesium concentrations of 0.5 to 2.5 yg/ml can be used for many

types of plant tissues, but due to the nonlinearity

of the calibration curve, the unknown samples must

be closely bracketed with standard solutions.)

Transfer duplicate test portions of the sample to

25- or 100-ml volumetric flasks and select an aliquot

of the sample that will provide a final concentration

of 0.2 yg Mg/ml. Add 0.1 percent lanthanum solution

60
to all samples. To one volumetric flask, add a

concentration of magnesium that will provide a

final concentration of 0.1 yg Mg/ml . Proceed as

directed in paragraph 10, (page 59), except set

the wavelength to 285.2 nm and the magnesium hollow

cathode lamp at 15 mA.

Calculations

Proceed as directed in Calculations, page 54, except

plot absorbance instead of percent transmission

yersus concentration.

12. Determination of Manganese by AAS

Prepare a standard stock solution of manganese as

described in reference [2] , and prepare a series of

working solutions of 0.5, 1.0, 1.5, 2.0 and 2.5 yg

Mn/ml in 0.1 N HClOi^ that contain 300 yg K/ml and

20 yg Ca/ml. (Note: For some animal tissues, the

manganese concentration may be a factor of 10 lower.

In this case the analyst should use the nonflame

technique with Mn standard solutions of 0.005 to

0.025 yg/ml.) Transfer duplicate test portions of

the sample to 25-ml volumetric flasks and select an

aliquot of the sample that will provide a final concentration of 1.0 to 1.5 yg Mn/ml. Add a known

concentration of manganese to one of the samples

as described previously. Proceed as directed in

paragraph 10 (page 59) except set the wavelength

to 279.5 nm and the manganese hollow cathode lamp

at 18 mA.

Calculations

Proceed as directed in Calculations, page 54, except

plot absorbance instead of percent transmission

versus concentration.

61
13. Determination o£ Zinc by AAS

Prepare a standard stock solution of zinc as described in reference [2] , and prepare a series of

working solutions of 0.1, 0.2, 0.3, 0.4 and 0.5

yg Zn/ml in 0 . IN HCIO.* that contain 300 yg K/ml

and 200 yg Ca/ml. Transfer duplicate test portions

of samples to 25-ml volumetric flasks and select an

aliquot of the sample that will provide a final concentration of 0.2 to 0.3 yg Zn/ml. Add a known concentration of zinc to one of the samples as described

previously, and proceed as directed in paragraph 10

(page 59) except set the wavelength to 213.8 nm

and the zinc hollow cathode lamp at 10 mA.

Calculations

Proceed as directed in Calculations, page 54, except

plot absorbance instead of percent transmission

versus concentration.

14. Determination of Mercury by Cold Vapor AAS Sample

Preparation

The following procedure is applicable for the dissolution of a variety of tissue samples such as NBS-SRM Orchard

Leaves and Bovine Liver for the determination of mercury.

The equipment used in this procedure is described by Rains

and Menis [11]

.

Transfer a 2 to 3-g sample to a digestion flask,

add 25 ml of 18N H2SO4 and 20 ml of 7N HNO3 and

connect the Allihn condenser. Using a heating mantle,

gently heat the flask for one hour at 150 °C. Cool

the flask, rinse the inside of the Allihn condenser

and add 20 ml of HNO3 - HCIO4 (1+1) through the condenser. Drain the water from the condenser and continue heating the flask until dense fumes of perchloric

acid appear. Cool the flask and rinse the inside of

62
the Allihn condenser with 5 ml of water. If the sample

shows a tendency to char, add an additional 10 ml of

HNO3 and reheat to fumes of perchloric acid. Continue

heating the flask until dense fumes of H2SO4 appear,

cool, and rinse the condenser with 1:10 HNO3. Heat

to boiling, cool, and rinse the condenser again.

Transfer the solution to a 100-ml volumetric flask,

add 5 drops of one percent solution of KaCraOy and

dilute to volume. Proceed with the determination

of mercury by the cold vapor method as described

in the next paragraph.

Determination

Prepare a standard stock solution of mercury as described in reference [2] , and prepare a series of

standard working solutions that contain 0.005,

0.010, 0.015 and 0.020 yg Hg/ml in O.IM HNO3. Add

five drops of one percent KaCraO? and dilute to

volume. Allow samples and standards to digest one

hour after the addition of KaCraOy before proceeding

with the analysis. Prepare "reducing" and "diluting"

solutions as described in Appendix 15. Turn on the

Perkin-Elmer 403 AAS unit as described in reference

[12], insert the mercury hollow cathode lamp, and

set the current at 10 mA. (Note: Any commercial

hollow cathode lamp will give adequate sensitivity;

however, vapor discharge lamps are not recommended

due to severe line broadening.) Attach a quartz

absorption cell (10 cm x 0.8 cm) to the burner head.

This absorption cell can be constructed from a 0.8 cm

internal diameter and 10 cm long Vycor tubing with

gas inlet and outlet ports and quartz windows. The

quartz windows are secured to the Vycor tubing with

epoxy cement. The cell is wrapped with a heating

tape and attached to the burner head. Turn on the

63
D2 arc lamp and allow it to warm up for 15 minutes,

then balance the hollow cathode and D2 arc lamps as

recommended in the instruction manual. Heat the

absorption cell to 200 °C with the aid of a variable

autotransformer as directed in Appendix 15. Attach

the reducing cell and set argon gas flow at 0.3 1/min.

Transfer an aliquot of standard mercury working

solution to the reducing cell. (Note: A mercuryconcentration of 0.2 yg will give an absorbance of

0.800.) Dilute to 25 ml with diluting solution, and

turn on recorder. Add 20 ml of reducing solution

and immediately close the system. Record maximum

absorbance. (See comments under calibration in

Appendix 15.) Obtain a net absorbance by substracting the value obtained for the reagent blank from

the standard solution. Repeat the calibration

curve until the absorbance values are within ±2

percent. Transfer an aliquot of unknown sample that

has a mercury concentration of O.Cl to 0.1 yg into

the absorption cell. Proceed as directed above.

Bracket the unknown sample with standard samples

since the mercury calibration curve is nonlinear.

Calculation

Proceed as directed in Calculations, page 54, except

plot absorbance instead of percent transmission

versus concentration.

References

Dean, J. A., and Rains, T. C, eds . , Flame Emission

and Atomic Absorption Spectrometry, Vol. 3^, Elements

and Matrices, (Marcel Dekker, New York, 1975).

Dean, J. A., and Rains, T. C. , eds.. Flame Emission

and Atomic Absorption Spectrometry, Vol. 2^, Components

and Techniques, (Marcel Dekker, New York, 1971). Chap-
ter 13, Standard Solutions for Flame Spectrometry is

reproduced as Appendix 14.

[3] Dean, J. A., and Rains, T. C, eds . , Flame Emission and

Atomic Absorption Spectrometry, Vol. 1_, Theory, (Marcel

Dekker, New York, 1969).

[4] Mavrodineanu , R. , ed. , Analytical Flame Spectroscopy,

(Macmillan, London; Springer, New York, 1970).

[5] Rains, T. C, Chemical Aspects of Atomic Absorption,

ASTM STP 443,, pp. 19-36, American Society for Testing

and Materials, 1969 (see Appendix 11).

[6] Rains, T. C, Atomic Absorption. Spectrometry - General

Consideration for the Application of Experimental Techniques, ASTM STP 564, pp. 50-67, American Society for

Testing and Materials, 1974 (see Appendix 12).

[7] Snelleman, W. , Rains, T. C, Yee, K. W. , Cooks, H. D. , and Menis, 0., Anal. Chem. , 4^, 394 (1970) (see

Appendix 13)

.

[8] Kuehner, E. C, Alvarez, R. , Paulsen, P. J., and

Murphy, T. J., Anal. Chem., £4, 2050 (1972) (see

Appendix 1)

.

[9] Scott, R. 0. and Ure, A. M. , Proc. Soc. Anal. Chem.,

December, 1972, pp. 288-293.

[10] Christian, G. D. , and Feldman, F. J., Atomic Absorption

Spectroscopy: Application in Agriculture, Biology, and

Medicine, (Wiley- Interscience , New York, 1970).

[11] Rains, T. C, and Menis, 0., JAOAC, 5_5 , 1339 (1972)

(see Appendix 15) .

[12] Analytical Methods for Atomic Absorption Spectrometry,

Perkin-Elmer , Norwalk, CT , March 1973, Reviewed

September, 1974, and March, 1976.

65
B. Determination of Calcium, Potassium, and Sodium in

Flour by Emission Flame Spectrometry.

J. R. Baldwin and R. Mavrodineanu

1. Objective

The purpose of the procedure described in this paper is

to determine calcium, potassium, and sodium in flour using

emission flame spectrometry.

2. Principle

Emission flame spectrometry is based on the well known

principle that chemical species, when properly energized,

may re-emit the absorbed energy as radiations.

These radiations are specific for every emitting species

and their intensities are proportional to the number of

particles, atoms, ions, molecules, or free radicals, participating in the radiation process. These two properties

are at the foundation of qualitative and quantitative spectral

analyses [1]

In the present case, the excitation energy is supplied

by a pre-mixed acetylene-air combustion flame into which the

analytical sample is introduced, in a constant and reproducible

manner, as a suspension of fine droplets using compressed

air and a pneumatic sprayer.

3. Procedure

Instrumentation

The equipment needed to perform the analysis consists

in principle of the analytical flame, with its sample supplying system, an adequate spectrometer followed by a photodetector, and a data acquisition and presentation system. A

detailed description of the equipment assembled and used for

this purpose at NBS, is given in reference [3], which is reproduced in Appendix 16. A pre-mixed acetylene-air flame

66
with pneumatic sprayer was used in this work. Further information on the instrumentation and methodology used in general

for Na, K, and Ca determinations by emission flame spectroscopy will be found in reference [2]

.

4. Reagents

Distilled water

Nitric acid

Perchloric acid

Lanthanum perchlorate, 10 mg La/ml, made by dissolving

11.727 g LaaOs , reagent grade, in 83 ml of perchloric

acid, diluted to 1 liter with distilled water.

Sodium, potassium and calcium stock standard solutions,

each containing 1000 yg/ml. These solutions were made

by dissolving for:

Sodium : 2.5447 g sodium chloride 99.9 percent pure,

SRM 919, in 1000 ml distilled water to which 5-ml nitric

acid were added; Potassium : 1.9087 g potassium chloride

99.9 percent pure, SRM 918, in 1000 ml distilled water to

which 5-ml nitric acid were added; Calcium : 2.4997 g

calcium carbonate 99.9 percent pure, SRM 915, in 1000 ml

distilled water to which 5 ml nitric acid has been added.

This basic standard solution as well as the composite

diluted solutions were kept in polyethylene bottles.

Composite Standard Solutions:

Suitable dilutions of the above stock solutions are

made with distilled water as indicated below to produce the

standards needed to calibrate the emission flame spectrometer :

67
Composite vg/ml

Solution Na K Ca

1 0.35 50 8

2 0.50 80 10

3 0.70 100 16

4 1.00 120 24

These composite standard solutions were used in the determination o£ Na and K.

For the determination of Ca, a lanthanum solution was

added to the same concentrations of Na, K, and Ca, to produce solutions containing a final concentration of 1000

yg/ml La each.

5. Sample Preparation

The weight loss at 85 °C is determined in separate 1-g

flour samples taken in duplicate. Each sample is weighed

separately into a glass weighing bottle provided with a

cover. The bottles are placed uncovered in an oven at 85 °C

for 20 hours, then placed in a desiccator. After cooling,

they are weighed with the cover on.

For the analysis, 5 g of flour sample is weighed in

five replications at the same time the samples for weight

loss are taken. The weighing is made to the nearest 0.1 mg.

Each sample is transferred to a 250-ml Teflon beaker provided

with a Teflon cover, and 10 ml of distilled water is added

to each sample. The samples are treated with a mixture of

30 ml of HNO3 and 5 ml of HClOit on a hot plate until all

organic matter is destroyed. During this time, HNO3 is

added, periodically, in 5 ml portions, as needed to remove

all organic matter, and the walls of the beaker are washed

periodically with a minimum of water. Finally, the walls of

the beaker are washed for the last time, and the contents

68
o£ the beaker is evaporated to about 1 ml. This operation

can take approximately one day, and depends on the type o£

the flour.

To this residue, 10 ml of water are added, the solution

is transferred in a 50-ml polypropylene volumetric flask,

and diluted to mark v/ith water. This solution is used for

the determination of Na and K. Calcium is determined in a

similar solution to which lanthanum was added to minimize

the interfering action of phosphorous. Thus, 5 ml of the

lanthanum stock solution are added to the sample to provide

a final concentration of 1000 yg/ml La. A corresponding

amount of lanthanum stock solution is added to each standard

solution to provide a final concentration of 1000 yg/ml La.

The spectrometer is calibrated in the usual manner,

before and after measuring the sample solutions, and the

o o measurements are made at 5889.9 A for sodium; 7664.9 A for

o potassium; and 6214 A for calcium (CaOH molecular band

maximum) . The samples are measured each in six replications

and their content of Na, K, and Ca is found by interpolation

on the standard calibration curves which are prepared by

plotting as the abscissa the concentrations of the standard

solutions (yg/ml) and the corresponding photocurrent values

(volts on the digital voltmeter) as the ordinate.

Calculation and Results

The amount of element contained by the flour is expressed

as yg/g and is referred to the material on an "as received"

basis. An example is given in Table 1 for a wheat flour

sample and in Table 2 for a rice flour including the percent

loss at 85 °C.

69
Table 1. Determination of sodium, potassium, and calcium

(yg/g) in a wheat flour sample on an "as

received" basis.

Sample No. Sodium Potass ium Calcium

1

2

3

4

5

Average yg/g

in Aliquot

Standard

Error (SE)

Percent SE

0. 695

0. 695

0.658

0.650

0.670

0.674

0.01

1.4

116

115

109

105

108

110.6

2.1

1.9

16.44

16.33

16. 22

16. 77

15.34

16. 22

0. 24

1.5

Average yg/g

in Sample 6. 71 1102.0 161.6

Loss: Twenty hours at 85 °C; 8.6 percent.

70
Table 2. Determination of sodium, potassium, and calcium

(yg/g) in a rice flour sample on an "as

received" basis.

Sample No. Sodium Potass ium Calcium

1

2

3

4

5

0.430

0.425

0.420

0.465

91

92

86

92

94

Average yg/g

in Aliquot

Standard

Error (SE)

Average yg/g

in Sample

0.435

0.01

4. 32

91

1.3

905. 0

11.55

11 .10

10.45

11 . 10

11 . 78

11. 20

0 . 23

111 . 6

Loss: Twenty hours at 85 °C; 8.3 percent.

6. References

[1] Mavrodineanu , R. , and Boiteux, H. , Flame Spectroscopy,

(John Wiley, New York, 1965).

[2] Mavrodineanu, R. , Bibliography on Flame Spectroscopy,

Analytical Applications, 1800-1966. NBS Misc. Publ.

281, (1967), continued from 1966 to 1968 in Analytical

Flame Spectroscopy, Selected Topics, R. Mavrodineanu,

ed. (MacMillan - Springer, London, New York, 1970).

[3] Mavrodineanu, R. , Discussion of Some Experimental and

Fundamental Conditions in Analytical Flame Spectroscopy,

Revue du GAMS (Paris) 1971 , No. 3, 39-60; reproduced in

Appendix 16.

71

SECTION V

MOLECULAR ABSORPTION SPECTROMETRY

Procedures for the Determination of Arsenic, Copper, and

Nickel by Molecular Absorption Spectrometry

R. W. Burke and B. I. Diamondstone

1. Objective

The procedures to be described have been used in the

Analytical Chemistry Division for several years for the

routine determination of trace concentrations of arsenic,

copper, and nickel in a variety of tissue samples. Through

appropriate selection of experimental conditions, analyses

for these three elements can be readily performed at the

0.01 part-per-million (ppm) level.

Immediately following are detailed descriptions of the

procedures currently used and recommended for the determination of these trace elements in biological matrices. To

demonstrate the broad applicability of these methods, typical

results are given for the analyses of some recently issued

and pending Standard Reference Materials (SRM's).

2. Principle

The determination of arsenic is based on the formation

of a deep red color when arsine is reacted with a pyridine

solution of silver diethyldithiocarbamate ; copper is determined as the yellow complex of Cu(II) - dibenzyldithio

carbamate; and nickel is determined as a- furildioxime chelate.

3. Spectrophotometric Procedure for Arsenic

Samples are wet-ashed in a mixture of high purity nitric

and perchloric acids prepared at NBS (Appendix 1) . To

eliminate volatilization losses all dissolutions are performed

under reflux in a Bethge apparatus. The arsenic (V) formed

73
is reduced to arsine (AsHa) in a special generator flask and

the latter is absorbed in a pyridine solution of silver

diethyldithiocarbamate . The deep red color which develops,

and whose exact nature is not completely understood, is

measured spectrophotometrically and compared to a series of

known arsenic standards.

4. Apparatus

Bethge Dissolution Apparatus. Available from G.

Frederick Smith Chemical Company, Columbus, Ohio.-^

Arsine Generator/Absorption Tube. Commercially available from Arthur H. Thomas or Fisher Scientific.

Spectrophotometer. Any stable single or double beam

instrument having reproducible cuvette positioning and a

thermos tatable sample compartment which will accept 10 mm

rectangular cuvettes. Readability and reproducibility

should be 0.001 absorbance or better.

Cuvettes. Rectangular, semi-micro, thick-walled, of

glass or quartz construction fitted with Teflon stoppers and

having a light path of 10 mm and an internal width of 4 mm.

Shaker-Water Bath. A Dubnoff type or its equivalent.

5. Reagents

Silver Diethyldithiocarbamate, 0.5 percent. Dissolve

1 g of silver diethyldithiocarbamate in 200 ml of pyridine.

Store in an amber bottle.

Potassium Iodide, 15 percent. Dissolve 15 g of KI in

100 ml of distilled water and store in an amber bottle.

^ In order to describe materials and experimental procedures

adequately it was occasionally necessary to identify commercial products by manufacturer's name or label. In no

instance does such identification imply endorsement by the

National Bureau of Standards, nor does it imply that the

particular product or equipment is necessarily the best

available for that purpose.

74
Stannous Chloride, 21 percent. Dissolve 25 g of

SnCl2'2H20 in 100 ml of concentrated hydrochloric acid.

Arsenic(V) Standard Stock Solution, 100 ppm. Dissolve

0.1320 g of AS2O3 in 10 ml of water containing 1 g of NaOH.

Oxidize As (III) to As (V) by addition of 2 ml of saturated

bromine water. Warm on a hot plate or steam bath to remove

excess bromine. Cool and dilute to 1 liter with distilled

water. This solution contains 100 yg As(V)/ml.

Arsenic(V) Working Standard, 1 ppm. Prepare 1 ug

As(V)/ml working standard by diluting 10 ml of the standard

stock solution to 1 liter with distilled water. Prepare

fresh weekly.

Zinc. Use granulated zinc that is 20 mesh or less and

whose arsenic content does not exceed 0.02 ppm.

6. Analysis Procedure

Dissolution. Depending upon the expected arsenic content, weigh a 1 to 5 g sample of dried^ tissue and transfer

to the round bottom flask of the Bethge apparatus. Add

30 ml of a 1:1 mixture of high purity nitric and perchloric

acid and connect the flask to the reflux condenser. With

cold tap water running through the condenser, gradually

raise the temperature of the flask and contents to 120 °C by

use of a variable transformer to control the power supplied

to the heating mantle. The stopcock should remain open and

the temperature maintained near 120 °C until all of the

sample has dissolved. Close the stopcock and complete the

oxidation by slowly raising the temperature to 210 °C.

Initially no increase in transformer power will be necessary

since the temperature will rise as the nitric acid distills

up the side arm and condenses above the stopcock. Additional

heat may be needed near the end of the oxidation to attain

^ See Section 1, Sample Preparation, page 5 for recommended

drying procedures.

75
the 210 °C reading. Normally, 15 to 20 minutes are required

to go from the 120 °C setting to 210 °C. At the latter

temperature, the sample should be clear and exhibit a yellow

or pale orange color. Lower the heating mantle, allow the

flask and contents to cool and drain the nitric acid back

into the sample solution. Wash down the inside of the

Bethge apparatus with distilled water and transfer the

contents of the flask to a 100-ml Teflon beaker. If silica

is present add 0.5 to 1 ml of hydrofluoric acid. Evaporate

to strong fumes of perchloric acid. Cool, add 5 ml of

distilled water and evaporate to approximately 2 ml . If

aliquots are to be taken for arsenic analysis, transfer the

sample to a 25-ml volumetric flask and dilute to volume with

distilled water.

Arsine Generation and Collection. Transfer the entire

sample or a suitable aliquot (As >^ 0 . 1 yg) to the generator

flask and add sufficient concentrated HCl to give a total

acid volume of 10 ml. Add 35 ml of water, 2 ml of 15 percent KI solution and 1 ml of 21 percent SnCla solution. Mix

and allow to stand at room temperature for 15 minutes in

order to reduce As (V) to As(III). Cool the assembled

generator in an ice bath for 30 minutes and transfer to a

25 °C shaker bath. Add 4.00 ml (Note A) of the pyridine

solution of silver diethyldithiocarbamate to the collection

tube. Disconnect the collector tube and add 2.5 g of zinc

to the generator flask, using a wide-mouth funnel to keep

zinc off the walls of the ground glass joint. Immediately

connect the collection tube to the flask by clamping the

ball joint together. Allow one hour for the evolution of

Note A. For determining arsenic in the range of 0.1 to 0.5 yg,

the amount of absorber solution used is reduced from 4.00 to

2.00 ml. In the latter instance, glass beads approximately

1 mm in diameter are used to produce the same length of column of absorber solution as was present in the normal run.

76
arsine to be completed. To avoid leaks, both the standard

taper joint of the flask and the ball joint should be

lubricated. "Nonaq" stopcock grease, available from Fisher

Scientific, has proven satisfactory for this purpose.

Disconnect the collection tube and, through either a rocking

motion or by use of a disposable Pasteur-type pipette, rinse

the walls of the tube with the absorber solution. Any

residue must be redissolved and the pyridine solution

thoroughly mixed. Transfer an aliquot to the 10-mm cuvette

and measure the absorbance at 535 nm against a reagent blank

prepared in the same way as the sample. Determine the

amount of arsenic present by comparing the measured absorbance

with the absorbances of a series of arsenic standards (0.5,

1, 2, 4, and 6 yg As/4.00 ml pyridine reagent or 0.1, 0.2,

0.3, 0.4 and 0.5 yg As/2.00 ml pyridine reagent - see Note

A). Both systems obey Beer's law and the slopes of the two

calibration curves are in the exact ratio of 1:2. The

amount of arsenic present in the tissue sample is given by

the relationship:

. . yg Arsenic Found Arsenic, ppm = t tttz

' Sample Wt . , g

Sample Absorbance, A^

Arsenic, ppm = slope of Calibration Curve (A^/yg) x Sample Wt . , g

7. Results

The spectrophotometric arsine-s ilver diethyldithiocarbamate procedure for arsenic has been applied to the

analysis of a number of NBS SRM's. Some typical results are

given in Table 1.

77
Table 1. Spectrophotometr ic determination of arsenic in

tissue samples by the arsine-silver

diethyldithiocarbamate procedure

.

oampi

e

IN CO oKM JNO . oampi e wr . , g Arsenic, ppm

Orchard Leaves 1571 0.5 10.2;

9.9;

10.3;

10.1

Wheat Flour^ 1567 5.0 0.03;

0.03;

0.03;

0.02;

0.03;

0.02

Rice Flour^ 1861 5.0 0.09;

0.10;

0 . 0 0 ;

0.10;

0.08;

0.09

Spinach 1570 2.0 0.17;

0.16;

0.18;

0.18

Pine Needles 1575 2.0 0.22;

0.20;

0.20;

0.22

Tomato Leaves 1573 2.0 0.30;

0.28;

0.29;

0.29

Currently available from the NBS Office of Standard

Reference Materials.

^ In preparation.

Remarks

Antimony is the only potential interference in the

spectrophotometric determination of arsenic in tissue by the

arsine-silver diethyldithiocarbamate procedure. Upon reduction with zinc, stibene (SbHs) is evolved which reacts similarly to arsine (AsHs) when absorbed in a pyridine solution

of silver diethyldithiocarbamate. Fortunately, the absorbance maxima of the two products are sufficiently different

fx for Sb = 505 nm; \ for As = 535 nm) and the pres- ^ max ' max ^ ^ ence of antimony can be checked by measurement of the

505/535 absorbance ratio. If only arsenic is present, this

78
ratio should not exceed 0.80. If this ratio is exceeded,

simultaneous equations can be used to correct for antimonyinterference since the absorbance of the colored arsenic and

antimony products are additive at these two wavelengths. No

corrections were necessary for any of the tissue samples

analyzed to date.

8. Spectrophotometric Procedure for Copper

This procedure is based on the spectrophotometric

measurement of the yellow complex formed in moderately

strong acidic media between copper(II) and dibenzyldithio

carbamate after its isolation and preconcentration by extraction into a non-polar organic solvent. The selectivity and

acid stability of the carbamate reagent is enhanced by using

it in the form of its zinc chelate which is dissolved directly

in the carbon tetrachloride extracting solvent. When this

solution is equilibrated with an aqueous solution containing

copper(II), the corresponding copper complex is formed and

extracted almost instantaneously as a result of its chelate

exchange reaction with zinc.

9. Apparatus

Spectrophotometer. Any stable single or double beam

instrument having reproducible cuvette positioning and a

sample compartment provided with temperature control which

will accept 10 mm rectangular cuvettes. Readability and

reproducibility should be 0.001 absorbance or better.

Cuvettes. Rectangular, semi-micro, thick-walled, of

glass or quartz construction, fitted with Teflon stoppers

and having a light path of 10 mm and an internal width of

4 mm.

Separatory Funnels. 60-ml capacity with Teflon stoppers and stopcocks. Stems are shortened to approximately

10 mm and the ends fire-polished.

79
10. Reagents

Zinc Dibenzyldithiocarbamate , 0.01 percent. Dissolve

20 mg of zinc dibenzyldithiocarbamate in 200 ml of carbon

tetrachloride. Store in an amber bottle.

Copper Standard Stock Solution, 100 ppm. Dissolve

0.100 g of copper metal in 5 ml of 1:1 nitric acid. Evaporate

to dryness and dissolve residue in IN hydrochloric acid.

Transfer to a 1 liter volumetric flask and dilute to the

mark with IN hydrochloric acid.

Copper Working Standard, 2 ppm. Dilute 5.00 ml of the

standard copper stock solution to the mark with 0 . IN hydrochloric acid in a calibrated 250-ml volumetric flask.

11. Analysis Procedure

Wet-ash 1 to 5 g of dried tissue in a 1:1 mixture of

nitric and perchloric acids. Use 15 ml of the mixed acids

for a 1-g sample and increase proportionately to 30 ml for 1

to 5-g of tissue. Dissolution may be carried out in a

Bethge apparatus, using the conditions previously described

for arsenic, or in a Teflon beaker. If a beaker is used, it

should be covered with a Teflon lid until all reaction with

the nitric acid has ceased. The cover is removed and rinsed

with distilled water and the solution is heated to fumes of

perchloric acid. Fuming is continued until approximately

one-half of the perchloric acid is removed. The sample is

cooled, the walls of the beaker are rinsed with distilled

water and the fuming is repeated. When the sample volume

has been reduced to 1 to 2 ml the beaker is removed from

heat, cooled and reserved for copper analysis. If the total

sample is to be used, it is transferred directly to a 60 ml

separatory funnel and diluted to 25 ml with distilled water.

If an aliquot is taken, the sample is first transferred to a

25-ml volumetric flask. In either instance the sample

weight or aliquot size should be chosen so as to contain 1

80
to 10 yg of copper and a final acid concentration of 0.1 to

IN. Add 5.00 ml of the carbon tetrachloride solution of

zinc dibenzyldithiocarbamate and extract the copper by

shaking vigorously for one minute. While the phases are

clearing, insert a loosely- fitting paper plug in the stem of

the separatory funnel. Drain a portion of the carbon tetrachloride solution containing the yellow copper complex into

a 10-mm cuvette and measure the absorbance of this solution

at 435 nm against a reagent blank carried through the entire

procedure. Determine the amount of copper present by comparing

the net absorbance with the absorbances of known amounts of

copper taken through the extraction procedure. Beer's law

is obeyed over the range 1 to 10 yg of copper. When

absorbance is plotted as a function of micrograms of copper

added, the slope of the resulting calibration curve should

be approximately 0.051 absorbance units per microgram of

copper.

12. Results

The results obtained for copper in a series of new NBS

botanical SRM's are summarized in Table 2. All values are

reported on a dried basis, i.e., samples were heated in an

air oven for one hour at 85 °C. Typical weight losses were

of the order of five percent.

Remarks

Bismuth and thallium(III) are potential interferences

in the spectrophotometric zinc dibenzyldithiocarbamate procedure for copper. However, neither of these elements has

been found in sufficiently high concentrations to cause

difficulty in any of the tissue samples analyzed.

81
Table 2. Spectrophotometr ic determination of copper in

three new NBS botanical SRM's.

Sample

Spinach

Pine Needles

Tomato Leaves

NBS SRM No.

1570

1575

1573

Sample Wt . , g

0.5

1.0

0.5

Copper, ppm

12.03; 12.08;

12.07

2.94; 2.93;

2.95

10.81; 10.79;

10.83

3- Available from the NBS Office of Standard Reference

Materials

.

13. Spectrophotometric Procedure for Nickel

The preferred reagent for the spectrophotometric determination of nickel in tissues is a- furildioxime . It is

superior to dimethylglyoxime because the wavelength of

maximum absorbance of its nickel chelate lies in the visible

region of the spectrum.

Nickel forms, with an excess of a- fur ildioxime , an

insoluble chelate which can be quantitatively extracted [1]

from aqueous media into chloroform in the pH range of 8.5 to

9.4. This extraction can be made very selective because the

interference of cobalt and copper can be eliminated by

backwashing the chloroform extract with ammonia. Citrate or

tartrate is used to prevent the hydrolysis and subsequent

interference of such common elements as iron and aluminum.

14. Apparatus

Spectrophotometer. Any stable single or double beam

instrument having a sample compartment provided with

temperature control which will accept 50-mm cuvettes.

Readability and reproducibility should be 0.001 absorbance

or better.

82
Cuvettes, 50-mm cylindrical or rectangular cuvettes

having a maximum volume o£ 15 ml and equipped with snuglyfitting Teflon stoppers or covers.

Separatory Funnels. 60-ml capacity and fitted with

Teflon stoppers and stopcocks. Stems are shortened to

approximately 10 mm and fire-polished.

pH Meter. Equipped with individual or a combination

glass -calomel electrode.

15. Reagents

a-Furildioxime , 1 percent. Dissolve 1 g of a-furildioxime in 100 ml of ethanol.

Nickel Standard Stock Solution, 100 ppm. Dissolve

0.100 g of nickel metal in 5 ml of 1:1 nitric acid. Evaporate

to dryness and dissolve residue in 0 . IN hydrochloric acid.

Transfer to a 1-liter volumetric flask and dilute to the

mark with 0 . IN hydrochloric acid.

Nickel Working Standard, 1 ppm. Dilute 2.00 ml of the

standard nickel stock solution to the mark with O.IN hydrochloric acid in a calibrated 200-ml volumetric flask.

Ammonium Citrate, 10 g/100 ml. Dissolve 10 g of

ammonium citrate in distilled water and dilute to 100 ml.

16. Analysis Procedure

Wet-ash, in a mixture of nitric and perchloric acids, a

sufficiently large sample to give a minimum nickel content

of 0.5 yg (see copper procedure for details). Dilute the

acidic sample, or an appropriate aliquot, to 20 ml with distilled water and add, with mixing, 10 ml of ammonium citrate

solution and 1.0 ml of alcoholic 0.1 percent a- furildioxime

reagent. Adjust pH to 9.0 ± 0.3 with 1:1 NH^OH and extract

with 10.00 ml of chloroform. Repeat the extraction with a

second 10.00 ml portion of chloroform and backwash the

combined extracts with 10 ml of 1:40 NHi+OH. Place a looselyfitting paper plug in the shortened stem of the separatory

83
funnel and drain the chloroform extract into a 50-iiim cuvette.

Measure its absorbance at 435 nm against a reagent blank

carried through the entire dissolution and extraction

procedures. Determine the micrograms of nickel present from

a calibration curve prepared from 0.50, 1.00, 2.00, 3.00 and

4.00 yg nickel standards.

17. Results

Table 3 shows some typical results for the determination

of nickel in Orchard Leaves, NBS SRM 1571.

Table 3. Spectrophotometric determination of nickel

in Orchard Leaves, NBS SRM 1571.

Sample Wt., g Nickel, ppm

2 1.35; 1.41; 1.37; 1.33

Remarks

The use of the backwash step in the extraction procedure

effectively removes the interference of copper and cobalt.

Tests show that at least 50 yg of copper and 2 yg of cobalt

can be tolerated in the determination of 1 yg of nickel.

18. Reference

[1] Peshkova, V. M. , Bochkova, V. M. , and Lazareva, L. I.,

Spectrophotometric Determination of Traces of Nickel

in Pure Indium and Aluminum, J. Anal. Chem. USSR (Eng.

Trans.) 15^, 699-702 (1960). Original Article, Zhur.

Analit. Khim. 15. 610-613 (1960).

84
SECTION VI

FLUORESCENCE SPECTROMETRY

Determination o£ Beryllium by Fluorescence Spectrometry

S. A. Wicks and R. W. Burke

1. Objective

The procedure described in this

Analytical Chemistry Division to det

tissue samples at the submicrogram 1

spectrometry.

2. Principle

The classical procedure for the determination of beryllium by fluorescence spectrometry is based on its reaction

with morin. This method enjoys considerable popularity

because of its high sensitivity. However, it suffers from

two disadvantages: (a) the reagent is unstable, being

particularly prone to oxidation even by atmospheric oxygen,

and (b) the method lacks selectivity. These disadvantages

necessitate the use of a carefully controlled, and hence

tedious, procedure in order to obtain satisfactory results.

In our experience, a more useful method for determining

submicrogram amounts of beryllium, especially in tissue

samples, utilizes the fluorescence of the beryllium

2-hydroxy-3-naphthoic acid complex in water near pH 7.5.

This method has been described by Kirkbright, West and

Woodward [1] . We have examined this method in detail and

have subsequently made three modifications which significant

improve its practicality . (a) The need for silicone-coatin

the flasks to prevent absorption losses of beryllium has

been eliminated by developing the complex under weakly

acidic media (pH 2 to 3). The pH is then increased to 7.5

and the fluorescence measured at the end of five minutes.

85

section is used in the

ermine beryllium in

evel by fluorescence
(b) Phenol Red is used as an internal visual indicator for

adjusting pH and (c) Preseparat ion of beryllium by extraction

with acetylacetone [2] is used to improve selectivity and

sensitivity

.

3. Apparatus

Spectrofluorometer . Equipped with stabilized xenon

source, variable slits and excitation and emission monochromators

.

4. Reagents

Beryllium Stock Solution, 100 ppm. Dissolve 0.100 g

of beryllium metal in 10 ml of 1:2 hydrochloric acid. Add

10 ml of perchloric acid and evaporate to HClOi^ fumes.

Cool, transfer to a 1-liter volumetric flask and dilute to

the mark with distilled water.

Beryllium Working Standard A, 1 ppm. To 10 ml of the

beryllium stock solution, add 10 ml of perchloric acid and

dilute to the mark in a 1-liter volumetric flask with distilled water. Prepare fresh daily.

Beryllium Working Standard B, 0.1 ppm. Dilute 10 ml of

beryllium standard A and 1 ml of perchloric acid to the

mark, in a 100 ml volumetric, with distilled water. Prepare

fresh daily.

2-Hydroxy- 3-Naphthoic Acid, 2 x lO'^'M. Dissolve 38 mg

of 2-hydroxy-3-naphthoic acid in 5 ml of 1:10 ammonium

hydroxide, transfer to a 1-liter volumetric flask and dilute

to the mark with distilled water.

Masking Agent, Calcium-CDTA (cyclohexanediaminetetraacetic acid). Prepare the following solutions:

A. CaCl2, O.IM. Dissolve 7.35 g of CaCl2*2H20 in

500 ml of distilled water.

86
B. CDTA, O.IM. Dissolve 3.46 g CDTA in 90 ml of

distilled water by addition of 1:10 ammonium

hydroxide. Adjust to pH 7.5 and dilute to 100 ml

with distilled water.

Combine 25 ml of O.IM calcium solution and 6 ml of O.IM

CDTA in a 100-ml volumetric flask and dilute to volume with

distilled water.

Buffer Solution, pH 7.5. Dilute 50 ml of concentrated

ammonium hydroxide to 300 ml with distilled water. Add

sufficient glacial acetic acid to adjust pH to 7.5 and

dilute to 1 liter with distilled water.

Ethylenediaminetetraacet ic Acid, Disodium Salt Solution

(EDTA) , 10 Percent. Dissolve 10 g of the disodium salt in

100 ml of distilled water.

Acetylacetone , 5 Percent. Transfer 5 ml of acetylacetone to a 100 ml volumetric flask and dilute to the mark

with distilled water.

Phenol Red Indicator, 0.1 Percent. Dissolve 100 mg of

Phenol Red in 100 ml of distilled water.

Quinine Sulfate Standard, 5 ppm. Dissolve 5 mg of

quinine sulfate dihydrate in 1 liter of 0 . 5M sulfuric acid.

5. Analysis Procedure

Wet-ash 1 to 5 g of dried tissue in a 1:1 mixture of

nitric and perchloric acid in a platinum dish. Use 10 ml of

the acid mixture for a 1-g sample and increase proportionately

to 30 ml for sample weights in the range 1 to 5 g. If

silica is present, add 1 ml of hydrofluoric acid. Cover the

dish with a platinum lid and digest on a hot plate until

oxides of nitrogen are no longer evolved. Cool, remove and

wash the cover and the sides of the dish with 10 ml of

distilled water. Heat to fumes of perchloric acid and

continue fuming until volume has been reduced to approximately

1 ml. Dilute to 50 ml and add 2.0 ml of 10 percent EDTA

87
solution. Add 2 to 3 drops of Phenol Red and adjust pH to 7

to 8 with 1:10 NHi+OH (until indicator just turns pink). Add

5 ml of five percent acetylacetone and readjust pH to 7 to

8. Allow the solution to stand for five minutes and then

extract it with three 5-ml portions of chloroform. Combine

the extracts in a platinum dish and cover with 10 ml of

water, 2 ml of concentrated nitric acid and 2 ml of concentrated perchloric acid. Evaporate off the chloroform on a

steam or water bath, transfer the dish to a hotplate and

continue the evaporation to dryness. Add 2 ml each of

nitric and perchloric acid and repeat evaporation to dryness.

Dissolve the residue in 10 ml of 0 . IN perchloric acid and

transfer to a 100-ml volumetric flask. Dilute to approximately

70 ml with distilled water and add 5.0 ml of the calcium

CDTA solution and 2 to 3 drops of Phenol Red solution.

Adjust to pH 7 to 8 with 6N NH^OH. Add 5.0 ml of pH 7.5

buffer and 5.0 ml of 2 -hydroxy- 3 -naphthoic acid reagent.

Dilute to volume with distilled water and mix. Transfer to

a 10-mm cuvette and, after exactly five minutes, excite the

beryllium complex at 373 nm and measure its fluorescence at

460 nm. Use a 0.5 ppm quinine sulfate solution in 0 . IN

sulfuric acid to correct for short term fluctuations in

instrument sensitivity. This solution should be prepared

fresh daily by ten-fold dilution of the stock quinine sulfate

with distilled water. Determine the amount of beryllium

present by comparing the measured fluorescence with that of

a series of beryllium standards covering the range

0.05-0.5 yg Be.

6. Results

Table 1 summarizes the results obtained for beryllium

in three SRM's which are currently available through the NBS

Office of Standard Reference Materials.

88
Table 1. Beryllium content of three SRM's as determined by

the acetylacetone extraction-2-hydroxy-3-naphthoic

acid spectrofluorometric method.

Sample SRM No. Beryllium, ppm

Orchard Leaves 1571 0.026; 0.025; 0.025; 0.026

Bovine Liver 1577 0.021; 0.013; 0.020; 0.015

Coal 1632 1.52; 1.46; 1.50

Remarks

Some preliminary experiments carried out during the

analysis o£ the orchard leaves suggest strongly that the

preparative wet oxidation should be performed in platinum

rather than Teflon. When 100-ng spike additions were made

to 1-g samples of orchard leaves and the dissolution subsequently performed in 100 ml Teflon beakers, the recovery

of the added beryllium was low and non-reproducible, usually

in the range of 10 to 40 percent. The reason for this

behavior is not understood at present. Recoveries from

borosilicate glass container and platinum, however, were

much superior to those observed from Teflon. Spike recoveries

from platinum through the entire analysis procedure were 95

percent or better.

7. References

[1] Kirkbright, G. F., West, T. S. and Woodward, C,

Spectrofluorometric Determination of Submicrogram Amounts

of Aluminum and Beryllium with 2-Hydroxy- 3-Naphthoic

Acid, Anal. Chem. 37^, 137-143 (1965).

[2] Adam, J. A., Booth, E. and Strickland, J. D. H. , The

Determination of Microgram Amounts of Beryllium Using

Acetylacetone, Anal. Chim. Acta 6, 462-471 (1952).

89

SECTION VII

POLAROGRAPHY

Determination o£ Aluminum, Bismuth, Cadmium, Copper, Iron,

Lead, and Nickel.

Analysis of Botanical Standard Reference Materials by

Cathode Ray Polarography

E. June Maienthal

1. Objective

The procedure described in this section was developed

and applied to the quantitative determination of traces of

aluminum, bismuth, cadmium, copper, iron, lead, and nickel

in botanical material by cathode ray polarography

.

2. Principle

The principle of polarography is based on the unique

characterization of the voltage - current curves, or polarograms , produced when a microelectrode is immersed in an

electrochemically active solution. The abscissa, or voltage

parameter, of the polarogram identifies the reducible or

oxidizable chemical element present in a complex solution.

The ordinate, or current parameter of the polarogram gives

an information on the quantity of a particular element

present in the solution.

The method is specific and sensitive, and requires only

small volumes of solution for the analysis.

^ The material used in this section was taken from a work

published by the author in the Journal of the Association

of Official Analytical Chemistry, Vol. 55, No. 5, p. 1109

to 1113 (1972)

.

91
3. Procedure

Instrumentation: The Davis Differential Cathode Ray

polarograph was used in performing the measurements described

in this section.

Reagents: All solution are aqueous unless otherwise

specified

.

(a) Cupferron solution — 6 percent.

(b) Sodium acetate solution — 2M.

(c) Sodium acetate (2M) + EDTA (disodium salt)

solution (0 . IM)

.

(d) Solochrome Violet RS solution — 0.1 percent.

(e) Sodium tartrate solution — 20 percent.

(f) Potassium cyanide solution — 20 percent.

(g) Sodium diethyldithiocarbamate solution — 0.1 percent.

(h) Sodium citrate solution — 10 percent.

(i) Hydroxylamine HCl solution — 20 percent.

(j) Dimethylglyoxime solution — one percent in alcohol.

Iron and Aluminum, Acid-Soluble. Transfer 250 mg

sample which has been dried 24 hours at 90 °C to 50 ml

Erlenmeyer flask. Add 5 ml HNO3, 1 ml HCl, and 1 ml perchloric acid; cover and heat cautiously until vigorous

frothing ceases. Treat blank and standard solutions for the

calibration curve similarly. Digest and fume to perchloric

fumes, adding more HNO3 if some organic material remains,

and evaporate to dryness. Dissolve residue in 1 ml HCl and

evaporate to dryness at <100 °C. Dissolve residue in 1 ml

perchloric acid (1 + 1) , transfer to 125 ml separatory funnel

and dilute to ca 20-25 ml. Add 1 ml of six percent cupferron

solution which has been filtered and cooled in ice and

extract with four 10 ml portions of cold CHCI3, adding a few

drops of cupferron before each extraction. (If a reddish

brown precipitate is formed more cupferron is needed.)

Reserve combined organic extracts for determination of iron.

Add 10 ml 2M sodium acetate solution to aqueous layer, add

92
1 ml six percent cupferron solution, wait 1-2 minutes and

extract with 15, 10, and 5 ml portions of CHCI3. Reserve

combined organic extracts for determination of aluminum.

(a) Iron. Let CHCI3 extract evaporate to dryness in

stream of filtered air and destroy residual organic material

by evaporation to dryness with a few ml HNO3 and 1 ml

perchloric acid. Add 1 ml HCl and evaporate to dryness at

<100 °C. Dissolve residue in 0.4 ml H2SO4 (1+1); then add

5 ml 2M sodium acetate +0.1M EDTA . (disodium salt) solution

and adjust pH to ca 4 with NHi+OH. Dilute to 10.0 ml.

Transfer portion of solution to polarographic cell, deaerate

with nitrogen, and measure iron reduction peak subtractively

with differential cathode ray polarograph at ca —0.2V vs.

mercury pool anode, using background blank in second cell.

(b) Aluminum. Remove CHCI3, destroy organic material,

and evaporate with HCl as described above for iron. Dissolve

residue in 1 ml perchloric acid (1+1) and 5 ml of 2M sodium

acetate solution. Add 5 ml 0.1 percent Solochrome Violet RS

solution. Dilute to 50.0 ml and transfer to polyethylene

bottle fitted with cap. Heat in water bath five minutes at

55-70 °C, cool, transfer to polarographic cell, deaerate,

and measure aluminum-dye complex reduction peak with cathode

ray polarograph at ca -0.65V vs mercury pool anode. For

aluminum, better peak resolution is obtained if blank signal

is measured separately and then subtracted from that of

sample

.

Iron and Aluminum—Total . Proceed as above except

filter the solution remaining after sample dissolution

through Whatman No. 42 paper. Wash with HCl (1+9). Transfer

residue to platinum crucible, dry, and ignite over burner.

Add two drops perchloric acid and 10 drops hydrofluoric

acid, and evaporate to dryness. Fuse residue in small

amount of sodium carbonate. Add water and then HCl dropwise

to neutralize sodium carbonate and transfer solution to

93
reserved filtrate which has been evaporated to dryness.

Proceed as described above for determination of iron and

aluminum

.

Lead and Bismuth. Wet-ash 1 g samples of dried

material and evaporate as already described, except that

ca 5-10 ml more of HNO3 may be necessary to destroy all

organic material. Treat blank and standard solutions for

calibration curve similarly. Dissolve residue in 10 drops

HCl and ca 10 ml water. Add ca 100 mg ascorbic acid to

reduce iron and transfer solution to 125 ml separatory

funnel. Add 10 ml 20 percent sodium tartrate solution,

30 ml HNi^OH, 10 ml 20 percent potassium cyanide solution,

and 10 ml 0.1 percent sodium diethyldithiocarbamate . Extract

lead and bismuth complexes with 15, 10, and 5 ml portions of

CHCI3. Evaporate CHCI3 extracts and destroy organic material

as already described. Dissolve residues in 0.5 ml HNO3

(5+95) and 2.0 ml 20 percent sodium tartrate solution.

Deaerate solution and measure polarographically as already

described. The bismuth reduction peak occurs at ca -0.5V

and lead at ca -0.7V vs. mercury pool anode.

Nickel. Wet-ash 1 g samples of dried material and

evaporate as described for lead and bismuth. Treat blank

and standard solutions for calibration curve similarly.

Transfer solution to 30 ml separatory funnel with water,

keeping final volume <10 ml. Add 1 ml 10 percent sodium

citrate solution, 2 ml 20 percent hydroxylamine HCl, and

1 ml one percent dimethylglyoxime in alcohol. Make solution

ammoniacal to methyl red plus three drops excess. Extract

nickel complex with 15, 10, and 5 ml portions of CHCI3.

Evaporate CHCI3 and destroy organic material as already

described. Dissolve residue in 0.2 ml H2SO4 (1+1) and a

small amount of water. Add 1 ml pyridine, dilute solution

to 5.0 ml, and measure nickel polarographically as already

described at ca -0.9V vs. mercury pool anode.

94
4. Discussion

The need for certified botanical standards both in

research and in industrial and field applications is leading

to the development and production of a number of new types

of Standard Reference Materials by the National Bureau of

Standards. Workers in the botanical and agricultural areas

have long needed standards to assist in trace element determinations in leaves and plants. These standards would also

be useful in the analysis of environmental samples such as

air particulates and contaminated vegetation, and for calibration of much of the automated equipment now being used in

these fields. Efforts have first been directed toward

orchard, alfalfa, citrus, and tomato leaves, aspen chips,

and pine needles.

The first of these botanical standards to be prepared,

analyzed, and certified was Orchard Leaves SRM 1571 [1]

.

This standard was prepared from fruit leaves, mostly peach,

with some apple and cherry leaves, gathered from an orchard

near Lansing, Michigan.

In order to certify any given element in a Standard

Reference Material, two completely independent methods or

an accepted referee type method by two different laboratories must be used. The methods must be both sensitive and

accurate

.

The most frequently used methods of analysis by workers

in botanical areas have included spectrophotometry, atomic

absorption, emission spectroscopy, and classical wet chemical methods. Polarography has been utilized very little.

Some of the polarographic methods have been described by

Nangniot [2,3].

It is difficult to understand why polarography has not

been more widely used when the sensitivity, accuracy, and

resolution of the technique are considered. Differential

cathode ray polarography has been applied successfully in

95
this laboratory for the past few years for the determination

of about 30 elements in many different matrices such as

steels and alloys [4-10], food [9,10], environmental samples

[5,7,9-11], and high purity reagents [6,7,9,10] covering

concentration ranges from 3 x 10 ^ to 100 percent. The

reliability of these polarographic determinations has been

verified through interlaboratory comparisons with many other

techniques

.

The proposed scheme was first to determine polarographi

cally iron and aluminum in the orchard leaves; later lead,

bismuth, copper, nickel, and cadmium were included.

Typical fruit leaf compositions range from about 100 to

300 ppm for both iron and aluminum, so sensitivity would be

no problem. Previous work here has resulted in the determinations of as little as 0.1 yg iron or aluminum. In these

cases the limitation on sensitivity was not instrumental but

rather due to the blank coming from the reagents and the

environment. For the iron and aluminum levels in the orchard

leaves, the blank would probably be low enough with respect

to the samples to be of no consequence.

Iron could be determined polarographically with no

separations, but aluminum would have to be separated. Cupferron extraction at pH 4.5 has been used often here for

separation of aluminum from the matrix [6] . This involves a

preliminary extraction of iron cupferrate from an acid

solution, so it seemed advantageous to reserve the extracted

iron for its subsequent polarographic determination.

The best supporting electrolyte for the polarographic

determination of aluminum is obtained by forming an aluminumdye complex with Superchrome Garnet Y [8,12] or Solochrome

Violet RS [12-15]. Iron could be determined with fewest

interferences in an EDTA-sodium acetate supporting electrolyte

Prior to the extractions, the organic material must

first be removed by wet ashing. After this step a small

amount of insoluble siliceous residue, which undoubtedly

96
contained iron and aluminum, was always present; bismuth,

cadmium, copper, lead, and nickel are not present in

measurable amounts in this type of residue. This was

probably the result of external sand or soil contamination

of the fruit leaves and its content would have no relation

to the actual leaf composition. Some nondestructive techniques, however, would only determine the total iron or

aluminum, so it seemed advisable to determine separately the

acid-soluble portion and then the total iron and aluminum on

a separate sample by treating the residue with hydrofluoric

acid, fusing it with sodium carbonate, and adding the fused

residue to the original solution.

For lead and bismuth, a simplification of a procedure

for their simultaneous determination, used in this laboratory

for steels and cast iron [5,16], appeared feasible, since

the orchard leaf matrix is much simpler for polarography

than that of steels.

For the determination of nickel, a polarographic method

developed for its determination as a trace element in Glass

SRM 614 [1,10] appeared to be equally feasible, since the

nickel in the orchard leaves was expected to be at about the

1 ppm level. The procedure described in this section was

also used to determine iron, nickel, lead, and cadmium in

tomato leaves and spinach Standard Reference Materials.

5. Results

The results for acid-soluble iron are shown in Table 1.

The results for iron on the first set were not reported

because of the extreme variation. The values on this set

ranged from 195 to 359 ppm which is a far greater variation

than has ever occurred before in this laboratory and could

only be ascribed to gross and erratic environmental contamination or sample inhomogeneity . The normal blank for

this procedure was in the vicinity of 0.3 ppm. The overall

97
average value of 262 ppm is in good agreement with a value

of 278 ppm later obtained by isotope-dilution, spark-source

mass spectrometry [R. Alvarez, P. J. Paulsen, and

C. W. Mueller (1970)], and the recommended value which

appears on the provisional certificate [1] is 270 ppm.

Table 1. Polarographic determination of iron

(acid soluble) in orchard leaves.

- - - Fe, ppm - - -

S amp 1

e

Set II Set III

2 260 258

9 257 282

10 263 272

11 257 259

12 257 255

13 254 253

15 258 282

19 262 267

Average 259 256

s , ppm 3.0 11.7

Pooled average 262

Average s 9.1

HCl-HNOa-HClO^

.

The results obtained for aluminum on the same samples

are shown in Table 2. No other values for acid-soluble

aluminum have been reported, hence this value has not been

certified.

98
Table 2. Polarographic determination o£ aluminum

(acid-soluble)^ in orchard leaves.

- Al , ppm -

Sample Set I Set II Set III

2

0 0 /:r 226 lost Zl 0

9 226 2 2 2

Oil 211

10 228 241 224

1

1

J. X 7 ? 223 225

12 220 226 204

13 225 229 222

15 223 226 222

19 230 228 225

Average 225 228 219

s , ppm 3.0 6.3 7 .

6

Pooled average 224

Average s 6.7

^ HCI-HNO3-HCIO4 •

The results for total iron determined on new samples

are shown in Table 3. From comparison with the acid-soluble

iron value, it is seen that the residue contains about 50 ppm

of the total sample equivalent of iron. Although isotopedilution spark-source mass spectrometry analysis was not

used for total iron, the insoluble residue was examined by

semiquantitative emission spectroscopy [V. C. Stewart (1970)]

and was found to contain iron in the 0.1-1 percent range

(estimated to be about 0.5%). This corresponds to about

25 ppm in the total sample, which, added to the mass

spectrometric value of 278 ppm, gives a total iron value

of 303 ppm, in good agreement with the polarographic value

of 312 ppm.

99
Table 3. Polarographic determination of iron

(total) in orchard leaves.

- - - Fe , ppm - - -

Sample Set II Set III

L 0 L L

7 Q 7

Q 0 L 0 J J. H

0 J D

11 317 310

12 320 310

13 305 295

15 318 317

19 316 300

Average 320 305

s , ppm 8 . 7 9 . 0

Pooled average 312

Average s 11.4

HCI-HNO3-HCIO1, -HF, NazCOs fusion.

The total aluminum was then measured and the results

are shown in Table 4. From comparison with the acid-soluble

aluminum value of 224 ppm (Table 2) , it is seen that the

residue contains 123 ppm of the total sample equivalent of

aluminum. The results obtained so far are not in good

agreement and aluminum has not been certified. A value of

420 ppm was obtained by activation analysis [T. E. Gills

(1970)]. In a round-robin study [P. D. LaFleur and

T. W. Hears (1971)], values ranging from 99 to 401 ppm were

reported by 11 non-NBS optical emission spectroscopy

laboratories

.

100
Table 4. Polarographic determination of aluminum

(total)^ in orchard leaves.

- - - Al , ppm - - -

S amp 1

e

Set II Set III

2 359 344

9 340 349

10 340 341

11 340 348

12 345 348

13 341 350

15 346 350

19 350 368

Average 345 350

s , ppm 6.7 8.0

Pooled average 347

Average s 7.5

^ HCl-HNOa-HClOu-HF, NazCOs.

The results for lead and bismuth are shown in Table 5.

The lead average of 44.6 ppm was considerably higher than

the anticipated value of about 5 to 10 ppm; however these

results were later confirmed by two other techniques. A

value of 43 ppm was obtained, using isotope-dilution, sparksource mass spectrometry, and a value of 45 ppm, using photon

activation analysis [G. J. Lutz (1971)]. Another set was

analyzed by the polarographic procedure, utilizing the

hydrofluoric acid treatment, in order to verify that no

lead was in the siliceous residue. An average of 44 ppm

was obtained, thus the residue contained an insignificant

amount of lead.

101
Table 5. Polarographic determination of bismuth

and lead in orchard leaves.

Sample Bi , ppm Pb, ppm

2 0\j • A-10yj 41 8

q u • J. u 46 3

1 0 0 1 0 15 44 2 44 8

11 0.11 44.2

12 0.11, 0.11 43.6, 44.2

13 0.12 43.0

15 0.09, 0.10 47.9, 47.9

19 0.09, 0.10 43.6, 44 . 2

Average 0.11 44.6

s , ppm 0.02 1.7

Each sample from bottle 15 gave significantly higher

lead values than did those from the other bottles; this

indicated that some lead inhomogeneity might exist.

Lead has now been certified at 45 ppm, but no other

values have been reported for bismuth, hence it is not

certified

.

The results for nickel are shown in Table 6. The

average of 1.4 ppm is in good agreement with an average of

1.2 ppm obtained by isotope-dilution, spark-source mass

spectrometry. Nickel has not been certified at 1.3 ppm [1]

.

102
Table 6. Polarographic determination of

nickel in orchard leaves.

Sample Ni, ppm

2 J- • •+

9 1 5

10 1.2

11 1.5

12 1.3

15 1.5

19 1.5

Average 1.4

s , ppm 0.1

The investigation was extended to determine cadmium.

Cadmium is also extracted by carbamate along with lead and

bismuth. The peaks were very small and poorly defined for

a 1 g sample but gave approximate values of 0.1 ppm. It was

decided to take larger samples and try to determine cadmium,

copper, lead, and bismuth simultaneously. By omitting the

potassium cyanide during the extraction, copper would also

be extracted. Five gram samples were taken through the

procedure and bismuth, copper, lead, and cadmium were measured simultaneously in a 10 percent hydroxylamine hydrochloride supporting electrolyte.

An average value of 0.11 ppm was obtained for bismuth

which is in perfect agreement with the 0.11 obtained earlier.

A value of 11.5 ppm copper was obtained which agrees with the

12 ppm value by neutron activation analysis [D. A. Becker

(1971)] and isotope-dilution, spark-source mass spectrometry,

and copper has been certified at 12 ppm. A value of 44.5 ppm

lead was found which is also in good agreement with the

previous values. The cadmium content was found to be

103
0.10 ppm which is in good agreement with the 0.11 ppm obtained

by atomic absorption [T. A. Rains (1971)].

Both the polarographic methods described here and the

new Standard Reference Material which has now been certified

for 18 elements with information given for nine others should

be very useful in many kinds of analytical problems, particularly those concerned with trace metals in foods, plants,

and other environmental samples.

6. References

[1] Catalog of Standard Reference Materials, NBS Special

Publication 260, July 1970 (and July 1971 supplement),

Office of Standard Reference Materials, National

Bureau of Standards, Washington, D. C. (available from

U.S. Superintendent of Documents, U.S. Government

Printing Office, Washington, D. C. 20402).

[2] Nangniot, P., J. Electroanal. Chem. 7_, 50-59 (1964).

[3] Nangniot, P., J. Polarogr. Soc. 11, 42-52 (1965).

[4] Maienthal, E. J. and Taylor, J. K. , Anal. Chem. 37_,

1516-1519 (1965).

[5] Maienthal, E. J., NBS (U.S.), Tech. Note 273, 8-13

(1965) .

[6] Maienthal, E. J., NBS (U.S.), Tech. Note 403, 22-40

(1966) .

[7] Maienthal, E. J., NBS (U.S.), Tech. Note 425, 23-41

(1967) .

[8] Maienthal, E. J., NBS (U.S.), Tech. Note 455, 19-40

(1968) .

[9] Maienthal, E. J., NBS (U.S.), Tech. Note 505, 17-36

(1969) .

[10] Maienthal, E. J., NBS (U.S.), Tech. Note 545, 41-69

(1970) .

104
[11] Maienthal, E. J. and Taylor, J. K. , in Advances in

Chemistry Series, Vol. 73, "Trace Inorganics in Water,"

American Chemical Society, Washington, D. C. pp. 172-

182 (1968).

[12] Florence, T. M. and Izard, D. B. , Anal. Chim. Acta 2_5,

386-389 (1961).

[13] Willard, H. H. and Dean J. A., Anal. Chem. 1264-

1267 (1950).

[14] Perkins, M. and Reynolds, G. F., Anal. Chim. Acta, 1_8,

616-631 (1958).

[15] Rooney, R. C, Analyst, 8_3, 546 (1958).

[16] Rooney, R. C, Analyst, 83, 83-88 (1958).

105
ACKNOWLEDGMENT

The editor acknowledges the permission received from

the various scientific journals to reproduce the papers

contained in the Appendix. Thus:

Appendices 1, 3, 6, 7, 9, and 13 were reprinted by permission

of Analytical Chemistry; Appendix 5 was reprinted by permission of the Journal of Radioanalytical Chemistry; Appendix 8

was reprinted by permission of Spectrochimica Acta; Appendix

10 was reprinted by permission of Applied Spectroscopy;

Appendices 11 and 12 were reprinted by permission of the

American Society for Testing and Materials; Appendix 13 was

reprinted by permission of Marcel Dekker, Inc.; Appendix 15

was reprinted by permission of the Journal of the Association of Official Analytical Chemists; Appendix 16 was

reprinted by permission of the Revue du GAMS.

The editor further acknowledges with gratitude the

assistance received during the preparation of this work

from the following organizations and persons:

-The Human Studies Laboratory of the Environmental

Protection Agency for encouragement, and support given

under Contract No. EPA AG-DS-0568, which made this

publication possible;

-The Office of Standard Reference Materials of the

Institute for Materials Research at NBS, J. P. Call,

Chief, for continuous support throughout various phases

of the experimental work;

-Philip D. LaFleur, Chief, Analytical Chemistry Division,

Institute for Materials Research at NBS, for advice and

critical review of this publication;

-H. L. Rook and T. E. Gills, Neutron Activation Analysis

Section, Analytical Chemistry Division at NBS, for

continuous guidance and help during the preparation of

this work;

106
-J. R. DeVoe, Chief, Special Analytical Instrumentation

Section, and T. J. Murphy, Analytical Spectrometry

Section at NBS , and J. I. Shultz, ASTM Research Associate

at NBS, for critically reviewing the manuscript;

-Mrs. Rosemary Maddock, Technical Information Service,

Institute for Materials Research at NBS, for editorial

assistance and for ascertaining that the terminology used

in this work conforms to NBS style and format;

-The Analytical Chemistry Division Text Editing Facility

for typing the manuscript.

107
APPENDIX

This Appendix contains sixteen reproductions from

papers describing various phases of the analytical

procedures assembled in this volume.

It was intended to provide, in a convenient manner,

additional relevant scientific information which could not

be included in the main body of this work.

It is hoped that this information will be useful to

the analyst.

The titles and names of the authors of the papers

contained in this Appendix are listed below:

1. Production and Analysis of Special High-Purity Acids

Purified by Sub-Boiling Distillation, by E. C. Kuehner,

R. Alvarez, P. J. Paulsen, and T. J. Murphy.

2. The Role of the Analytical Blank in Accurate Trace

Analysis, by T. J. Murphy.

3. Evaluation of Lyophilization ' for the Preconcentration

of Natural Water Samples Prior to Neutron Activation

Analysis, by S. H. Harrison and P. D. LaFleur.

4. Activation Analysis with the Nuclear Reactor, by

P. D. LaFleur and D. A. Becker.

5. Characterization of a Nuclear Reactor for Neutron

Activation Analysis, by D. A. Becker and P. D. LaFleur.

6. Method for Determination of Mercury in Biological

Materials by Neutron Activation Analysis, by H. L. Rook,

T. E. Gills, and P. D. LaFleur.

7. Simultaneous Determination of Trace Elements in Platinum

by Isotope Dilution and Spark Source Mass Spectrometry,

by R. Alvarez, P. J. Paulsen, and D. E. Kelleher.

8. Determination of Trace Elements in Zinc by Isotope Dilution Spark Source Mass Spectrometry, by P. J. Paulsen,

R. Alvarez, and D. E. Kelleher.

108
9. Spark Source Mass Spectrographic Analysis of Ingot Iron

for Silver, Copper, Molybdenum, and Nickel by Isotope

Dilution and for Cobalt by an Internal Standard Technique,

by P. J. Paulsen, R. Alvarez, and C. W. Mueller.

10. Trace Element Determinations in a Low-Alloy Steel Standard Reference Material by Isotope Dilution, Spark

Source Mass Spectrometry, by P. J. Paulsen, R. Alvarez,

and C. W. Mueller.

11. Chemical Aspects of Atomic Absorption, by T. C. Rains.

12. Atomic Absorption Spectrometry - General Considerations

for the Application of Experimental Techniques, by

T. C. Rains.

13. Flame Emission Spectrometry with Repetitive Optical

Scanning in the Derivative Mode, by W. Snelleman,

T. C. Rains, K. W. Yee, H. D. Cook, and 0. Menis.

14. Standard Solutions for Flame Spectrometry, by J. A. Dean

and T. C. Rains.

15. Determination of Submicrogram Amounts of Mercury in

Standard Reference Materials by Flameless Atomic

Absorption Spectrometry, by T. C. Rains and 0. Menis.

16. Discussion of Some Experimental and Fundamental Conditions in Analytical Flame Spectroscopy, by R. Mavrodineanu

.

109
APPENDIX I Production and Analysis of Special High-Purity

Acids Purified by Sub-Boiling Distillation

Edwin C. Kuehner, Robert Alvarez, Paul J. Paulsen, and Thomas J. Murphy

Analytical Chemistry Division, Institute for Materials Research, National Bureau of Standards, Washington, B.C. 20234

Sub-boiling distillation from pure quartz or Teflon

(Du Pont) stills has been Investigated for the production

of high-purity inorganic acids and water. Nitric, hydrochloric, hydrofluoric, perchloric, and sulfuric acids

produced by this method contained significantly lower

cationic impurities than high-purity acids from commercial sources. A complete system, including the

Class 100 environment, production, and storage of

these high-purity reagents is described. A method

based on spark source mass spectrographic isotope

dilution analysis has been developed for the simultaneous determination of 17 elements in these materials. Results of the analyses of both the acids purified by sub-boiling distillation and the ACS reagent

grade acids used as starting materials are reported.

The sum of the common Impurity elements determined

in the purified acids ranged from 2.3 ppb in nitric acid

to 27 ppb in sulfuric acid. No element in any of the

purified acids exceeded 10 ppb and most were well below the 1-ppb level.

The Analytical Chemistry Division at the National Bureau of Standards has become increasingly involved with

the analysis of samples and Standard Reference Materials (SRM's) requiring the determination of elements at the low

parts per million (ppm, 10~« g/g) to the parts per billion (ppb, 10~' g/g) concentration range. This is illustrated by

recent analyses which include lead at 3.31 ppm in lunar samples (/), uranium at 72.1 ppb in Trace Elements in Glass

(SRM 616) (2), nickel at 1.3 ppm in Orchard Leaves (SRM

1571) (2), and strontium at 0.14 ppm in Bovine Liver (SRM

1577) (2). The low level of these elements puts stringent

requirements on the purity of reagents used in the analytical

procedure.

Inorganic acid purity is of particular importance because of

the relatively large amount of these acids required for sample

dissolution and other chemical operations. These procedures can require quantities of acids in excess of ten times the sample weight. To take full advantage of the sensitivity and

accuracy of an analytical technique, the reagent blank should

be held to no more than a few per cent of the amount being

determined. Therefore, the accurate determination of an

element near the 1-ppm level will depend on the availability of

acids containing no more than 1 ppb of the element. High-purity inorganic acids have been available for some

time from commercial sources. Although these acids are

satisfactory for many trace element determinations, they are not always adequate for low level trace work either through a lack of purity or high upper limit specifications. [Emission

(1) \. L. Barnes, B. S. Carpenter, E. L. Garner, J. W. Gramlich,

E. C. Kuehner, L. A. Machlan, E. J. Maienthal, J. R. Moody,

L. J. Moore, T. J. Murphy, P. J. Paulsen, K. M. Sappenfield,

and W. R. Shields, "Isotopic Abundance Ratios and Concentrations of Selected Elements in Apollo 14 Samples," Proc. ApoUo

14 Lunar Sci. Conf., Geochim. Cosmochim. Acta Suppl. 3, 2, MIT Press, Cambridge, Mass., in press.

(2) Office of Standard Reference Materials, NBS Spec. Pubt. 260

(1970).

spectrography (i), the technique usually applied to the analyses of high-purity acids, lacks sensitivity causing upper

limits to be set at 1 ppb or higher.] The determination of the

isotopic composition and concentration of lead in lunar samples can be cited as a recent example. A reagent blank

of 340 ng Pb was calculated from the producer's values for lead in commercial high-purity acids and the volume of these

acids required to dissolve one gram of lunar rock and separate

the lead from other constituents. Since the expected Pb concentration was only a few ppm, the introduction of this quantity of external lead (of a different isotopic composition)

would have precluded a reliable determination of the Pb

present either as to amount or isotopic composition. Using

acids purified by sub-boiling distillation, the total blank was

determined to be 5 ng for the actual analysis. Consequently,

because of this experience and other problems associated with

the determinations of trace metals in a variety of materials, the Analytical Chemistry Division at NBS recently set up

facilities for the "in-house" purification, analyses, and dis- tribution of inorganic acids. Sub-boiling distillation was selected as the method of

purification for the inorganic acids. No single purification

procedure is capable of removing all classes of impurities

from these acids. Since the trace element program at NBS is mainly concerned with trace metals, the technique which ap- peared to be most efficient in removing metallic or cationic

impurities, sub-boiling distillation, was studied. In subboiling distillation, infrared radiators vaporize the surface without boiling the liquid in the vaporizer compartment.

The vapor is condensed on a tapered cold finger and the

distillate is collected in a suitable container. Sub-boiling

distillation was selected over conventional or boiling distillation since studies have shown that in the latter method

significant contamination of the distillate occurs from creeping

of the unrectified liquid and entrainment of particulates in the vapor stream formed during bubble rupture (4). Subboiling distillation completely eliminates the entrainment

problem since no bubbles are formed. Creeping of unrectified liquid from the vaporizer is minimized by the position

of the cold finger type condenser. It should be pointed out

that while this is an extremely efficient still for the separation

of impurities of low vapor pressure such as metal ions, it offers

little purification from impurities of high vapor pressure such

as organic matter or many of the anions. Commercial quartz sub-boiling stills which were designed

for the production of high-purity water have been used for the production of high-purity mineral acids of low-level lead content at the Carnegie Institution of Washington (5). Similar sub-boiling quartz stills (Quartz Products Corp.,

Plainfield, N.J.) were installed at NBS for the production of

(3) N. A. Kershner, E. F. Joy, and A. J. Barnard, Jr., Appl. Spectrosc.,25, 542(1971).

(4) R. C. Hughes, P. C. Murau, and G. Gundersen, Anal. Chem.,

43, 691 (1971), and references cited therein.

(5) K. D. Burrhus and S. R. Hart, ibid.. 44, 432 (1972).

Reprinted from ANALYTICAL CHEMISTRY, Vol. 44, Page 2050. October 1972 American Chemical Society

110
hydrochloric, nitric, perchloric, and sulfuric acids as well as water. An all Teflon (Du Pont) sub-boiling still was designed

and constructed at NBS for the production of high-purity

hydrofluoric acid.

Efficient purification by the still only partially solves the problem of producing and distributing pure acids. Airborne

particulate contamination and container contamination must

also be minimized to ensure a high quality product. Air- borne particulate contamination can be virtually eliminated by

enclosing the still and distillate containers in a Class 100

clean air chamber. Teflon FEP bottles which have been

vigorously cleaned with nitric and hydrochloric acids are used

as containers for the purified acids and high-purity quartz is used for water. To evaluate the purity of the acids and water produced by

this process, an analytical method based on isotope dilution- spark source mass spectrometry (SSMS) was developed. This

method has limits of detection as low as 0.01 ppb for common

impurity elements. Seventeen elements were determined

simultaneously in each of the high-purity acids and water pro- duced by sub-boiling distillation and the ACS grade acids used

as starting materials.

APPARATUS FOR THE SUB-BOILING DISTILLATION

OF HIGH-PURITY ACIDS AND WATER

Quartz Sub-Boiling Still. The commercially available subboiling still shown in Figure 1 is made of quartz. This type

still is used for the production of HCl, HNO3, HCIO4, H2SO4,

and H2O. Heating of the liquid being distilled is done by a

pair of infrared radiators positioned on both sides of the condenser. These elements, inside quartz tubes, heat the sur- face of the liquid and evaporate it without causing it to boil. This positioning also serves to heat the walls above the liquid

tending to keep them dry which minimizes creep of liquid

along the walls between the liquid reservoir and the condensing

cold finger. The condenser is tilted downward toward the

distillate outlet to allow the condensed liquid to flow to the

tip above the outlet. The still is fed by a 6-8 pound bottle of ACS reagent grade acid through a liquid level control which maintains the liquid to just below the overflow height.

Approximately 400 to 500 ml of liquid are thus maintained

inside the still at all times. A three-way stopcock on the

liquid level control is used to drain the still after the consumption of each bottle of ACS reagent grade feed acid. All parts

of the liquid feed system are made of Teflon. Teflon Sub-Boiling Still. An all-Teflon (Du Pont) subboiling still having all the essential features of the quartz still was designed and constructed for the production of hydrofluoric acid. This still, shown in Figure 2, was fabricated

starting with a commercial 2-liter Teflon bottle. The heaters, condenser, acid inlet, and the overflow were inserted into what was originally the bottom of the bottle and the distillate outlet was inserted at the bottle cap. For the heaters. Teflon

rods are machined to form 19-mm o.d. closed-end tubes. Heating coils inside glass tubes are then inserted into the heater tubes. The maximum operating temperature of the

still is limited by the softening point of the Teflon around the

heater. The glass tubing serves to support the heating tube. The cold finger condenser is similarily machined into a 25- mm o.d. tube from solid rod. Both the condenser and heater tubes are threaded at the open ends to fit into conversion

fittings which are in turn threaded into tapped holes in the bottom of the bottle. The tapped hole in the condenser

fitting is machined at an angle such that the condenser is tilted downward toward the distillate outlet. Because HF

and other liquids do not adhere readily to Teflon, the distillate drips ofi" instead of flowing down along the condenser.

Therefore, a trough is secured under the condenser to catch the HF and direct it to the distillate outlet. A piece of tubing

FEED BOTTLE

t DRAIN

Figure 1. Pure quartz sub-boiling still

FEED BOTTLE

i DRAIN

Figure 2. All-Teflon sub-boiling still

inserted through the bottle cap using commercial Teflon

fittings serves as a collector for the distillate. The tube is provided with an umbrella at the end to protect the collection

bottle from possible fall-out contamination. The acid feed and overflow tubes are attached with Teflon fittings threaded

into the bottle bottom. Teflon tape is used when necessary on the threads of the fittings to ensure a liquid tight seal. Clean Air Chamber. Each sub-boiling still is housed in a

clean air chamber (Environmental Air Control, Inc., Annandale, Va.) to protect the distillation process from external particulate contamination. All parts on the clean air side of the chamber including the HEPA filter frame and diffuser are constructed of aluminum or plastic. The chambers meet

Class 100 specifications—that is, they remove 99.97% of all particulate matter larger than 0.3 ixm. The air flow through

the chamber is adjusted to match the exhaust rate through a plenum assembly at the back-bottom of the unit. By this means, the acid purification operation is protected from room

contaminates and at the same time, acid fumes are minimized

in the room.

Containers for Reagent Storage. Acid distillates are col- lected and stored in Teflon-FEP bottles. These bottles are cleaned by soaking for 24 hours first in hot 8M HNO3 and

then in hot 6M HCl for 24 hours. They are then rinsed with

sub-boihng distilled water and finally with the acid being

stored. The containers are reused for the same acid after

first being rinsed with distilled water and then rinsed with the

acid. Water from the sub-boiling still is stored in quartz containers which are cleaned in the same manner as the Teflon

bottles.

Ill
Figure 3. Clean environment evaporator and transfer system

APPARATUS AND REAGENTS FOR ANALYSIS OF

ACIDS AND WATER

Enriched Isotope Spike Solutions. The individual isotopic

spiices for the elements being determined were on hand as 1 mg/ml solutions. Compatible spiking elements were combined to give 50 Mg/ml concentrations of each element in a

pair of master spiking solutions. One solution contained

the HCl soluble elements, the other solution the HNO3 soluble elements. Clean Air Exhaust Hood. In the analysis of the acids,

evaporations were performed on a hot plate in a Class 100 exhausted fume hood. The basic unit is similar to the clean chamber except that the air is exhausted down through a grill of Teflon that serves as a "floor" for the hood and then out through a standard acid fume exhaust system. Teflon is used

extensively throughout those parts of the hood exposed to acid fumes.

Evaporator and Liquid Transfer System. The apparatus,

shown in Figure 3, provides a clean environment for evaporating a spiked sample and transferring the residual volume onto

high-purity gold wires. Nitrogen flowing through a 0.1 -;nm

cellulose filter provides a clean atmosphere, sweeps out evaporating acid, and prevents entry of particulate contamination.

A pure quartz pipet inserted off center in a Teflon stopper

at the top is used to transfer the last few drops of evaporating

acid onto pure gold wires positioned from the side joint.

PROCEDURE FOR ANALYSIS OF ACIDS AND WATER

BY ISOTOPE DILUTION SSMS

The isotope dilution analyses were performed on the acids and water produced by sub-boiling distillation and on the ACS

reagent grade acids used as starting materials. In addition, we have also included results obtained in previous analyses of

commercial high-purity acids. This variety in the type and

source of acids analyzed has resulted in some changes of the procedures used. Hydrofluoric Acid. Due to its reactivity with quartz and

glassware, HF was handled exclusively in Teflon-FEP containers, otherwise it was treated the same as the other acids from a similar source. Water from the Sub-Boiling Still. Because the purity of the sub-boiling distilled water was higher than that of the acids, the water was preconcentrated before spiking. Two liters of water were evaporated to approximately 100 ml in the

original storage container before being removed and spiked.

The water was not spiked in the storage container because of our reluctance to contaminate the container with the enriched

spike isotopes. The spiked 100-ml water sample was then

handled in the same manner as the acids purified by subboiling distillation. Sample Spiking. All samples were spiked using 1 ngjml

solutions freshly diluted from the 50 jug/ml master spike solutions. Acids purified by sub-boiling distillation were all spiked for a nominal concentration of 1 ppb ; nitric and hydrochloric acids were also spiked for 0.1 ppb. The nominal

1-ppb spike contained 100 ng of each spike element in 100 grams of acid (1 ng/g); the 0.1 ppb spike had 30 ng/300

grams of acid. The 1 ng/g spike gives an altered isotopic

ratio of approximately one at a concentration of 1 ppb for most of the impurity elements. Concentrations up to ten times higher and more than ten times lower than the spiked

for value can be determined from the measured ratios, but

with greater uncertainty.

The ACS reagent grade acids used as starting materials

for the sub-boiling distillation were spiked at 1 ppb, 10 ppb,

and 100 ppb in order to cover the expected concentration range for the diff'erent impurity elements. An element was

only measured for the sample where the spike came closest to the actual concentration. One half of the spiked sample was used for each analysis.

The commercial high-purity acids analyzed previously were

spiked for many but not all of the elements surveyed in the ACS reagent grade and sub-boiling still acids. One hundredgram samples were spiked for concentrations in the 1- to 100- ppb range. Because of the higher level of impurities only 20

grams of the lOO-gram sample were required for the analyses.

Sample Evaporation and Transfer to Pure Gold Wires.

The clean environment evaporator shown in Figure 3 was

used to vaporize commercial high-purity acids in a standard

laboratory fume hood prior to our obtaining a Class 100

clean air fume hood. A single sample at a time was evaporated to a few tenths of a milliliter and then transferred with the pure quartz pipet to the tip of "six nines grade" gold

wires and dried. The ACS reagent grade acids and the acids purified by subboiling distillation were evaporated in the Class 100 fume

hood in open quartz and/or Teflon beakers on a hot plate.

The clean environment evaporation dish was used however to

transfer the last few drops of the spiked sample to the gold

wires for drying. The gold wires were then heated to 425

°C for 15 minutes to drive off' hydrocarbons and occluded

acid. At this point partial loss due to incomplete transfer or

loss on heating can be ignored inasmuch as the quantitative

analytical data are a function of the altered isotopic ratio and the only requirement is that enough of each element be

retained for an adequate measurement. Spark Source Mass Spectrographic Determination of Altered

Isotopic Ratios. The pair of gold wires was mounted in the spark source such that several millimeters of the wire ends,

which were coated with the spiked sample residue, would be

overlapping and parallel to each other. When a source vacuum of 1 X 10~' Torr was reached, a graded series of

exposures was made. The electrodes were moved relative to each other during sparking in order to spark new spiked

sample residue on each exposure. One half of the surface of each electrode was sampled during this first graded series of exposures. A second identical series of graded exposures was then made on the remainder of the sample. A single

photographic plate was used for each sample.

The photographic plates were processed using the bleach and internal image developer for reducing plate fog developed

by Cavard (6). The photoplates were examined visually to select exposures which would give optimum sensitivity for each element deter- mined and for evidence of interferences at the same nominal mass such as unresolved doublets, abnormally wide lines, or

isotopic ratios which varied from one exposure to the next.

If an interference was indicated for the -|- 1 lines of an ele- ment, the less sensitive +2 or +3 lines were considered for measurement. The selected exposures for each element were

then densitometered measuring both the spike and natural isotopes on each exposure. From four to six exposures were measured for each element if available.

(6) A. Cavard in "Advances in Mass Spectrometry," E. Kendrick,

Ed., The Institute of Petroleum, London, 1968, pp 419-29.

112
Calculation of Impurity Concentrations. The amounts of

impurities present in each acid were computed from the sample

weight, the weight of each spike added, and the measured

altered isotopic ratios using the usual formula for isotopic dilution analysis as described in earlier papers (7, 8). Impurity concentrations were calculated for each isotopic ratio measured for an element. These values were then averaged.

This system gives a more realistic evaluation of the uncertainty

in the results than using an averaged ratio to calculate one

concentration value. This is especially critical when the measured (altered) isotopic ratio is close to that of either the spike or natural element. When only the spike isotope for an element was detected, an upper limit of concentration was computed by substituting twice the noise level of the photographic plate at the position

of the natural isotope for the intensity of the natural isotope.

The concentration of mononuclidic sodium was estimated by

assuming it has the same sensitivity as potassium and measuring intensity-areas of the ^'Na compared to that of either the ''K or the ""K isotope of the same exposure. The results of these analyses of the acids and water are presented in a later section describing the operating procedure

used for the production of each of the acids by sub-boiling

distillation.

DISCUSSION OF ANALYSIS PROCEDURE

The acids purified in the sub-boiling still and the ACS

reagent grade starting materials were analyzed for 17 elements

simultaneously by the spark source. The elements measured

represent elements found as impurities in reagents, elements commonly determined in many types of analytical samples,

and elements from different groups of the periodic table. It was felt that this selection of elements would give a representative view of the overall reagent purity.

The analysis technique itself is subject to blank problems

which cannot be properly evaluated. Care was taken during

the analysis to prevent contamination of the samples from

containers and the external environment. Based upon the lowest values found in repeated analysis of the acids produced

by sub-boiling distillation, estimates can be made of the probable blank contributions. In general any element

reported near the 0.1 -ppb level or below may have a considerable blank contribution and the value should therefore be considered as an upper limit. Concentrations signifi- cantly above this level represent actual reagent impurities

with uncertainties ranging from ± 10 to 30 % depending on the value of the altered ratio. Interfering lines having the same nominal mass as either the spike or natural abundance isotope of an element being

determined were sometimes encountered during these analyses.

These interferences are normally most prevalent for the lower mass elements and for the more impure acids. The three major sources of these lines are hydrocarbons, anions, and

anion fragments, and various molecular combinations of the major impurities with themselves and the acid anion. Heating of the sample to 425 °C before sparking and the heating

that occurs during sparking greatly reduced interference from

the hydrocarbon and anion lines. Examples of anion fragment interferences are "CP^O+

with the "Cr+ spike and 3'Cli«02+ with the "Zn+ spike in perchloric acid analysis, and '^S"02 with natural "Zn+ in

sulfuric acid. The perchloric acid interferences can be evalua- ted by monitoring the line from the other chlorine isotope.

(7) R. Alvarez, P. J. Paulsen, and D. E. Kelleher, Anal. Chem.,

41,955(1969).

(8) P. J. Paulsen, R. Alvarez, and D. E. Kelleher, Spectrochim. Acta, 24B, 535 (1969).

Figure 4. Laboratory for production of high-purity acids

ACS reagent grade HCIO4 with high levels of Na and K

showed fines for: NaO+, NaK+, Na2+, K2+, Na2Cl+, K2CI+,

Na2C10+, K2CIO+, Na2C102+, etc. Some of these lines represented potential interferences; however, this type of

interference was not significant for the acids purified by subboifing distillation since their cationic impurities were always

at a very low level. Measurement of the isotopic ratio of an element at the +2

or -j-3 charge state will eliminate all three types of interfering

lines since these molecular species fragment rather than form a

multiply-charged ion. When such interferences existed, Ag,

Cu, and K isotope ratios were measured on +2 ions and Zn,

Ni, Cr, and Ca isotope ratios were measured on +3 ions.

PRODUCTION AND ANALYSIS OF HIGH-PURITY

ACIDS AND WATER

Figure 4 shows the laboratory setup for the production of

high-purity acids. Each quartz sub-boiUng still and distillate container is housed in a separate Class 100 clean air

unit to prevent particulate contamination during distillation. Any extraneous acid vapor produced is exhausted through the balanced plenum assembly which serves four clean air units. The Teflon sub-boiUng still is housed in a separate clean air chamber and extraneous acid vapors are exhausted by a standard laboratory hood. The ACS reagent grade acid to be

purified is fed to each still from a container just outside the

clean air chamber through a liquid level control which maintains the pre-set liquid level. In this manner, acid is constantly fed to the still as distillate is removed. Since impurities are concentrated in the pot Uquid, the still is drained

after the consumption of each 6- to 8-pound feed bottle. After distillation, each acid was allowed to stand in its Teflon-FEP container for at least two weeks before analysis

to allow for any impurities in or on the container walls to

react with the acid.

113
Table I. Hydrochloric Acid

Impurity concentrations ppb by wt (ng/g) Acid from

sub-boiling ACS reagent Commercial

still grade high purity

Pb 0.07 0.5 <1

Tl 0.01 0.1 Ba 0.04 2 Te 0.01 0.1 Sn 0.05 0.07 <6

In 0.01 Cd 0.02 0.03 6.5 Ag 0.03 0.05 0.2

Sr 0.01 0.05 Zn 0.2 2 4 Cu 0.1 4 1 Ni 0.2 6 3 Fe 3 20 7 Cr 0.3 2 0.3 Ca 0.06 70 24 K 0.5 200 10 Me 0.6 10 20 Na 1 500 S 6.2 ppb S 820 ppb S 70 ppb

Table II. Nitric Acid

Impurity concentrations ppb by wt (ng/g) Acid from

sub-boiling ACS reagent Commercial

still grade acid high purity

Pb 0.02 0.2 0.3

TI 0.2 Ba 0.01 8 Te 0.01 0.1 Sn 0.01 0.1 1 In 0.01 Cd 0.01 0.1 6.2 Ag 0.1 0.03 0.1

Sr 0.01 2 Se 0.09 0.2 Zn 0.04 4 8 Cu 0.04 20 4 Ni 0.05 20 3 Fe 0.3 24 55 Cr 0.05 6 130 Ca 0.2 30 30 K 0.2 10 11 Mg 0.1 13 Na 1 80 2 2.3 ppb 2 220 ppb 2 240 ppb

The purified acids were then analyzed by the isotope

dilution mass spectrometric method described previously.

These analyses characterize the acid as delivered to the user

rather than as produced, since this is what is of interest to the

analyst.

The Teflon-FEP bottles will be used repeatedly over again

to contain the same high-purity acid. It is reasoned that the container should become cleaner with use and cause less con- tamination to future lots of acids. Any acid not used within a few months will be replaced with freshly distilled reagent.

Hydrochloric Acid. Hydrochloric acid and water form a

constant boiling mixture at 6N hydrochloric acid. Initial experiments were with starting acid of this concentration. Further experiments showed that the starting concentration

could be increased to ION HCl without causing bubble for- mation and that the distilled product was also ION. However, concentrated 12A'^ acid could not be used because of

bubble formation on heating. Apparently, acid of higher

Table III. Perchloric Acid

Impurity concentrations ppb by wt (ng/g) Acid from

sub-boiling ACS reagent Commercial

still grade acid pure acid Pb 0.2 2 16 TI 0.1 0.1 Ba 0.1 >1000 10 Te 0.05 0.05 Sn 0.3 0.3 <1

Cd 0.05 0.1 4 Ag 0.1 0.1 0.5

Sr 0.02 14 Zn 0.1 7 17 Cu 0.1 11 3 Ni 0.5 8 0.5 Fe 2 330 10 Cr 9 10 18 Ca 0.2 760 7 K 0.6 200 9 Mg 0.2 500 4 Na 2 600 2 16 ppb 2 > 3400 ppb 2 100 ppb

concentration than constant boiling 6A'^ can be produced be- cause the azeotropic mixture that condenses on the cold finger

absorbs HCl from the HCl-rich vapor to reach the concentration of the acid in the pot. ACS reagent grade hydrochloric acid diluted to lOA'^ or

31 weight per cent with high-purity water was used as the feed

acid. The still heaters were adjusted to 225 W of power. About 2 liters of lOA^ high-purity hydrochloric acid were pro- duced per day under these conditions. Table I shows the analysis of the high-purity acid produced

by sub-boiling distillation, the starting ACS reagent grade

acid, and a lot of commercial high-purity acid. A summation

of these impurity elements shows that the sub-boiling distilled acid contained 6.2 ppb, the ACS reagent grade contained

820 ppb, and commercial high-purity 70 ppb. The only

element found in the sub-boiling distilled acid at levels higher than 1 ppb was iron at 3 ppb. The other elements were at the sub-ppb level, generally lower than 0.1 ppb.

Nitric Acid. The sub-boiling distillation of concentrated 70% ACS reagent grade nitric acid required a low heater temperature to prevent the HNO3 vapor in the proximity of

the heater from disproportionating to form nitrogen dioxide

with resulting discoloration of the distillate. Experiments

showed that at a heater power setting of 107 W, little or no

discoloration of the distillate took place. As a result, only

about 500 ml per day of nitric acid was produced. Titration showed the distillate to be 70% HNO3, the same concentration as the starting acid. Table II shows the results of the analysis of the sub-boiling

distilled acid, ACS grade starting acid, and a lot of commercial

high-purity acid. The totals of the impurity elements deter- mined were 2.3 ppb for the sub-boiling distilled acid, 220 ppb

for the ACS starting acid, and 240 ppb for the commercial

high-purity acid. The sub-boiling distilled nitric acid was the purest of the acids produced. No element was found at a

concentration greater than 1 ppb and only sodium was

detected at that level, and this accounts for almost half of the

total impurities found. Most elements were in the 0.05- to

0.01 -ppb range.

Perchloric Acid. Initial experiments on the sub-boiling

distillation of concentrated 70% perchloric (HC104-2H20)

showed that high heater temperatures caused the perchloric

114
Table IV. Sulfuric Acid

Impurity concentrations ppb by wt (ng/g) Acid from ACS reagent

sub-boiling still grade acid Pb 0,6 0.5

Tl 0.1 0.1 Ba 0.3 0.2 Te 0.1 0.1 Sn 0.2 0.6 Cd 0.3 0.2 Ag 0.3 0.6

Sr 0.3 0.4

U . J £, Cu 0.2 6 Ni 0.2 0.5 Fe 7 6 Cr 0.2 0.2 Ca 2 123 K 4 9 Mg 2 4 Na 9 50 2 27 ppb 2 200 ppb

acid in the still to turn bright yellow, probably because of the formation of chlorine dioxide. At lower power settings, little or no color was noted and the distillate was colorless. Another

problem with this distillation was that in the earlier work,

crystals would occasionally build up on the cold finger condenser. This material was probably perchloric acid monohydrate (HCIO4 H2O) which has a melting point of 50 °C. Crystal formation could be controlled by changing either the flow rate of the condenser water or the heater temperature.

However, this problem was noted only in the earlier work. For the last six months, no crystal formation on the cold

finger has been noted even when the still is operated continuously for weeks. The power setting used for the pro- duction of purified perchloric acid was 240 W, and the pro- duction rate was about 600 ml of perchloric acid per day.

Titration has shown that the distillate is 70 wt % of HCIO4,

the same as the starting ACS grade acid. Table III shows the results of the analyses of the subboiling distilled acid, the starting ACS grade acid, and a lot of commercial high-purity acid. The total of the impurity

elements found was 16 ppb for the sub-boiling distilled acid, 3400 ppb for the starting acid, and 100 ppb for the commercial

high-purity acid. These analyses demonstrate the efficiency

of sub-boiling distillation when the concentration of impurity

elements in the purified product is compared to the starting

acid. The concentration of barium which was greater than 1000 ppb was reduced to 0.1 ppb, a purification factor of

greater than 10,000. Another example is calcium which was

reduced to 0.2 ppb in the purified acid from 760 ppb in the

starting acid, a factor of 3,800. It should also be noted

that chromium which was present at 10 ppb in the starting acid was not significantly changed in the purified acid. This

is probably due to the fact that chromium can be volatilized from hot perchloric acid solutions as chromyl chloride,

Cr02C]2, so little, if any, purification can be expected by subboiling distillation. Chromium accounts for over half of the

total impurities found in the purified acid. Sulfuric Acid. Initial experiments with the sub-boiling

distillation of sulfuric acid showed that, because of the density

of H2SO4, the acid that condensed on the cold finger tended

to drop off before reaching the collection tube. Another

problem was that even at the highest power setting, the distillation rate was extremely slow. Modification of the still

Table V. Hydrofluoric Acid

Impurity concentration ppb by wt (ng/g) Acid from ACS reagent

sub-boiling still grade acid Ph U.Uj n 0

U. Tl11 u. z Da n 1 U. J

f\ 1 U. 1 0x1 11/

Ae n ns nu 1

. 1 n 1 nU s, J Zn 0.2 AH Cu a Ni 0.3 12 Fe 0.6 110 Cr 5 20 Ca 5 14 K 1 28 Mg 2 10 Na 2 100 S 17 ppb 2 320 ppb

Table VI. Water

Impurity concentrations ppb by wt (ng/g) Water from sub-boiling still Pb 0.008

Tl 0.01 Ba 0.01 Te 0.004 Sn 0.02 Cd 0.005 Ag 0.002

Sr 0.002 Zn 0.04 Cu 0.01 Ni 0.02 Fe 0.05 Cr 0.02 Ca 0.08 K 0.09 Mg 0.09 Na 0.06 2 0.5 ppb

by increasing the angle of incline of the cold finger condenser

solved the condensate problem, and the rate of distillation was increased by using a platinum foil reflector around the

still. At the maximum power setting of 427 W, the rate of

distillation is now about 300 ml per day. Titration has

shown that the purified acid is 96 wt % H2SO4, the same as

the starting ACS grade acid. Table IV shows the results of the analyses of the subboiling distilled acid and the starting sulfuric acid. The total of the impurities found in the starting acid was 200 ppb, the lowest of any of the ACS grade acids, and the purified acid contained 27 ppb, the highest of the purified acids. Sodium

was the principal contaminant in the high-purity acid at 9 ppb

with iron right behind it at 7 ppb. These two elements account for over half the total impurities found in the purified

sulfuric acid. Hydrofluoric Acid. Hydrofluoric acid was purified in the

all-Teflon sub-boiling still shown in Figure 2 which was

designed and built at NBS. Hydrofluoric acid and water form a constant boiUng mixture at 36% HF. Because of the previous experience with hydrochloric acid, the sub-boihng

distillation of concentrated 48 % HF was attempted. Titra- tion showed that the distilled product was also 48 % HF. Ap -

115
Table VII. Summary of Experimental Conditions for Producing High-Purity Acids and Water by

Sub-Boiling Distillation

Concentration of Concenstill feed tration of Production

acid, % otiil neater distillate. rate. Reagent by wt power, W % by wt ml/24 hr HCl 31 225 31 2,000

HNO3 70 107 70 500 HCIO, 70 240 70 600 H2SO4 96 427 96 300 HF 48 164 48 300 H2O 200 4,000

parently, as in the case of hydrochloric acid, the azeotropic

mixture that condenses on the cold finger absorbs HF from

the HF rich vapor to reach the same concentration as the acid

in the still. Since the heaters of the Teflon still for the purification of

hydrofluoric acid are enclosed in Teflon tubes, the temperature

of the heater had to be kept below the softening point of

Teflon. As a result the maximum power setting for this still was 164 W which produced a distillation rate of 300 ml per 24 hour day.

Table V shows the results of the analyses of the starting

ACS grade HF and the purified product. The total of the impurities in the purified acid is 17 ppb compared to 320 ppb

in the starting ACS grade acid. The two principal impurities

in the purified acid are Ca and Cr, both at 5 ppb, and they account for over half the total impurity found. The only

other elements found at a concentration of greater than 1 ppb

are K at 1 ppb, Mg at 2 ppb, and Na at 2 ppb. Water. Water has been included in this study because it is the most commonly used chemical reagent and because subboiling distillation has been used to produce high-purity water both here at NBS and elsewhere. Initially, water of higher

purity than the distilled water from the laboratory distribution system was obtained by re-distillation in a commercial,

continuous-feed still. This still had a tin-coated evaporator

and baffle and was equipped with a quartz condenser. This

unit was designed to disengage CO2 and other gases from the

distillate and vent them olT. Water produced by this still was used as the feed water to the sub-boiling still. The entire assembly of feed apparatus, quartz sub-boiling still, and

quartz collector was contained in a Class 100 clean air chamber. The distillation rate at a power setting of 200 W was 4

liters per day.

The analysis of the sub-boiling purified water is given in Table VI. This water contained a maximum of 0.5 ppb total of the 17 elements determined. No element was detected at a

concentration of greater than 0.1 ppb. Since no correction

for an analysis blank could be made, the values must be

regarded as upper limits for the purity of the water. The

water may in fact be more pure than the analysis indicates. The optimum conditions for the production of each highpurity acid and water by sub-boiling distillation described above are summarized in Table VII. The impurity elements determined in the acids and water

(Tables I-VI) are tabulated in order of decreasing mass. This reveals that the major impurities for the acids purified

by sub-boiling distillation are all low mass elements. None

of the NBS purified acids have an impurity level as high as

1 ppb for elements above iron in mass. Elements from iron on down sometimes exceed 1 ppb but none exceeded 10 ppb.

A number of the major impurities found in the ACS reagent

grade and commercial high-purity acids are elements common

to their glass storage containers. It would appear that the means of storing the acids are as responsible for the acid

purity as the purification method. It should be noted that of all the acids analyzed, only the ACS grade perchloric acid showed an element (Ba) that exceeded the 1-ppm level.

Received for review March 27, 1972. Accepted May 30, 1972. Certain commercial instruments or chemicals are mentioned in this paper; this does not imply recommendation

or endorsement by the National Bureau of Standards.

116
APPENDIX II

REPRINTED IN PART FROM

NATIONAL BUREAU OF STANDARDS SPECIAL PUBLICATION 422.

Accuracy in Trace Analysis: Sampling, Sample Handling, and Analysis,

Proceedings of the 7th IMR Symposium,

Held October 7- 11, 1974, Gaithersburg, Md. (Issued August 1976).

THE ROLE OF THE ANALYTICAL BLANK

IN ACCURATE TRACE ANALYSIS

Thomas J. Murphy

Analytical Chemistry Division

Institute for Materials Research

National Bureau ofStandards

Washington, D.C. 20234 U.SA.

The inability to control the analytical blank, i.e., contamination from all sources external to the sample, has seriously affected the accuracy of low level trace determinations. Most of the sources of the blank are variable and it is this variability that determines the uncertainty of the blank correction and, therefore, the lower limit of trace element concentration that can be determined

with reliability. To improve both the accuracy and lower limit of trace deter- minations, it is imperative to control the variability of the analytical blank. The

only practical way to accomplish this is to reduce the size of the blank itself by

controlling the sources of the blank. The analytical blank is composed of contamination from four principal sources, namely: the environment the analysis is performed in, the reagents

used in the analysis, the apparatus used, and the analyst performing the analysis.Environmental contamination is caused by particulates and gaseous compounds in the ambient air. Methods for reducing the blank from this source by

the use of isolation chambers or "absolute" filters are described.

While commercially available high-purity reagents have helped the anailyst

to control the blank from reagents, they are frequently not low enough in trace element concentration and must be purified before use. Methods for the preparation of high-purity reagents, especially water and the mineral acids, are reviewed.

Contamination from beakers, containers, mortars and other apparatus can

seriously affect the blank. Techniques for controlling this type of contamination are described with emphasis on the purity of materials. The last source of contamination is that caused by the analyst during the

analysis due to carelessness or poor technique. Suggestions are given to aid the

analyst in this regard.

Keywords: Accuracy; analysts; apparatus; blank; contamination; environ- ment; purity; reagents.

117
Accuracy in Trace Analysis

I. Introduction

The analytical blank may be considered the "Achilles' heel" of trace

analysis. As the Greek warrior of Homer's Iliad had his vulnerable point, so also does trace analysis in that the size and variability of the analytical

blank may render useless the information from the analysis. First, let me

define what is meant by the analytical blank. The analytical blank is simply the contamination by the element or compound being determined

from all sources external to the sample. I am not including in this definition the so-called instrumental blank or noise level which really refers to

the sensitivity or level of detection of the analytical method. Also not included in this definition is the so-called negative blank, that is loss of an

element during analysis by physical processes such as adsorption. This is really a separate subject unto itself. Obviously, this definition includes

both inorganic and organic contamination, but my remarks will deal

mainly with trace inorganic cation contamination.

Modem methods of analysis have lowered the threshold of determining

trace elements to the low parts per billion (ng/g) for many elements but the

inability to control the analytical blank has seriously affected the accuracy

of these methods. The blank is a problem common to all trace element

techniques with the exception of certain nuclear techniques such as neutron activation analysis and fission track analysis.

Let's look at the effect of the blank on the accuracy of a trace analysis.

During the analysis, the analyst normally carries "blanks" thru all the

steps of the analysis and makes a "blank-correction" of the analysis based

on the estimation of the contamination from these blank determinations.

Figure 1 shows the effect on accuracy of a 10 ng blank that exhibits a

variability of ± 5 ng. This represents a very low blank with a reasonable

uncertainty. It is not uncommon, even in very careful work to have blanks an order of magnitude higher. Notice that at the microgram level, the effect on accuracy of a blank of this size is small, but that it increases

rapidly until at the 0.01 Aig or 10 ng level, a 50 percent uncertainty would

be introduced by the blank. If the blank were an order of magnitude higher

with the same percentage variability, a determination at the 0. 1 /xg level

would have a 50 percent uncertainty due to the blank alone.

It is the variability or the uncertainty in the blank correction [ 1 ] and

not the absolute value of the blank that affects the accuracy of the analysis. As the variability becomes a significant percentage of the sample

amount, more and more determinations of the blank are necessary so that

the variability can be estimated with some degree of reliability. It is a common mistake of analysts to base the blank correction on a single

118
Murphy

10

0 10 20 30 40 50

PERCENT UNCERTAINTY

Figure 1 . Effect of a 1 0 ± 5 ng blank on accuracy of analysis.

determination and have no information on the approximate variability or

effect on accuracy.

One might argue that, since it is the variability and not the absolute

value of the blank that affects accuracy, the way to increase accuracy is to

have large blanks but with reduced variability. This is certainly not true in

the real world of trace analysis. Firstly, it is inherently inaccurate to base

a result on the difference between two signals or numbers of relatively the

same size. But secondly, and more importantly, real blanks, large as well

as small, do exhibit marked variability since most of the sources of the

blank are not constant. The only practical way to lower the variability and

increase the accuracy of the blank correction is to reduce the size of the

blank. It is obvious from figure 1 that as one goes to lower levels of trace

concentrations the blank becomes more and more of a factor. Figure 2

119
Accuracy in Trace Analysis

COMPARISON of INTERIABORATORY RESULTS for an AQUEOUS STANDARD and WHOLE BLOOD

Aqueous Solution 140 ng Pb/;

75 60

Porcine Blood 30 n{ Pb/g

+ 40r

+ 20

-20

-40*- '

456 109 453 801 62

90

234567 1234567

LABORATORY NUMBER

Figure 2. Comparison of interlaboratory results for an aqueous standard and whole blood.

represents two parts of a study to develop a referee method for lead in

blood by atomic absorption spectrophotometry. The left portion

represents the results from different laboratories for the analysis of an

acidified aqueous standard. No chemistry was necessary and the results

fluctuate, not very accurately, around the "base-line" value. But notice

what happens when the same laboratories analyze a real blood sample as shown on the right portion. ^ The level of lead was a factor of five lower

than the standard and some chemistry was necessary. The results varied

from 30 to 450 percent high, in all probability due to lack of control of the

analytical blank.

This is but one example of the fact that as one goes to lower trace concentrations, the blank becomes the dominant factor on accuracy and

determines the lower limit of trace element concentration that can be

determined with reliability. It is imperative to control and reduce the

analytical blank if improvements in precision and accuracy of trace determinations are to be made.

This is for porcine blood which contains much lower lead than human blood.

120
Murphy

II. Sources of Contamination

How does one go about lowering a blank? The sources of the blank

must be considered and steps taken to reduce the contamination from

each source. The analytical blank is composed of contamination from four principal

sources, namely, the environment the analysis is performed in, the re- agents used in the analysis, the apparatus used, and the analyst performing the analysis. Of these factors only the contamination from reagents is relatively constant while the contamination from the other factors is variable and determines the variability of the analytical blank.

A. Environment

Let us consider environmental contamination. Fall-out or absorption of

particulates or gases from the laboratory air can cause significant and

variable amounts of contamination of the sample or a solution of the sample. As much as 200 /ig of particulates per m^ of air have been reported in

the air of an analytical laboratory [2] and it was shown to contain large

amounts of calcium, silicon, iron, aluminum, sodium, magnesium potassium, thallium, copper, manganese and lesser amounts of several other elements. Analysts in Dortmund, Germany, were not able to lower iron

blanks lower than 0.04 fxglml because 20 tons of ferric oxide were

dispersed into the air from nearby plants [ 3 ] . Table 1 shows the lead particulate concentrations from three different locations. The lead concentration from downtown St. Louis, 18.84 figlm^, was much higher than rural

Missouri, 0.77 /tg/m^, [4] due to automotive exhaust pollution. Our

Table 1 . Examples of lead concentration in air

Site Lead concentration

Downtown, St. Louis, Missouri* 18.84

Rural Park, Southeastern,

Missouri" 0.77

Laboratory air, NBS,

Gaithersburg, Maryland'' 0.4

" See reference [4]. See reference [5].

121
Accuracy in Trace Analysis

laboratory air at NBS, Gaithersburg, Maryland [5] compares favorably

to rural air but still contains too much lead for low level work. Wide variations in the particulate content of air is to be expected. Useller [6] has

stated that in a typical rural area airborne particulate counts larger than

0.5 fim of l,400,000/m3 are not unusual and that in metropolitan areas counts of 5 3,3 00,000/m^ are normal.

The use of evaporation chambers has been one approach to reducing

airborne contamination. Evaporations are performed in a closed system

under filtered air or nitrogen. Figure 3 shows some different evaporation

chambers from the literature. Chamber (a) was designed by Thiers [7,8]

.

Filtered air is passed over the solution and out the bottom while the sample is heated from below with a hot-plate and above with a heat lamp.

Chamber (b) is a German design by Koch [9] in which filtered air or

nitrogen is passed over the sample solution. A heated quartz dome

IR lamp

Pyrex K^y r ^1

^

Hot plate

(a)

Filtered

air in

-Out

Quartz surface evaporator

Glass

Heater

(b)

Borosillcate glass

Out

Filtered

Teflon

(0

Stainless steel <d)

Filtered N, in

Figure 3. Evaporation chambers from the literature, reference 35. Reprinted with permission of John Wiley & Sons, Inc.

122
Murphy

prevents condensation of liquids. The last two chambers, (c) and (d),

were used by Chow and McKinney [10] for the analysis of high-purity

HCl (Glass) and HF (Teflon).

Figure 4 is a drawing of an evaporation chamber that has been used by

Alvarez at NBS [11]. The solution is evaporated by heating from above

and below the chamber while filtered nitrogen passes over the solution

until only a drop remains. The drop is then drawn up into the capillary and

loaded onto gold wire for analysis by spark source mass spectrometry.

Table 2 shows the resuhs of Chow and McKinney [ 10] for the analysis

of 500 ml samples of high-purity hydrochloric acid which was evaporated

Figure 4. Clean environment evaporator and transfer system.

Table 2. Lead contamination of labware after exposure to air'

Beaker Condition Laboratory Time

(days)

Lead

M(g)

Teflon Open Ordinary 8 4.07

2.32

Nitrogen-flushed container

Open

Ordinary

Pure air

881.13

0.44

Nitrogen-flushed container Pure air 8 0.18

.13

Borosilicate glass

Nitrogen-flushed container

Nitrogen-flushed container

Pure air Pure air

1

1

0.02

.03

» See reference [10].

123
Accuracy in Trace Analysis

slowly under different conditions. When the HCl was evaporated over a

period of 8 days from an open Teflon beaker, the lead contamination was

high and variable. Using a clean air chamber under nitrogen reduced the

lead contamination only by a factor of about three. Both of these evaporations were performed in a normal fume hood. (Since the volume of air

moving across an area in a fume hood is many times greater than the air

moving across the same area in the laboratory, the contamination is greater in the hood than elsewhere in the laboratory.) When the same

evaporations were performed in a clean air laboratory in which dusts and

aerosols were removed by electrostatic precipitation and filtration, the

lead contamination was reduced by a factor of 10 when the HCl was

evaporated open and by a factor of 20 when it was evaporated in an

evaporation chamber flushed with nitrogen.

The major development to date for providing particulate free air is the

high efficiency particulate air filter, better known as the HEPA filter

[ 12] . This filter was developed during World War II for the Manhattan

Project to remove fissionable particulates from air and to this date

remains the most efficient means of air filtration. The HEPA filter has an

efficiency of 99.97 percent for 0.3 /xm particles and are routinely employed for supplying air that meets the Class 100 specification [ 13] , that

is less than 1 00 particles per cubic foot of air larger than 0.5 /^m. These filters, coupled with the laminar airflow principle [14] in which the "clean"

air is moved unidirectionally, have been used for the last decade to provide clean particle-free work areas. Figure 5 is a view of the clean room located in the Analytical Chemistry

Division at the National Bureau of Standards. The HEPA filters are located on the far wall. In fact the entire wall is composed of HEPA filters

causing a wall of clean air to pass unidirectionally down the room. This

room is really a particulate-free room and cannot be used as a clean

laboratory since it was not designed for acid vapor.

Figure 6 is a cut-away view of the clean laboratory also located in the

Analytical Chemistry Division at NBS. This room was designed for trace

inorganic analysis and can be used for chemical operations such as acid

dissolutions and evaporations. All of the air entering the room is passed

thru prefilters and then thru the HEPA filters located over the center

work-bench of the room. The Class 100 clean air is directed

unidirectionally down and out across the bench to prevent particles from

entering the area. Plexiglas dividers prevent cross-contamination. The

air is recirculated thru the filters by drawing air at the base of the outside

walls. In addition to the "clean" area under the center bench, there are several Class 100 hoods that provide clean work areas on the outside

bench. Acid dissolutions and evaporations are performed in the two

124
Murphy

Figure 5. Particulate free clean room —National Bureau of Standards.

hoods at the rear of the room. These are Class 100 clean air hoods lined

with sheet Teflon and are so designed that all air 1 2 cm from the front is exhausted while a curtain of air moves out the front to prevent particulates from entering the hood.

Figure 7 is a schematic of a laboratory used at Bell Telephone Laboratories for trace analysis [15]. The inlet air to the room is non-laminar

flow clean air which has been filtered thru HEPA filters. The air to the

clean hood is further purified by recirculation thru HEPA filters and the

room is kept under positive pressure. Using this room, no significant contamination of water samples could be detected by spark source mass spectrometry or activation anaysis.

Table 3 shows a comparison of particulates in the air of an ordinary

laboratory, a clean room, and a Class 100 hood, all at NBS [5 ] . Both the

clean room and Class 1 00 hood air showed a dramatic reduction in contamination. Lead concentrations were reduced by a factor of over 1000,

iron by a factor of 200 and the other elements, which were low to start with, by a factor of 10.

While HEPA filters are a powerful tool in reducing particulate contamination, they are not the complete answer, to environmental con125
Accuracy in Trace Analysis
Murphy

WALL CUMET

Figure 7. Schematic of clean laboratory - Bell Telephone Laboratories, reference 22. Reprinted with permission of Analytical Chemistry.

Table 3. Particulate concentration in laboratory air^

Concentration, ng/m^

Iron Copper Lead Cadmium

Ordinary Laboratory 0.2 0.02 0.4 0.002

Clean Room .001 .002 .0002 n.d.

Clean Hood .0009 .007 .0003 0.0002

» See reference [5].

tamination. Figure 8 shows the particle size of some common contaminants. Since HEPA filters are efficient for the removal of particles 0.3 /xm

or larger, particles as small as bacteria are removed, but some common

particulates such as tobacco smoke are not efficiently removed since they

are smaller than 0.3 /xm. Obviously gases, both organic and inorganic, are not removed. To remove these contaminants, the air must be passed thru

absorbants and scrubbed, which is an expensive operation. Most laboratories either live with the small amount of contamination that results from

clean room operation or resort to the evaporation chamber technique in

the clean room to further reduce environmental blanks.

127
Accuracy in Trace Analysis
Murphy

B. Reagents

The chemical reagents used during the course of an analysis are an important source of the analytical blank. The first order of priority for a

trace laboratory in regard to reagents is a readily available supply of ex- tremely high-purity water. There are several commercial systems on the

market for producing "conductivity water"; that is, water whose conductivity approaches the theoretical conductivity of 0.055 micromho/cm at 25 °C [15,16]. These systems are based on the use of mixed-bed or monobed ion exchangers, in which cation and anion exchange resins are mixed in a single unit, and sub-micron filtration. Measured conductivity

of deionized water has been reported as low as 0.05 micromho/cm (temperature not specified) [ 16] . However, only ionized compounds are detected by conductivity measurements and soluble unionized compounds

or particulates are not detected. Both soluble organic materials and particulates are generated by ion-exchange columns. Studies at the Philips'

Laboratories [17] have shown that deionized water with a conductivity

of 0.06 micromho/cm contained up to 100 ppb of cation contamination.

They postulated that the detected impurities were present in particulate or colloidal form and were not appreciably ionized. Another possibility is that the cations form unionized complexes with dissolved resin material.

In fact there is some literature evidence [18-20] to show that small

amounts of heavy metals can be masked in deionized water and lost to

polarographic analysis due to chelation by soluble resin. Whatever the

reason, there is ample evidence in the literature [17] to prove that conductivity is not a reliable measure of trace element concentration in water.

Dr. Ralph Thiers [8] of Harvard wrote in 1957, "Since non-ionic

materials are not held by ion-exchange resins, and since traces of

nitrogeneous compounds are leached off the resin by water, probably the

best treatment for obtaining purest water in all ways is ion-exchange followed by distillation from fused quartz." Nothing has happened in the last

two decades to change the validity of that statement and this position has

been supported by other investigators [2 1-23 ]

.

The system used by our group at NBS consists of taking the "building

distilled" water as it is supplied to the laboratory and redistilling it in a

conventional still equipped with a quartz condensing system. The distillate from this still is fed to a quartz "sub-boiling" still (Quartz Products

Corp., Plainfield, N.J.) where the water is again distilled, but by a nonboiling technique which will be described later. The "building distilled"

water consists of ordinary city water that has been passed thru a mixed129
Accuracy in Trace Analysis

bed deionizer, distilled in a tin-lined still, stored in a tin-lined tank and distributed thru tin pipes to the laboratory.

Table 4 shows the results of the analysis of the "sub-boiling" distilled

water and the "building distilled" water by spark source isotope dilution

mass spectrometry (SS-IDMS). There was better than a tenfold reduction

in impurities from "building distilled" water which is really good quality

water for a high-volume operation. Copper at 3 ppb is the principal impurity in this water. The analysis of the "sub-boiling" distilled water, which

totaled 0.5 ppb for the elements determined, really represents an upper

limit for most impurities since no attempt was made to correct for a blank

during the evaporation of the water. This water compares favorably with

the best water reported in the literature. While it is one problem to

produce high quality distilled water, it is quite another to keep it pure before use. We have been using quartz for storage and this analysis is for

water stored in quartz for 2 weeks. We are currently studying the storage

of water in Teflon.

Studies in the literature have shown that the trace element content of

water stored in Teflon or polyethylene increased by a factor of 5 to 1 0 for

some elements after 30 days storage [22]. As a general rule, distilled

water should be used as soon as possible after preparation.

Table 4. Impurity concentration in distilled water

Sub-boiling Building

distilled distilled (ng/g) (ng/g)

Pb 0.008 0.1

Tl .01 <.01

Ba .01 .004

Te .004 .02

Sn .02 .2 Cd .005 .03 Ag .002 .005

Sr .002 .002

Zn .04 1 Cu .01 3 Ni .02 0.1 Fe .05 .4 Cr .02

Ca .08 1 K .09 0.06

Mg .09 2 Na .06 0.1

Total Impurity 0.5 ppb 8.0 ppb

130
Murphy

The next most important group of reagents for the trace analyst are the

mineral acids. In recent years high-purity acids have been available from

commercial suppliers. While these acids may prove quite satisfactory for a number of trace applications, the concentrations of some trace elements

are too high for low level trace analyses so the analyst will have to purify

his own acids in these cases. High-purity hydrochloric acid, hydrobromic

acid, and hydrofluoric acid have been produced by saturating pure water

with the pure gaseous compound [7,8,24-30] or by isopiestic or isothermal distillation [31,32]. The two-bottle sub-boiling method of Mattinson

[33] has been used to produce high-purity HCl and HF [34]. Nitric

acid, perchloric acid and sulfuric acid are usually purified by repeated

distillations [35].

We have been purifying these acids by "sub-boiling" distillation from

quartz or Teflon stills [ 1 1 ,36] . Figure 9 is a schematic of a quartz "subboiling still which is used for the production of high-purity water and the

mineral acids with the obvious exception of hydrofluoric acid. The still is fed by a 6- to 8-lb bottle of ACS Reagent Grade acid through a liquid level

control. Heating of the liquid being distilled is accomplished by a pair of

infrared radiators positioned on both sides of the condenser.

The surface of the liquid is heated and evaporated without causing the

liquid to boil. This positioning of the heaters also serves to heat the walls

above the liquid, tending to keep them dry which minimizes creep of

FEED BOTTLE

. .

DRAIN

Figure 9. Pure quartz sub-boiling still.

131
Accuracy in Trace Analysis

liquid between the cold finger condenser and the Hquid reservior. The

cold finger condenser is tilted downward to allow the condensed liquid to

flow to the tip above the outlet. The distillate is caught in Teflon FEP bottles which have been rigorously cleaned with nitric and hydrochloric

acids.

Figure 10 is a schematic for the all Teflon sub-boiling still [ 11 ] . It was

fabricated from a 2-liter TFE Teflon bottle and is designed on the same

principal as the quartz still except that a trough of Teflon was hung from

the cold finger to catch the drops of hydrofluoric acid dropping from the

condenser.

Each "sub-boiling" still is housed in a clean air chamber, that is, a Class

100 hood, to protect the distillation process from external contamination.

Figure 1 1 is a view of the laboratory for producing these high-purity

acids. We produce enough high-purity acid in this facility to supply the entire Analytical Chemistry Division and some other laboratories with special needs.

Table 5 shows the results of the analysis of sub-boiling distilled

hydrochloric acid, the starting ACS reagent grade acid, and a lot of commercial high-purity acid. A summation of these impurity elements shows

that the sub-boiling distilled acid contained 6.2 ppb, the ACS reagent

grade 820 ppb, and the commercial high purity, 70 ppb. The only element

found in the sub-boiling distilled acid at a level higher than 1 ppb was iron

FEED BOTTLE

f >

DRAIN

Figure 10. All-Teflon sub-boiling still.

132
Murphy

Figure 1 1 . Laboratory for production of high-purity acids.

133
Accuracy in Trace Analysis

Table 5. Impurity concentration in hydrochloric acid

Sub-boiling ACS reagent Commercial

distilled grade acid high purity

(ng/g) (ng/g) (ng/g)

Pb 0.07 0.5 <1

Tl .01 .1 —

Ba .04 2 —

Te .01 0.1 —

Sn .05 .07 <6

In .01 — —

Cd .02 .03 0.5

Ag .03 .05 .2 Sr .01 .05

. 4

Cu .1 4 1 Ni .2 6 3 Fe 3 20 7 Cr 0.3 2 0.3

Ca .06 70 24

K .5 200 10

Mg .6 10 20

Na 1 500

Total impurity 6.2 ppb 820 ppb 70 ppb

at 3 ppb. The other elements were at the sub-ppb level, generally lower

than 0. 1 ppb.

The results for the analyses of sub-boiling distilled nitric acid, the ACS

reagent grade acid, and a lot of commercial high-purity acid are shown in

table 6. The totals of the impurity elements determined were 2.3 ppb for

the sub-boiling distilled acid, 220 ppb for the ACS starting acid, and 240

ppb for the commercial high-purity acid. No element was found in the

sub-boiling distilled acid a concentration greater than 1 ppb and only sodium was detected at that level. Most of the elements were in the 0.05 to

0.01 ppb range.

Similar analyses for perchloric acid are shown in table 7. These

analyses demonstrate the efficiency of sub-boiling distillation when the

concentrations of impurity elements are compared to the starting acid.

The concentration of barium which was greater than 1000 ppb was

reduced to 0.1 ppb, a purification factor of greater than 10,000. Another

example is calcium which was reduced to 0.2 ppb from 760 ppb in the

starting acid, a factor of 3,800. It should also be noted that chromium

which was present at 10 ppb in the starting acid was not significantly

changed in the purified acid probably due to the formation of volatile

134
Murphy

Table 6. Impurity concentration in nitric acid

Sub-boiling ACS reagent Commercial

distilled grade acid high purity

(ng/g) (ng/g) (ng/g)

Ph \j . n 9 XI • ^2 Ba .01 8 Te .01 0.1 —

Sn .01 .1 1 In .01 Cd .01 0.1 0.2

Ag .1 .03 .1 Sr .01 2

Se .09 0.2

Zn .04 4 8

Cu .04 20 4

Ni .05 20 3 Fe .3 24 55 Cr .05 6 130

Ca .2 30 30

K .2 10 11 Mg .1 13 Na 1 80

Total impurity 2.3 ppb 220 ppb 240 ppb

Table 7. Impurity concentration in perchloric acid

Sub-boiling ACS reagent Commercial

distilled grade acid high purity

(ng/g) (ng/g) (ng/g)

Pb 0.2 2 16 Tl .1 0.1

Ba .1 >1000 10

Te .05 0.05

Sn .3 .03 <1

Cd .05 0.1 4

Ag .1 0.1 0. Sr .02 14

Zn .1 7 17 Cu .1 11 3 Ni .5 8 0. Fe 2 330 10

Cr 9 10 18 Ca 0.2 760 7

K .6 200 9

Mg .2 500 4

Na 2 600

Total impurity 16 ppb >3400 ppb 100

135
Accuracy in Trace Analysis

Table 8. Impurity concentration in sulfuric acid

Sub-boiling

distilled (ng/g)

ACS reagent

grade acid

(ng/g)

Pb

Tl

Ba

Te

Sn

Cd

Ag

SrZn

Cu

Ni

Fe

Cr

Ca

K

Mg

Na

4

2

9

0.6

.1.3

.1.2

.3.3

.3

.5

.2

.2 70.2

2 123

9

4

50

0.5

6

0.5

.1.2

.1.6

.2

.6

.4 2

0.2

6

Total impurity 27 ppb 200 ppb

chromyl chloride, Cr02Cl2. Chromium accounts for over half of the total

impurities of 16 ppb found in the purified acid.

The analyses of sub-boiling distilled sulfuric acid, shown in table 8,

showed that it contained 27 ppb of the elements determined. Sodium and

iron were the principal contaminants, accounting for over half of the total

impurities.

Hydrofluoric acid was purified using the all-Teflon still. Table 9 shows

the results of the analyses of the starting ACS grade acid and the purified

product. The total of the impurities in the sub-boiling distilled acid was 17

ppb compared to 320 ppb in the starting acid. The two principal impurities

in the purified acid were calcium and chromium at 5 ppb, which accounted

for over half the total impurities found.

The situation for salts and other reagents is not as favorable as for the

acids. Again, there are special high-purity grades of some of these re- agents available from commercial sources which are a major improvement

over ACS reagent grade materials. However, there are relatively few of

these reagents available and they often contain impurities at a level too

high for use in low level trace analysis [22,23,37] . Hume [38] has stated

in a recent article that natural sea water contains much lower concentrations of many heavy metals than artificial sea water made up from the

136
Murphy

Table 9. Impurity concentration in hydrofluoric acid

Sub-boiling ACS reagent

distilled grade acid

(ng/g) ' (ng/g)

Pb 0.05 0.8

Tl .1 .2 Ba .1 .5 Te .05 .1 Sn .05 11 Cd .03 2

Ag .05 0.1 Sr .1 .5 Zn .2 4

Cu .2 3 Ni .3 12 Fe .6 110

Cr 5 20

Ca 5 14

K 1 28

Mg 2 10

Na 2 100

Total impurity 17ppb 320 ppb

purest available reagent chemicals. An example of a high-purity, commercially available reagent that has met a need is phosphorus pentoxide. The

specifications for the ACS reagent grade would allow for up to 1 00 ppm

of lead. One supplier of high purity reagents has a limit of 0. 1 ppb for lead

in this material and it has found wide use in the silica gel method for lead

by isotope dilution mass spectrometry [39]. On the other hand, the

specifications for high-purity sodium acetate allow a maximum of 0.05

ppm of lead. A total of 3 g of sodium acetate is sometimes used during the

sample processing for the polarographic determination of lead, so the lead

contamination could be as high as 1 50 ng Pb from this reagent alone. In

cases like this the analyst will have to purify his reagents before use. The

purification method used will depend on the nature of the reagent and the

impurities to be removed. Mercury cathode electrolysis, extraction with

dithizone or cupferron, ion-exchange, and crystallization have all found

use in this regard [ 7 ,8 ,2 1 ,3 5 ]

.

C. Apparatus

The third important source of contamination is that due to apparatus

that comes into direct contact with the sample or sample solution, such as beakers Mttles, filters, mortars, etc.

137
Accuracy in Trace Analysis

Table 10 shows the results of Vasileyeskaya [23] for the analysis of

high-purity hydrofluoric acid, hydrochloric acid, and nitric acid after

evaporation in Teflon, platinum, and quartz containers. The results show

higher trace element concentrations when the evaporations were performed in either platinum or quartz than in Teflon with the evaporation in

quartz showing the highest values. Note the increase in aluminum, iron,

calcium, and magnesium when the evaporations were performed in quartz

compared to Teflon.

Table 1 1 shows the data compiled by Hetherington, Stephenson, and

Witherbum [40] for the maximum recorded impurity levels in various

types of fused silica or quartz. There are two main types of vitreous silica,

translucent or opaque quartz, and transparent quartz. Translucent quartz

has the highest trace element concentrations and should be avoided.

Transparent Types I and II quartz are made from naturally occurring

quartz crystals or sands. Type I is made by electric melting and Type II by flame melting. Type II quartz has a somewhat lower trace element

content because some impurities are partially volatilized in the flame.

Type III quartz is a synthetic quartz made by vapor phase hydrolysis of

pure silicon compounds such as SiCU. It has a much lower trace element

content than natural quartz with the exception of chloride which runs about 50 ppm and it contains about 0. 1 percent hydroxide. There is yet

another type of transparent quartz. Type IV, which is also a synthetic

quartz made by the oxidation of SiCLj and electrical fusion. It has about

the same trace element content as Type III, is virtually free from hydroxide but has a chloride content of several hundred ppm. Notice that the

principal impurities in Types I and II quartz are aluminum, calcium, iron,

and magnesium, the same elements that increased when the pure acids

were evaporated in quartz. Obviously synthetic quartz should be used

whenever possible. One problem is that it is usually not available in the

form of laboratory ware such as beakers, dishes, or crucibles.

Table 12 shows the trace element concentrations of some container

materials from the literature [7,48,53,54]. While variations of impurity

content from batch to batch are to be expected, the compilation is useful

for comparative purposes. Borosilicate glass can seriously contaminate

solutions [8] and should be avoided unless the analyst is sure that the

contamination of the element being determined is negligible such as for

rhenium. In addition to these elements contamination from arsenic and

lead have been reported. The quartz reported here is a Type 1 quartz made

from the melting of pure domestic quartz sand which is claimed to be

purer than the imported quartz crystal. Both polyethylene and Teflon are purer than natural quartz. The polyethylene reported here is produced by

138
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>-i O >o O Q TO 2S ^^S

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139
Table 11.

Accuracy in Trace Analysis

Maximum recorded impurity levels in various types of vitreous silica'^

Translucent Concentration (ppm) in Element vitreous transparent vitreous silica silica Type I Type II Type III

A 1 Al jUU 1

A

£0oo <0. 25

DO U. 3 A 1 0.

1

. 1 As XTr\ NU ND XTNDr\ < .02

D y 4

•>

. 3 . 1 Ca zlHJ lo A /I 0.4 < . 1 cr U. 1 VTrv ND .03 Co ND ND ND .0001 Cu ND 1 •1 <1

Ga ND ND ND <0.02

Au ND ND ND <.l

Fe 77 7 1.5 <.2

Li 3 7 1 ND

Mg 150 4 ND ND

Mn ND 1 0.2 <0.02

Hg ND ND ND <.l

P ND 0.01 0.005 <.001

K 37 6 <1 .1 Na 60 9 5 <.l

Ti 120 3 2 ND

U ND ND 0.0006 ND

Zn ND ND ND <0.1

Zr 15 3 ND ND

« See reference [40]. ND = not detected.

Table 12. Trace element concentration of container materials

Element Borosilicate Quartz Polyethylene Teflon

glass

(Mg/g) (Mg/g) (Mg/g) (Mg/g)

Al Major 50 0.3

B Major 0.5 .09 Ca 1,000 12 .2 0.002

Cr .015 <.03O

Cu .004 .022

Fe 3,000 5 .6 .035

K 3,000 4

Mg 600 2 .08 Mn 1,000 .01 Na Major 4 .17 Sb 2.9 .005 .0004

Ti 2

Zn 0.73 .093

Zr <1 .09

140
Murphy

the high pressure noncatalyzed process which is superior in trace element

content to the low-pressure process catalyzed by organic compounds of

aluminum or oxides of transition metals [ 3 ,8 ,4 1 ]

.

Table 1 3 lists an order of preference for container materials based on

the purity of the material. Teflon FEP bottles and beakers have been used

at NBS for the past several years with favorable results after thorough

cleaning with nitric and hydrochloric acids to remove contaminants introduced during fabrication. The high-purity acids described earlier were

all stored in Teflon FEP bottles for at least 2 weeks and no significant

levels of contamination were observed.

Filter paper is another source of trace element contamination. Quantitative ashless filter paper has been shown to contain 20 trace elements

at greater than 1 ppm [42]. Membrane filters, while not free from trace

element impurities are significantly better than paper [41 ] . All plastic fil- tration assemblies are available commercially which have been designed

to minimize contamination. Glass filtration assemblies have been shown

to contaminate pure water [43]. The trace element concentration of

membrane filters which are fabricated from cellulose acetate,

polypropylene, fluorocarbons, or polycarbonates can be lowered by acid

leaching before use [41,44]. Maienthal [44], for example, has shown,

table 14, that the concentrations of Fe, Cu, and Pb can be lowered by acid

washing of cellulose ester filters, but the contamination is still too high for

low level work. Whenever possible, centrifugation in plastic tubes should

be substituted for filtration.

The grinding and sieving of samples prior to analysis is another serious

contamination problem [7,41,45]. Mortars made from single crystal alumina, tungsten carbide or pure molybdenum have been used for this purpose since these mortars introduce significant contamination of only one

Table 13. Container materials in order of increasing impurities

1. Fluorocarbons

Teflon TFE and FEP; Tefzel; Halar; Kel-F

2. Quartz—Synthetic

3. Polyethylene

High Pressure—Conventional

4. Quartz—Natural

5. Platinum

6. Borosilicate

141
Accuracy in Trace Analysis

Table 14. Reduction of contamination of celluolus acetate filters by acid leaching'^

/ig/47mm sheet

Fe Cu Pb

Unwashed 0.30 0.50 0.10

Acid washed .036 .040 .041

• See reference [44].

or two elements which would be neglected in the analyses. Standard

sieves made of metal should not be used for sieving [46] . Instead fine silk

cloths have been recommended as a substitute. Whenever possible, it is best to avoid grinding and sieving altogether by dissolving large samples

and aliquoting.

Rubber stoppers and tubing are to be avoided as they are extremely

dirty materials with regards to trace element concentrations [48].

Likewise, polyvinyl chloride tubing is high in trace elements. Tygon tubing, for example, has been shown to contain lead and nickel in excess of

200 ppm and five other elements in excess of 10 ppm [48 ] . The choice of

materials for stoppers or tubing would be Teflon or polyethylene, both of

which are low in trace element content. Heating by gas using metallic bur- ners is a possible source of contamination. Ceramic electric hot plates are the least contaminating heat source since metal hot plates corrode and are attacked by acids. Ovens and muffle furnaces are also possible sources of

contamination. Fused silica liners are available for muffle furnaces which

help to reduce contamination [7 ]

.

D. Analyst

This brings us to the fourth source of contamination, the analyst. Careless manipulations can cause serious contamination. The touching of sur- faces that will come into contact with either the sample or a solution of the

sample can cause contamination with a number of elements besides sodium and chlorine. Sweat has been shown to contain potassium, lead, ammonium, calcium, magnesium, sulfate and phosphate ions [49]

.

Table 1 5 shows the results of a test for lead from the fingers of 1 6 individuals. Two fingers were soaked for two minutes in 15 ml of IN HNO3

and the lead determined by isotope dilution mass spectrometry. The

results, which showed an average 3. 1 /xg of lead, indicate that one must be

very careful in handling solutions containing trace amounts of lead.

142
Murphy

Table 15. Lead leached from fingers of 16 individuals by dilute nitric acid

Lead found

(Mg)

High reading 13.1

Low reading 0.8

Average 3.1

The use of cosmetics by analysts can cause serious trace element contamination since they can contain high concentrations of Al, Be, Ca, Cu,

Cr, K, Fe, Mn, Ti, and Zn [23,50]. Some hair dyes contain lead acetate

and eye make-up may contain mercury as a preservative. Trace analysts

would be well advised to avoid the use of such products. Even medications can be a problem. Calamine lotion, for example, which is used to

treat skin irritations such as poison ivy, is practically pure zinc oxide

when dry and has been known to cause contaminations. Watches and

jewelry such as rings and bracelets should not be worn in the trace laboratory because of contamination hazards.

To keep blank levels as low as possible the analyst must exercise care

in keeping the volume of reagents to a minimum, and the time or temperature of evaporations or reactions to a minimum. Excess of reagents, time, or temperature, all cause higher and more variable blanks.

Unless great care is exercised, cross-contamination from other work in

the laboratory can contaminate a sample. The analyst must be aware of

the history of beakers and other containers so that vessels that have been

exposed to major concentrations of one element are not used for trace

determinations of that element even after rigorous cleaning treatment.

Great care must also be exercised in the handling of ultra-pure reagents

least they become contaminated due to careless acts such as opening a

container in a unclean atmosphere or pouring a liquid from the container

into another solution.

And, finally, the analyst must "think blank"; that is, he must be aware

as to the effect on the blank of every step of the procedure. He must ask

himself "If I do this, what is the effect on the blank?" and avoid those

operations which tend to increase the blank or whose effect is not known,

whenever possible.

The importance of the analyst in this regard has also been noted by

others. Burriel-Marti [51] has stated that the analyst must really work

under "chemically aseptic" conditions. Libby [21] has stated that "low

level radiochemistry (and low level trace analysis as well) is something

like the discipline of surgery —cleanliness, care, seriousness, and prac143
Accuracy in Trace Analysis

tice." Alimarin [52], has written that "a major factor is also the ex- perience and the skill of the analyst, which obviously cannot be described

statistically." One last quote is from Ruzicka and Stary [21], "It is interesting to note that often an analytical chemist trained in trace determinations by spectrophotometry or spectrography is more successful in

this field (substoichiometry) than a radiochemist who is used to working

on neutron-activation analysis where the value of the reagent blank can be

neglected."

III. Conclusion

In order to decrease the variability of the analytical blank and increase

the accuracy of the blank correction, the analyst must reduce the contamination from each source, (1) by working in a Class 100 clean air en- vironment, (2) by using specially purified reagents, (3) by selecting apparatus materieds with the lowest possible impurities, and (4) by being

constantly aware of his own effect on the blank.

The methods discussed above for the control of the analytical blank

have been applied at NBS for the reduction of blank values and hence the

variability of the blank correction. Table 1 6 shows our experience with

lead in the Trace Elements in Glass (TEG) Standard Reference Materials over a period of 4 years. Since the lead blanks have been reduced to about

2 ng with an uncertainty of ± 1 ng, we have been able to analyze lunar

rocks and other materials containing sub-ppm amounts of lead with less

than 1 percent uncertainty. Table 17 shows our experience with silver.

Again by lowering the silver blank from 970 ng to 3 ng, the variability was

also reduced. These two examples illustrate the fact that control of con- tamination is not a hopeless task but does take special care, patience, and

financial support.

Table 16. Example of lead blank reduction

' TEG = Trace element in glass, SRM's 610 through 619.

Lead found (jug)

Initial analysis of TEG» standard 330 ± 250

TEG analysis using selected acids 260 ± 200

TEG analysis in Class 100 hood 20 ± 8 TEG analysis using special acids in Clean Room

(Lunar also) 2 ± 1

144
Murphy

Table 17. Example of silver blank reduction

Silver found Gug)

Initial analysis of TEG» standard 970 ± 500

TEG analysis using Class 100 hoods 207 ± 200

TEG analysis using special acids in Clean Room 3 ± 2

TEG = Trace element in glass, SRM's 610 tiirough 619.

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Accuracy in Trace Analysis

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[3 1 ] Irving, H. and Cox, J. J., Analyst 87, 207 (1962).

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pp. 26-33, Interscience, New York, N.Y. (1970).

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Techniques, Trace Metals in the Environment, Kneip, T. J., Ed., p. 346, Plenum

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1881-84(1973).

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Dekker, New York, N.Y. (1972).

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Publishers, New York, N.Y. (1959).

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[49 ] Handbook of Clinical Laboratory Data, 2d edition, Faukner, W. R. , King, J. W. , and

Damm, H. C. , Eds. , pp. 342-344, The Chemical Rubber Co. , Cleveland, Ohio ( 1 968).

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146
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[51] Burne\-Mani,F., Experientia Suppl. 5,7 1 (1956).

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I. P., Ed., p. 1 1 , Israel Program for Scientific Translations, Jerusalem, Israel (1968).

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147
APPENDIX III Evaluation of Lyophilization for the Preconcentration of Natural

Water Samples Prior to Neutron Activation Analysis

S. H. Harrison and P. D. LaFleur

Analytical Chemistry Division, National Bureau of Standards, Washington, D.C. 20234

W. H. Zoller

University of Maryland , College Park, Md.

Neutron activation analysis (NAA) has been applied successfully to a variety of complex environmental matrices

for trace element determination. Because of its excellent

sensitivity for many elements and general lack of matrix ef- fects, instrumental neutron activation analysis (INAA) has

been used to determine up to 30 elements in rocks {1), soils

(2), atmospheric particulates (3), sediments (4), and biological tissues (5). Although NAA is presently used to analyze various

aquatic samples, the applicability of the technique to the water matrix is hindered by two main factors: very low concentrations of the elements of interest, and interferences from more concentrated dissolved species.

Except for Na and CI in sea water, most natural waters

contain such low concentrations of most elements that few

can be accurately determined directly on bulk water samples by completely instrumental techniques. The sensitivi- ty of analysis can be greatly improved by concentration of the dissolved species from the water into a different medi- um, or removal of water to leave a residue of dissolved material that can be irradiated in a small sample container. The latter approach was used by Mamuro et al. (6) who

evaporated the water prior to irradiation. Since evaporation is generally a slow process carried out in the open, it is subject to possible difficulties. These difficulties include

contamination by fallout particles from laboratory atmosphere, dissolution of ions from container surfaces, and loss of volatile species if performed at an elevated temperature.

Other workers have devised chemical methods for re- moval of specific elements from water. Selective anion ex- change was used by Robertson (7) to determine Ag in sea water and by Becknell et al. (8) to measure Hg in fresh water. Nagatsuka et al. (9) used an initial iron hydroxide

coprecipitation step for concentration of rare earths from

natural waters. Fujinaga et al. (10) found that elements

producing short to medium lived isotopes when irradiated,

including V, Mo, and U, could be concentrated from water by liquid-liquid extraction followed by organic coprecipitation. In all of these treatments, there is a risk of contamination from reagent and container walls. Some reactors are equipped with facilities for irradiation of very large volume samples. However, this is not a totally

satisfactory solution to the problem of analysis of natural water for several reasons. First, there may be substantial

variations of flux over large volumes requiring very careful

flux mapping and monitoring. Second, the handling and

containment of very large samples may cause severe blank

problems. Finally, the counting of large volume samples

will necessarily involve an unusual (and inefficient) geo- metrical configuration for which accurate calibration may

be difficult. The work presented here deals with the preconcentration of water by freeze drying using a method which virtually eliminates sample contamination and trace element

losses. Freeze drying is a method commonly used for drying

biological tissues in which trace elements may be bound organically. Also, it has been used for drying natural, water al- though without associated evaluations of the technique

{11-14). However, questions may be raised regarding the

suitability of this method of concentration that point to the

possibility of losses of volatile elements during the drying

process and the ability to transfer quantitatively the dry

residue from the freeze drying container to an irradiation container. The first question arises because it is known that some

trace elements can exist in chemical forms that have signif- icant vapor pressures at room temperature, with respect to that of water. When a sample containing such compounds

is placed in a vacuum system for an extended length of

time, trace element losses from the sample will most likely occur. A major object of this work was to determine if there

are significant losses of various elements in water when the water is subjected to freeze drying.

To evaluate this question, known quantities of radioactive tracers for 21 elements (including many known to be

volatile) were added to water, the solutions freeze dried

and the tracer residues counted. The results confirm that

at least 95% of all but the most volatile elements studied

(Hg and I) were retained in the residue. The second question, concerning sample transfer, was eliminated by designing a freeze drying container which also would serve as an

irradiation and counting container. The material chosen was linear polyethylene film (Phillips Petroleum's Marlex).

This preconcentration and containment system has been

used successfully in our laboratory for the INAA of river and estuarine water samples.

EXPERIMENTAL

Apparatus. The freeze dryer used for the tracer studies consists of a high capacity two-stage pump, glass vacuum manifold, and

three freeze drying units. Each freeze drying unit was made up of a

glass sample chamber and cold trap. An 0-ring gasket on a flat flange served to seal the sample chamber when under vacuum. All other connections were made with ground glass stopcocks, ball

joints, and straight standard taper joints. An extra liquid nitrogen

cold trap was used between the oil pump and the manifold to pre- vent back diffusion of oil vapor. The counting equipment used to determine tracer yields were a

7.6 X 7.6-cm Nal(Tl) 7-ray detector, and a 13% efficient Ge(Li) detector which produced a peak with full width at half maximum of

3.0 keV for the ^°Co 1332-keV line. The Nal(Tl) detector was used

to obtain good counting statistics on samples which contained only one radioactive tracer. The Ge(Li) detector was used to resolve the photopeaks from the solutions spiked with two or more radioactive

tracers or from single isotope solutions that had a significant amount of radioactive daughter present. The bags used to contain the water samples were made of 0.0025-cm (1-mil) linear polyethylene film. The film was cut and

heat sealed to make approximately cubic shaped, open topped

bags. Each bag was soaked in 50% HNO3 for 24 hours to remove

surface contamination. Although HNO3 is probably not the ideal leaching agent, it was felt that HCl followed by HNO3 could pres- ent CI contamination difficulties. Different cleaning procedures

are presently being investigated. Linear polyethylene was chosen over conventional polyethylene because of its superior performance with respect to structural degReprinted from ANALYTICAL CHEMISTRY, Vol. 47, Page 1685, August 1975 American Chemical Society ..q
Table I. Trace Element Content of Linear

Polyethylene Film

Concentrations in ng/g

Element Average Range Na 3400 580-6600

CI 5300 3700-6900

Al 1000 870-1200

Br 990 790-1300

Sm 0.26 0.18-0.31 Co 14 0.90-27

Zn 410 150-820

Fe 2200 1000-4700

Sc 0.34 0.13-0.73 Sb 11 2.2-30

La 4.5 3.0-6.8 Cr 1300 1200-1500

Th 4.0 0.95-13 Sr < 180 <180

Au 5.3 2.4-7.0 Ag 4.4 < 3-5.1 Ba <250 <250

Mn < obO 140—300

Ce 35 6.0-92 Eu <0.14 <0.14

V 210 210

Se <3.5 <2.5-5.6 Mg < 60,000 <60,000

Ca < 100,000 < 100,000

Table II. Trace Element Content of Conventional

Polyethylene Film

Element Concentration, ng/g

Na 30,000 CI 24,000

Al 19,000

Br 120

Sm 1.4 Co 81 Zn 1,600

Fe 11,000 Sc 6.9 Sb 33 La 8.1 Cr 170

Th 3.0 Ba 28,000

Mn 110

Ce 27 Eu 4.2 V 150

Se 43 Mg 12,000

Ca 7,700

radation during long neutron irradiation at high flux. Experience

in the laboratory shows that a 4-hr irradiation at a thermal neutron flux of 5 X 10^^ n cm~^ sec"' renders a thin film of conventional polyethylene almost to a powder whereas, after the same

dose linear polyethylene film is in a manageable form, somewhat

more rigid and with some chance of cracking. It was anticipated

that, in future work, it would be useful to be able to reopen the bags after irradiation and add chemicals to redissolve the residue;

therefore, the need for a non-degrading film. The linear polyethylene film chosen was quite clean compared

to conventional polyethylene film which has been analyzed in this laboratory. Typical trace element concentrations in the acid washed bags, made exactly as for sample containment, are shown

Table III. Trace Element Retention Yields for Freeze-Drying Water

Element Isotope 71 Range, % Average, %

Na" Na 3

Qc. o_qq 4 Q7 4 -I- 1 R Sc So 5 07 q . 4 q V

48Tr 2 11 on 0—1 no ft \J\J .V 1 \J\J ,0 1 nn 4 4- n Cr 4

QA 0—QQ t: qRy l) .yq 4-±

0ci .un Fe 1 1 01 1 Co 6 Q4 9—1 00 0 Q7 . O 0

Z>n 1 1 09 R As As 6 94 8—98 S Se ^Se 6

Q'^ ^1 on n Qfi 9 4- 1 ft Br" 3 QI 4—1 no P qqy y ,0 -1- ± 11,17 Rb 4 QQ ^1 0*^ fi 101 9 4-1 ft Sr 1 • • • 11 01Ul Ag 1 qft yo 1

. 1 Cd 4 y^t ,y—i 1

0

1 09 J- fl Sb 6 Qfi 7_QQ 7 Qft yo 4- 1 1 .o ± X .1

I

12 Gj 3 lU ,I7 OO .U Rft DO 4-± 91£tl Cs '"Cs 1 qq Ba 6 91.8-100.6 97.7 ± 3.0 Au 4 92.1-95.0 94.0 ± 1.3 Hg 4 50.1-72.0 61 ± 9 Tb '«°Tb 1 98.7 Ce '"Ce 2 97.5-99.1 98.3 ± 1.1

" Tracer solution used was not acidified.

in Table I. A representative trace element profile of the conventional film is given in Table II. The results in Table I compare

quite favorably to those in Table II with the exception of Cr.

If the new cleaning procedures being investigated do not substantially reduce the trace element content of the bags used, other

materials must be sought. Solutions. Inorganic radioactive tracers of one to three elements were added to 500 ml of filtered river water, tap water, or deionized water (generally filtered river water was used). Each element was added in amounts that would give concentrations less than 1 mg/liter in the resulting solution. In some cases, known volatile ox- idation states of elements were used as tracers, i.e., ""^VOCla. The pH of the prepared tracer solutions was adjusted to about

1.5 with HNO3. The purpose of the pH adjustment was to follow the common practice of preserving water samples by acidification

(12). In most cases, the stock tracer solutions were allowed to equilibrate for at least 24 hours in a polyethylene bottle prior to a freeze drying run. Procedure. One-hundred-ml aliquots of tracer solution were

transferred to the polyethylene bags. Each sample was then frozen

in liquid nitrogen. Concentration gradients due to zoning when

samples are frozen slowly probably do not occur when they are

"flash" frozen in this manner. An additional 100 ml of tracer solu- tion was transferred to a polyethylene bottle and set aside as a ref- erence sample.

The frozen samples were placed in the freeze-drying chambers

which then were evacuated. The freeze drying process is essentially a low temperature, low pressure distillation of water which sublimes from the surface of the ice and condenses in the cold trap. The heat of sublimation removed from the sample, together with

the low pressure conditions and the insulation provided by the vacuum, is sufficient to keep water with salinity below 9 mg/g fro- zen during the drying period even when the apparatus is at room

temperature. Two to three days under a vacuum of 1.3 to 6.5 pascals (10 to 50 millitorr) is needed to dry 100 g of ice to a residue, which remains on the bottom of the bag. Depending on the substances present in the water used, the residue may appear granular

or as a film. To determine the retention yields of the tracers added, the bags

containing freeze-dried residue were transferred to the same size polyethylene bottle used for the reference aliquot. Concentrated

acid was added to redissolve the dry residue. The volume in each

bottle was brought up to 100 ml. The activity from each isotope was then measured in the reference sample and compared to that

in the freeze-dried sample.

In order to verify the radiochemical purity of samples spiked with only one isotope, one sample in each such set was counted on

149
Figure 2. Diagram of single freeze-drying unit

(a) Teflon valve, (b) 15-mm 0-ring joint, (c) 75-mm 0-ring joint, (d) Styrofoam

flasi< lid, (e) cold trap. (0 liquid nitrogen, (g) stainless steel insulating flask, (h) freeze drying chamber, and 0) gasket of polyettiylene film

Table IV. Results of INAA of Freeze Dried Residue

of Back River Sample

r- ^ t A A ng in sample t-onccntration in trecze dried ^ ' Element residue, ng/g of water ng in Poly bag

Figure 1. Nine-chamber freeze dryer made of glass and Teflon with 0-rings and polyethylene gaskets, installed in Class 100 clean room

a Ge(Li) detector. If no significant radioactive impurities or daughters were present, all samples in the set were assayed with

the Nal(Tl) detector. For all samples, the maximum error due to counting statistics was 1%. To minimize geometry counting errors due to possible incomplete dissolution of residue, the samples were counted at a distance

as far from the detector as practicable, generally 5 to 10 cm from

the Ge(Li) detector and 20 cm from the Nal(Tl).

RESULTS AND DISCUSSION

The results of this investigation (Table III) indicate that,

for all but two or three of the observed elements, freeze drying is a quantitative preconcentration technique for in- organic trace elements in water. No consistent retention value was obtained for those elements (Hg and I) which

were lost substantially during freeze drying. Bromine,

added as a bromide salt, was lost from acidified solutions but quantitatively retained in neutral solutions. The re- sults obtained with the three different water types (filtered

river, tap, and deionized) were indistinguishable, and

therefore are not specifically identified in Table III. In general, single determinations were carried out using a filtered river water tracer solution and multiple results contain one

or two runs each of river, deionized, and tap water. A critical factor in obtaining good retention yields, particularly for volatile elements, is to keep the sample frozen

solid during the lyophilization process. Loss of vacuum is the primary reason for sample melting except, as previously mentioned, in the case of saline samples. In addition to leaks in the system and malfunction of the pump, a com- mon cause of vacuum loss was blockage of cold traps. This

Eu 0.24 >20

So 0.31 118

Th 0.089 2.5 Cr 5.4 0,51

Fe 530 30

Co 1.2 10

Sr 440 >300

Ag 0.24 7.7

•Zn 12.3 3.2 Ba <35

Sb 0.64 6.0 Se 0.33 10

Na" 0.53 mg/g

CI" 0.84 mg/g

Mn" 0.23 Mg,/g

" The results for Na. CI. and Mn were obtained on a liquid

sample and are presented for informational purposes only.

results because the water vapor which is frozen in the trap

is not uniformly distributed on the glass surface but accu- mulates at a small area around the level of the liquid nitro- gen. If the cryogenic flask has a tight fitting cover, the ice

in the trap will accumulate at the level of the cover rather than the level of the liquid nitrogen. Since such a small

portion of the total capacity of the trap is used to freeze out

the water vapor, the ice condensing from a large sample

may collect until it fills the available space, thus blocking

the path to the sample chamber. A standard size trap (used

in the system described) which has 13-mm o.d. tubing and

a 38-mm diameter body has to be changed twice when

freeze drying a 100-g sample. The use of a Dry Ice-acetone mixture as a coolant does not alleviate the problem.

To avoid the inconvenience of changing traps every 24

hours, an improved freeze-drying system has been devel- oped. This system is fitted with 75-mm traps which provide

adequate area to freeze dry at least two 100-g water samples consecutively before changing traps. Other improved

features of this system are glass 0-ring joints for all connections and Teflon valves in place of greased stop cocks. With these changes, contamination originating from con150
struction materials of the freeze dryer itself are minimized.

The entire new system, with nine chambers for samples is shown in Figure 1. An expanded diagram of one of the units

is shown in Figure 2. Since this freeze drier was installed in a Class 100 clean room facility, contamination of the dry

residue from laboratory air particulates is minimized (15). Under normal laboratory conditions, it is recommended

that air which is bled through inlet valves be filtered.

It was previously stated that one of the advantages of

this method of freeze drying water was the elimination of the need to transfer the dry residue. The bag containing

the residue after freeze drying may be heat sealed, and

then folded to make a small sample (1 cm X 2 cm X 0.5 cm)

ready for irradiation and counting.

The results of a typical analysis of dissolved species in water by INAA following the described preconcentration

procedure appear in Table IV. This sample was taken from

the Back River, just east of Baltimore, Md. The ratio of

trace element content in the sample to the average trace el- ement content in a polyethylene bag is sufficiently high for most elements to result in reliable data, particularly considering errors due to the sampling procedure alone. Work

carried out in this laboratory to determine the variations in trace element concentration attributable to sampling tech- niques, suggest that such errors may easily exceed ±20%.

The use of neutron activation for the analysis of trace el- ements in the aquatic environment is now well established. The multielement analyses of fresh to moderately saline water is greatly aided by the ability to handle relatively

large samples with a minimum of pre-irradiation manipulation, as reported here. In addition, the use of freeze drying

does not preclude subsequent post-irradiation chemical

separations for elements that cannot be observed instru- mentally.

LITERATURE CITED

(1) R. H. Filby, W. A. Haller, and K. R. Shah, J. Radioanal. Chem., 5, 277

(1970). (2) H. D. Buenafama, (Conf-721069-3), 1972, 10 pp. Dep NTIS.

(3) W. H. Zoller and G. E. Gordon. Anal. Chem., 42, 257 (1970).

(4) A. J. De Groot, K. H. Zschuppe, M. De Bruin, J. P. W. Houtman, and P. Amin SInggih. "Modern Trends in Activation Analysis ", J. R. DeVoe and

P. D. LaFleur, Ed., Vol. I, 1969, p 62.

(5) E. L. Kanabrocki, L. F. Case. L. Graham. T. Fields. Y. T. Oester, and E. Kaplan, J. Nucl. Med.. 9, 478 (1968).

(6) T. Mamuro, Y. Matsuda. A. Mizohata. T. Matsunami, and T. Tal<euchi, "Annual Report of the Radiation Center. Osal<a Prefecture", Vol. II, 1970, p 23

(7) D. E. Robertson, "Pacific Northwest Laboratory Annual Report for 1972

to the USAEC Division of Biomedical and Environmental Research". Vol.

II, part 2, p 24.

(8) D. E. Becknell, R. H. Marsh, and W. Allie, Jr., Anal. Chem.. 43, 1230

(1971). (9) S. Nagatsuka, H. Suzuki, and K. Nakajima. Radioisotopes. 20, 305

(1971).

(10) T. Fujinaga. Y. Kusaka, M. Koyama, H. Tsuji, T. Mitsuji, S. Imai, J. Okuda, T. Takamatsu, and T. Ozaki, J. Radioanal. Chem., 13, 301

(1973).

(11) D. F. Schutz and K. K. Turekian. Geochim. Cosmochim. Acta, 29, 259

(1965).

(12) A. P. Grimanis, G. Pantazis. C. Papadopoulos, and N. Tsanas. "Third United Nations International Conference on the Peaceful Uses of Atomic

Energy", 1964.

(13) P. Schramel and K. Samshal, Int. J. Environ. Studies, 5, 37 (1973).

(14) W. F. Merritt, "Proceedings of International Conference on Identification and Measurement of Environmental Pollutants". 1971, p 358.

(15) T. J. Murphy, "Proceedings of 7th Materials Research Symposium". National Bureau of Standards, 1974.

Received for review November 21, 1974. Accepted March

26, 1975. Certain commercial equipment, instruments, or materials are identified in this paper in order to adequately

specify the experimental procedure. In no case does such

identification imply recommendation or endorsement by

the National Bureau of Standards, nor does it imply that the material or equipment identified is necessarily the best

available for the purpose.

151
APPENDIX IV

REPRINTED IN PART FROM

TECHNICAL NOTE 548

ISSUED DECEMBER 1970

Not. Bur. Stand. (U.S.), Tech. Note 548, 164 pages (Dec. 1970)

CODEN: NBTNA

Activation Analysis Section:

Summary of Activities, July 1969 to June 1970

Philip D. LaFleur and Donald A. Becker, Editors

Activation Analysis Section

Analytical Chemistry Division

Institute for Materials Research

National Bureau of Standards

Washington, D.C. 20234

152
3. ACTIVATION ANALYSIS WITH THE NUCLEAR REACTOR

A . Introduction

During this year we have continued our efforts to attain

a high degree of versatility in activation analysis and have

spent much of our time in developing a greater expertise in

nondestructive activation analysis, particularly coupled with

computer data reduction. We intend, of course, to continue

our efforts in obtaining the highest possible versatility in

both destructive and nondestructive activation analysis and

in the investigation of parameters affecting accuracy and

precision in activation analysis.

We have spent a great deal of time in characterizing the

two pneumatic tube facilities which became available this

year. All four of the originally Installed facilities are

now available and have been very carefully studied in terms

of temperature effects, relative neutron fluxes and to a more

limited extent on neutron energy spectra. We have maintained

a very satisfactory relationship with the Reactor Operations

Section and with the Health Physics Section.

A greater portion of our activities this year than in

past years has been spent in service analyses for organizations

outside the Bureau of Standards and for other groups within

the Bureau of Standards. Particularly notable are our efforts

in the analysis of lasers and silicon chips. We are presently

beginning a service analysis program for the Department of

Agriculture, involving the determination of mercury in wheat

and peas and with the National Institutes of Health in the

determination of iodine in blood serum samples. It is our

hope that contacts with other governmental organizations in

the Washington area, such as these, may continue and that

interesting cooperative research programs may develop therefrom.

153
B . Facilities

1 . Characterization of the NBSR for Neutron Activation

Analysis

Since the last report [1] much additional information

has been obtained on the irradiation characteristics of the

NBSR. This information has included thermal neutron flux

variations within the irradiation areas, neutron energy

distribution in the various irradiation positions, temperature effects, and measurements of the gas production from

several different biological materials. These different

types of measurements will be treated separately here, with

the techniques used presented along with the results obtained.

For comparative purposes, all results obtained to date will

be included here.

The current status of the NBSR irradiation facilities

are given in table 1. The operational facilities include

Table 1. NBSR irradiation facilities.

Pneumatic Facilities

Ident

.

Position Attitude

Access

Diameter Status

RT-1 Core gap edge Horiz

.

1.5 in

.

Operational

RT-2 In core Horiz

.

1.5 in

.

Operational

RT-3 Core gap, D^O Horiz

.

1.5 in

.

Operational

RT-4 D^O reflector Horiz

.

1.5 in

.

Operational

RT-V Vertical thimble Vert

.

0.75 in. Under const.

RT-C

1 Vertical thimble] (Vert

.

0.75 in. Under develop.

(RT-V-2)

j

(Converter) J-4

J

jvert

.

0.75 in. Under const.

RT-T Thermal column Vert

.

1.5 in. Under const.

Manual Access Facilities

Vertical

thimble

In core, G-2 Vert

.

1.5 in

.

Operational

Vertical

thimble

Reflector, V-4 Vert

.

(7

1 in

.

Positions

)

Planning

154
the four pneumatic tubes (fig. 1) and the manually accessed

Long Term Irradiation Facility (LTIF) located in the G-2 v

vertical thimble. The program of characterization has

included measurements on all of these operational facilities,

but with particular emphasis on the pneumatic facilities

which are used for the majority of the activation analysis

irradiations

.

a . Pneumatic Tube Irradiation Facilities

The four operational pneumatic tube facilities have

diameters of 1 1/2 inches, are powered by CO^, and terminate

in significantly different neutron environments (fig. 1).

TANK WALL

RT-1

RT- 2

RT- 3

/

/

/ — /

/ — ~— /

/

/

/

/

RT - 4

Figure 1 Positions of the pneumatic tube terminals

in the NBSR. (Terminology; The proximal

end of the rabbit is that end closest to

the reactor core vertical centerline; the

distal end is that furthest from the core

centerline .

)

155
Linear polyethylene rabbits, which have a capacity of approximately 42 cm ( 2 . 6 cm dia x 8 cm long) are used in these facilities [1].

( 1

) Neutron Flux Measurements

Initial measurements in the pneumatic tubes were for the

thermal neutron flux. Of interest was the absolute thermal neutron flux, and the variation of this neutron flux within each

irradiation position. Some measurements also have been made

of the neutron energy distribution characteristics within

each facility.

( 2 ) Thermal (Sub-cadmium) Neutron Flux

The thermal (sub-cadmium) neutron flux measurements were

made using the thin foil technique [2]. Two different

detector foils, gold and copper, were used for these measurements. Copper foils were preferred, due to the absence of

significant resonance peaks in its neutron cross section

(fig. 2). Some measurements were also made on gold, foils.

CB)

10-

10"

10-

10'

10'

10'

COPPER

GOLD

• COBALT

ll»!

1

1

- 1

1

• . • i a 1 \

r. 1 !• 11

0.1 10 iOO

x|0- xlO^

100

x|o3

I clO*

10

x|0'

100

xlO*

Figure 2.

NEUTRON ENERGY CEV)

Neutron cross sections of copper, gold and

cobalt as a function of neutron energy.

156
since gold is often used in other laboratories. However,

since gold has significant neutron self-absorption problems,

slight variations in foil thickness can lead to appreciable

errors in flux determinations

.

The technique used consisted in irradiating a thin foil

(0.005 in Cu foil; 0.001 in Au foil) of the element of

interest in the appropriate facility, then counting the

radioactive species formed on a large volume Ge(Li) detector.

This detector has been calibrated for absolute photopeak

counting efficiency using NBS calibrated radioactive sources

.

Neutron fluxes were then calculated from nuclear constants.

The thermal (sub-cadmium) neutron flux was obtained after

correction for epicadmium contribution to the foil activity.

For copper, this correction did not exceed however, for

gold this correction was in excess of 25^. No neutron selfshielding corrections were made.

The results of the flu^c determinations are found in

table 2 . It is apparent that the thermal neutron flux

in tubes RT-1, RT-2 and RT-3 are very similar, with a significantly lower flux in RT-4. The gold foils gave higher

flux numbers (relative to those from Cu measurements) for

the tubes RT-1, RT-2 and RT-3, where a larger epicadmium

neutron flux exists, but a value very close to that for

copper in RT-4, where the neutrons are highly thermalized.

Repeated determinations of these flux measurements gave

results agreeing well with the original values.

Table 2. Experimental neutron flux-NBSR pneumatic facilities

Sub-Cadmium Neutron Flux-Proximal End

I -2 -1.

( n. cm sec ) Detector

Foil RT-1 RT-2 RT-3 RT-4

Gold 7.0x10^^ 6.9x10^^ 5.8x10"^^ 1.4x10^^

Copper 5.6x10^^ 6.3x10^^ 4.9x10^^ 1.3x10"^^

157
( 3 ) Neutron Flux Variation Measurements

In order to obtain accurate and precise results in

neutron activation analysis using a nuclear reactor, the

geometrical variation of the flux within each irradiation

position must be accurately evaluated. Previous studies of

the NRL Nuclear Reactor [3] revealed serious neutron flux

variations within the sample irradiation area, and thus emphasized the need for such measurements

.

The flux measurements for the NBSR were made on the

inside dimensions of the polyethylene rabbits for the 1 1/2

in. pneumatic tube system (RT-1 to RT-4). Since the irradiation space available for each facility approximated a right

circular cylinder 2.6 cm in diameter and 8 cm long, both

the circumferential flux distributions' and the axial flux

distributions were measured. The procedure developed used

3/l6 in. disks of copper foil or 20 mil copper wire with gross

gamma counting of the copper-64 radioactivity using an automatic sample changer. The copper activity was checked for

extraneous radioactivities by counting on a 47cc Ge(Li) detector and 4096 channel PHA and by following the decay characteristics of the foil on the gross gamma counter for over 8

half-lives. Both procedures revealed no detectable radioactivity in either the wire or the foil other than copper-64.

All samples from a particular type of measurement were counted

under identical conditions.

The results obtained for the circumferential flux distribution of the pneumatic facilities are found in table 3-

In all cases copper foils were used. These results are also

shown graphically in figure 3- The foil placement diagram for the circumferential flux measurements is shown in

figure 4. It can be readily seen from these results that

there is no significant circumferential flux variation at

any plane perpendicular to the axis of the right circular

cylinder comprising the rabbit irradiation area.

158
Table 3. Circumferential flux distribution in

NBSR pneumatic tube facilities.

Pneumatic Tube

Facility

Location in

Rabbit

No . of

Monitors

Relative

Neutron Flux

RT-1 Proximal 7 l.OOO±0.OO3

RT-1 Middle 7 0 .9^7±0 .004

Kl —1 Uls tai 7 O.o/l+O.UO/^

RT-2 Proximal 7 1 .000±0 .005

RT-2 Middle 6 0 .902±0 .002

DTI o jjis uai 7 U.o/4±U.UUJ

RT-3 Proximal 7 1 .000±0 .004

RT-3 Middle 6 0 .928±0 .003

RT-^ Di R ta 1 7 0 82Q + 0 00"^

RT-4 Proximal 7 1 . 000 + 0 . 010

T>rn )i RT - 4 Middle 7 0 . 7o7±0 . 007

RT-4 Distal 6 0 .603±0 .014

3,^Normalized to proximal end E 1.000.

Uncertainties are ts//n at 95% confidence level

.

Figure 3. Diagramatic representation of the circumferential flux distribution in the NBSR pneumatic

tubes at three positions inside the rabbit.

159
Figure 4 Copper foil placement for circumferential flux

measurements in the NBSR pneumatic tubes

.

The axial flux measurements were made using the copper

wire. A single length of the wire was centered along the

rabbit axis and held securely in position with polyethylene

foam. After irradiation, the wire was cut into 1 cm sections,

weighed, and counted. The results, shown in figures 5 and

6, reveal a serious neutron flux dropoff along the axis

of the rabbit. The magnitude of this dropoff over the

length of the rabbit ranged from approximately 11^ for RT-1

1.00

X

D

.80

Z

0

D

lU

Z

uj .40

>

<

.20 _RT-4

2 3 4 5 6

DISTANCE FROM PROXIMAL END (CM)

8

Figure 5. Axial flux distribution in the NBSR pneumatic tubes.

160
Figure 6. Axial flux dropoff inside the rabbits (normalized to proximal end =1.00 for each pneumatic

tube )

.

to over hO% for RT-4. The flux dropoff was linear in all

pneumatic facilities except RT-2.

( 4 ) Neutron Energy Distribution Measurements

Some preliminary neutron energy distribution measurements have been made, consisting almost entirely of cadmium

ratio measurements . These cadmium ratios were made at both

ends of all four pneumatic facilities. Three different

elemental foils were used, 1 mil thick gold foil, 5 mil copper

foil, and 2 mil cobalt foil. Interlocking 20 mil cadmium

covers were used for the cadmium shielded measurements.

The results of these measurements are found in table

4. The values for gold and cobalt were obtained primarily

161
Table 4. Cadmium ratios for the NBSR

pneumatic tube facility.

a Experimental Cadmium Ratios

Pneumatic Location

Facility In Rabbit Au(Cd) Cu(Cd) Co(Cd)

RT-1 Proximal 7-3 ^6 29

RT-1 Distal 10 64

RT-2 Proximal 3-7 15-7 H

RT-2 Distal 4.0 15.8

RT-3 Proximal 10.3 65 42

RT-3 Distal 16.5 80.5

RT-4 Proximal 87 536 395

RT-4 Distal 111 582

^For elemental foils using 20 mil cadmium covers.

for intercomparisons with other laboratories, since these two

elements are the ones generally used for flux and cadmium

ratio measuarements . However, as mentioned previously, the

large resonance neutron cross sections (fig. 2) casts some

doubt on usefulness of measurements using these foils.

The copper foils, which behave as true "infinitely thin"

foils throughout the neutron energy range seen in a nuclear

reactor, should yield results which can be compared directly

to neutron fluxes above and below the cadmium cutoff.

Additional efforts will be expended in the direction of

neutron energy measurements in these facilities by the use

of threshold detectors. These threshold detectors yield

integrated neutron fluxes above their reaction thresholds,

and will be used to plot diagrams of the integral neutron

flux vs. neutron energy similar to the work done by us at

the NRL nuclear reactor [3].

( 5 ) Temperature Measurements

An extensive series of measurements were made in the

pneumatic facilities using stick-on type temperature sensors.

162
These sensors were described previously [1] as were the

various techniques employed and some preliminary results.

Essentially all of the measurements have been completed at

this time, and the results obtained are presented in tables

5, 6, 7 and 8. The equilibrium temperatures were lower than

those expected, and therefore maximum irradiation times are

based on factors other than temperature, e.g, radiation

Table 5. Radiation damage and temperature

measurements at 10 MW in RT-1.

Material and Configuration Wt . (g)

Length of

Irrad

.

Results

Empty rabbit

Quartz ampoule, empty

Lead in quartz ampoule

Iron in quartz ampoule

3.5g

0.6g Pb

3.5g Quartz

0.6g Fe

3.5g Quartz

6 h Radiation

damage not

excessivet)

20 m 170±10°F

20 m 225±5°F

20 m <180°F

^20 minutes required to reach equilibrium temperature

In this and succeeding tables, this phrase means the rabbit

is not so brittle as to break upon deceleration in receiver.

Table 6. Radiation damage and temperature

measurements at 10 MW in RT-2.

Length of

Materials and Configuration Wt . (g) Irrad. Results

Empty rabbit - 20 m 155±5°F

Empty 2 dram poly vial 3.25g poly 10 m 205±5°F

Empty heat sealed sections 0.7g poly -^^q ^ 205±5°F of poly tubing (total)

163
Table Radiation damage and temperature

measurements at 10 MW in RT-3.

Material and ConfIguration Wt . ( g)

Length of

Irrad

.

Results

Empty rabbit

Empty rabbit

Lead pieces (6) in sealed

quartz ampoule

"Quartz block

Quartz block

Lead and quartz - lead

(single piece) in sealed

quartz ampoule

Empty rabbit, containing

two 2/5 dram poly vials,

with polyvial packing

0.6g Pb

3.7g Quartz

6.0g

7.0g

l.Og Pb

3.2g Quartz

20 m

8 h

4 h

15 m

20 m

10 m

8 h

<110°P

Radiation

damage not

excessive.

195±5°F

170±10°F

190±10°F

>220°F

Radiation

damage not

excessive to

rabbit or

polyvials

.

Poly foam

crumbly

.

Table 8. Radiation damage and temperature

measurements at 10 MW in RT-4.

Length of

Material and Configuration Wt . ( g ) Irrad

.

Results

Empty rabbit

Empty rabbit

Lead in sealed quartz

ampoule

0.6 g Pb

3.5 g quartz

16 h

16 m

15 m

Radiation

damage not

excessive

105±5°F

<130°F

damage to the sample carrier. Irradiations of up to 8 hours

are made routinely in the polyethylene rabbits at a thermal

13 -2 -1

neutron flux of 5x10 -^n.cm sec

164
Initially, reactor operations would permit temperatures

of up to 200°F in the rabbit facilities. However, a few

tests were made of the melting point of these rabbits and

the effect of irradiation on the melting point. The results

of these tests are given in table 9- As a result of these

tests, the maximum permissable temperature in the pneumatic

facilities was raised to 240°F.

Table 9. Heat resistance of polyethylene rabbits:

effect of irradiation timea.

Length of Effect of Temperature (°F)

Irradiation 230°P 240°F 260°P 280°P

none none none slight

softening

completely

me Ited

10 sec none none slight

softening

mostly

melted

1 min none none slight

softening

completely

deformed

10 min none none slight

softening

transparent

and starting

to deform

Melting point of conventional polyethylene is '^^250°F; high

l^denslty polyethylene '^^270°F.

Rabbits Irradiated in RT-1.

( 6 ) Pressure Measurements

When the request was made for the irradiation of freeze

dried biological materials in the NBSR, a number of questions

were raised which could not be definitively answered. One

of these questions involved the danger of explosion of a

quartz ampoule due to the pressure generated by gaseous

decomposition products of the material under irradiation.

To effectively answer this question, and to establish criteria

for the safe irradiation of a variety of aqueous solutions,

165
organic materials, and biological materials, a series of pressure measurements was initiated. The pressure testing system

used to test irradiated samples is shown in figure 7. It

consists of an appropriate sized rubber balloon, a section of

polyethylene tubing which will fit inside the balloon but is

still large enough to hold the irradiated quartz ampoule, and

a gas-tight clamp for the open end of the balloon. To make

a pressure measurement, an irradiated sample (inside the

quartz ampoule) is inserted into the tubing and the balloon,

the end of the balloon clamped shut and the ampoule struck

smartly with a hammer. The polyethylene tubing prevents the

shattered quartz from puncturing the balloon, and the pressure

inside the ampoule estimated from the degree of expansion of

the balloon.

Tests with a hypodermic syringe (less needle) snugly

fitted to the balloon opening indicated that 1/2 cm of gas

could be readily detected. The quartz ampoules being tested

had volumes of 1 cm or greater, therefore excess pressures

of 1 atm could most certainly be measured. Since even poor

quality glass blowing produces ampoules which can readily

withstand 2 atm excess pressure, and good quality quartz

glass blowing can produce ampoules which will withstand

internal pressures in excess of 5 atm excess pressure, a

preliminary limit for testing purposes of 1 1/2 atm excess

pressure was established.

RUBBER BALLOON

POLYETHYLENE TUBING^

Figure 7- Device used to measure pressures generated from

irradiated samples.

166
The results of these pressure tests are found in table

10. From these results it can be seen that freeze-dried

biological materials produce only small quantities of gaseous

products upon irradiation in the pneumatic facilities RT-1,

RT-3 and RT-4. For example, 1/2 gram of freeze-dried orchard

leaves, irradiated for 6 hours in RT-3 at a neutron flux of

13 -2 -1 3 5x10 n.cm sec produced only about 1 cm of gaseous decomposition products (which produced approximately 1 atm of

excess pressure in the ampoule size used) . These conditions

of excess pressure are well within the tentative limits set

up for these tests.

Table 10. Pressure measurements on neutron

irradiated samples

.

Sample

Orchard leaves

freeze dried

Alfalfa, freeze

dried

Blood, freeze

dried

Liver, freeze

dried

Water, in heatsealed polyethylene

Orchard leaves,

freeze dried

Orchard Leaves,

freeze dried

Length

Wt (g) of Irrad. Facility Results'

0.25

0.25

0.25

0.25

2.0

0.50

0.50

2 h

2 h

2 h

2 h

5 m

8 h

6 h

RT-1

RT-3

RT-3

RT-3

RT-3

RT-4

RT-3

Small pressure

buildup

No significant

pressure buildup

No significant

pressure buildup

No significant

buildup

No significant

pressure buildup

Small pressure

buildup

Small pressure

buildup

^Limit of detection ^1/2 cm^ ; "small pressure buildup"

estimated to be 1/2 - 1 cm-^

.

167
b . Long Term Irradiation Facility (LTIF)

The flux measurements made on the LTIF Vertical Thimble

(see [4] for details of construction) Included only neutron

flux measurements and cadmium ratio determinations. Since

this facility has its own recirculating D^O cooling system,

the temperature measurements were not immediately essential.

The measurement of pressures due to sample decomposition have

not yet been made on this facility, at least partly due to

the probable applicability of those measurements made with

the pneumatic tubes.

The flux variation measurements on this facility were

made using two different techniques. For the vertical flux

measurements a copper wire inside of sealed polyethylene

tubing was quickly inserted the entire length of the vertical

irradiation facility and irradiated for 30 m at a power level

of 10 KW . At the end of the irradiation time, the copper

wire was quickly removed from the reactor, the polyethylene

tubing removed, and the copper wire cut into small pieces for

gross Y counting of the 12. 9h copper-64 radioactivity. At

the lower end, where the activity level was relatively high,

the wire was cut into 1/2 inch lengths. As the induced radioactivity decreased in the upper portions of the wire (above

the core area), sections were increased to 1 inch, and finally

to 6 inches at a point considerably above the upper core area.

After this vertical flux diagram was obtained, the piece

having the highest specific activity (from the core gap area)

was taken for an absolute flux determination.

Circumferential flux measurements were made during

irradiations for the cadmium ratios. These measurements were

taken using suitably weighted polyethylene rabbits at the end

of a nylon fishing line. The copper wire was wrapped around

the rabbit in which the bare copper foil was irradiated. The

cadmium covered copper foil had its separate irradiation.

This set of two irradiations was necessary for each area

measured. Four areas were measured this way, as shown in

168
figure 8 and table 11.

The results of these measurements are shown in figure

8 and tables 11 and 12. Figure 8 shows the vertical

distribution of the total neutron flux in the LTIF, and

demonstrates the effectiveness of the core gap area as a

resion of maximum thermal neutron flux. The maximum neutron

/]

(.

\

/•

/

/

/

/

/

/

/

NEUTRON FLUX DISTRIBUTION

IN VERTICAL THIMBLE G-2

^Z*^ UPPER CORE

<

^ CORE GAP

\

\ LOWER CORE

100 90 80 70 60 50 40 30 20 10

PERCENT OF MAXIMUM FLUX

70

65

60

55

50

45

40

35

30

25

20

15

10

5

0

HEIGHT

(inches)

Figure 8. Vertical distribution of the total neutron flux

in the NBSR LTIF.

169
Table 11. Cadmium ratios in the LTIP.

Cadmium Ratio

Area Height (in. ) ( Copper)

Below core 0.5 84.1

Lower core 11 32.8

Core gap 19.5 54.9

Above core 56 I830

Table 12. Circumferential flux variation in the

NBSR LTIP irradiation facility.

Location in LTIF

Location in No. of Relative , Area Height (in)^ Rabbit Monitors Neutron Flux

On bottom 0. 5 Bottom 9 l.OOOtO.OlO^

On bottom 1. 8 Middle 9 1.122±0.008

On bottom 3. 0 Top 9 1 . 238±0 .009

Lower core 11. 5 Bottom 9 1.000±0.018

Lower core 12. 8 Middle 9 1.055±0.026

Lower core 14. 0 Top 9 1.088±0.015

Core gap 20 . 0 Bottom 9 1.000±0.021

Core gap 21. 3 Middle 9 0 .997±0 .016

Core gap 22. 5 Top 9 0.980±0.018

Upper core 28. 5 Bottom 9 1.000±0.08l

Upper core 29. 8 Middle 9 0.837±0.170

Upper core 31. 0 Top 9 0.725±0.092

Above core 56. 5 Bottom 9 1.000±0.011

Above core 57. 8 Middle 9 0.885±0.013

Above core 59. 0 Top 9 0.774±0.011

b

^Measured from bottom of polyethylene rabbit (see figure 8).

Normalized to bottom = 1.00.

"^Uncertainties are ts//n at 95^ confidence level.

170
flux for 10 MW operation was found to be approximately

l4 -2 -1

I. 1x10 n.cm sec . The cadmium ratios are given in table

II. The core gap cadmium ratio of 55 is significantly

higher than the core area, which has a cadmium ratio of

approximately 33- All of these cadmium ratios are somewhat

higher than would normally be expected, due to the highly

thermalizing effect of the D^O filled thimble. The copper

cadmium ratio of I83O at a hight of 56 inches in the thimble

12 —2 —1 (with a 10 MW neutron flux of '^^5xlO n.cm sec ) suggests

the potential use of this facility for obtaining a highly

thermalized neutron environment. The results of the circumferential flux distributions in the LTIP reveal a small but

significant variation in the neutron flux from side-.to-side

in the thimble. This variation is strongly dependent upon

the area chosen for the irradiation, but usually is less

than 10^ over the outer diameter of the polyethylene rabbits

used for the measurements.

c , Conclusions

The NBSR has been characterized for many of its irradiation parameters, both for the pneumatic facilities RT-1, RT-2,

RT-3 and RT-4, and for the LTIP (G-2 Vertical Thimble). While

some additional measurements remain to be made, the irradiation facilities available for use are revealed as a set of

extremely useful and convenient facilities under which a

wide variety of irradiation environments can be obtained.

(D. A. Becker and P. D. LaFleur)

171
APPENDIX V

REPRINTED FROM JOURNAL OF RADIOANALYTICAL CHEMISTRY, Vol. 19, 1974, pp. 149-157.

CHARACTERIZATION OF A NUCLEAR REACTOR

FOR NEUTRON ACTIVATION ANALYSIS

D. A. BECKER, P. D. LaFLEUR

Activation Analysis Section, Analytical Chemistry Division,

National Bureau of Standards, Washington, D. C. 20234 (USA)

Ouelques caracteristiques du reacteur nucleaire NBSR utilise pour I'analyse par activation

neutronique ont 6t6 6valuees; tes techniques de mesure d^veloppees sont d6crites. La discussion

porte sur les caracteristiques suivantes: (1) le flux de neutrons thermiques absorbes par le cad- mium, (2) la distribution energetique des neutrons mesuree grace aux rapports cadmium et a des

detecteurs a seuil. L'exces de pression dans les echantillons engendre lirradiation.

Introduction

Optimum utilization of a nuclear reactor for neutron activation analysis requires a careful and complete characterization of all the available irradiation facilities. This information

is important both for reactor and personnel safety, and for complete control of the irradi- ation parameters affecting the precision and accuracy of the technique. When the NBS

Nuclear Reactor (NBSR) started up several years ago, an extensive program of characterization was initiated. During this reactor characterization a number of techniques and procedures were developed which were felt to be of interest to other activation analysis laboratories.

The design and operating characteristics of the NBSR have been reported previously.*'^

These reports also contain some of the preUminary results on the characterization studies.

Due to space limitations, this paper will be limited to new or improved techniques developed or results obtained during these characterization studies. The are presented in the

hope that other activation analysis laboratories wiU find them useful in the evaluation and

characterization of their own nuclear reactors. The techniques discussed below are the thermal (sub-cadmium) neutron flux determination, neutron energy distribution measurements using cadmium ratios and threshold foil- detectors, and the determination of excess sample pressures generated during irradiation.

J. Radioanal. Chem. 19 (1974)

172
D. A. BECKER, P. D. LaFLEUR: CHARACTERIZATION

Thermal (sub-cadmium) neutron fluxes

In all measurements reported here, the thermal (sub-cadmium) neutron flux was determined utihzing the thin foil technique, in which the measurement is made with minimum

disturbance of the surrounding neutron environment.^ For most of these measurements,

reagent grade copper foil or wire was used. The advantages of this material include easy

availabihty at adequate purities, low cost, and lack of high resonance neutron cross section

peaks. Some measurements were also made with 0.0025 cm thick gold foils, but the neutron self-shielding occurring due to the very high resonance neutron cross sections made

these results subject to large corrections. The technique used consisted in irradiating a thin

foil or wire of the element of interest in the appropriate facility, then counting the radioactive species formed on a large volume Ge(Li) detector at a reproducible counting geometry. This detector had been calibrated for absolute photopeak counting efficiency at

this geometry using NBS caUbrated radioactive point sources. Neutron fluxes were then

calculated from the nuclear datas shown in Table 1 . For the flux determinations using

copper foils, care must be taken to insure total annihilation of the positrons in close proximity to the detector foil. In this case, covering the foil with approximately 0.1cm of aluTable 1

Data for nuclear calculations^

Nuclear

reaction

Isotopic

abundance

Oeff

(0th)

Effective threshold energy, MeV

E7

Gamma-ray abundance

HalflifeDetector

efficiency ^ Sample

form

Sample pu- rity,%

"Cu(n,7) 64 Cu .691 (4.5b) thermal 0.511 0.38 12.8 h 0.00161 Cu foil 99.9+

'»'Au(n,7)i98 Au 1.00 {98.8b) thermal 0.412 0.95 2.70 d 0.00200 Au foil 99.99+

5»Ni(n,p) 58 Co 0.678 420 mb^ 2.9" 0.810 0.99 71.3 d 0.00096 Ni wire 99.98

^TeCn.p) 54 Mn 0.0S84 610 mb'' 4.2*' 0.835 1.00 303 d 0.00093 Fe wire e

•'Ti (n,p) 46 Sc 0.0799 2 30 mb*^ 0.889 1.00 83.9 d 0.00087 Ti wire

e

1.120 1.00 0.00068

^'^Mg(n,p) 24 Na 0.786 60 mb'' 6.3" 1.369 1.00 15.0 h 0.00054 Mg foil e

1.120 1.00 0.000235

s6Fe(n,p) 56 Mn 0.917 110 mb*' 7.5" 0.847 0.99 2.58 h 0.000915 Fe wire e

"Al(n,Q:) 24 Na 1.00 1 30 mb'' 8.4"^ 1.369 1.00 15.0 h 0.00054 Al wire 99.9+ 2.754 1.00 0.000235

^Nuclear data taken from Ref.'' unless otherwise noted,

"oata taken from Ref.'

*^Data taken from Ref.' ^^Experimentally determined using NBS calibrated radioactive sources; 47 cm^ Ge(Li) detector at a distance of 5.0 cm.

^See text.

/ Radioanal. Chem. 19 (1974)

173
D. A. BECKER, P. D. LaFLEUR: CHARACTERIZATION

minum was sufficient. Also it must be noted that the positron decay fraction of 19% for

copper-64 leads to a 0.5 1 1 MeV photon abundance of 38% for the nuclear calculation. In

all cases, the thermal (sub-cadmium) neutron flux was obtained after correction both for

epicadmium contribution to the foil activity and for n';iitron self-shielding. For copper,

the epicadmium correction did hot exceed 7% in the worst case (RT-2), however, for gold

this correction was much greater.

The results of these flux measurements for the pneumatic tubes are foimd in Table 2. This table includes values obtained using both the copper foils and the gold foils. It is apparent that the thermal neutron flux in tubes RT-1 , RT-2 and RT-3 is very similar, with a

significantly lower flux in RT-4. As expected, the thermal column tube RT - T had a neutron flux two orders of magnitude lower. The gold foils gave higher flux numbers (relative to

those from Cu measurements) for the tubes RT-1 , RT-2 and RT-3, where a larger epicadmium

neutron flux exists, but a value very close to that for copper in RT-4, where the neutrons

are highly thermalized. Several redeterminations of these flux measurements gave results

agreeing well with the original values. A later, independent flux determination using a multiple foil activation iterative method to evaluate the reactor neutron flux spectra,^ produced

a thermal neutron flux value for RT-4of 1.33-10*^ n*cm'^ 'sec'' ,^ in good agreement with

the 1 .3* 10'^ n'cm'^ - sec'* reported here using the copper foil and simple calculation from

nuclear constants.

Table 2

Experimental neutron flux-NBSR pneumatic facilities

Detector

foil

Maximum sub-cadmium neutron flux n-cm"' -sec''

RT-1 RT-2 RT-3 RT-4 RT-T

Copper

Gold

5.9-10'*

7.0-10'*

5.6-10'*

6.9-10'*

4.9-10'*

5.8-10'*

1.3- 10'*

1.4- 10'*

1.6-10"

Neutron energy distribution measurements

Preliminary neutron energy distribution measurements consisted almost entirely of cadmium ratio measurements.^ These cadmium ratios were made in all five pneumatic facilities. A total of three different elemental foils were used. They were 0.0127 cm thick copper

foil, 0.0025 cm thick gold foil, and 0.005 1 cm thick cobalt foil. Interlocking 0.05 1 cm cadmium covers were used for the cadmium shielded measurements. The results of these

/. Radioanal. Chem. 19 (1974)

174
D. A. BECKER, P. D. LaFLEUR: CHARACTERIZATION

Table 3

Cadmium ratios for the NBgR pneumatic tube facilities

Pneumatic

facility

Location

in rabbit

Experimental cadmium ratios^

Cu(Cd) Au(Cd) Co(Cd)

Kl -1 Proximal 46 7.3 29

RT-] Distal 64 10

RT rrOAJJlIal 15.7 3.7 11 RT-2 Distal 15.8 4.0

RT-3 Proximal 65 10.3 42

RT-3 Distal 80 16.5

pT-4 Proximal 540 87 390

RT-4 Distal 580 111

RT-T Bottom 3400

^For elemental foils using 20 mil cadmium covers, where

subcadmium + epicadmium neutrons Bare foil activity at T zero Cd Ratio = =

epicadmium neutrons Cd covered foil activity at T zero

measurements are found in Table 3. The values for gold and cobalt were obtained primarily

for intercomparisons with other laboratories, since these two elements are the ones generally used for flux and cadmium ratio measurements. However, as mentioned previously,

the large resonance neutron cross sections casts some doubt on the usefulness of measure- ments using these foUs. The copper foUs, which behave more closely to true ,4nfmit9ly

thin" foils throughout the neutron energy range seen in a nuclear reactor, yield results

which should be more representative of the neutron fluxes above and below the cadmium

cut-off. The copper cadmium ratios for the pneumatic tube facilities (Table 3) range from

less than 16 for RT - 2 to over 3000 for RT - T, the thermal column rabbit. Variations in

the cadmium ratio from end-to-end of the irradiation container (rabbit) were especially

large for both RT - 1 and RT-3.

More recently, a program has been initiated to extensively characterize the fast neutron

spectra present in all the NBSR irradiation facilities. Besides the useful information this

would inevitably provide for NBSR users, it was felt that an evaluation of suitable threshold

foils in the well characterized NBSR facilities would assist other activation analysis laboratories in their own reactor evaluation programs.

/. Radioaml Chem. 19 (19 74)

175
D. A. BECKER, P. D. LaFLEUR: CHARACTERIZATION

At present six different nuclear reactions have been used to determine the integrated

fast neutron flux, with threshold energies of 2.9 MeV to 8.4 MeV. These thieshold foils and wires were irradiated in the pneumatic tubes RT - 1 through RT- 4, for periods of 15

seconds to 30 minutes. In all cases interlocking 0.102 cm thick cadmium covers were

used to reduce the thermal neutron activation. In order to estabUsh that no significant attenuation of fast neutrons occurred through

the use of the cadmium covers, several experiments were made using elemental nickel.

The nuclear reactions used were ^^Ni(n, 7)^^Ni and ^^Ni(n,p)^^Co, for thermal and fast flux measurements respectively. The results are shown in Table 4, and indicate that above

the 2.9 MeV threshold for the fast neutron reaction, no significant neutron attenuation

occurred due to the cadmium covers. This result is not comphcated by cobalt-58 bumup problems, since the conditions employed here result in less than 1% bum-up for a bare nickel foO.^

The nuclear reactions and sample materials used for the neutron energy distributions

as well as the nuclear data used for the calculations are found in Table 1 . After each irradiation, the radioisotope of interest was counted using the same caUbrated 47 cm^

Ge(Li) semiconductor detector used for the thermal neutron flux determinations, and

the fast flux calculated from the nuclear constants and threshold values given. Copper

foil flux monitors were included with each irradiation at the opposite end of the rabbit

from the cadmium to normalize the values obtained for each irradiation position. It was

determined experimentally that the thermal neutron flux depression 8 cm distant from

6.5 grams of cadmium was only 2.0±0.5%.

The purties of the foils and wires used are also given in Table 1 if known. Whenever

the product nuclide could be formed by an (n, 7) nuclear reaction, the radioactivity

produced both with and without a cadmium cover was observed. In all cases, with a simple post-irradiation wash of the sample material, no interference was noted from the

(n,y) product. Initial work with the iron reaction forming manganese-56 did indeed reTable 4

Variation of neutron flux with cadmium thickness using elemental nickel

Wire No,

Cadmium

thickness

Relative

fast flux^

Relative

thermal flux

1 bare 1.00 1.000

2 .020 in 0.981 .0033

2 .040 in 0.987 .0011

^Absolute value 1.00 = 5.21-10' n-cm'^ -sec'' ; integral neutron flux above 2.9 MeV for RT-4.

/. Radioanal. Chem. 19 (1974)

176
D. A. BECKER, P. D. LaFLEUR: CHARACTERIZATION

suit in a significant interference from the (n,7) product, even with a cadmium cover. However,

a sample of extremely pure iron wire was obtained which did not produce an interference,

and this material was used for the threshold reactions. Analysis of the iron wire later revealed

a concentration of only about 2 ppb manganese.

The resulting integral neutron flux spectra for the four pneumatic tubes RT - 1 through

RT - 4 are shovra in Fig. 1 , along with a spectrum of a pure fission neutron flux.^** The data

points obtained fall very close to a straight line, with a slope similar to that of the fission

spectrum. The fast flux levels observed for each of the pneumatic facilities are close to

those expected, showing increasing thermaUzation of the neutron flux with distance from

the core area. For comparison purposes, the only other fast neutron data available for the NBSR is

that of McELROY et al., for RT - 4 from thermal up to 6 MeV.^ His data show a very similar spectrum shape,, with an absolute magnitude somewhat lower. At present, only data

points for neutron energies greater than 2.9 MeV have been determined. Additional work

is in progress to lower this to 1.0 MeV, using the **^In(n/i')"^'"In nuclear reaction,

and even lower with other nuclear reactions. With the use of additional suitable threshold

Neutron energy , MeV

Fig. 1 . Integral fast neutron flux for the NBSR pneumatic tube facilities

/. Radioaml. Chem. 19 (1974)

177
D. A. BECKER, P. D. LaFLEUR: CHARACTERIZATION

reactions, it should soon become quite simple to make 3 to 8 irradiations with readily obtainable materials, and obtain a reasonably good approximation of the fast neutron

spectrum in any reactor facility.

With the increased emphasis on the analysis of trace elements in organic materials,

aqueous solutions, and freeze dried biological materials in the NBSR, a number of questions were raised which could not be definitively answered. One of these questions in- volved the danger of explosion of a quartz ampoule due to the pressure generated by

gaseous decomposition products of the material under irradiation. To effectively answer

this question, and to establish criteria for the safe irradiation of such materials, a series

of pressure measurements was initiated. The pressure testing system used to test irradiated samples shown ini Fig. 2. It consists of an appropriate sized rubber balloon, a section

of polyethylene or other tubing which wiU fit inside the balloon but is stUl large enough to

hold the irradiated quartz ampoule, and a gas-tight clamp for the open end of the balloon.

To make a pressure measurement, an irradiated sample (inside the quartz ampoule) is inserted into the tubing and the balloon, the end of the balloon clamped shut, and the

Pressure measurements

Clamp

Rubber

balloon

Polyethylene

tubing

Sample

Quartz

ampoiie

Pig. 2. Iiiadiated sample pressure test apparatus using rubber balloon

/. RadioanaL Chem. 19 (1974)

178
D. A. BECKER, P. D. LaFLEUR: CHARACTERIZATION

ampoule struck smartly with a hammer. The polyethylene tubing prevents the shattering

quartz from puncturing the balloon, and the pressure inside the ampoule estimated from

the degree of expansion of the balloon.

Tests with a hypodermic cyringe (less needle) snugly fitted to the baUoon opening

indicated that 0.5 cm^ of gas could be readily detected. The quartz ampoules being

tested had volumes of 1 cm^ or greater, therefore excess pressures of 1 atm could most

certainly be measured. Since even poor quaUty glass blowing produces ampoules which

can readily withstand 2 atm excess pressure, and good quaUty quartz glass blowing can produce ampoules which will withstand internal pressures of more than 5 atm excess

pressure, a preliminary limit for testing purposes of 1 .5 atm excess pressure was established.

From the results of these pressure tests it can be seen that freeze-dried biological

materials produce only small quantities of gaseous products upon irradiation in the pneumatic fadhties RT - 1, RT- 3 and RT- 4. For example, 1/2 gram of freeze-dried orchard

leaves, irradiated for 6 hours in RT - 3 at a neutron flux of 5*10*' n-cm'^ 'sec'* , produced only about 1 cm^ of gaseous decomposition products (which produced approximately 1 atm of excess pressure in the ampoule size used). These conditions of excess

pressure are well within the tentative limits set up for these tests, and in many cases permit irradiations of these materials in heat sealed polyethylene instead of quartz.^

A modification of the balloon technique was recently developed, which permits a more quantitative measurement of the excess pressure in irradiated samples. This modification uses heavy walled rubber tubing as the fracture chamber, with the volume

change determined using a caUbrated water manometer. Using this modified system, a

large number of various materials are in the process of being checked for pressure generation during irradiation. These results, when completed, wiU be reported in the literature.

Condusion

In conclusion, the NBSR has been characterized for many of its irradiation parameters, some of which are reported here. While additional measurements remain to be made,

especially in the area of threshold foil measurements of neutron flux energies, the irradiation faculties available for use are revealed as a set of extremely useful and convenient

facilities imder which a wide variety of irradiation environments can be obtained. In

addition, it is hoped that the procedures and techniques developed will be found to be

useful in the characterization of other irradiation fadhties.

The authors wish to acknowledge very substantial assistance of the NBS Reactor Operations staff in the performance of this work. They also wish to acknowledge the very helpful and able assistance of a summer intern, Mr. R. SPERBURG, for much of the threshold foil work reported here.

/ Radioaml. Chem. 19(1974)

179
D. A. BECKER, P. D. LaFLEUR: CHARACTERIZATION

References

1. P. D. LaFLEUR, Editor, NBS Tech. Note 508, 1969.

2. P. D. LaFLEUR, D. A. BECKER, Editors, NBS Tech. Note 548, 1970.

3. ASTM PubUcation E—262—65T, ..Measuring Thermal Neutron Flux by Radioactivation Techniques," published by the American Society for Testing and Materials (ASTM), Philadelphia, Pa.,

1965.

4. C. M. LEDERER, J. M. HOLLANDER, I. PERLMAN, Table of Isotopes, 6th ed. Wiley, 1967.

5. J. MOTEFF, Nucleonics, 20 (December 1962) 56.

6. Data taken from literature of Reactor Experiments, Inc., San Carlos, California. Not further

identified as to source.

7. W. N. McELROY, J. A. ULSETH, S., BERG, G. GIAS, T. B. CROCKETT, „Proceedings, Neutron Cross Sections and Technology," edited by D. T. Goldman, NBS Special PubUcation 299,

Volume 1, 1968 p. 235.

8. W. N. McELROY, . L. S. KELLOGG, W. Y. MATSUMOTO, J. L. JACKSON, Hanford Eng.

Develop. Lab. Progress Report, p. HEDL-1, June-July, 1971.

9. ASTM PubUcation E-264-65T, „Measuring Fast-Neutron Flux by Radioactivation of Nickel,"

pubUshed by the American Society for Testing and Materials (ASTM), Philadelphia, Pa., 1965.

10. B. E. WATT, Phys. Rev., 87 (1952) 1037.

180

/. Radioanal. Chem. 19 (1974)
APPENDIX VI

Method for Determination of Mercury in

Biological Materials by Neutron Activation Analysis

Harry L. Rook, Thomas E. Gills, and Philip D. LaFleur

Activation Analysis Section, Analytical Chemistry Dioision, National Bureau of Standards, Washington, D.C. 20234

A method is described for the determination of mercury in a variety of matrices. Three significant improvements over currently used procedures have

been achieved: The procedure has a minimum of chemical manipulations, thereby minimizing technique related errors; the radiomercury separation is quantitative, thus eliminating unnecessary errors

involved in the determination of chemical yields; the

procedure is based on a simple combustion separation

of radiomercury allowing the same technique to be

used on a wide variety of matrices. The method was

tested using radiotracer experiments and by analyzing

samples of flour and coal which had been independently analyzed. The procedure has been subsequently used to determine the mercury concentration

of two new Standard Reference Materials being offered

by the National Bureau of Standards.

In recent years, mercury contamination of the environment

has been recognized as a significant health problem. Because

of the severe toxicity of some organic mercury compounds,

notably methyl mercury, levels of less than 1 part per million (ppm) must be routinely determined to ensure the safety of human and animal life. Because of high sensitivity, neutron activation is an ideal technique for this analytical

problem. Nondestructive neutron activation analysis has been reported for the determination of mercury above the 0.1 ppm level {1-4). However, both useful isotopes of mercury

have low energy gamma emissions, usually necessitating a

chemical separation of mercury from the activated matrix. Most separation methods reported in the literature involve

acid dissolution followed by a chemical separation specific for mercury, coupled with Nal(Tl) counting {5-22). These

(1) L. A. Rancitelli, J. A. Cooper, and R. W. Perkins, "Proceedings

of the 1968 International Conference, Modern Trends in Activation Analysis," Nat. Bur. Stand. Spec. Publ., 312, 101 (1969).

(2) W. A. Haller, R. Filby, L. A. Rancitelli, and J. A. Cooper,

ibid., p 177.

(3) R. H. Filby, A. L. Davis, K. R. Shah, and W. H. Haller, Mikrochim. Ada, 1970, 1 1 30.

' (4) V. P. Guinn and R. Kishore, Paper presented at ANS Topical

Meeting, Nuclear Methods in Environmental Research, Columbia, Mo., 1971.

(5) K. Ljunggren and T. Westermark, Pure Appl. Chem., 1, 127

(1960).

(6) T. Westermark and B. Sjostrand, Int. J. Appl. Radial. Isotopes,

9, 1 (1960).

(7) C. Kellershohn, D. Comar, and C. LePoec, J. Lab. Clin. Med.,

66, 168 (July 1965).

(8) H. Hamaguchi, R. Kuroda, and K. Hosohara, J. At. Energy

Soc.ya/7., 2, 317(1960).

(9) C. K. Kim and J. Silverman, Anal. Chem., 37, 1616 (1965).

(10) D. Comar, C. LePoec, M. Joly, and C. Kellershohn, Bull. Sac. Chim. Fr., 1, 56 (1962).

(11) H. Smith, Anal. Chem., 35, 635 (1963).

(12) B. Sjostrand, ii:d.. 36, 814 (1964).

(13) K. Samsahl, ibid., 39, 1480 (1967).

(14) H. A. Das, J. G. VanRaaphorst, D. Hoede, and J. Zunderhuis,

Int. J. Appl. Radial. Isotopes, 17, 252 (1966).

(15) H. D. Livingston, H. Smith, and N. Stojanovic, Talania, 14, 505

(1967).

(16) T. H. Handley, Anal. Chem., 36, 153 (1964).

(17) R. Kuroda, T. Kiriyama, and K. Ishida, Anal. Chim. Ada,

40, 305 (1968).

methods were usually developed for a specific matrix type

and are not easily adapted to changed composition. This

type of activation analysis is often time-consuming, requires

yield corrections, and gives results which are often techniquedependent. Recent separation methods reported use combustion tech- niques with mercury absorbed either on selenium paper {23) or in various solutions {24). In each case, however, the separated mercury was again required to be radiochemicaUy

pure. In this work, a rapid, quantitative separation of volatile elements was carried out followed by counting using a high

efficiency, high resolution Ge(Li) detector and gamma ray

spectrometer.

Using this method, samples of flour and coal containing

from 0.015 to 5 ppm were analyzed to test the accuracy of the procedure. Samples of Standard Reference Material 1571 and 1577, Orchard Leaves and Beef Liver, were also analyzed

for mercury, and concentrations of 0.155 ppm and 0.0145 ppm, respectively, were found. It is of interest to note that

excellent agreement was obtained later by flameless atomic

absorption spectrometry and isotope dilution spark source mass spectrometry.

EXPERIMENTAL

Apparatus and Reagents. The sample combustion train (Figure 1) was made from an outer 19-38 standard taper

Vycor joint which had been extended to a total length of 30 cm. The mercury condenser was made from an inner 19/38 standard taper Vycor joint, extended to 25 cm, which was in- serted through a liquid nitrogen cold trap made from a small polyurethane acid shipping container. Standard solutions were made by dissolving a weighed

quantity of triply distilled metallic mercury in 1 : 1 high purity

nitric acid and diluting to volume. The concentration used was 1.0065 mg/ml which was dilute enough to prevent any

significant neutron self absorption, but concentrated enough

to negate loss due to wall absorption of mercury in the storage

container. Procedure. For analysis, 0.5-gram samples were en- capsulated in cleaned, heat treated quartz tubes. Standard

solutions were encapsulated in a like manner. Five sample

tubes were arranged in a concentric ring with a standard in the

center in a polyethylene rabbit and irradiated in the pneumatic

transfer facility RT-3 of the NBS Reactor for 2 to 4 hours at a

thermal neutron flux of approximately 6 X 10"n cm~^ sec~'. Flux monitors were not used as the radial flux variation is less

(18) F. Kukula and M. Kirvanek, Isotopenpraxis, 4, 57 (1968).

(19) G. V. S. Rayuda, B. Tiefenbach and R. E. Jervis, Trans. Amer.

Nucl. Sac, 11, 54(1968).

(20) O. Johansen and E. Steinnes, Int. J. Appl. Radiat. Isotopes, 20, 715 (1969). (21) B. R. Petersen, Dan. Kemi., 49 (11), 171 (1968).

(22) E. Steinnes and O. Johansen, Nord. Hyg. Tidskr., 50(2), 71 (1969).

(23) L. Kosta and A. R. Byrne, Talanta, 16, 1297 (1969). (24) R. R. Ruch and H. J. Gluskoter, Environ. Geol. Notes, 43, (Feb. 1971).

Reprinted from ANALYTICAL CHEMISTRY, Vol. 44, Page 1114, June 1972 American Chemical Society |gj
than ±2% in RT-3. The samples were allowed to decay 3 days to minimize personnel radiation exposures. The quartz tubes were washed in 1 : 1 nitric acid, cooled to

liquid nitrogen temperature, and broken 5 mm from the top. The samples were weighed into a ceramic combustion boat,

and 25 mg of mercuric oxide carrier added. The boat was

inserted into the combustion chamber, an oxygen flow of ~30

cc/min passed over the sample and the sample ignited with an

oxygen-gas torch. The sample was allowed to burn freely with the downstream quartz wool plug heated to red heat to ensure complete combustion of volatile hydrocarbons and

carbon produced during the sample burn. After combustion

ceased, the ash was heated to approximately 800 °C for 5 minutes and the entire combustion tube flamed down to the standard taper joint to drive all volatile components of the sample into the condenser. The combustion tube was allowed to cool for 5 minutes, the

liquid nitrogen cold trap removed, and the condenser placed

joint end down into a 4-oz polyethylene bottle. The con- denser was warmed to room temperature and the mercury

washed into the bottle with 10 ml of concentrated nitric acid and

three 10-ml portions of distilled water. The total volume of

solution was adjusted to 50 ml and counted on a 60-cc Ge(Li)

detector. The 77-keV gamma ray of mercury- 197 was used

for the determination. Primary standards were made by sealing 1-ml aliquots of

the previously mentioned standard solution in quartz vials and

irradiating them in the same configuration as that of the sample. After irradiation, the standard was frozen in liquid nitro- gen, broken in a beaker, allowed to melt, and decanted into a

1-liter volumetric flask. The broken quartz vial and beaker were washed with six 5-ml portions of distilled water and the

final volume adjusted to 1 liter. One milliliter of the dilute solution was pipetted into a 4-oz polyethylene bottle and di- luted to 50 ml. The specific activity at the end of irradiation was compared to that of the sample for quantitative results. A method of additions analysis also was carried out to deter- mine if mercury was lost during analysis. This procedure is described in a following section. Bromine Separation. The only isotope to interfere after the combustion separation is bromine-82 and then only when

its concentration is at least a factor of 100 times that of mercury. To eliminate this interference, either of two courses of

action may be employed. Bromine-82 has a half-life of 35.3 hours while mercury-197 has a half-life of 65 hours. Thus,

allowing the sample to decay 10 to 15 days optimizes the reduction of bromine activity while maintaining usually adequate mercury sensitivity. If decay does not give the necessary im- provement of the mercury signal relative to bromine, a silver bromide separation can be carried out with quantitative re- covery of mercury-197 due to the relatively high solubility of mercury bromide in warm water. The procedure is as fol- lows: The mercury-197 plus carrier was washed from the con- denser into a 40-ml centrifuge tube with 10 ml of concentrated HNO3 followed by four 5-ml portions of distilled water. The

solution was warmed to 40 °C in a hot sand bath, 2 mg of

bromide carrier were added, and bromide was precipitated with AgNOj. The solution and precipitate are stirred at 40 °C to ensure complete dissolution of HgBr2 and precipita- tion of AgBr. The solution was centrifuged, decanted info a 4-oz polyethylene bottle, and the walls of the tube were washed

down with 3 ml of 2A' HNO3, The precipitate was washed

twice with 2-ml portions of warm 2A' HNO3 followed by

centrifugation . The wash solutions were added to the original

solution. The final solution was adjusted to 50-ml total volume and counted as before. A reduction to less than 5%

of the initial bromine activity was obtained with quantitative retention of mercury-197 in the solution. Method Evaluation. TTie method was first tested for complete recovery of mercury by burning non-irradiated samples

of orchard leaves with mercury-203 tracer added. The re-

-LN,

O2 INLET SAMPLE

QUARTZ

WOOL \

1 JOINT

Figure 1. Combustion apparatus

Table I. Determination of Mercury in Characterized Materials

Sample

IAEA flour

Nondestructive

Destructive Average

FDD flour

Destructive

Hg found, ppm Comparative value

4.82 4.90

4.88

4.87 ± 0.04

0.0105

0.0107

0.011

4.9 ±0.3(25)

0.013 ± 0.003 (25)« Average 0.011 ± 0.003 (27f

" Results obtained by flameless atomic absorption

' Results obtained by neutron activation.

covered mercury-203 was brought to 50-ml volume and compared to the activity of a like quantity of tracer diluted di- rectly to 50-ml volume and counted in a standard configuration. The results of these tests indicated complete recovery of

the mercury-203 tracer. A total of eight separations was

carried out with a mean recovery of 99.52% and a relative standard deviation of ± 1 .68 %.

The procedure described above did not, however, test the recovery of mercury incorporated into an organism. To

ascertain that the mercury could, in fact, be recovered in this case, mercury-203 nitrate was added to the water in a tank

containing goldfish. The fish survived for varying periods

from one day to three weeks. The dead fish were removed

from the tank, frozen, and lyophilized. The fish were then counted whole, followed by analysis for ^'Hg using the combustion technique. After burning, the solution from the cold

trap was placed in a vial that approximated the length and

diameter of the fish, and the solution was counted. In all cases, counting was done using a 7.6 cm X 7.6 cm Nal(Tl)

detector about 25 cm from the detector surface. As a further check, one fish was counted, dissolved in nitric acid, and

diluted to the standard 50-ml geometry. The combustion

effluent from a second fish was also diluted to this volume.

The comparative results still showed quantitative recovery. The experimental results showed a mean recovery of 98.9%

with a relative standard deviation of ±1.0%. Although one

of the fish had lived for three weeks, speciation of the mercury

present in the fish was not determined. It is not anticipated,

however, under the conditions of the analysis, that organo- mercury compounds would behave diff'erently than inorganib mercury.

(25) R. A. Nadkarni and W. D. Ehmann, Radiochem. Radioanal. Uil.,6, 89(1970).

(26) E. Somers, "Proceedings of the Symposium, Mercury in Man's Environment, Ottawa, Canada," 99 (1971).

(27) J. T. Tanner, M. H. Friedman, D. N. Lincoln, M. Jaffee, and L. Ford, FDA Preprint (1971).

182
Table II. Mercury Content of SRM-1571 Orchard Leaves

Sample Hg, ppm

OL-01 0.1S8 01^2 0.149 OL-11 0.156 OL-12 0.154 01^13 0.152 OH4 0.154 OL-15 0.155 OL-17 0.157 OL-10 0.155 01^20 0.155 OL-21 0.159 Mean = 0.155. 2s = 0.0056.

tsjVJi = 0.0019.

RESULTS AND DISCUSSION

The work undertaken in this laboratory was to develop a

single analytical method, preferably with quantitative re- covery of mercury, which could be applied without change

to a large number of sample types. Since all mercury compounds are volatile above 400 °C, sample combustion or heat- ing in an oxygen stream seemed the best approach. The

collection of mercury was carried out with a simple liquid

nitrogen condenser which quantitatively trapped mercury and was easily washed clean after combustion. With this system, complete (.oUection of mercury was accomplished on

such diverse matrices as orchard leaves, wheat flour, coal, fuel oil, freeze dried fish, and river sediment. After the previously mentioned preliminary tests, the method was checked by analyzing samples of flour and coal having mercury concentrations ranging from 0.01 to 5 ppm.

Where mercury levels were high enough (>1 ppm), samples were analyzed nondestructively by irradiation followed by

direct counting of the mercury-197. These samples were

then analyzed by the described combustion technique and the

results compared. The analyses were carried out on the following samples: a flour sample prepared by the Inter- national Atomic Energy Agency which had been doped with approximately 5 ppm of mercury; a sample of flour contain- ing approximately 0.01 ppm of mercury analyzed by the U.S. Food and Drug Administration by both atomic absorption

and activation analysis and by the Food and Drug Directorate

Table IV. Mercury in Beef Liver Hg, ppm

0.0140

0.0141 0.0118 0.0140

0.0173

0.0137 0.0152

0.0138

0.0169 Mean = 0.0145. 2 s = 0.0034. tslVn = 0.0013.

of Canada using atomic absorption. The results of these analyses are given in Table I. With the procedure giving satisfactory results, an intensive

analytical program was carried out on a new Standard Reference Material—SRM 1571, Orchard Leaves. The intent was to determine the mercury content using two or more

non-related analytical techniques with an accuracy in the

final results of better than ±10% at a 95% confidence level. A total of 1 1 samples from three diff'erent bottles was analyzed

by the procedure described. The samples were found to contain 0.155 ± 0.006 ppm of mercury (Table II). The absolute value of the mercury concentration was

checked by carrying out a method of additions analysis on

the orchard leaves. Four 40-gram samples of the base leaves were spiked with standard mercury solutions to give samples

containing approximately 0.0, 0.5, 0.5, and 1.0 ^g added

mercury per gram of leaf, respectively. The samples were

freeze dried, tumbled for four hours, and mixed in a ball mill for a minimum of 6 hours. Two extra samples were pre- pared in the same manner and spiked with mercury-203 tracer

to ascertain that no losses occurred during freeze drying and

that the mixing was uniform. The tracer experiments in- dicated complete mixing with an inhomogeneity of less than ±1%.

Ahquots of each group were analyzed by the described procedure and the base concentration of mercury in the orchard leaves was determined by fitting a least squares

regression to the individual results. The least squares fit gave a base concentration of 0.161 ± 0.010 ppm of mercury. The individual results are given in Table III.

Table III. Method of Addition of Hg in Orchard Leaves

Sample Hg added, ppm Hg found, ppm least squares.

5-1 0 0.151 0.161 5-2 0 0.156 0.161

5-3 0 0.169 0.161 5-4 0 0.154 0.161 5-5 0 0.154 0.161

1-1 0.495 0.699 0.667

1-2 0.495 0.665 0.667

4-1 0.485 0.621 0.657 4-2 0.485 0.702 0.657

2-1 1.012 1.168 1.196 2-2 1.012 1.207 1.196 Y A + Bx. A = 0.1606 ± 0.910. B = 1.024 ± 0.0201. Base mercury content = 0 .161 ± 0 .010

Hg calculated by [PPM HgJ_|" PPM Hg ~\ L Found J LCalculatedJ -0.0096

-0.0046

-f0.0084 -0.0066

-0.0066

-1-0.0316 -0.0023

-0.0361 +0.0439

-0.285

-1-0.0104

183
The mercury concentration in the SRM Orchard Leaves

also was determined using flameless atomic absorption spec- trometry and isotope dilution spark source mass spectrometry. The results obtained by these techniques were 0.160 ± 0.012 and 0.145 ± 0.022 ppm of mercury, respectively.

A second new Standard Reference Material, Beef Liver, with a considerably lower mercury content was also analyzed

by this technique. Nine samples were analyzed using the bromine separation. The samples contained 0.0145 ±

0.0034 ppm of mercury. The individual results are given in Table IV. The mercury concentration in the SRM Beef

Liver was determined using flameless atomic absorption

spectrometry. The results obtained by this technique were

0.0160 ± 0.0012 ppm of mercury.

ACKNOWLEDGMENT

The cooperation of T. C. Rains, P. J. Paulsen, and Robert

Alvarez is gratefully acknowledged. Their investigations

and analyses gave special credence to this work. The Authors

would also like to thank H. P. Yule for his help in the data

reduction.

Received for review October 26, 1971. Accepted February

7, 1972. In order to specify procedures adequately, it has

been necessary to identify commercial materials and equipment

in this report. In no case does such identification imply

recommendation or endorsement by the National Bureau of

Standards, nor does it imply that the material or equipment

identified is necessarily the best available for the purpose.

184
PLATINUM WIRE ANODE^

APPENDIX VII

Simultaneous Determination of Trace Elements in Platinum by

Isotope Dilution and Spark Source Mass Spectrometry

Robert Alvarez, Paul J. Paulsen, and Daniel E. Kellelier Spedrochemical Analysis Section, National Bureau of Standards, Washington, D. C. 20234

An isotope dilution technique using the spark source mass spectrograph was investigated for applicability

to simultaneous multi-element trace determinations.

Isotopically altered Ag, Cu, Ni, Pb, and Pd were separated from a 250-mg sample of platinum by cation

exchange, and eiectrodeposited onto gold electrodes

for sparking in the spectrograph. Electrodeposition

provided the spiked analytes in an adherent form,

relatively free of anions and organic residue from the

ion exchange resins, both of which could complicate

the spectrum. Contamination, the principal limitation

of the method, was minimized by isothermally distilling

the reagent acids and by isolating the operations from

the laboratory environment. After determining the

isotope ratios, the concentrations were computed,

ranging from 0.5 ppm (^g/gram) for Ni to 14 ppm for Pb. Improved precision and accuracy was obtained

compared to conventional spark source analysis. The

results were compared with those of other methods

and the platinum material was issued by NBS as Standard Reference Material 681.

Stable isotope dilution analyses are usually performed

using mass spectrometers with thermal ionization sources. These methods depend on the chemical form of the element

for optimum results and are generally not suitable for simul- taneous multi-element trace determinations. This is particularly true if elements having high ionization potentials are to be included in the analysis. Webster (1) presents an excellent description of the general method including applications.

The spark source mass spectrograph has been employed by

Leipziger (2) to determine copper in Nickel Oxide NBS

Standard Reference Material (SRM) 673 and antimony in Tin SRM 835 using isotope dilution techniques. The average

values obtained, 21 and 121 ppm, respectively, agreed with the NBS certificate values. Because the high voltage spark source ionizes all elements

with approximately the same efficiency, isotope dilution spark

source mass spectrometry (ID-SSMS) should enable the simultaneous determination of many trace elements in a sample. The object of this investigation was to apply this method

to determine five trace elements in a sample of platinum.

Isotopically altered Ag, Cu, Ni, Pb, and Pd were separated

from the platinum matrix by cation exchange and eluted with

HCl into an electrolysis cell. After neutralizing the acid, the spiked analytes were eiectrodeposited onto gold wires and

sparked in the spectrograph. The mass spectra were photographically recorded, isotopic ratios determined, and the

results calculated. After comparing these results with those of other analytical methods, this platinum material, previously

examined for homogeneity, was subsequently issued by the National Bureau of Standards as SRM 681. The authors have also developed an ID-SSMS method for determining trace elements in each of two NBS zinc standard

reference materials (i). Compared to this zinc method, the

(1) R. K. Webster in "Advances in Mass Spectrometry," J. D. Waldron, Ed., Pergamon Press, New York, N. Y., 1959, pp 103-119.

(2) F. D. Leipziger, Anal. Chem., 37, 171 (1965).

(3) P. J. Paulsen, R. Alvarez, and D. E. Kelleher, Spectrochim.

Acta, in press.

ANODE COMPARTMENT

POROUS VYCOR ROD

GOLD AIRE CATHODES

Figure 1. Cell for acid neutralization and electrodeposition

platinum procedure required a greater number of analytical

operations using special techniques and apparatus to reduce

contamination to an acceptable level.

EXPERIMENTAL

Reagents. Deionized water is made immediately before use by passing distilled water from the laboratory distribution system through a mixed bed resin column. HCl and HNO,-)

are prepared by isothermal distillation. Aqueous ammonia

is prepared by bubbling NH3 into deionized H2O, after passing the gas into a gas washing bottle. Evaporator for Sample Solutions. A borosilicate glass

chamber, similar to the one described by Thiers (4), provides a clean environment for evaporating solutions. The ap- paratus is fabricated from a crystallizing dish 150 mm in diameter by 75 mm high, and a culture dish of the same

diameter. A lateral displacement of the coincident edges of

the two units is prevented by three lengths of glass rod that are fused to the outside of the culture dish (lower unit) and project

upward. Filtered nitrogen is introduced through a tube

sealed at the center of the crystallizing dish, and escapes from

between the edges of the modified dishes to prevent entry of

particulate contaminants. The gas is filtered by a cellulose

filter which has a mean pore size of 0.1 /xm and is supported

by porous polypropylene disks in a Nylon filter holder. Glove Box. A fiber glass glove box is employed to reduce

airborne contamination during ion exchange and miscellane- ous operations. Ion Exchange Column. The column consists of an 8-mm

i.d. borosilicate glass tubing drawn to a fine tip, containing a

resin bed 4 cm in height. The resin has a 100 to 200 mesh

size and is of an analytical-grade, sulfonic acid type with an 8% divinylbenzene crosslinkage. Before use, the column is

(4) R. E. Thiers in "Trace Analysis," John H. Yoe and H. J. Koch,

Eds., John Wiley and Sons, Inc., New York, N. Y., 1957, p 637.

Reprinted from ANALYTICAL CHEMISTRY, Vol. 41, Page 955, June 1969

185
Table I. Natural and Spike Isotopic Abundance of Isotopes

Isotopic abundance

Natural abundance, in spilce, Isotope atomic per cent atomic per cent

'»'Ag 51.83 0.9

io»Ag 48.17 99.1 63CU 69.17 0.30 «Cu 30.83 99.7 »Ni 67.88 0.38 "Ni 3.66 99.02

26.71 10.6 "«Pd 11.81 87.5

206p^a 92.15

1.255

" Lead exhibits isotopic variability in nature and is therefore not listed.

Table II. Trace Element Concentrations ih Platinum Standard

Reference Material 681 Determined by

Isotope Dilution Analysis

Concentration 95% Confidence

in ppm No. of limits in ppm for Element (/ig/gram) samples the concentration" Ag 2.09 7 ±0.15

Cu 5.04 6 ±0.54

Pd 5.92 4 ±0,32

Pb 13.6 6 ±0.9

Ni 0.46 4 ±0.06

» These confidence limits are based upon tS-, where S-; is an esti- mate of the uncertainty in the average concentration and is based upon the observed variation between the k samples. The I is Student's / factor for 95% confidence and k — 1 degrees of freedom.

pretreated with 50 ml of 6M HCI and washed with 5 ml of

water.

Neutralization and Electrolysis Cell. The cell shown in Figure 1 was designed for neutralization of the acid eluate and

subsequent electrodeposition of the isotopically-altered trace

elements.' The components are fabricated of polytetrafluoroethylene (PTFE) except as otherwise indicated. The

cell body has a 25-mm i.d. and a volume of 13 ml. For the

neutralization step, the cylindrical compartment, shown

suspended by a reentrant centerpost threaded into the lid, is

filled with 1 ml of aqueous NHs. This assembly is removed

prior to electrodeposition. Two high-purity gold wires, inserted into a removable PTFE plug, serve as the cathode

during electrodeposition. When this plug is fitted tightly into the body of the cell, the 0.7-mm diameter cathode wires

project 2 mm into the cell body. An isolated anode compartment is formed by inserting a 3.5-mm diameter by 6-mm long porous Vycor (5) rod into an undersize hole at the side of the

cell and connecting this rod to a glass reservoir with plastic

tubing. A platinum wire makes an electrical contact with

diluted HNO3 added to this compartment as an electrolyte.

During electrodeposition, the cells are housed in a plastic enclosure under a slight positive pressure of nitrogen to minimize airborne contamination.

Enriched Isotope ("Spike") Solutions. With the exception

of ^*Pb, the enriched isotope materials in amounts equivalent

to 10 mg of the element were obtained from the Oak Ridge

National Laboratory. Solutions containing one mg per ml

of ""Ag, "Cu, "^Ni, and ""Pd were prepared by weighing

each material to ±0.02 mg, dissolving it in the appropriate

acid, and diluting the solution to volume in a 10-ml volumetric

flask. The possible presence of impurities in these materials

(5) R. A. Durst, /. Chem. Educ., 43, 437 (1966).

and the small sample weights used made it advisable to verify the solution concentrations. However, because these solu- tions were diluted to add the required submicrogram additions of the spikes to the sample solution, the concentrations of the

diluted spike solutions were determined by isotope dilution

as follows: Measured volumes containing 5 /xg of each spike were mixed with solutions containing known amounts of the normal elements. The pure clement additions were computed from Equation 1 to yield isotope ratios approximating one for all elements except Ag (see Calculations and Results). The solution was electrolyzed for one hour at a current of 2 mA in the previously described cell. The gold wire elec- trodes were then sparked, the isotope ratios were determined,

and the spike weights were computed from Equation 1. A

reliable ^""Pb solution could be prepared by weighing 100.0 mg of the NBS radiogenic lead isotopic standard, SRM 983

(6), dissolving it in HNO3, and diluting to volume with water

in a 100-ml volumetric flask. Mass Spectrograph. A double-focusing Mattauch-Herzog

design, spark source instrument (Consolidated Electrodynamics Model 21-110) that had been modified to give 22 exposures

per plate is operated at an ion accelerator voltage of 24 kV.

Microphotometer-Analog Computer. A microphotometer

having a precision screw (Jarrell-Ash Model 23-500) and

equipped with an analog computer to measure peak areas

(7) is used. A function generator, which is the basic component of the analog computer, provides an output proportional to relative ion intensities when the input is a voltage

proportional to per cent absorption. An integrator follows the function generator to give signals proportional to peak

intensity areas as the line is scanned. Procedure. A 250-mg sample of platinum was treated with warm aqua regia for several minutes and rinsed in water to remove possible surface contamination. The sample was

weighed into a 25-ml quartz erlenmeyer flask and heated with

3 ml of HCI and 1 ml of HNO3 until dissolved. After adding measured volumes of the spike solutions, the platinum

solution was transferred to a 20-ml low form, quartz crucible and evaporated almost to dryness in the chamber described. The resulting salts were dissolved in 5 ml of H2O and the

solution was passed through the cation exchange column

using the full flow rate of the column. During this step, the

isotopically altered trace elements were adsorbed on the

resin and the chloroplatinate solution was the effluent. The

column was washed with 4 ml of H2O, and eluted with 5 ml

of 3M HCI into the electrolysis cell after the washings had

been discarded. To neutralize the acid eluate with a minimum

of contamination, aqueous ammonia was pipetted into the

cylindrical compartment and the cell was closed with its tightly fitted lid. This distillation step yielded a small

solution volume that was favorable for the electrodeposition

of the analytes. After removal of the cylindrical compartment, the acidity of the solution was adjusted to read approximately 6 with indicating pH paper. The acidity is not critical. The solution was then electrolyzed for 5 hours at a current of

2 mA, while stirring at approximately 200 rpm. After with- drawing the solution, the gold electrodes were rinsed in water and transferred to the mass spectrograph.

A graded series of exposures was made with the pair of

electrodes at integrated beam currents of 0.3 X 10~'° to

1 X lO"' coulomb to obtain optimum intensities for the

isotope lines of the various amounts of electrodeposited elements. By moving one electrode so as to expose a fresh surface, the electrodeposit was consumed more effectively

and a greater signal was obtained. The ion-sensitive photographic plates (Ilford Q-2), were developed in a high contrast

(6) E. J. Catanzaro, T. J. Murphy, W. R. Shields, and E, L. Garner,

J. Res. Natl. Bur. Stand., 72A, 261 (1968).

(7) P. J. Paulsen and P. E. Branch, NBS Technical Note 401, Washington, D. C, Sept. 1966.

186
Table III. Comparison of Results by Isotope Dilution with Other

Analytical Methods for Platinum Standard Reference Material 681

Concentration in ppm (/ig/gram)

Isotope Activation Optical emission spectrochemical" Element dilution analysis" Polarography' Lab A Lab B LabC

Ag 2.1 1.8 1.4 2.0 2 Cu 5.0 5.0 5.2 6.3 3.0 2 Pd 5.9 5.6 5.0 6.1 4.0 Pb 13,6 10.3 8.5 15.0 10 Ni 0.46 <1 0.8

12.3 0.44

» Activation analyses by Michel Crambes and Gilbert W. Smith, NBS Analytical Chemistry Division.

' Polarographic analyses by E. June Maienthal, NBS Analytical Chemistry Division.

' Speclrochemical analyses by outside laboratories cooperating with NBS.

'' N.D. Not Detected.

developer (Kodak D-19) for 2V2 min, placed in a short stop solution for 10 sec, fixed for 1 min, washed 15 min in tap

water, and rinsed in distilled water. The intensity areas of

the lines for the isotopes listed in Table I were measured and

their ratios were calculated. From 6 to 20 ratio measure- ments were made for each element depending on the availability of lines having suitable intensity.

To correct the determination for contamination, the

entire analytical procedure was performed using the same

sample size of high-purity platinum, SRM 680. A direct mass spectrographic comparison of SRM 680 with SRM 681 had indicated that the elements of interest except for Ni were

substantially lower in SRM 680 than in the material being

analyzed. Consequently, to obtain favorable intensity ratios, the SRM 680 solution was spiked with one tenth of the amount added to SRM 681 and both materials were analyzed

concurrently. Use of the higher-purity matrix to evaluate contamination, which did not exceed 10% of the reported

values, yielded more reproducible results than were obtained

by spiking the reagents alone. For Ni however, this latter procedure had to be used.

CALCULATIONS AND RESULTS

The concentrations were computed from Equation 1.

MiBR- A) ^

'

where C is the concentration in ppm (/jg/gram), IV is the weight of isotopically enriched material ("spike") added in

/xg, M is the weight of sample in grams, A and B are the

natural abundances of the analyte isotopes a and b, Agp and

Bsp are the abundances of isotopes a and b in the spike, R is the measured altered ratio of isotope a to isotope b, and K is

the ratio of the natural atomic weight to the atomic weight of

the spike. In the method, it is desirable to alter the isotopic compositions of the analytes to obtain isotope ratios, R, approximating one whenever possible. This minimizes the effect of photographic emulsion calibration errors, and could be done for all analytes except Ag. For that element, ""Ag spike was added

to change the '"'Ag/'"Ag ratio fro.Ti its natural value of 0.93

to 2. To obtain the desired R values, the spike additions were computed from Equation 1 using the elemental concentrations estimated from a direct sparking of the platinum

sample. This photographically recorded spectrum was also used to determine the ^'"^Pb and -"'Pb natural abundances in the platinum material. These determinations were necessary because of the isotopic variability of Pb in nature (6). For

the other analytes, an experimental determination of the

natural abundances of the isotopes was not required ; and the

values listed in Table I, supplied by the NBS Analytical Mass

Spectrometry Section, were used. The isotopic analyses

provided by the Oak Ridge National Laboratory for its isotopically enriched materials and by Catanzaro et al., (6) for the radiogenic lead isotopic standard, SRM 983, are also

listed in this table. These isotope analyses are of sufficient accuracy that errors from this source do not significantly

affect our results. In all, seven samples were run, the first three using experimental conditions that did not satisfactorily preconcentrate

all of the elements. However, using the experimental conditions described under Procedure, the last four samples

yielded all of the analytes for simultaneous determination. The elemental concentrations obtained for the first three samples ate also included in the results. The determination

of Ag was satisfactory for all of the samples. Table II lists the average concentrations obtained, the number of samples,

and the 95 % confidence limits. A comparison of the isotope

dilution results with the results obtained by other analytical

methods is given in Table III. Except for Pb, the agreement

among methods having high precision was satisfactory.

DISCUSSION AND CONCLUSIONS

Unlike most trace analytical methods, isotope dilution

analysis does not require analyzed or synthesized standards of

the same matrix material for reliable determinations. As

applied to spark source mass spectrometry, the simplest

procedure consists of evaporating a portion of an isotopically

spiked solution of the sample on a suitable substrate for sparking in the spectrometer. Compared to direct sparking

of the sample, poorer limits of detection will be obtained. Moreover, a trace element may be undeterminable if the only isotope lines suitable for measurement are interfered with by isotope lines of the matrix elements or complex

ionized species of the matrix and anion fragments. For

example, copper could not be determined in platinum by this direct-evaporation technique because one of its two stable isotopes, «'Cu+, is interfered by i93Pt'+. To obtain improved limits of detection and remove these

interferences, the isotopically spiked analytes can be separated

from the matrix before sparking. A preconcentration pro- cedure should be selected that minimizes contamination which

is the principal limitation of these methods. These proce- dures can be relatively simple or incompletely characterized because determinations by isotope dilution methods are

unaff'ected by analyte losses once equilibrium has been

established with the added spikes. However, to obtain

isotope lines of sufficient intensity for precise ratio measurement, a high recovery of the spiked analytes is desirable.

187
Concentration limits of detection comparable and even

superior to those obtained by the direct sparking technique are

possible if contamination can be reduced to an acceptable

value. Although the experimental procedure should be

designed to achieve a low method blank, its variability is an

important factor affecting the precision of the results. Therefore, the least number of manipulations is desirable and should

be performed in a clean environment to reduce spurious contamination. In the platinum method, contamination can

occur during sample dissolution, evaporation, ion exchange,

and transfer of the spiked analytes to the substrate for sparking. Gold is a suitable substrate for it does not react with the solution being electrolyzed ; it is mononuciidic thus producing

a simple spectrum ; and it is obtainable in high purity. The

gold wire that was used has the composition of NBS Standard

Reference Material 685-W listed in the Certificate of Analysis.

For the dissolution step, minimal quantities of isothermally

distilled acids are used. The contribution to the total blank

from the reagents is relatively constant and can be corrected

for. The spiked platinum sample solution is evaporated in a quartz vessel surrounded by filtered nitrogen. The ion exchange step is performed in a glove box using methylmethacrylate rods and holders to support the ion exchange

columns. The last stage, during which the submicrogram

amounts of the analytes are being transferred to the wire substrate for sparking, is particularly sensitive to spurious contamination. Although the eluate solution can be evaporated

to a small volume and pipetted onto the gold wires, it is

preferable to electrodeposit the analytes. This procedure

yielded an adherent form for sparking, relatively free of

anions and organic residue from the resin, both of which

would complicate the spectrum. Moreover, electrodeposition

required less handling and, therefore, was less subject to contamination. The electrolysis cell design incorporated a

replaceable porous Vycor plug to prevent cross-contamination from the eluate solutions. To determine elements that are not ordinarily electrodeposited, evaporation of the eluate would have been required. In

this case, the electrodeposition step could be followed by

evaporation and possible ignition of the residue on fresh electrodes. Ignition would remove the organic residue, one

of the disadvantages of this evaporation procedure.

In conclusion, isotope dilution techniques provide spark

source mass spectrometry with methods for independently

analyzing i\omogeneous materials that can serve as standards

for the direct sparking comparison procedures. In general, these standards of the matrix either are not available or often have values that are not as precise as required. Although

ID-SSMS is not a rapid method (for the procedure developed,

four samples required approximately 8 days to run), its capability of simultaneous multi-element determinations and

applicability to most of the elements having two or more stable

isotopes, makes it very attractive for reliable trace element

determinations.

Received for review March 3, 1969. Accepted April 4, 1969. Presented in part before the Division of Analytical

Chemistry, 155th National Meeting, ACS, San Francisco,

Calif., April 1, 1968. Certain commercial materials and

equipment are identified in this paper in order to adequately

specify the experimental procedure. In no case does such

identification imply recommendation or endorsement by the National Bureau of Standards, nor does it imply that the material or equipment identified is necessarily the best avail- able for the purpose.

188
APPENDIX VIII

Reprinted from SPECTROCHIMICA ACTA, Vol. 24B, pp. 535-544. Pergamon Press 1969, Printed in Northern Ireland.

Determination of trace elements in zinc by isotope dilution

spark source mass spectrometry*

Paul J. Paulsen, Robeet Alvarez and Daniel E. Kellehee

Spectrochemical Analysis Section, Analjrtical Chemistry Division,

National Biireau of Standards, Washington, D.C. 20234, U.S.A.

(Received 11 April 1969)

Abstract—The applicability of the isotope dilution technique in spark source mass spectrometry

has been investigated for the determination of Pb, Cu, Cd, Ag, Tl, and Sn in zinc metal. In the

procedure developed, a solution of the zinc is spiked with known amounts of stable isotopes of

the elements to be determined, and the elements are electrodeposited onto gold wires that

subsequently serve as electrodes for the mass spectrographic isotopic analysis. Chemical

operations, including isothermal distillation of reagents, dissolution of the zinc, and the electrodeposition step are conducted in a closed system to minimize contamination. The results of the

analysis of two NBS zinc standard reference materials include determinations ranging from

11 ppm (11 /ig/g) for Pb to as low as 0.02 ppm (20 ng/g) for Sn. It is concluded that this method

is of general utility for simultaneous multi-element trace analysis and is especially applicable in

providing much needed standardization in spark source mass spectrometry.

Inteoduction

Spaek source mass spectrometry (SSMS) is potentially capable of determining all

elements in a sample at ppbf concentrations. In practice this capability is limited

by specific interferences and background generated by the matrix being analyzed,

instrument memory, and instrument background for gases in the source (C, N, O,

CO, hydrocarbons, etc.). The accuracy of analysis depends on the availability of

suitable reference standards for each material to be analyzed. If standards are not

available, the analysis is based on estimated relative sensitivity factors for the

elements; this type of analysis will have an uncertainty of a factor of three or more.

If a standard is available [1] or if the sample is amenable to the preparation of

synthetic standards [2], accuracies to within 5-10% can be obtained under favorable

conditions. For many materials, however, standards of the same matrix are not

readily available and, for high-purity metals, synthetic standards are not practical.

This situation points up the need both for standards at the ppm level suitable for

SSMS and for additional techniques that can give accurate analyses without

comparison to a reference standard.

Stable isotope dilution analysis using SSMS to measure isotopic ratios offers

the possibility of meeting these needs. With this technique the sample to be

analyzed is mixed with a known amount of a separated stable isotope (spike) of

the element to be determined. The spike must have a known isotopic composition

significantly different from that of the element in the sample. Measurement of the

* Contribution of the National Bureau of Standards, not subject to copyright,

t In this paper the abbreviations ppm and ppb represent respectively, parts per million by

weight (/tg/g) and parts per billion by weight (ng/g).

[1] R. K. Skogerboe, a. T. Kashtjba and G. H. Morrison, Anal. Chem. 40, 1096 (1968).

[2] G. D. NiCHOLLS, A. L. Graham, E. Williams and M. Wood, Anal. Chem. 39, 584 (1967).

1

189
Pattl J. Paulsen, Robekt Alvarez and Daniel E. Kelleher

resulting ratio of isotopes in the mixture by mass spectrometry, enables the calculation of the amount of the element present in the original sample. Analysis by

stable isotope dilution requires no reference standards, and is specific and sensitive. It can be applied to most elements having two or more stable isotopes.

Thermal ion source mass spectrometry has been applied to stable isotopic dilution

analysis for many years and is a widely accepted means of analysis [3]. In applying

the method to SSMS the procedure is basically the same. However, the generation

of ions by a high voltage r.f. spark combined with photographic plate detection of

ions gives the SSMS considerably different capabilities in terms of sensitivity

and element coverage than the thermal ion instruments in which ions are detected

electronically. Stable isotope dilution analysis with SSMS has been applied by

Leipziger [4] to the determination of Cu in NBS 673 nickel oxide and Sb in

NBS 835 tin standard reference materials.

The object of this work has been to develop a procedure to apply the techniques

of preconcentration, combined with stable isotope dilution and SSMS, to the

determination of ppm and lower level trace elements. Of particular interest is its

application to the analysis of Standard Reference Materials (SRM) which could

serve as composition standards for SSMS. Analysis of zinc samples proposed as NBS standard reference materials, SRM 728 and SRM 683, offered an opportunity

for comparison of results of SSMS isotope dilution with those of other trace

analytical techniques.

Experimental

Reagents

Distilled water from the building distribution system was passed through a mixed bed ion exchanger prior to use. Hydrochloric acid was isothermally distilled just before it was used to dissolve

the zinc samples. Open beakers of A.C.S. reagent grade HCl were placed in a

sealed container overnight with a quartz dish containing deionized water. This

procedure yielded approximately 9ilf HCl in the quartz container when 3 vol. of

HCl were used for 1 vol. of water.

The separated stable isotopic spikes used in these experiments were obtained

from Oak Ridge National Laboratory. The material selected for spiking an element

is always an enriched sample of the isotope that has a low natural abundance.

When an element has more than two isotopes, the selection of the spiking isotope is based on freedom from interferences from isotopes of other elements, its abundance

with respect to the major isotope of the element, the available isotopic purity of

the enriched isotope, and its cost. Enriched ^oepb, 203-^1, nvSn, ^Cd, ^^^Ag, and

®5Cu were obtained in 10 mg quantities and stock spike solutions were prepared

at concentrations of approximately 1 mg/ml using accurately weighed samples.

A.C.S. reagent grade HCl and HNO3 were used to prepare the spike solutions.

Subsequent dilutions of these spike stock solutions with distilled water were so large that impurities from these reagents could not contribute significantly to

the analysis blanks.

[3] R. K. Webster, Advances in Mass Spectrometry (Edited by J. D. Waldron), pp. 103-119.

Pergamon Press (1959).

[4] F. D. Leipziger, Anal. Chem. 37, 171 (1965).

190
Determination of trace elements in zinc

The diluted stock solutions employed for analysis were assayed by reverse spiking to determine the concentration of the element. In this operation, solutions

containing known amounts of the pure natural elements were mixed with precisely

measured volumes of the spike solutions, and the altered isotopic ratios were

measured. The procedures and calculations were similar to those for the zinc

analysis described under Procedure. Analysis of the diluted solutions avoids

errors that might occur during the dilution step.

Electrodeposition cell Figure 1 shows the electrodeposition cell used to preconcentrate the trace

elements in zinc. The tetrafluoroethylene polymer (Teflon) cell had been employed

previously [5] to electrodeposit silver onto the surface of a spectroscopic graphite

electrode. For the present work the graphite electrode was replaced with a plug

of the same diameter through which two high purity (6-9's grade) gold wires,

0.63 mm dia., were inserted to serve as cathodes. The anode compartment consists of a 15 mm length of porous Vycor sealed to Vycor tubing. The compartment

contains dilute HNO3 and a high purity platinum wire as anode. The electrodeposition cell is protected from external contamination by placing it inside a

small plastic box having a flow of filtered nitrogen gas, and mounting this box

and its external stirring motor inside a portable clean box through which filtered

air is passed. Clean environments for all stages of sample handling are provided by

either clean boxes or covered containers flushed with filtered nitrogen.

Spark source mass spectrograph

A double-focussing Mattauch-Herzog design, spark source instrument (modified Consolidated Electrodynamics Model 21-110 [6]) was operated at an ion

accelerator voltage of 24 kV. The ion source pumping system is a combination of

an ion pump and a titanium sublimation pump. The instrument has been modified

to give 22 exposures per plate.

Microphotometer

A microphotometer (Jarrell-Ash Model 23-500) having a precision screw had

been equipped in our laboratories with an analog computer to measure peak areas

[7]. The analog computer is basically a function generator the output of which can be adjusted to conform to the plate response curve to ions. Thus when its input is a

voltage proportional to percent absorption profile of a line image from the photographic plate, its output is a voltage proportional to the intensity profile of the ions

that generated the image on the plate. The function generator is followed by an

[5] R. Alvakez, Electrodeposition of trace constituents as a preconcentration technique for

optical emission spectroscopy—deposition of silver from zinc solution. Symp. Trace

Characterization, National Bureau of Standards, October 3-7 (1966).

[6] P. J. Pauxsen and P. E. Branch, Instrument modifications. NBS Tech. Note 286, pp. 2-6

(1966); NBS Tech. Note 4.01,^^. 70-73(1966); iVBS Tec7t. iVo<e 422, pp. 50-55 (1968); P.J.

Paulsen and D. E. Kelleher, NBS Tech. Note 452, pp. 45-47 (1968).

[7] P. J. Paulsen and P. E. Branch, Electronic analog computer for measuring intensity areas

of peaks. NBS Tech. Note 401, pp. 62-70 (1966).

191
PauIj J. Paulsen, Robert Ajlvabez and Daniel E. Kellehiir

Gold cathodes

I cm

Fig. 1. Electrodeposition cell.

integrator which measures the peak area under the intensity profile. The area thus

recorded is proportional to the total number of ions generating the line image.

Preliminary experiments

Since most of the trace elements of interest in the zinc samples are more noble

than zinc, electrodeposition was selected as a means of preconcentration. Electrodeposition of a neutralized zinc solution obtained by dissolution of the zinc in HNO3

yielded a deposit which, when analyzed by SSMS, proved to be mostly zinc containing

increased concentrations of the trace elements Ag and Cu. Electrodeposition of a sample obtained by dissolution of the zinc in HCl yielded a zinc deposit with Pb, Tl,

Sn, Cd, Ag, and Cu highly enriched over their concentrations in the original zinc

sample. For this reason HCl was selected as the solvent.

Procedure

One gram samples of zinc were etched lightly (<1% removal) in dilute HCl to

remove surface impurities, rinsed in distilled water, dried, and weighed. The samples

were then dissolved in a minimum volume of HCl (about 4 ml HCl to 1 g of zinc) that

had been purified by isothermal distillation. Measured quantities of the separated

stable isotopes were added to the zinc samples while they were in the early stages of

192
Determination of trace elements in zinc

dissolution. The solution was placed in the electrodeposition cell where the more noble

impurities were electrodeposited onto two high purity gold cathodes by passage of a

2 mA current for 2-4 hr. At the end of the electrodeposition period the sample

solution was removed and the gold cathodes were washed with distilled water with

the cell potential still applied. The gold cathodes were then loaded into the spark

source mass spectrograph.

Two plates (Ilford Q-2) were exposed by sparking half the available plated surface area of the gold cathodes for each plate, using a graded series of exposures starting

with an integrated beam current of 0.3 x 10"^" coulombs and increasing to 1 x 10~'

coulombs. The electrode surfaces were moved during the exposures to spark as much

of the deposit as possible. The combination of electrode movement and increasing

exposure levels insured isotope ratio measiirements over a wide range of impurity

concentration levels as well as optimum usage of all the available sample for elements

present in low amounts. The plates were developed in high contrast developer

(Kodak D-19) for 2| min, placed in short stop solution for 10 sec, fixed for 1 min,

washed 15 min in tap water and rinsed in distilled water. One plate was selected for

isotope ratio measurements based on the level of line intensities and the relative

freedom from fog and scratches on the plate. The isotope ratios' measured were

Pb 208/206*, Tl 205/203*, Sn 120/117*, Cd 112/111*, Ag 109*/107, and Cu 65*/63

(*indicates the stable isotopic spike). In general the ratios of the singly charged ions

were measured ; however, ratios of multiply charged ions also were measured when

appropriate to obtain additional data.

The formula for calculating concentrations is shown in equation (1).

ppm (by weight) =

j^^ ^^R - A)

In this equation W is the weight of the stable isotopic spike in fig, M is the weight

of the sample in grams, B is the measured "altered" isotope ratio (isotope a/isotope h),

K is the ratio of natural atomic weight/atomic weight of the spike, A and B are the

natural abundances (atomic fraction) of isotopes a and h, and A^^, B^^ are the abundances (atomic fraction) of isotopes a, h in the spike. This is a general equation which

applies to elements with two and more isotopes.

Calculation of concentration from the measured isotopic ratios normally requires

two additional measured quantities : (a) the weight of the sample and (b) the weight

of the separated stable isotope added. The Oak Ridge values for the isotope abundances in the enriched spike, which accompany each spike purchased, and table

values of natural abundances were used in calculation. The one exception was for

lead, the abundance of which varies in nature [8]. The natural isotopic abundance of

the lead in the zinc was measured from SSMS spectra of the original zinc sample.

When the natural ratio of the major isotope to the minor isotope (the spiking

isotope) is approximately two or larger the amount of spike isotope added was

adjusted to give a final ratio in the spiked sample of close to one. This procedure

minimizes the errors in the determination of isotope ratios that may arise from errors

in the photographic plate calibration. If the natural ratio is close to one, as is the

[8] R. D. Russell and R. M. Fabquhab, Lead Isotopes in Geology. Interscience (1960).

193
Paui- J. Paulsen, Robert Axvabez and Daniel E. Kelleher

case for silver, the element was spiked to give a ratio of approximately two. An

initial spike of the sample was made based on the results of a normal SSMS analysis of

the sample. The concentrations found from this spiking were used to calculate the

exact amount of spikes to add in order to obtain the optimum isotope ratios for

measurement.

Reagent and system blanks were evaluated by adding stable isotopic spikes to a

zinc sample of much higher purity and following the same analysis procedure. All

elements except tin gave blanks too small to affect the results. An evaluation of the

tin blank by this technique was not possible since a direct analysis of the higher

piirity zinc by the spark source showed it to have almost the same tin content as the samples being analyzed. Blank determinations using the same procedures and

volumes of reagents as in a normal determination but without the dissolution of a

zinc sample gave inconclusive results.

The time required for carrying out this procedure in practice has been estimated.

We assume that reagents, spike solutions, and apparatus have been readied for the

analysis and a group of 4-6 samples is to be run. The time required is 2-3 days per

sample for the six listed elements, or less than one-half day per element for each

sample.

Results

Table 1 gives the results of analysis of the two ends of two zinc bars for silver and

tin. The two sets of values listed are from completely independent runs starting

with new spiking solutions. After the first set of results for tin was obtained (run 1)

there was some concern that sorption losses could have occurred from the solution

used to make the tin 117 spike addition. This solution contained 0.04 //g/ml of tin 117

and 1 ml of this solution was added to provide only 4 x 10~^ g of tin 117 to the zinc

sample. In the second set of samples (run 2) two modifications were made in order

to minimize any possible sorption losses. First, high purity zinc was added to the tin

117 spike to serve as a carrier with 100 parts zinc to 1 part tin. This ratio was

selected so that the impurities from this zinc sample would be too low to cause

interferences with any of the elements being determined. Second, the tin spike

additions were made from a 10-fold higher concentration of tin, 0.4 //g /ml, using

only 0.1 ml. All spikes were added to the samples immediately after being diluted

from the 0.1 mg/ml working solutions. The agreement between the two sets of tin

results indicated that sorption losses were not a problem for either spike concentration. This was further confirmed when the 0.4 figlml solution was analyzed by

reverse spiking with pure tin and found to be 93% of its original concentration 11 days

after it was used to spike the samples.

Three lots of zinc in the form of shot (SRM 728) and both ends of two zinc bars

(SRM 683) were analyzed for Pb, Tl, Sn, Cd, Ag, and Cu. Both the shot and the bars

were found to be homogeneous for all elements tested withm the precision limits of

the method. Therefore, the data for each standard were pooled to give an average

value.

The analyses of both the zinc bar and the zinc shot by isotope dilution SSMS

are given in Table 2. Determinations of Pb, Cd, Cu, and Ag by other techniques are

also listed for comparison. The close agreement between the isotope dilution results

194
Determination of trace elements in zinc

Table 1. Independent duplicate determinations of Ag and Sn in zinc SRM 683

(bar form)

Bar 1 Bar 2

End A End B End A End B

(ppm by wt.)

Silver

Run 1 1.18 1.06 1.13 1.02

Run 2 1.14 1.18 1.33 1.25

Average 1.16 1.12 1.23 1.14

Tin

Run 1 0.019 0.018 0.013 0.016

R\in 2 0.023 0.019 0.014 0.016

Average 0.021 0.019 0.014 0.016

Table 2. Comparison of results by isotopic dilution and other analytical techniques for

zinc standard reference materials 683 and 728

SRM 683 SRM 728

Isotope dilution Other Isotope dilution Other

SSMS techniques SSMS techniques

(ppm by wt.)

Lead 11.2 ± 0.5* 10.8t 11.3 ± 0.4 10.8t

Copper 5.6 ± 0.3 6.4t 5.5 ± 0.5 5.8t

Cadmium 1.17 ± 0.09 1.12t 1.13 ± 0.07 1.13t

Silver 1.16 ± 0.12 1.46$ 1.16 ± 0.05 1.09$

Thallium 0.1 72 ± 0.015 O.I63 ± O.OI2

Tin 0.017 ± 0.001 0.021 ± 0.009

* The ± values are the 95 % confidence limits for the limiting mean expressed in ppm. This

is equal to tQ_Q^S\/n, where S is the standard deviation of the mean and n is the number of

independent determinations, ranging from 6 to 8, and t is student's t for 95 % confidence.

t Polarography, E. J. Maienthal (NBS).

J Neutron activation analysis, B. A. Thompson and D. A. Becker (NBS).

for the two standards for all six elements arises from the fact that both were produced

from a common lot of zinc. This agi'eement is also a measure of the precision of the

isotope dilution technique. Good agreement was found with the other methods of

analysis for all determinations except the Cu and Ag in SRM 683. These differences

on Cu and Ag are not excessive considering the low concentrations involved. It

should be noted that isotope dilution SSMS provided values for six of the seven

elements that were certified for these two standards.

195
PauIi J. Paulsen, Robert Alvakez and Daniel E. Kelleher

Discussion

Preconcentration

Measurement of the isotopic ratios for the impurities in the zinc without the

electrodeposition-preconcentration step is a possibility. Direct evaporation of the

zinc chloride solution onto high purity gold wires might serve this purpose ; however,

the amount of sample on the surface of the gold wires would be relatively small

compared to that available when zinc metal is sparked directly. Larger samples

could be loaded but this would probably involve pelletizing large amounts of zinc

chloride with a high purity conducting powder (Ag, Au, graphite, etc.); this binder

would in effect dilute the zinc chloride sample. Either system of loading the sample

for SSMS measurement of isotopic ratios would result in poorer sensitivity for the

isotope dilution analysis than would exist in a direct analysis of the metal by SSMS.

It is estimated that sample evaporation onto gold wires would give detection limits

of about 1-10 ppm for isotope dilution as compared to 1-10 ppb found for a direct

SSMS analysis.

The chloride ions introduced in the dissolution step act to dilute the zinc sample

as well as to introduce additional lines into the mass spectrum. An example of the

complications that these additional lines could cause was observed when the electrodeposition procedures were being investigated. Zinc plated electrodes which were

not washed with distilled water retained sufficient zinc chloride on their sm-faces to

contribute significantly to the spectrum. The mass spectra of these samples contained

lines for ZnCl+, ZnOCl+, and ZnC10H+, as well as C1+. The ZnOCl+ Imes were

intense enough to interfere with the tin lines which have the same nominal mass. Washing the electrodeposit with distilled water removed this interference. Electrodeposition thus served two functions: it removed the chloride interferences and it increased the sensitivity of the method. Preconcentration in part compensates for

the sensitivity lost because of the sample dissolution which is necessary to perform

the spiking step.

Preconcentration of a 10 ppb impurity from a Ig sample into a consumable

sahiple size of about 10 mg or less would provide 1 x 10~^ g of the impurity. This is

1000 times the 1 x 10""^^ g that gives a just detectable line and should give sufficient

signal for measuring isotopic ratios on several exposures with reasonable precision.

These figures would predict quantitative determinations down to 10 ppb for most

elements with preconcentration, provided of course that reagent and system blanks

did not set a higher limit. In our experiments a comparison of the line intensities

obtained from direct SSMS analysis of the zinc metal to those from the samples

preconcentrated onto the gold electrodes indicates that electrodeposition results in a 1000 fold increase in the trace element concentrations in the zinc deposit. From the

intensities of the lines obtained for the different elements reported it has been con- cluded that all six elements could have been determined at the 20 ppb level that was

found for tin, and with comparable precision.

The electrodeposition technique applied to the preconcentration of the impurities

in the zinc represents one of the simplest possible separation methods. No chemical

other than the easily purified HCl required to dissolve the zinc sample was used.

Little physical handling was involved in the process. These conditions plus the fact

that all the impurity elements determined were separated from a single sample

196
Determination of trace elements in zinc

simultaneously should keep the sample contamination (blank level) to a minimum.

Modifications of this method should be applicable to preconcentration from most

active metals. More complex preconcentration procedures have also been used

successfully for isotope dilutions SSMS analysis. In a separate investigation [9]

five elements were preconcentrated from a platinum matrix using a cation exchange

column to separate these impurities from the platinum complexed as a chloride

anion. All five elements were determined simultaneously with the concentration

ranging from 0.5 to 14 ppm by weight.

The six trace elements determined in zinc represent a range of more than 500

(Sn to Pb) in concentration for elements that could be simultaneously determined

from a single sample. There is of course no upper limit on the concentrations that

could be covered by isotope dilution, and it should therefore be possible to simultaneously determine elements from a single sample over a concentration range from

the ppb level to the per cent level. The ppb to ppm range, however, will encompass

the levels where this technique should prove most useful.

Comparison to other mass spectrometric methods

SSMS analysis by direct comparison to a standard reference material must be

carefully controlled in order for the precision of the measurements to approach the

photographic plate limiting value of 3-5% relative standard deviation. The precision is normally affected by the variations in the amount of the impurity elements

reaching the plate from one exposure to the next. These variations can arise from

inhomogeneities in the sample on a micro scale, changes in the point of sparking with

respect to the "optical" axis of the instrument, distance between sample electrodes,

rf and dc voltages of the instrument, etc. None of the factors however, have any

detectable effect on the measured isotopic ratio for an element ; but only affect the

absolute amount of the impurity measured. The precision of the analysis by stable

isotope dilution SSMS is therefore independent of the forementioned instrumental

factors and is controlled only by the limitations of the photographic plate. Isotope

dilution analysis could therefore be expected to give better precision than that of

direct analysis by SSMS.

The comparison of spark source to thermal ion source mass spectrometry for

analysis by stable isotope dilution is of interest. Thermal ion instruments can measure

isotopic ratios of many elements with precisions of 0.01% relative standard deviation

while the photographic plates used by SSMS limits its precision to about 3% at best.

In an actual analysis the difference in precision will be less than indicated by the

ability to measure isotope ratios since the precision of a determination will also be

aifected by other factors such as sample homogeneity relative to the amount of

sample dissolved for spiking, the accuracy of spike additions, reproducibility of

blanks, etc. The sensitivity of SSMS is relatively independent of the ionization

potential of an element. Sensitivity varies markedly with ionization potential for a thermal ion source; its sensitivity is substantially better than the SSMS for low

ionization potential elements, but is much poorer for the elements with high ionization potentials.

[9] R. AiiVABEZ, P. J. Patxlsen and D. E. Kelleher, Anal. Chem., 41, 955 (1969).

197
Paul J. Paulsen, Robert Alvarez and Daniel E. Kelleher

Thus, the spark source can extend the range of elements determined by stable

isotope dilution to elements not attainable with the thermal ion source instrument,

but would not be competitive for the other elements if precisions of better than

3-5% are needed. If the analysis requirements or blank problems make this precision

acceptable, the ability to analyze for many elements simultaneously, over wide

ranges in concentration, gives the spark source a definite advantage.

Summary

Isotope dilution spark source mass spectrographic analysis when combined with

preconcentration techniques has yielded reliable determinations, down to the 20 ppb

level. A comparison of isotope dilution determinations to those of polarography and

neutron activation has shown good agreement for the trace elements. This technique

gives absolute values without the use of a reference standard and therefore should

be particularly applicable to the analysis of standard reference materials that are needed for direct trace analysis by SSMS. The recording of the entire mass range by

SSMS on each exposure makes it possible to determine a large number of elements

simultaneously from a single sample. This not only represents a saving of time for

the analysis but also allows for simple methods of preconcentration which should

minimize reagent blanks. The nearly constant detection sensitivity of SSMS for all elements would predict that most elements should be measureable at the 20 ppb level

found for tin, providing, of course, that preconcentration can be accomplished and

blanks do not set higher limits of detection. This should be particularly important for

elements with high ionization potentials where, in general, isotope dilution analysis

using thermal ion sources is not possible. The preconcentration procedure adopted

here for the determination of trace elements in zinc should be applicable to most other

active metal matrices.

198
APPENDIX IX

Spark Source Mass Spectrographic Analysis of Ingot Iron for

Silver, Copper, Molybdenum, and Nickel by Isotope Dilution and

for Cobalt by an Internal Standard Technique

Paul J. Paulsen, Robert Alvarez and Charles W. Mueller

Spectrochemical Analysis Section, Analytical Chemistry Division, National Bureau of Standards, Washington, D. C. 20234

A NUMBER of trace elements are known to significantly

affect the properties of ferrous materials. As compositions

are being changed to meet the need of new applications, additional trace elements are becoming important. The increased

interest in these trace elements makes it desirable to provide more reliable methods for their determination by using new

analytical instrumentation and techniques. Analysis by

isotope dilution spark source mass spectrometry (ID-SSMS)

enables many trace elements in a matrix to be determined

simultaneously with high sensitivity and precision (/, 2). Moreover, the methods do not require a quantitative recovery or even a determination of the recovery. Because of the in- herent high sensitivity of the spark source mass spectrograph,

simplified separation procedures that have low recoveries but

are relatively contamination-free can be used. Contamination is often the limiting factor in reliable trace determinations. The object of this investigation was to develop an ID-SSMS

method for simultaneously determining trace amounts of

silver and other elements in ingot iron, a matrix having a

greater number of elements at higher concentrations than the high-purity platinum (/) and zinc (2) analyzed previously. In this method, silver, copper, molybdenum, nickel, and cobalt were satisfactorily electrodeposited onto gold wire elec- trodes after extracting the iron from a solution of ingot iron, NBS-SRM 1165. The aqueous layer had been neutralized before electrolysis. By spiking the sample solution with known amounts of enriched isotope solutions, the elements Ag, Cu, Mo, and Ni could be determined. Co, a mononuclidic element, was also determinable because of its chemical and electrochemical similarity to Ni.

EXPERIMENTAL

Reagents. Deionized water was prepared by passing

distilled water from the laboratory distribution system

through a mixed bed resin column. Hydrochloric and nitric acids, previously prepared by isothermal distillation and

stored in quartz flasks, were used for sample dissolution. Ammonia gas was bubbled into a gas washing bottle and

then into deionized water to produce aqueous ammonia.

Reagent grade isopropyl ether, with a reported boiling range

of 67.8 to 69.0 °C and a nonvolatile matter content of 0.003%, was shaken before use with an equal volume of IM HCl in a separatory funnel. Enriched isotope materials, in amounts

equivalent to 10 mg of the element, were obtained from the Oak Ridge National Laboratory. Spike solutions containing one mg per ml of ""Ag, ^^Cu, '^Ni, and "Mo were pre- pared by weighing each material to ±0.02 mg, dissolving it in nitric acid, adding aqueous NH3 to the Mo solution, and

diluting each solution to volume in a 10-ml volumetric flask. Standard solutions of natural Ag, Cu, Ni, Mo, and Co were

(1) R. Alvarez, P. J. Paulsen, and D. E. Kelleher, Anal. Chem.,

41, 955(1969).

(2) P. J. Paulsen, R. Alvarez, and D. E. Kelleher, Spectrochim.

Acta, TAB, 535 (1969).

similarly prepared by weighing 100 mg of each high-purity metal to ±0.05 mg, dissolving in ACS reagent grade nitric acid, adding aqueous NH3 to the Mo solution, and diluting the solutions to volume in 100-ml volumetric flasks. Evaporator for Sample Solutions. The borosilicate glass

chamber, which uses filtered nitrogen to prevent entry of

particulate contaminants and provides a clean environment,

has been described previously (/). It is simUar to that described by Thiers (i). Electrolysis Cell. The polytetrafluoroethylene cell, which

has two 0.7-mm diameter high-purity gold wire cathodes and

an isolated anode compartment, has been described in detail previously (7). Mass Spectrograph. The double-focusing MattauchHerzog design, Spark Source Mass Spectrograph and the microphotometer with its analog computer have been described previously (/). Procedure. Approximately 100-mg samples of ingot iron,

NBS-SRM 1165, were weighed to 0.1 mg and introduced into 30-ml borosilicate beakers. The "'Ag, "Cu, and ^^Ni

solutions were diluted and combined to make a multiplespike solution having elemental concentrations ranging

from 7 Mg/ml for Cu, to 0.3 Mg/ml for Ag. The "Mo solution was diluted to 3 Mg/ml. Immediately after preparing these

solutions, 2 ml of 7M HCl were added to each of the beakers. While the samples were dissolving, 1 ml of the multiplespike solution and 100 ii\ of the diluted "Mo solution were

pipetted into the beakers. In addition, for future verification of the elemental concentrations in the multiple and diluted "Mo spike solutions, aliquots of these solutions were combined with solutions containing known amounts of the

natural elements. The Co solution was diluted to 50 Mg/ml

and 250 jul of this solution were pipetted into half of the sample

beakers. The covered beakers were heated to dissolve the samples and several drops of HNO3 were added. The solu- tions were then evaporated to dryness in the clean-environ- ment chamber and the resulting residues were dissolved by

adding 9 ml of 7M HCl to each beaker and warming.

The solutions were cooled in an ice bath and introduced

into 60-ml separatory funnels. Each beaker was rinsed twice with 0.5 ml of IM HCl and the washings were added to the appropriate separatory funnel. Ten milliliters of cold iso- propyl ether was added to each separatory funnel and the

iron was extracted into the ether layer by shaking for one

minute. The aqueous layer containing the isotopically altered analytes was withdrawn into the original beaker

used for sample dissolution. The extraction step was repeated

using the same ether which had been back-extracted with

two 10-ml portions of water to remove the iron and shaken

with IM HCl. The aqueous layer was then transferred to

the original beaker, and evaporated to 2-3 ml in the clean- environment chamber. Aqueous NH3 was added until a

precipitate formed from the residual iron remaining in solution. The solution was then heated to remove excess NH3,

transferred to the electrolysis cell, and electrolyzed for 3 hrs

(3) R. E. Thiers in "Trace Analysis," John H. Yoe and H. J. Koch, Eds., John Wiley and Sons, Inc., New York, N. Y., 1957, Chap. 24.

Reprinted from ANALYTICAL CHEMISTRY, Vol. 42, Page 673, May 1970

199
Table I. Average Concentrations in Ingot Iron NBS-SRM

1165 Determined by Spark Source Mass Spectrometry

Compared to NBS Certified Values

Concentration ( % by weight)

NBS

certified

CALCULATIONS AND RESULTS

The concentrations of Ag, Cu, Mo, and Ni were computed

from Equation 1

Method This work"

Ag Isotope 0.00050 ± 0.00003 C

dilution Cu Isotope 0.0172 ± 0.0008 0.019

dilution Ni Isotope 0.0247 ± 0.0009 0.026

dilution Mo Isotope 0.0042 ± 0.0003 0.005

dilution Co Standard 0.0092 ± 0.0013 0.008

addition

Uncertainty is expressed as 95% confidence limits on the

limiting mean U.uSI\/n. For Ag, Cu, Ni, and Mo, the value

includes uncertainties associated with the variations among the

individual samples and the analysis of the spike solution. For Co,

it includes the variations in the concentrations computed from

Equation 2 and the variations in determining Ri. These values are considered reliable to ± 1 in the last digit.

" Not certified.

at 4 mA while stirring at 200 rpm. After withdrawing the

solution, the gold electrodes were rinsed by introducing ap- proximately 7 ml of water into the cell and stirring. The pair of electrodes was sparked in the mass spectrograph and a graded series of exposures was made from

3 X 10"" to 1 X 10^* coulomb. The ion-sensitive photographic plates (Ilford Q-2) were processed using the bleach and internal image developer MK-7 described by Cavard (4). They were placed in the dichromate bleach for 4 minutes,

rinsed in water for Va min, developed for 7 min, fixed for 1 min, and washed for 15 min. The intensity areas of the

lines on the photographic plate for the elements were measured and the ""Ag/'O'Ag, "Cu/«»Cu, <!2Ni/«»Ni, and "Mo/

'^Mo ratios were determined for all samples. The "Co/'^Ni

ratios were also determined for samples with added Co,

and the ^'Co/^'Ni ratios determined for samples without the Co addition. From 6 to 14 ratio measurements were made

for each element depending on the availability of lines having

suitable intensity.

To correct the determinations for contamination, the

entire analytical procedure was performed under the same

experimental conditions using the same quantities of reagents

and 5 mg of high-purity iron. To obtain more favorable

isotope ratios, the iron was spiked with one-tenth of the amounts added to the ingot iron samples and both materials were analyzed concurrently. The blanks were found not to exceed 2 % of the reported values. To verify the elemental concentrations in the multiplespike and diluted "Mo solution, aliquots of the solutions

that had been reserved for this purpose were pipetted into the

polytetrafluoroethylene electrolysis cell. After electrolyzing the stirred solutions for 3 hours at a current of 4 mA, the gold wire electrodes were sparked in the mass spectrograph.

Except for Mo, the altered isotope ratios were measured and

the spike weights computed from Equation 1. The isotope

lines of Mo were too weak to be measured and therefore an evaporation procedure was used. Approximately 0.5 ml

of the combined diluted "Mo and natural Mo solution was

evaporated to one tenth of the volume in a quartz crucible. The evaporated solution was pipetted onto gold wires and

dried. After sparking the wires in the mass spectrograph,

the "Mo spike weight was computed from the measured

isotope ratios.

(4) A. Cavard in "Advances in Mass Spectrometry," E. Kendrick,

Ed., The Institute of Petroleum, London, 1968, pp 419-429.

C =_ WK{A^^-B,„R)

M (BR - A) (1)

were C is the concentration in yug/g, W is the weight of isotopically enriched material (spike) added in fig, M is the weight

of sample in grams, A and B are the natural abundances of

the analyte isotopes a and b, A,p and Bsp are the abundances of

isotopes a and b in the spike, R is the measured altered ratio of

isotope a to isotope b, and K is the ratio of the natural atomic

weight to the atomic weight of the spike.

The spike isotopes provided by Oak Ridge National Laboratory were all highly enriched; the abundances of the isotopes

""Ag, "Cu, "Mo, and ^^Ni being 99.1, 99.7, 92.8, and 99.0 atomic per cent, respectively. The quoted accuracy of the abundances was such that errors from this source do not sig- nificantly affect our results. To minimize the effect of photographic emulsion calibration

errors, it is desirable in determining R to measure isotope lines that have approximately the same intensity. This was done

for all elements except silver. Silver has only two naturally

occurring isotopes, "'Ag and "'Ag, which have almost the same abundance. Therefore, an amount of ""Ag spike was

added to change the value of the ""Ag/""Ag ratio to 2. Table I lists the average concentrations for Ag, Cu, Mo, and

Ni obtained by isotope dilution, the 95 % confidence limits, and a comparison of the results with the NBS certified values. The results were derived by running ten samples, in two groups

of five. Separate spike solutions were prepared and added to each group. The individual results from each group did not

vary significantly and therefore the data could be pooled for

statistical evaluation. In addition to the elements being determined by isotope

dilution, cobalt, a mononuclidic element, was also found to be

electrodeposited on the gold electrodes. Because of the

similar chemical and electrochemical behavior of cobalt and

nickel, it was postulated that both elements would be preconcentrated on the electrodes with a constant relative recovery. To test this hypothesis, the ratios of the ''Co to '"Ni line intensities were measured on each exposure together with the

altered isotope ratios used for the isotope dilution determinations of Ag, Cu, Mo, and Ni. The ratios were found to be

constant among the samples. We concluded that under these experimental conditions, Ni would serve as an internal stan- dard to monitor the recovery of Co which could vary from

sample to sample. Co would then be determinable by a

standard addition of Co. The ratio of the ''Co to "Ni line

intensities were determined for each sample having a known

and constant amount of Co added. By using the more abundant ''Ni instead of the '"Ni, a "Co/"Ni ratio approximating

unity could be obtained by adding the appropriate amount of Co.An average "Co/^'Ni ratio, of 1.04 ± 0.05 was obtained

for seven samples without added cobalt. The cobalt content,

Co(s), of a sample was computed from the following equation: ^ Co(a)/^.°°Ni

where R2 is the measured "Co/'Wi ratio for a sample with

added cobalt, Co (a) is the added cobalt, and ""Ni, '«Ni are

the nickel contents including the small amounts added with

the '^Ni spike. All concentrations are in /zg-atom/gram of

ingot iron. The average of six determinations of Co, the 95 %

200
confidence limits, and the NBS certified value are given in Table I. Recovery Experiments. One of the advantages of the

isotopic dilution technique of analysis is that only enough

spiked sample required for an accurate isotope ratio deter- mination need be recovered from the original sample. A

knowledge of the recovery efficiencies is not required for computation of results of analysis. The efficiency of re- covery of a preconcentration technique however, is of interest because the amount of recovery will effect the detection

limits for the method. Experiments were performed in order

to determine the recovery for the procedures up to and includ- ing the electrodeposition step. A set of samples was run using the previously described experimental procedure but omitting the spike additions. For

comparison, electrodeposition periods of one hour were also used in addition to the 3-hour periods used in the procedure.

The material recovered on the high-purity gold cathodes was

redissolved in 0.2 ml of warm HNO3, spiked with the stable isotopes, and neutralized with aqueous ammonia. The re- covered trace elements were then electrodeposited onto the same gold electrodes. The electrodes were sparked in the SSMS and the amounts of material recovered on the elec- trodes were computed from Equation 1 . This procedure gave

results for Ag, Cu, and Ni; however, Mo could not be detected in this second plating.

Increasing the plating time from one to three hours increased

the total recovery for Ag, Cu, and Ni from 20 to 40 %. The

high sensitivity of the spark source mass spectrograph method

is illustrated by realizing that only 0.2 /ig of silver on the gold

electrodes was more than adequate for the determination of

silver.

Received for review December 2, 1969. Accepted February

24, 1970.

201
APPENDIX X

REPRINTED FROM APPLIED SPECTROSCOPY, Vol. 30, No. 1, 1976, pp. 42-t6.

Trace Element Determinations in a Low-Alloy Steel Standard

Reference Material by Isotope Dilution, Spark

Source Mass Spectrometry*

PAUL J. PAULSEN, ROBERT ALVAREZ, and CHARLES W. MUELLER

Institute for Materials Research, National Bureau of Standards, Washington, D.C. 2023^

A stable isotope dilution procedure using the spark source mass spectrograph was developed for the simultaneous deReceived 16 May 1975; revision received 10 September 1975.

* Certain commercial materials and equipment are identified in this paper in order to adequately specify the experimental pro- cedure. In no case does such identification imply recommendation or endorsement by the National Bureau of Standards, nor

does it imply that the material or equipment identified is necessarily the best available for the purpose.

termination of cerium, copper, neodymium, selenium, sil- ver, tellurium, and zirconium in a low-alloy steel material.

Except for copper, the elements were present at trace concentrations that are difficult to determine by most analytical methods. In the procedure, known amounts of the isotopically enriched elements were added to 100-mg samples,

which were dissolved in perchloric and hydrofluoric acids.

After evaporation of the solutions almost to dryness, dissolution of the residues, and addition of hydrofluoric acid

Volume 30, Number 1, 1976 202 APPLIED SPEaROSCOPY
to complex the iron, the resulting solutions were electrolyzed in poly tetrafluoroethylene cells. The electrodeposit on high purity, gold cathode wires was sparked in the mass

spectrograph and the isotope ratios of the isotopically

equilibrated elements were measured. The concentrations,

ranging from 0.041% for copper to 3.1 Mg/g (ppm) for neo- dymium, were calculated from the general isotope dilution

equation. The low^alloy steel is available from NBS as Standard Reference Material 1261. Index Headings: Analysis, for trace elements; Mass spec-

'-"""""v, spark source; Methods, analytical.

INTRODUCTION

A set of low-alloy steel Standard Reference Materials

(SRMs) was issued by the National Bureau of Standards

in 1957 primarily for use in optical emission and x-ray

spectrometric methods of analysis. At that time, several trace elements present in this set were not certified either

because they were at extremely low concentrations or be- cause they were generally regarded as having an innocuous

effect on the physical properties of steel. Consequently,

the anticipated benefits of providing certified values for these elements would not have justified the cost of developing analytical methods having the requisite reliability.

3ince this set of SRMs was issued, new applications of

ferrous products, mainly for the nuclear and aerospace

industries, have necessitated more stringent specification

limits and quality control determinations of additional

trace elements. Coinciding with an increased interest in the effects of trace elements on the properties of ferrous

materials has been the increased use of spark source mass

spectrometry as a powerful tool for estimating the concentration of nearly every element in a sample down to a

few nanograms per gram (parts per billion). However for

reliable quantitative determinations, SRMs were needed

having certified values for the trace elements of interest

and having concentrations of the major constituents, similar to those of the samples being analyzed.

A new set of iron and low-alloy steel SRMs was made in the late 1960's to replace the earlier set. The SRMs were

fabricated in the form of rods, 3.2 mm in diameter by 51

mm long, for spark source mass spectrometry and microchemical techniques as well as in the form of 32-mm diameter discs for x-ray and optical emission spectrometry.

This replacement set was intended to provide concentration ranges for approximately 40 elements.

Certification of the trace elements in these materials

required sensitive and reliable methods. Moreover, for the

efficient characterization of these materials with respect

to trace elements, analytical methods that would yield

results for a number of elements simultaneously were desirable. Isotope dilution procedures using the spark source mass spectrograph (ID-SSMS) has been found to meet

these requirements.'"'

The object of this investigation was to develop an IDSSMS method for determining a number of analytically

difficult trace elements such as selenium, tellurium, and

neodymium in a low-alloy steel material SRM. In a previ- ous application of an ID-SSMS method to a ferrous material, copper, molybdenum, nickel, and silver had been

determined in an ingot iron, SRM 1165.' Although the

low-alloy steel material was a more complex matrix than

the ingot iron, a relatively simple ID-SSMS procedure was

developed which enabled a number of chemically diverse

elements to be determined simultaneously.

I. EXPERIMENTAL

A. Reagents and Special Solutions. Deionized water was prepared by passing distilled water from the laboratory distribution system through a mixed-bed, resin col- imn. Commercially produced, high purity, perchloric and

lydrofluoric acids were used for sample dissolution; similar

ligh purity sulfuric, nitric, and hydrochloric acids were

ised for preparing solutions of the enriched isotope materials (spikes) and natural elements. The purity of the

perchloric, nitric, and hydrochloric acids had been evaluated previously by an ID-SSMS procedure developed by

the authors.^ Materials, isotopically enriched with respect

to '«Nd, '«Ce, '"Te, '"'Ag, «'Zr, «Se, and «Cu, were obtained from the Oak Ridge National Laboratory. The

>sotopic abundance, which was the highest available at the

time of purchase, was at least 89 atomic percent. Each

spike solution was prepared to contain 1 mg/ml of the

"lement by weighing the spike material to ±0.02 mg, dis- solving it in the appropriate acid, and diluting the solution

to volume in a 10-ml volumetric flask. A standard solution

of each natural element under investigation was prepared

by weighing 100 mg of the high purity metal to ±0.05 mg,

dissolving it in the appropriate acid, and diluting the

solution to volume in a 100-ml volumetric flask. For

cerium and neodymium, the equivalent weight of oxide was used.

B. Evaporator for the Sample Solution. The evaporator is similar in function and design to the borosilicate

glass unit previously described' except that the principal

components are constructed of polytetrafluoroethylene

TFE) and translucent fluorinated copolymer (TFEEP). The 14.5-cm diameter TFE base is 13 mm thick

nd has four cyHndrical cavities into which 30-ml TFEFEP beakers fit snugly. The 10-mm deep cavities are 90°

apart with centers on a 3.5-cm diameter radius. A 12.5-cm

diameter by 9.0-cm high cover of TFE-FEP fits tightly

into a cylindrical groove in the base. Near the top of the cover are two 8-mm diameter TFE tubes, which are

press-fit into holes approximately 180° apart. Filtered air enters the apparatus through one of the tubes to remove

the acid vapors and prevent entry of particulate con- taminants.

C. Electrolysis Cell. The TFE electrolysis cell consists

of two parts: a cylindrical body and a base. The cell body

has a 33-mm height, a 17-mm inside diameter, and a 3-mm

wall thickness. The base makes a liquid-tight fit into the

cell body and has four holes, 90° apart, on a 12-mm diameter circle. The holes are drilled undersize for inserting

0.9-mm diameter high purity gold wire electrodes. The

gold wire had been examined by direct sparking in the mass spectrograph for the presence of impurities that

could affect the measurements. None were found. The

overall purity was determined to be comparable to that

of the high purity gold, SRM 685.« The ends of the four

wires are dipped in aqua regia, rinsed in water, and ad203 APPLIED SPEaROSCOPY
justed to project approximately 2 mm into the cavity of

the assembled cell. Two opposite wires serve as the

cathode; the other two as the anode. During electrodeposition, the solutions in the cells are stirred with TFE stirrers at approximately 200 rpm. The cells are housed in a plastic

enclosure under a slight positive pressure of nitrogen to minimize airborne contamination.

D. Mass Spectrograph. A double-focusing, MattauchHerzog design, spark source instrument (Consolidated

Electrodynamics model 21-110) that had been modified to give 22 exposures per plate is operated at an ion accelera- tor voltage of 24 kV. The ion-source sample chamber is evacuated with a high-speed, high-conductance ion pump

having added titanium sublimation. A pressure of 7 X

10~* Pa (5 X 10~' Torr) can be obtained in approximately

20 min after loading a sample into the chamber. Less than

60 min is required to attain a pressure in the range of

10~° Pa (10""' Torr). The pressure is measured using an ion

gauge mounted directly in the ion source chamber. The

chamber containing a sample from the isotope dilution

procedure is evacuated to a pressure of less than 1 X 10~^

Pa (1 X 10~' Torr) before sparking. The high conductance

and speed of the pumping system maintains pressures in the 10~^ Pa (lO"' Torr) range during sparking even when

relatively large amounts of gas are being released.

E. Microphotometer-Analog Computer. A microphotometer having a precision screw (Jarrell Ash model

23-500) and equipped with an analog computer to measure

peak areas' is used. A function generator, which is the

basic component of the analog computer, provides an

output proportional to relative intensities when the input

is a voltage proportional to percent absorption. An in- tegrator follows the function generator to give signals pro- portional to peak intensity areas as the line is scanned.

F. Procedure. A preliminary analysis of the steel sample was performed during the initial development of the

isotope dilution procedure. The elemental concentrations

determined were used to calculate the amount of spike

isotope necessary to obtain an optimum altered isotopic

ratio, which for the elements of interest except silver is 1. An altered isotopic ratio of approximately unity is desirable because it will minimize the effects of any errors in the

calibration curve for the photographic plate and will usually yield the maximum number of measurable isotopic

ratios. Silver, which has a natural isotopic ratio of approximately one, was spiked to yield an altered isotopic

ratio of 2. In the final procedure, a 100-mg sample was weighed to ±0.1 mg and transferred to a 30-ml TFE-FEP beaker.

The beaker was covered with a TFE lid. Aliquots of spike

solutions having compatible ions were combined and di- luted in volumetric flasks. Immediately after preparing

these solutions, 2 ml of 6 M HCIO4 and 0.5 ml of 26 M

HF were added to the sample. While the sample was dissolving, known volumes of the spike solutions were added

using calibrated plastic pipets. At the same time, aliquots

of the spike solutions were mixed with solutions containing

known amounts of the natural elements for future verifi- cation of the spike concentrations.

The beaker was then heated to completely dissolve the

sample, transferred to the evaporator, and heated further

until HCIO4 fumes were observed. After adding 0.5 ml

of 26 M HF, the contents of the beaker were heated al- most to dryness. The residue was dissolved and the iron was complexed by adding 2 ml of H2O and 0.5 ml of 26 M

HF, and heating the beaker. The solution was then elec- trolyzed for 4 h at a constant current of 6 mA while being

stirred at 200 rpm. The solution was withdrawn and the

electrodes were rinsed in water. The electrodeposit on the ends of the gold wire elec- trodes was sparked in the mass spectrograph and a graded

series of exposures was made from 3 X 10~" to 1 X 10~*

C. The ion-sensitive photographic plates (Ilford Q-2) were processed using the bleach and internal image developer MK-7 described by Cavard.' They were placed in the dichromate bleach for 4 min, rinsed in water for 3'^ min,

developed for 7 min, fixed for 1 min, and washed for 15

min. For all elements except zirconium, intensity areas of

both measured isotopes of an element were determined on

each suitable exposure using the microphotometer-analog

computer. Maximum peak intensities, corrected for background, were measured for zirconium. The isotopic masses measured for each element are: Ag 107, 109; Ce 140, 142; Cu 63, 65; Nd 144, 145; Se 80, 82; Te 126, 125; and Zr

90, 91. The second mass listed for each element is the

spike isotope ; the first mass is usually the isotope of highest abundance in the naturally occurring element. In general, the ratios of the singly charged ions were

measured, but ratios of the multiply charged ions were

also measured when appropriate to obtain additional data

or to avoid interferences existing at the singly charged

lines. The individual elemental concentrations were calculated

from the measured altered isotopic ratios (R) using Eq.

(!)•

^ _ WKU.p - B.^R) , . ^ ~ MiBR - A)

where C is the concentration in micrograms per gram, W

is the weight of isotopically enriched material (spike)

added in micrograms, M is the weight of sample in grams,

A and B are the natural abundances of the analyte iso- topes a and h, A^p and B,p are the abundances of isotopes a and b in the spike, R is the measured altered ratio of isotope a to isotope b, and K is the ratio of the natural

atomic weight to the atomic weight of the spike.

The spike concentrations in the solutions that had been

added to the samples were verified by preconcentrating

the isotopically equilibrated elements from the solution

that had been reserved for this purpose, determining the

isotopic ratios, and calculating W in Eq. (1). Two preconcentration procedures were used: electrolysis using the

experimental procedure described for the sample, and

evaporation of separate aliquots of the solution onto high

purity gold wires after which the wires were ignited at 450°C for 30 min. The evaporation procedure enabled us

to obtain data for elements that had not been electrodeposited satisfactorily.

The method blank was determined by performing all of the steps in the procedure using the same quantities of

acids but smaller quantities of the spikes.

Volume 30, Number 1, 1976 204
[I. DISCUSSION AND RESULTS

The graded series of exposures used in the analysis was

selected to obtain the optimum sensitivity for elements at concentration levels down to 1 Mg/g (ppm). Among other

considerations, the selection of isotope lines and charge

states was based on the required sensitivity and potential

interferences. Some of the specific considerations for the

determined elements are listed below.

1. Copper. Because of the high copper concentration,

0.041 %, in the sample, the exposures produced Cu+

lines, most of which were too dark to be measured reliably.

Therefore, the Cu-+ and Cu*+ isotopic ratios were also

measured to obtain an adequate number of ratios. For

these charge states, both *^Cu and "Cu isotopes occurred

at fractional mass/charge positions, and therefore were

free of interferences from + 1 ions occupying unit mass

positions in these m/e regions.

2. Selenium. Copper-containing samples normally yield

lines at mass positions 79 and 81 corresponding to [CuO]+,

and less intense lines at 80 and 82 corresponding to [CuOH]+. If [CuOH]+ lines exist, they can interfere with

the determination of the isotopic ratio, ^^Se/^^Se, when the

singly charged ions are used. This potential interference was indicated by faint lines of [CuO]+ being present at mass positions 79 and 81. Because molecular species, such

as [CuOH]+ will not form stable multiply charged ions,

the +3 charge states of the selenium isotopes were measured to obtain the isotopic ratio. Consequently, these

measurements, at fractional m/e positions, were also free

of interferences by any molecular species. Concentrations

calculated from the Se+ and Se'+ isotopic ratios were in agreement, indicating that [CuOH]+ did not in fact significantly interfere with the Se+ measurements.

3. Zirconium. Each of the zirconium isotope lines had a

faint line on the high mass side, which was attributed to hydrocarbons and could not be resolved when measuring

the customary intensity areas. Because the contribution

of the hydrocarbon line to the center of each Zr isotope

line was essentially zero, the maximum peak intensity,

corrected for background, was measured for each isotope

line. Measurements of maximum peak intensity are not

as sensitive as those of intensity area. However, the elec- trodeposited Zr yielded a signal of sufficient intensity for satisfactory precision with either mode of measurement.

4. Silver. Both the -fl and +2 ions were measured for

TABLE I. Selenium concentration in sample 1, NBS-SRM

1261, low-alloy steel calculated from replicate exposures.

Exposure "Se intensity '"Se intensity Concentration in %

No. area area by wt

TABLE II. Average selenium concentrations determined in

test samples of NBS-SRM 1261. low -alloy steel.

Sample Concentration in "vc by wt

6 713 825 0.003598

7 603 710 0.003682

8 387 470 0.003838

9 336 377 0.003455

10 378 479 0.004064

11 367 440 0.003772

12 333 402 0.003822

Average 0.00375

Relative standard deviation 5.2%

96% confidence limits of the mean 0.00018

Average

95% confidence limits of a single

determination

0.00375

0.00350

0.00360

0.00373

0.00370 0.00352

0.00366

0.0036 0.0002

TABLE III. Results obtained by ID-SSMS for seven ele- ments determined simultaneously in NBS-SRM 1261, lowalloy steel.

Element

Average

concentration

in % by wt

95 % confidence

limits of a single

determination

No. of samples

Copper 0.041 ±0.0020 7 Zirconium 0.0071 ±0.0011 5 Selenium 0.0036 ±0.0002

Cerium 0.0015 ±0,0001 t Tellurium 0.0007 ±0.0002 5 Silver 0.00036 ±0.00004 5 Neodymium 0.00031 ±0.00002 6

the '"^Ag and ""Ag isotopes to obtain an adequate number

of isotopic ratios.

5. Tellurium. The singly charged ionr were used to determine the isotopic ratios from which the concentration was calculated.

6. Cerium. The '^''Ce isotope line used in determining

the isotopic ratio also has a contribution from '''^Nd present

in the sample. This contribution was calculated for each

exposure from a measured '"Nd intensity and the natural

isotopic ratio, "^Nd/'"Nd. This correction of the intensity area at mass position 142 was always less than 10% of the

total area because of the low concentration of Nd compared

to Ce in the sample and the relatively low abundance of

the "^Nd isotope.

7. Neodymium. Singly charged ions were used to determine the altered isotopic ratio. A computer program was developed, incorporating Eq.

(1), that provides a print-out of concentration values for each pair of intensity area measurements without printing

the values of R used in the calculations. The program also

gives an average concentration and a statistical analysis

of the data.

As an example of the data, Table I shows the *^Se and

'"Se intensity areas that were measured for a series of ex- posures, and the selenium concentrations calculated for each pair of intensity areas. The average concentration of

selenium in samples 1, the relative standard deviation, and

the 95% confidence limits of the mean are also shown.

Table II shows the average selenium concentration in each

test sample, and the 95% confidence limits of a single

determination.

Table III shows the results obtained by ID-SSMS, for

205 APPLIED SPEaROSCOPY
TABLE IV. Comparision of results by ID-SSMS with other

methods of analysis for NBS-SRM 1261, low.alloy steel.

centr&tion

in % by wt

Analytical method

0.041 ID-SSMS

0.043 Neutron activation 0.042 Isotope dilution, thermal ion- source mass spectrometry

0.041 Spectrophotometry

Zirconium 0.0071 ID-SSMS

0.009 Gravimetry

0.008 Spectrophotometry

0.0036 ID-SSMS

0.003 Neutron activation

Cerium 0.0015 ID-SSMS

0.001 Spectrophotometry

0.0014 Isotope dilution, thermal ion- source mass spectrometry

Tellurium 0.0007 ID-SSMS

0.0005 Polarography

Silver 0.00036 ID-SSMS

0.00040 Atomic absorption

Neodymium 0.00031 ID-SSMS

0.00028 Isotope dilution, thermal ion- source mass spectrometry

seven elements in NBS-SRM 1261, low-alloy steel. The

table lists the average concentrations of the elements in percentage by weight, the 95 % confidence limits of a single

determination, and the number of samples for which results were obtained. The experimental conditions used for the first two samples did not preconcentrate all of the

elements satisfactorily, but the final conditions described

under "Procedure" yielded all of the analytes for simultaneous determination of the seven elements.

Table IV shows a comparison of the results obtained by

ID-SSMS with other methods of analysis. Considering the

difficulties in determining most of these trace elements, the

agreemient of results is considered satisfactory. Most other

methods are specific for a single element and often involve

complicated chemical separations and manipulations.

III. CONCLUSIONS

The major sources of error in trace element determina-i

tions are usually losses of the elements and adventitious

contamination, the blank. The use of isotope dilution in the procedure that was developed minimizes errors due

to trace element losses. The trace elements are brought to equilibrium with the spikes at an early stage of the pro- cedure, after which the concentrations obtained are independent of losses. Although the blank is not considered a problem in the determination of these elements, small

quantities of reagents and few handling operations ensure

that the blank does not contribute significantly to the

method error.

1. R. Alvarez, P. J. Paulsen, and D. E. Kelleher, Anal. Chem. 41, 955

(1969) .

2. P. J. Paulsen, R. Alvarez, and D. E. Kelleher, Spectrochim. Acta 248, 535 (1969).

3. P. J. Paulsen, R. Alvarez, and C. W. Mueller, Anal. Chem. 42, 673

(1970) .

4. R. Alvarez, P. J. Paulsen, and C. W. Mueller, NBS Technical Notei

642,70,(1970). I

5. R. E. Thiers, in Trace Analysis, John H. Yoe and H. J. Koch, Eds.,|

(Wiley, New York, 1957), Chap. 24, p. 637. I

6. Catalog of Standard Reference Materials, NBS Special Publ. 260,"

April 1973, Office of Standard Reference Materials, National Bureau

of Standards, Washington, D.C. (available from U.S. Superintendent

of Documents, U.S. Government Printing Office, Washington, D.C.

20402).

7. P. J. Paulsen and P. E. Branch, NBS Technical Note 401, (1966).

8. A. Cavard, in Advance in Mass Spectrometry, E. Kendrick, Ed., (The

Institute of Petroleum, London, 1968), pp. 419-429.

Volume 30, Number 1, 1976 206 APPLIED SPEaROSCOPY
APPENDIX XI

Reprint from

Special Technical Publication 443

American Society for Testing and Materials

1916 Race Street, Philadelphia, Pa. 19103

1969

T. C. Rains^

Chemical Aspects of Atomic

Absorption

REFERENCE: Rains, T. C, "Chemical Aspects of Atomic Absorption,"

Atomic Absorption Spectroscopy, ASTM STP 443, American Society for Testing and Materials, 1969, pp. 19-36. ABSTRACT: The phenomenal growth of atomic absorption spectrometry in the past twelve years can be attributed to many factors such as the high

sensitivity of the method for a large number of elements, advances in overcoming interferences, saving of time required for analysis, and the rapid

improvement of instrumentation. The major interferences in atomic absorption

are of a chemical nature; however, physical interferences which are associated with the chemical matrix often are present. To overcome chemical interferences and to eliminate or minimize their influences from phosphate, aluminum,

titanium, and any other elements producing nonvolatile compounds, releasing

agents, or high-temperature flames are used. Also, the use of a separation and

preconcentration technique often becomes necessary when determining

elements at the subtrace level. A list of organic Ugands and solvents used in solvent extraction for absorption is presented as a means of preconcentration

and removal of chemical interferences. KEY WORDS: atomic absorption, detection Umits, electrodeless discharge,

nebulizer burner, interferences, releasing agents, organic ligands, solvents,

analyzing, tests

Atomic absorption spectrometry (AAS) has made phenomenal growth

since its introduction by Walsh in 1955 [1].^ This growth can be attributed

to many factors such as the high sensitivity of the method for a large number

of elements, advances in overcoming interferences, speed of analysis, and the

rapid improvement in instrumentation. Today the analytical chemist has a

variety of atomic absorption instruments and components from which to choose. These include different light sources, nebulizer burners, oxidant-fuel

systems, optical arrangements, and associated electronics. All of these

parameters must be considered in any discussion of the chemical aspects of

AAS, and for that reason a brief discussion of these components also is presented.

The sensitivity and precision obtained by AAS are of concern to the

analyst; however, the requirements for sample preparation prior to making an

^Research chemist, Analytical Chemistry Div., National Bureau of

Standards, Washington, D. C. 20234.

^The italic numbers in brackets refer to the list of references appended to this paper.

207
ATOMIC ABSORPTION SPECTROSCOPY

absorption measurement can be of equal importance. Where the sample is a solution, the analysis, often times, can be performed on the sample as received.

The major interferences in absorption are of a chemical nature. However,

physical interferences often are present and are associated with the chemical

matrix. Scattered light wliich appears as molecular absorption may be due to high soUds or large droplets produced by the nebuUzer. This type of

interference is very pronounced in the far ultraviolet region of the spectrum

as exemplified by arsenic and selenium. Also, molecular fragments such as CH,

C2, and CN do exist and can contribute to the background as well as to

scattered Hght. Molecular absorption also is observed from the OH bands over

the wavelength region of 2800 to 3500 X.

Sensitivity and Detection Limit

The terms "sensitivity" and "detection Hmits" have led to some confusion

in the field of atomic absorption. Sensitivity is defined as the concentration

of an analyte in solution that produces an absorption of 1 percent. Detection

limits is not defined so readily. A number of papers have appeared which

discuss this issue [2-4]. Winefordner and co-workers [5-6] have derived

theoretical equations for the calculation of the detection limit based on

instrumental and spectral constants. In general the term is defined as the

concentration, micrograms per milliliter, that produces an absorption equal to twice the magnitude of the fluctuations of the blank or background noise.

The major problem is the selection of a blank. Usually, the blank is distilled

water which really cannot be considered a true blank in reporting a detection

limit in a sample. Table 1 presents the detection Hmits obtained with a variety

of instruments and under various conditions [7-9]

.

Instrumental Components

The instrumental components are best illustrated by a schematic diagram,

as shown in Fig. 1. The basic components are the light source, nebulizer

burner, monochromator, photodetector, and readout system.

Light Source

To attain maximum absorption the light source should emit radiation of

the desired wavelength that is sharp, intense, and stable. There are many

types of Une sources used in absorption work; however, the most commonly

used source is a hollow-cathode lamp. Another source which fulfiUs the first two requirements is the electrodeless discharge lamp. The lamp developed by

Meggers and Westfall [10] is an excellent source for use in absorption. In conjunction with the lamp various types of power sources and cavities are used [11-I3] . The purpose of the cavity is to transfer the power from the

microwave generator, such as a medical diathermy unit, to the inert gas within the lamp. The charged inert gas ions then bombard the metal present

producing an excited metaUic atom which emits its characteristic radiation.

208
RAINS ON CHEMICAL ASPECTS

TABLE \—Detection limits reported by atomic absorption spectrometry.

VV dVC- VP Ti + +

1

... W3VC~ T^P+ P^+ l ATI Ion rrf" r\ lenKin, Limit, lengtn, Liinit, E/lement A /^g/ml j-^iciTicrii A /xg/ml

AgA n . . . "^"^R

1

U.Uyj "31 n 1 A 1 u. 1 Mo U.UUJ A « 1 9^7 0 ic MH 2*

All 2676 O.oifl Mi 9^90

p 2497 u PV» 9 1 70

Ra 5536 0.1* 98^^ Z.OD J Be 2349 0.003* 947A U.J Bi 2231 0.02 Pr Ca 4227 0.003^ Pt 9A'^Q 0 5 Cb . . . 4059 20* RK 7snn 0 005-^ Cd . . . 2288 0.001<^ Rp 1.5* Co 2364 0.007^ RVi 0 f)3 Cx 3579 0.005 Rni\.U . • ^4QQ 0 3 Cs 8521 0.05* n 1 Cu 3248 0.005 ^919 0.2*

Dv . . 4211 0.2* ^p 1961 0.5 Er 4008 0.2* 2516 0 1*

Fii 4594 0.2* 5*

Pp nV/. X1^

VJa ... 2874 0 07 9r 4607 0 01

Gd 3684 4* Ta 2715 6*

Ge 2651 2* Th 4326 2*

Hf 3072 15* Tp 2143 0.3 H? 2537 0.25' Ti 3643 0.1*

Ho . , 4163 0.3* Tl 2768 0.2

In . .. . . . 3039 U.U5 Tm. . . A An A

Ir . . . 2850 4* U 3585 30*

K , . 7665 0.005* V 3184 0.02*

La , . , 3928 8* W 4008 3*

Li 6708 0.005 Y 4077 0.3*

Lu 3312 50* Yb 3988 0.04

Mg. . . 2852 0.0005 Zn 2139 0.002

Mn 2795 0.005 Zr 3601 5*

^ Laminar flow burner with preheater. * Nitrous oxide-acetylene.

<^ Argon (entrained air)-hydrogen. ^ Quartz tube. ^ High-brightness lamp.

/ Osram lamp.

S Mercury germicidal lamp.

One of the more effective cavities wliich meets the requirement for

absorption is the Evenson cavity [14] shown in Fig. 2. To match the

resonance frequency and impedance of the source, the cavity is provided with

a coupUng shde and tuning screw. To faciUtate tuning a RF power meter is

placed between the source and cavity. When the resonance frequency of the

cavity is tuned to the frequency of the diathermy unit and the impedance is then matched, the power reflected from the cavity is at a minimum.

209
ATOMIC ABSORPTION SPECTROSCOPY

LIGHT CHOPPER

SOURCE

POWER

SUPPLY —Iphotodetector]

MONOCHROMATOR

NEBULIZER- FLAME

lOCK-IN

amplifier

DIGITAL read-out recorder

FlG.l-Schematic diagram ofatomic absorption instrument.

FIG. 2-Evenson cavity and lamp.

The major objection to the electrodeless discharge lamp as a source for

absorption measurements is the instabihty or drift of the source. The effect

of intensity and stability of lead lamps have been studied as a function of

argon pressure as shown in Table 2. Lamps prepared for arsenic, bismuth,

copper, lithium, sodium, and zinc have exliibited similar characteristics.

TABLE 2-Effect of argon pressure in lead electrodeless discharge lamps.

Aigon Pressure, Intensity, Drift, torr arbitrary units %/min

<0.1 0

0.2 250 < 1 0.4 1100 1 1.0 200 < 1 2.0 1 5 3.0 1 5

210
RAINS ON CHEMICAL ASPECTS

A comparison of the intensity of electrodeless discharge lamps and

hollow-cathode lamps is given in Table 3. In nearly every case, the

electrodeless lamp was more intense than the hollow-cathode lamp, but in no

case was the absorption greater (see Fig. 3). The lower sensitivity for the

electrodeless lamp was attributed to self reversal. Measurement of the hne

width of the sodium lamp with an interferometer showed that self reversal

occurred, as shown in Table 4, for the sodium resonance hne at 5890 X when

the power of the RF generator exceeded 15 W. To ehminate the Hne reversal

of sodium, the RF generator is operated at a power of 10 W, and the air to the lamp is preheated 180 to 200 C. This approach is found to increase the

intensity of the lamps without any loss of absorption.

TABLE 3-Companson of intensity of electrodeless discharge and hollow-cathode lamps.

Electrodeless Hollow Cathode Discharge

Wavelength, Current, Power,

Element A mA 1 W I

Ag 3280 12 1100 30 1000

Bi 3068 10 400 30 200

Cu 3247 12 2300 30 8400

Fe 2483 30 200 30 200

Hg 2537 ... ... 30 5400

Na 5890 15 640 20' 900

Pb 2833 12 96 30 1100

Zn 2138 12 14 30 530

FIG. 3 -Comparison of sensitivities of hollow-cathode and electrodeless discharge

lamps.

Nebulizer-Burner System

The nebuhzer-burner system is probably the most important component in any absorption measurement. It is the function of this system to convert the

analyte in the sample solution to atomic vapor. This process includes the

211
ATOMIC ABSORPTION SPECTROSCOPY

TABLE 4-Absorbance for sodium with electrodeless discharge lamp.

Burner, triflame-air/C2H2

Wavelength, X: 5890

Na, /Lig/ml: 1.0

RF Generator, W Line Width, %" Absorbance

5 0.170

6 0.035 0.165

10 0.04 0.160

15 0.06 0.130

20 0.08 0.100

40 0.020

^Interferometer measurement, resolution: 0.012 X.

following steps: (1) dispersion of solution in the form of small droplets, (2)

evaporation to dryness, (3) decomposition of salt or oxide, and (4) formation

of atomic vapor. The extent to which these occur will depend upon the

matrix, type of nebulizer burner, oxidant, and fuel gases. The most commonly used system for the production of atomic vapor is the premixed gas or laminar flow burner. The solution is sprayed into a spray

chamber where the coarse and fine droplets are separated. The fine droplets

are mixed with the flame gases; they then pass into the flame where they are vaporized.

The major advantages of the laminar flow burner over the diffusion or

total consumption burner in absorption are increased sensitivity and a minimum of interference. Since only atoms in the ground state will absorb

energy at the resonance wavelength, the flame should be of sufficient

temperature to dissociate the molecules, but not so high as to produce ions.

Many elements in relatively cool flames such as argon (entrained air)-

hydrogen or air -propane are fully dissociated, others, such as the alkaUne

earth metals, require air-acetylene flame. Aluminum, silicon, and titanium

require a fuel-rich nitrous oxide-acetylene flame. Also, the length of time that

the atoms remain in the observed area of the flame will depend upon the gas

velocity. The flame front propagation rate for an oxygen-acetylene flame in a diffusion burner is very high (approximately 1200 cm/s); therefore, the

residence time of the atoms is very short. However, the flame velocity for the

air-acetylene or nitrous oxide-acetylene flame is 160 or 180 cm/s, respectively. The decrease in flame velocity will permit a longer residence time for the

atoms in the observed area and give an increase in sensitivity with a minimum

number of interferences. The temperature and burning velocities for various

gas mixtures are given in Table 5 [15-1 7]

.

Another method for producing atomic vapor is by use of a hightemperature furnace. Woodriff and Ramelow [18] report the use of a high-temperature graphite-tube resistance furnace capable of attaining a temperature of 3000 C. The solution is aspirated with argon into the furnace

212
RAINS ON CHEMICAL ASPECTS

TABLE 5 -Temperature and burning velocities of oxidant-fuel gases used in absorption.

Oxidant Fuel

Temperature

Measured, deg C

Burning Velocity,

cm/s

Air-natural gas Air-C2H2 . , Air-H2 . . . . 02-natural gas. O2-C2H2 . . O2-H2

N2O-C2H2 . . N2O-H2 . . .

1700 to 1900

2125 to 2400

2000 to 2050

2740

3060 to 3135

2550 to 2700

2867

2607

55

160

320 to 440

1100

2000

180

380

where the free atoms are produced. This system was reported to be useful for

eliminating those interferences due to refractory elements. The coupUng of an

ultrasonic nebuhzer with a high-temperature furnace undoubtedly would

provide optimum conditions for the determination of trace elements in the

presence of refractory oxides.

Optical Arrangement and Associated Electronics

Of equal importance in atomic absorption are the optical arrangement,

detection system, and the overall electronic components. These components

are essential and must be evaluated in any discussion of atomic absorption.

The monochromator should have sufficient resolution to separate the spectral

line under investigation from other spectral lines. Any emitted Une not

resolved by the monochromator, whether from the filler gas or from any

impurity in the metal in the hollow-cathode lamp, can produce a severe loss of sensitivity. Therefore, a spectrum of the desired region should be prepared

for each source of radiation. There are some special instances where a narrow

slit width is required to avoid this type of interference.

To detect the radiation a multipUer phototube with its associated power

supply and measuring system is essential for high sensitivity and precision.

Because the detection limit is affected directly by the stability of the

measuring circuit, the multiplier phototube, power supply, amplifier, and

readout system must be sensitive and stable. The most commonly used

phototubes for absorption as well as emission are given in Table 6.

Interferences

Chemical interferences occur in varying degrees for all elements in atomic

absorption spectrometry. A chemical interference is defined as any reaction

which affects the analyte or the element to be determined. This may occur in the liquid, sohd, or gaseous phase of the flame. Any cation or anion present

in the sample which will form compounds with the analyte and are not

dissociated easily will reduce the absorption. An example of this type of

interference which usually is cited is the effect of aluminum, phosphate.

213
ATOMIC ABSORPTION SPECTROSCOPY

TABLE 6 -Multiplier phototubes commonly used in absorption measurements.

Multiplier Spectral

Phototube Response Range, X

R-106 S-19 1600 to 6500

RCA-1P21 SA 3000 to 6000

RCA-1P28 S-5 2000 to 6500

RCA-6217 S-10 3300 to 8000

RCA-7102 S-1 4000 to 10000

EMl-6256 S-13 1650 to 6700

EMI-9558 Q S-20 1650 to 8500

FW-118 S-1 3200 to 10000

Dumont 6291 S-11 3200 to 6000

NOTE- R; Hamamatsu TV Co., Ltd., Hamamatsu-City, Japan.

RCA: Radio Corporation of America, Harrison, N. J. EMI: E.M.I. Electronics Ltd., Middlesex, England.

FW: International Telephone and Telegraph Corporation, Fort Wayne, Ind. Dumont: Alien B. Dumont Laboratories, Inc., Clifton, N. J.

titanium, or silica on the determination of alkaline-earth metals. The effects

of aluminum and phosphate on magnesium for the total consumption and

laminar flow burners are given in Figs. 4 and 5. The mechanism of this interference is discussed in great detail by many

investigators [19-21]. In general, most of the workers agree that the

suppression is caused by an interferent which affects the vaporization rate of

the aUcaUne-earth element such as magnesium from the solid to the gaseous

state. To eliminate or control chemical interferences in atomic absorption, it is

essential that various parameters be optimized. These include the type of

burner (laminar flow or total consumption), oxidant fuel, flame temperature,

flame region, and sample medium. The premixed gas or laminar flow burner is used primarily in absorption. This system suffers less from chemical

interferences than the diffusion or total consumption burner because smaller

droplets are introduced into the flame and the burning velocity of the

oxidant-fuel mixture is low.

The flame temperature and the flame region play a major role in the extent

of the interference. With a fuel-rich nitrous oxide-acetylene flame the

chemical interference of aluminum, titanium, or silicon on the alkaline-earth

metals is removed. However, this high-temperature flame usually introduces

another type of interference which is due to ionization of the analyte. The

alkali and alkaHne-earth metals are especially susceptible to ionization, but

this is controlled by the addition of a cation having a similar or lower

ionization potential than that of the analyte.

Another way to ehminate the suppression of the analyte due to formation

of involatile compounds is by the addition of releasing or protective chelating

agents. Releasing or protective chelating agents are defined as substances

214
RAINS ON CHEMICAL ASPECTS

FIG. A-Effect of phosphate on magnesium in absorption for three different burners: A and B -air-acetylene flame; C-oxygen-hydrogen. (a) H3PO4 (b) glycerol, 10 (v/vj%

plus HCIO4 0.1 M (c) lanthanum, 1 mg/ml (d) (NH4)2HP04 (3) (NH4)2HP04 plus

glycerol, 10 (v/v)%and HCIO4, 0.1 M.

215
ATOMIC ABSORPTION SPECTROSCOPY

0.1 10 10 100 1000- ALUMINUM, Mg/mi

FIG. S -Effect of aluminum on magnesium in absorption with and without releasing

agents: (a) aluminum (b) glycerol, 10 (v/v)% plus HCIO4, 0.1 M (c) lanthanum, 1 mg/ml

(d) EDTA, 0.05 M (e) lanthanum, 1 mg/ml; glycerol, 10 (v/v)%and HCIO4, 0.1 M.

which, when added in sufficient quantity in the presence of an interferent,

wUI restore the absorption of the analyte to its original value. The manner in

which these chelates and releasing agents prevent condensed phase type of

interference or an interference that occurs in the soUd state is not understood

clearly. Compounds such as ethylenediaminetetraacetic acid (EDTA) are known to form strong complexes with the alkaline-earth metals, and will

preferentially complex the alkaline-earth metals even in' the presence of

phosphate or aluminum. However, the similar action of the simple sugars and

glycerol has not been explained. A list of releasing and protective chelating

agents is given in Table 7. The effects of phosphate in the analysis for magnesium by atomic

absorption with and without releasing and protective chelating agents for

three different burner systems are given in Fig. 4. The suppression of

magnesium is far greater with the total consumption burner (Fig. 4C) than

with the laminar flow burner. This is due to the high-burning velocity of the

oxygen-hydrogen flame and the larger droplet size produced by this type of

burner. Also, a solution 10 percent in glycerol and O.l-M perchloric acid

(HCIO4) was more effective than lanthanum in releasing magnesium. With the

laminar flow burner, phosphate as orthophosphate (H3PO4) is not a major

interferent; however, ammonium orthophosphate (NH4)2HP04) or H3PO4

adjusted to a pH of 4 to 5 interferes strongly. In a solution containing

glycerol [10 volume/volume (v/v) percent] and 0.1 M in perchloric acid,

216
RAINS ON CHEMICAL ASPECTS

TABLE 7 -Releasing and protective chelating agents used in absorption.

Reagent Type Interferent Analyte Ref

La releaser Al, Si, P04^7S04^~ Mg, Ca 22,23

Sr releaser Al, B, Te, Se, NO3 Mg, Ca 24,23,25

P04^TS04^~2-

Mg . releaser Al, Si, P04^ , SO4 ^7

Ca releaser A1,P04^~ Mg, Sr 27

Glycerol, HCIO4 • • • protector [Al, Fe, Th, rare earths, [Mg, Ca, Sr,Ba] 28, 29

Si,B,Cr,Ti,P04

S04^ '! NH4CI protector Al Na,Cr 30

NH4CI protector Sr,Ca,Ba,P04^ 7 S04^~ Mo 31

Ethylene glycol . . . . protector P04^^ Ca 29

Mannitol protector P04^~ Ca 29

Dextrose, sucrose . . . protector PO4 Ca, Sr 29

EDTA chelate [Se, Te, B^ Al, _Si,_ Mg, Ca 26

N03^~,P04^~,S04^"

8-hydroxyquinoline . chelate Al Mg, Ca 32

recovery of 1 /^g/ml of magnesium was complete in the presence of 0.05-M

phosphate. By increasing the acidity to a pH of 0.5, complete recovery was

obtained up to 0.5 -Af phosphate.

Lanthanum is used frequently as a releasing agent for the alkaline-earth

metals in the presence of aluminum or phosphate. The releasing action of

lanthanum has been postulated to be due to the high heat of formation of

lanthanum phosphate [22] . Dinnin [33] suggested that the releasing action

is a mass action effect,

Ax + By::!Ay^-Bx

where:

A = analyte,

B = releasing agent,

X = anion which forms a refractory compound, and

y = anion which forms an easily dissociated compound.

As the concentration of B is increased, the reaction will be forced toward

Bx, thus leaving Ay free to dissociate at a lower temperature. This reaction

alone will not explain why some cations wiU act as releasing agents and others

will not. Since the process which occurs within the flame gases is rate

controlled, as indicated by studies with low-velocity flames and variable

heights within the flame, the correct explanation is most probably a combination of the two mechanisms.

Lanthanum was found to be more effective than glycerol or EDTA for the

determination of magnesium or calcium in the presence of aluminum, as shown in Fig. 5. However, a weight ratio of 10:1 for lanthanum-to217
ATOMIC ABSORPTION SPECTROSCOPY

aluminum was required to provide complete recoveries of magnesium or calcium. The high concentration of lanthanum required to release the

alkahne-earth metals in the presence of aluminum will often produce burner

clogging. If, however, glycerol is used in conjunction with the lanthanum, a lanthanum-to-aluminum ratio of only 2:1 is necessary for complete release of

alkahne-earth metals. Also, the addition of a 10 percent solution of glycerol to the standards, as well as to the unknown, has the advantage of improving

the burning characteristics and controlUng the viscosity, thus increasing the

accuracy and precision of the method. Rubeska and Moldan [21] have

observed that the releasing effect of lanthanum is facihtated by the addition

of 8-hydroxyquinohne. Instead of requiring a 1 percent solution of

lanthanum to overcome the suppression of aluminum, a 0.2 percent solution

of lanthanum in 1 percent 8-hydroxyquinohne suffices. Tliis corresponds to a lanthanum/aluminum molar ratio of 2. Similar results have been reported

using a combination of EDTA and lanthanum [34]

.

Role of Organic Solvent

The sensitivity and detection limit can be improved with organic solvents.

The increase in sensitivity is not as great in absorption as reported for flame

emission; however, the possibihty of concentrating the analyte by solvent

extraction tecliniques permits the determination of many trace elements.

Also, chemical, as well as physical interferences, often can be removed and

higher accuracy obtained. In absorption the addition of miscible organic

solvents such as alcohols, ketones, and organic acids to an aqueous solution is of limited value. Usually the increase in sensitivity is less than fivefold, and

the analyte is diluted inevitably by the addition of the solvent.

By the judicious selection of an organic ligand and solvent, many trace elements are removed from the sample matrix and then determined by atomic

absorption spectrometry. Table 8 lists many of the organic ligands and

solvents used in emission and absorption spectrometry.

Ammonium pyrrohdine dithiocarbamate (APDC), diethylammonium

diethyldithiocarbamate (DDDC), thenoyltrifluoroacetone (TTA), and

dithizone are used extensively in chelate extractions for absorption as well as flame emission. Many of these organic ligands are made selective by

controlling the acidity and the use of masking agents. The chelates formed

can be extracted readily with a variety of solvents.

The most frequently used organic solvent is methyl isobutyl ketone

(MIBK). Benzene and kerosene are inferior to MIBK because they produce a

large luminous and smoky flame. Carbon tetrachloride or chloroform can be

used; however, the enhancement is considerably less than with MIBK. Apart

from the fact that the solvent should extract the chelate, it should burn and

provide a stable flame. In addition, the physical properties of the solvent such

as viscosity, surface tension, boihng point, and mutual solubility in an aqueous medium must be taken into account. It should not produce toxic

products during combustion or give a high background in the flame.

218
RAINS ON CHEMICAL ASPECTS

TABLE S-Organic Ugands and solvents used in solvent extraction for atomic absorption

and flame emission spectrometry.

Organic

Ligand Solvent Element Material Ref

HAA MIBK Be air sample 35

APDC MIBK Co,Cu,Fe,Ni,

Pb,Zn

sea water 36

Cu, Zn plants, soils 37

Pb,Bi,Hg, Ni,Sb urine 38

Co,Cu,Pb,Cd,Fe, KCl 39

Mn Mi

Ld, 1 1, Hg blood, urme 40

Cupferron . . . MIBK Al

V Mg alloys,limestone 4142

DDC MIBK le 43

CHCI3 Cu, Nl, Mn Al alloys 44

DDDC .... CHCI3 y^Uy ivin 45

As cast iron, Se 46

Dithizone . . . CHCI3 Hg 47

Oxine . . . MIBK Mg natural water,brine 48

Butanol Mo Cb and Ta 49

3-methy 1- 1 -butanol Ca H3PO4 50

MIBK Ca, Mg, Fe,Ni Al 51

Naphthenic

Acid .... MIBK Ca 52

Salicylic acid,

di-n- butylamine. MIBK Ag 53

TTA MIBK Cu zinc 8

Al minerals,bronzes,

Zn-base aUoys

41

Fe,Cu,Pb,Bi 8

rare earths 54

Sb,Fe,Mo,Se, 55

Sn, Au, In, Pb,Cb

NOTE- HAA = acetylacetone

APDC = ammonium pyrrolidine dithiocarbamate

Cupferron = N-nitrosophenylhydroxylamine

DDC = sodium diethyldithiocarbamate

DDDC = diethylammonium diethyldithiocarbamate

Dithizone = diphenylthiocarbazone

Oxine = 8-hydroxyquinoIine

TTA = 2-thenoyltrifluoroacetone

The major objection to MIBK is the increased solubility in a highly acid

medium. By using a 3:1 mixture of MIBK-cyclohexane the solubility in the

aqueous phase is decreased without any significant change in the extraction

of the various chelates or burning characteristics of the ketone.

219
ATOMIC ABSORPTION SPECTROSCOPY

Preparation of Standards, Samples, and Evaluation of Data

The standard stock solutions are prepared best in hydrochloric or nitric

acids from liigh-purity metals or nonhygroscopic reagent grade chemicals. If

possible, sulfuric or phosphoric acids should be avoided as they produce an

adverse effect on many elements. The stock solutions are prepared generally

at concentrations of 1.000, 5.000, or 10.000 mg/ml. The standards are prepared by an appropriate dilution from the stock solution as needed. The

final anion concentrations of the standards and samples should be approximately the same. The final accuracy of the analysis may depend upon the

care in the preparation of the standard solutions.

In atomic absorption a curve is prepared to cover the appropriate

concentration range. Usually, this means the preparation of standards which

produce an absorption of 0 to 80 percent. The correct method for plotting

data derived from an atomic absorption instrument equipped with a linear

readout system is to convert percent absorption to absorbance and plot the

absorbance on linear coordinates against concentration. The following

relationsliip is used to convert absorption values to absorbance:

absorbance = log (1 00/%T) = 2 - log %T

where %T = 100 — % absorption.

Frequently, the curve is nonlinear especially at high absorption values.

These curves can be used; however, the number of standards should be

increased especially in the nonlinear portion of the curve. If the nonlinear

portion of the curve is used it is essential that the concentration and the

absorbance be very carefully read.

The sample should be prepared by a standard laboratory procedure. Many

of the high refractory oxides and silicates are dissolved with hydrofluoric acid

in a Teflon Uned bomb [56] . By this technique the silicates and aluminates

are dissolved within 4 h at a temperature of 120 to 140 C. The excess fluoride

is removed by fuming the sample with HCIO4. Tliis procedure eliminates the

need for a basic fusion to dissolve the refractory oxides and silicates which

contribute to high concentration of undesirable salts. The final choice and

concentration of acid, such as HCIO4, hydrochloric acid (HCl) or nitric acid

(HNO3) are important since the acid concentration may affect the rate of

atomization and the production of atomic vapor.

Frequently, a separation or preconcentration step is required before

making the final measurement. Ion-exchange resins are helpful in separating

trace elements from the matrix; however, ion-exchange procedures have a tendency to be time consuming. Preconcentration by coprecipitation is usually fast and very effective. An excellent example is the coprecipitation of

barium with lead chromate. Usually, a solvent extraction procedure is the

best method for separation and preconcentration.

To check interferences and to determine the need for separations or the

use of releasing agents, the method of standard additions can be utilized.

220
RAINS ON CHEMICAL ASPECTS

While there are only two basic methods (single and multiple) for standard

addition, the method has a variety of names such as the standard addition

method, mixture, incremental addition, autostandarization, and method of

additions. The multiple addition method generally is preferred to the single

addition; however, if improperly used the multiple method can lead to a

serious error. The curve must be linear, and corrections must be made for any

absorption which results from scattered Ught or reagents. This method

requires the transfer of three to five test portions of the sample to volumetric

flasks. One flask is diluted to volume, and incremental amounts of the

standard solution containing 1 to 4 times the concentration of the analyte are added to the remaining flasks and then diluted to volume. The absorbance is measured and plotted against the concentration on Unear graph paper. The

concentration is determined graphically by extrapolating the Unear curve through the Y axis until the Une intersects the X or concentration axis. The

concentration of the unknown is determined from the point of intersection

[-57]. In the single addition method dupUcate aliquots of the sample are transferred to flasks, A and B. An aHquot of a standard from a stock solution

is added to B and both solutions are then diluted to volume. The

concentrations of the standard and the analyte should be approximately

equal. If there is a suppression or enhancement due to the matrix, the

standard addition will be affected in a similar manner. The absorptions of

Samples A and B are determined, and their concentrations are determined

from a cahbration curve. If the recovery was either high or low, the true concentration is determined by the following equation \58\

.

S

X -=C

Y-X

where:

X =Mg per ml of analyte in unknown (A) as determined by the calibration

curve, 5 = standard added, Mg per ml,

Y = g per ml of analyte found in unknown (B) containing the standard

addition as determined by the cahbration curve, and

C = corrected concentration of analyte, /xg per ml.

Advantages of the single addition over the multiple addition include: (1)

only two test portions are required, (2) the method may serve as a dupUcate

determination when there is no interference, and (3) it is not essential to

prepare the standards for the cahbration curve in the same matrix as in the

unknown sample.

Acknowledgments

The author is indebted to O. Menis and J. I. Shultz for their assistance in

the preparation of this manuscript.

221
ATOMIC ABSORPTION SPECTROSCOPY

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Photometry," ^/zfl/yrica Chimica Acta, Vol. 37, 1967, pp. 421-428.

[22] Yofe, J. and Finkelstein, R., "Eliniination of Interferences in Flame Photometric

Determination of Calcium in the Presence of Phosphate and Sulphate," yl/!fl/>'rtC(2 Chimica Acta, VoL 19, 1958, pp. 166-173.

222
RAINS ON CHEMICAL ASPECTS

[23] Dickson, R. E. and Johnson, C. M., "Interferences Associated with the Determination of Calcium by Atomic Absorption," Applied Spectroscopy., Vol.

20, 1966, pp. 214-218.

[24] Elwell, W. T. and Gidley, J. A. F., Atomic Absorption Spectrophotometry, 2nd

ed., Pergamon, London, 1966.

[25] David, D. J., 'The Determination of Exchangeable Sodium, Potassium, Calcium,

and Magnesium in Soil by Atomic Absorption Spectrophotometry ,"y4«fl/>'s?, Vol.

85, 1960, pp. 495-503.

[26] Ramakrishna, T. V., Robinson, J. W., and West, P. W., "The Determination of

Calcium and Magnesium by Atomic Absorption Spectroscopy," /iMfl^vriciZ Chimica

Acta, Vol. 36, 1966, pp. 57-64.

[27] RocchiccioU, C. and Townshend, A., "Some Interferences in the Atomic

Absorption Spectrophotometry of Calcium," Analytica Chimica Acta, Vol. 41,

1968, pp. 93-98.

[28] Rains, T. C, Zittel, H. E., and Ferguson, M., "Flame Spectrophotometric

Determination of Micro Concentrations of Strontium in Calcareous Material,"

Analytical Chemistry, Vol. 34, 1962, pp. 778-781.

[29] Rains, T. C, Zittel, H. E., and Ferguson, M., "Elimination of Anionic

Interferences in Flame Spectrophotometric Determination of Calcium," Talanta,

Vol. 10, 1963, pp. 367-374.

[30] Barnes, L., Jr., "Determination of Chromium in Low AUoy Steels by Atomic

Absorption Spectrometry," /Ina/j^ftca/ Chemistry, Vol 38, 1966, pp. 1083-1085.

[31] David, D. J., 'The Suppression of Some Interferences in the Determination of

Molybdenum by Atomic-Absorption Spectroscopy in an Air-Acetylene Flame,"

Analyst. Vol. 93, 1968, pp. 79-82.

[32] Marshall, G. B. and West, T. S., "Determination of Traces of Calcium, Magnesium,

Iron, and Nickel in Aluminum Salts by Atomic-Absorption Spectrophotometry

with a Microwave-Excited Source and HoUow-Cathode Lamps," Talanta, Vol. 14,

1967, pp. 823-831.

[33] Dinnin, J. I., "Releasing Effects in Flame Photometry Determination of Calcium,"

Analytical Chemistry, Vol. 32, 1960, pp. 1475-1480.

[34] Adams, P. B. and Passmore, W. O., "Critical Factors in the Determination of the Alkaline Earth Elements in Glass by Atomic Absorption Spectrometry,"

Analytical Chemistry, Vol. 38, 1966, pp. 630-633.

[35] Bokowski, D. L., "Rapid Determination of BeryUium by a Direct Reading Atomic

Absorption Spectrophotometer," Atomic Absorption Newsletter, Vol. 6, 1967,

pp. 97-100.

[36] Brooks, R. R., Presley, B. J., and Kaplan, I. R., "APDC-MIBK Extraction System

for the Determination of Trace Elements in Saline Waters by Atomic-Absorption

Spectrophotometry," Talanta, Vol. 14, 1967, pp. 809-816.

[37] Allan, J. E., 'The Determination of Copper by Atomic Absorption Spectrophotometry," Specfroc/i/wzca /Icto, Vol. 17, 1961, pp. 459-466.

[38] WnUs, J. B., "Determination of Lead and Other Heavy Metals in Urine by Atomic

Absorption Spectroscopy," Chemistry, VoL 34, 1962, pp. 614-617.

[39] Sprague, S. and Slavin, W., "Determination of Very Small Amounts of Copper and

Lead in KCl by Organic Extraction and Atomic Absorption Spectrophotometry,"

Atomic Absorption Newsletter, No. 20, May, 1964.

[40] Berman, E., "Determination of Cadmium, Thallium and Mercury in Biological Materials by Atomic Ahsox^iion" Atomic Absorption Newsletter, Vol. 6, 1967,

pp. 57-60.

[41] Eshelman, H. C. et al, "Extraction and Flame Spectrophotometric Determination

of k\\xm\num,'" Analytical Chemistry, Vol. 31, 1959, pp. 183-187.

[42] Sachdev, S. L., Robinson, J. W., and West, P. W., "Determination of Vanadium by

Atomic Absorption Spectrometry," Analytica Chimica Acta, Vol. 37, 1967, pp.

12-19.

[43] Dean, J. A. and Simms, J. C, "Flame Spectrometric Determination of Tellurium,"

Analytical Chemistry, VoL 35, 1963, pp. 699-702.

[44] Dean, J. A. and Cain, C, "Flame Spectrophotometric Determination of Copper,

Nickel, and Manganese in Aluminum-Base k]loys" Analytical Chemistry, Vol. 29,

1957, pp. 530-532.

223
ATOMIC ABSORPTION SPECTROSCOPY

Wyatt, P. F., "Diethylammonium Diethyldithiocarbamate for the Separation and

Determination of Small Amounts of Metals," Analyst, Vol, 78, 1953, pp. 656-661.

Menis, O. and Rains, T. C, "Determination of Arsenic by Atomic Absorption

Spectrometry with an Electrodeless Discharge Lamp as a Source of ' Radiation'"

Analytical Chemistry, submitted for publication.

Feldman, C. and Dhumward, R., "An Atomic Absorption Tube for Use with an

Atomizer-Burner: Application to the Determination of Mercury," U.S. Atomic

Energy Commission TID-7655, 1962, pp. 379-389.

Suzuki, M., Yanagisawa, M., and Takeucki, T., "Determination of Magnesium by

Atomic Absorption Spectrophotometry after Extraction," Talanta, Vol. 12, 1965,

pp. 989-996.

Kirkbright, G. P., Peters, M. K., and West, T. S., "Determination of Small

Amounts of Molybdenum in Niobium and Tantalum by Atomic-Absorption

Spectroscopy in a Nitrous Oxide-Acetylene ¥\nmt" Analyst, Vol. 91, 1966, pp. 705-708.

Yanagisawa, M., Suzuki, M., and Takeuchi, "Determination of Traces of Calcium

in Phosphoric Acid and Its Salts by Atomic Absorption Spectrometry," Talanta,

Vol. 14, 1967, pp. 933-936.

Marshall, G. B. and West,T. S., "Determination of Traces of Calcium, Magnesium,

Iron, and Nickel in Aluminum Salts by Atomic Absorption Spectrophotometry

with a Microwave-Excited Source and Hollow-Cathode Lamps," Talanta, Vol. 14,

1967, pp. 823-831.

Szego, L. and Calvert, J. M., "Naphthenic Acid Extraction of Calcium for Determination by Atomic Absorption Spectrophotometry," Analytica Chimica

Acta, VoL42, 1968, pp. 1-6. Belcher, R., Dagnall, R. M., and West, T. S., "An Examination of the Atomic

Absorption Spectroscopy of Silver," Talanta, Vol. 11, 1964, pp. 1257-1263.

Rains, T. C, House, H. P., and Menis, O., "Flame Spectra of Sc, Y, and Rare-Earth

Elements," >4nfl/>'ricfl Chimica Acta, Vol. 22, 1960, pp. 315-327.

Menis, O. and Rains, T. C, "Sensitivity, Detection Limits, Precision and Accuracy

in Flame Emission and Atomic Absorption Spectrometry," chapter in Analytical

Flame Spectroscopy, Selected Topics, Mavrodineanu, R., ed., Centrix, Eindhoven,

1969.

Bernas, B., "A New Method for Decomposition and Comprehensive Analysis of

Silicates by Atomic Absorption Spectrometry," Analytical Chemistry, Vol. 40,

1968, pp. 1682-1688.

WiUard, H. H., Merritt, L. L., Jr., and Dean, J. A., Instrumental Methods of

Analysis, 4th ed., Van Nostrand, Princeton, N. J., 1965, pp. 342-344.

Dean, J. A.., Flame Photometry, McGraw-Hill, New York, 1960.

224
APPENDIX XII

Reprint from

Soecial Technical Publication 564

American Society for Testing and Materials

1916 Race Street, Philadelphia, Pa. 19103

1974

T. C. Rains^

Atomic Absorption Spectrometry—

General Considerations for tine

Application of Experimental

Techniques

REFERENCE: Rains, T. C, "Atomic Absorption Spectrometry—General Considerations for the Application of Experimental Techniques," Some Fundamentals

of Analytical Chemistry, ASTM STP 564, American Society for Testing and Materials, 1974, pp. 50-67.

ABSTRACT: In applying atomic absorption spectrometry to an analytical problem,

the analyst must have an understanding of the basic instrumental components. The

essential components of an atomic absorption instrument consist of a primary

source of radiation, a means of producing atomic vapor from the analyte, wavelength selection, signal detection, and readout. While several types of radiation sources are available, hollow cathode lamps are the primary choice for most workers in the field. The production of atomic vapor of the analyte is probably the most important

parameter in atomic absorption spectrometry. The selection of the oxidant-fuei or nonflame method of producing neutral atoms will depend upon the concentration

of analyte and the matrices. While the analyst has a wide choice of methods at his disposal, an understanding of the problems related to the production of atomic vapor is essential to obtain the maximum precision and accuracy. Also, inter- ferences, sample preparation, and methods for evaluation of data are discussed.

KEY WORDS: chemical analysis, accuracy, atomic spectroscopy, evaluation, inter- ferences, light sources, precision, atomic vapor

Atomic absorption spectrometry (AAS) is a unique anaytical technique

for the determination of metallic elements. To take advantage of the

method, it is essential that the analyst become familiar with the basic

components of his instrument and have a working knowledge of the

chemical processes ocurring in the production of atomic vapor. The results

obtained by AAS, for the most part, are dependent upon the ability of the

analyst to apply many of the experimental techniques which have been

' Research chemist, Analytical Chemistry Division, National Bureau of Standards, Washington, D.C. 20234.

225
RAINS ON ATOMIC ABSORPTION SPECTROMETRY

developed over the years. The principles and practices discussed here are not the only techniques that can be applied but are intended only to act as guidelines.

Essential Features of Atomic Absorption Spectrometry

Sources of Radiation

The basic components of all atomic absorption spectrometers are depicted in Fig. 1. The function of the primary source of radiation in AAS

Primary Source Production of Wavelength Radiation

of Radiation Atomic Vapor Isolator Detector Readout (HCL) (Flame or (Monochromator (Multiplier ^ (Digital or Non-Flame) or Filter) Phototube) Recorder)

FIG. 1Essential features of an atomic absorption spectrometer.

is to generate radiation at the wavelength at which absorption is to be

measured. This source should radiate the wavelength of the resonance

lines of the analyte without interference from other spectral lines. The

resonance lines should be sharp with very low background. At the present

state of the art, a number of light sources are commercially available

which fulfill these requirements in AAS, for example, hollow cathode,

vapor discharge, and electrodeless discharge lamps. Hollow cathode lamps

(HCL) are available for some 60 elements and, in general, are satisfactory

for most AAS work. Although the operation of hollow cathode lamps is extremely simple, one may sometimes obtain better results by deviating

from the recommended conditions. The choice of operating conditions

depends to a great extent on whether the operator is searching for a maximum precision at concentrations near the detection limit or over a wide analytical range. To determine the optimum conditions, it is necessary to examine the effect of absorbance of an analyte by varying the

lamp current and the spectral bandpass of the monochromator. The

principal effect of increasing lamp current is to increase the light

intensity; however, this may lead to an increase in line width as a result of

self-absorption or Doppler broadening. These effects are discussed in great

detail by Butler and Brink [1].^ In general, the best sensitivity is obtained

with a low current; however, a poor signal-to-noise ratio may be

experienced because of the resultant low intensity. As a result one should

balance the benefits of maximum signal-to-noise against sensitivity. Also,

the spectrum produced by the lamp should be scanned over the area of

analytical importance and all lines identified. This serves as a warning of

possible spectral interference due to the carrier gas or impurities in

^ The italic numbers in brackets refer to the list of references appended to this paper.

226
SOME FUNDAMENTALS OF ANALYTICAL CHEMISTRY

cathode material and as a means of correcting light scatter with a nonabsorbing line.

Batchelor [2] has reviewed the present state of development of the HCL.

The major emphasis by manufacturers has been on improvement in

stability and greater light intensity by electrode design and control of the

carrier gas pressure.

Other radiation sources used include vapor discharge lamps and

electrodeless discharge lamps. Vapor discharge lamps are known for their

high intensity; however, self-absorption is inherently present in these

lamps which limits their sensitivity.

Electrodeless discharge lamps (EDL) were originally described by

Jackson [3]. Meggers and Westfall [4] reported the construction of an EDL

using "^Hg as a spectroscopic source. With the single element isotope, the

lamp gave very sharp spectral lines which were measured to eight significant figures. The subsequent development of lamps using metallic halides

[5-9] has made available light sources for wavelength calibration, atomic

absorption, and fluorescence spectrometry. While EDL's are known for

their high spectral intensity, they lack reproducibility and ease of

operation. The difficulty encountered with lamp drift has been overcome

by controlled heating of the lamp [10,11]. More recently two commercial

manufacturers have made available EDL's having power supplies which

operated at radio-frequencies of 29 and 951 mHz. These lamps for a few

elements, for example, arsenic and selenium, produce relatively high

emission signals with excellent stability. However, the Hmiting factor for

these sources is the number of elements which can be excited at these

frequencies.

Sources ofAtomic Vapor Production

Historically a flame was the original means of producing atomic vapor

for AAS. The flame is still the basic source for the vast majority of AAS

measurements, and it will probably remain so for the foreseeable future.

While a lot of development work is in progress with nonflame techniques

(graphite furnace, carbon rod, and tantalum strip) they are not expected

to replace the flame except where additional sensitivity is needed.

There are two basic type of nebulizer burners. When the liquid is sprayed into a mixing chamber and the droplets are mixed with the

,combustion gas, the process is called indirect nebulization and is referred

to as a premixed chamber or laminar flow burner system. In the second

type the aerosol passes directly into the flame where the fuel and support

gas are mixed. This is the so-called turbulent (sprayer) burner. Of the two

types of nebulizer-burners, the premixed system is used generally in all commercial systems. Schematics of each type are given by Herrmann [12].

The aerosol produced by an indirect nebulizer is mixed in the spray

chamber with the combustible gas before entering the burner. This spray

227
RAINS ON ATOMIC ABSORPTION SPECTROMETRY

chamber also performs another function by removing the large droplets

before they reach the flame. To achieve optimum sensitivity in AAS the

spray chamber should produce uniform fine aerosol particles (less than 10

^m in diameter) which are mixed with the support gas. Normally, the

efficiency of this system is less than 10 percent. The tyoical spray chamber

used in AAS has varied very little from the original design used by Walsh

[13]. This can be attributed to one of two causes, either the original design

left nothing to be desired or there has been a surprising lack of initiative

by investigators.

Various ways to improve the efficiency of atomization in a flame have

been proposed. One is the so-called "sample boat" system. In this system,

a few microliters of the sample are placed in a small tantalum boat. The

boat is mounted on a sliding device that allows it to be pushed into the

flame in a reproducible manner. The sample is first dried in the boat near

the flame. After drying, the boat is inserted into the flame, and the metal

content is rapidly atomized to form a transient atomic vapor cloud above

the boat. The radiation source is focused just above the top of the boat. A

sharp peak of absorbance versus time is recorded, and the height is proportional to the concentration of metal vaporized. This technique is most successful for the more volatile elements. The major disadvantage of

the system is the inability to reproducibly dry the sample before introduction it into the flame. Delves [14] modified the sampling boat technique

for the determination of blood lead by vaporizing samples from nickel

crucibles into a quartz absorption tube. Blood lead results were superior

with this system. Also, there were fewer interferences with increased

sensitivity.

An entirely different concept of introducing the analyte into the flame is that of hydride generation. In this method, the sample in solution is reacted in a flask with stannous chloride and potassium iodide solutions

and finally with zinc metal [15] or sodium borohydride [16] to produce

hydrides such as ASH3, H2Se, and SbH3. The gas is swept from the

solution with argon carrier gas into an argon (entrained air)-hydrogen

flame in which the hydride is readily decomposed to give the metal

(atomic vapor). The sensitivity from this technique is much greater than

for direct aspiration of the solution into a flame. For arsenic the sensitivity

is 1 ng/ml/1 percent absorption.

In summary, the major advantages of the flame as a means of

production of atomic vapor are: (a) simplicity of the technique, (b) speed

with which a determination can be made, and (c) relative little maintenance of system. The major disadvantages are: (a) a relative large

quantity of sample is required for a determination, (b) a hostile

environment is created for the production of ground state atoms, (c) large

quantities of oxidant and fuel gases are required, and (d) sensitivity is limited for many elements.

228
SOME FUNDAMENTALS OF ANALYTICAL CHEMISTRY

Nonflame Absorption Devices

A relatively new technique of producing atomic vapor is with nonflame

devices. These methods differ from flames in that an environment is provided which is more conducive to production of atoms in their ground

state. Also, a relatively small quantity of the sample (1 to 50 \A) is necessary for an analysis which gives these methods improved sensitivity over flame methods. However, high precision is more difficult to attain.

Nonflame methods can be grouped into categories such as: {a) cathodic

sputtering, (b) graphite furnace or carbon rod devices, (c) tantalum ribbon,

and id) cold vapor. Cathodic sputtering was first described by Gatehouse

and Walsh [17\. It consists of making the sample the cathode in a demountable hollow cathode lamp. Atomization of the material of the

cathode is achieved by the process of cathodic sputtering. A major

disadvantage of the technique is the need to prepare a cathode from the

sample which tends to restrict it to metallic samples. Dogan et al [18] has

demonstrated the feasibility of mixing a nonmetallic sample with a

metallic powder and pressing the mixture into pellets. These pellets are buffed and used as the cathode in a demountable lamp. This technique

has a potential advantage over other nonflame devices, since it can be

used in conjunction with a vacuum monochromator to determine those

elements such as carbon, sulphur, phosphorus, and the halides which emit

in the far ultraviolet region of the spectrum. As of this date, only limited

work has been reported on this technique, but it does possess some

interesting analytical possibilities.

The first furnace device proposed for AAS was that described by L'vov

[19]. In his early work, a solution was placed on the end of the carbon

electrode and evaporated to dryness. The sample was then vaporized by a d-c arc into the carbon furnace containing a tantalum liner heated with an

a-c current. With this device very impressive absolute detection limits were

reported. Typical detection limits ranged from 10'^^ to 10"" g of the

analyte. However, the analytical capabilities of this device were limited be- cause of the power requirements and poor precision.

Massmann [20] constructed a somewhat simpler graphite furnace. In his

design, the sample was placed in the graphite tube through the hole in the

top, and then the tube was thermally heated to vaporize the sample.

Comparison of sensitivities reported by L'vov and Massmann devices are summarized in Table 1. L'vov has shown that the sensitivity is inversely

proportional to the square of the diameter of the graphite tube if small

amounts of the sample are analyzed. The sensitivity and detection limits in

these methods are expressed in absolute mass rather than concentration

since the solvent is evaporated before atomization.

Modifications to the graphite furnace have been made by Woodriff and

Ramelow [21]. West [22] described a slightly different approach at the

229
RAINS ON ATOMIC ABSORPTION SPECTROMETRY

TABLE 1Sensitivities obtained by atomic absorption with graphite cells.

Sensivity, weight/ 1% absorption

Element Wavelength, A L'vov Furnace Massmann Furnace

Ag 3281 0.8 pg

Al 3093 7-7 pg

As 1890 0.2 ng

Ba 5535 41.0 pg

Bi 3068 26.0 pg« 0.2 ng

Cd 2288 0.5 pg 2.0 pg

Co 3527 200.0 pg^

Cr 4254 41.0 pg

Cs 4555 3.3 ng^

Cu 3248 1.8 pg 9.0 pg

Fe 2%7/2483 17.0 pg 20.0 pg

Hg 2537 170.0 pg^ 200.0 pg

In 3039 3.0 pg 200.0 pg

K 4044 960.0 pg

Li 6708 20.0 pg

Mg 2852 0.5 pg

Mn 2795 0.5 pg" 0.8 pg

Mo 3133 11.0 pg"

Na 5890 6.0 pg

Ni 3525 180.0 pg" 70.0 pg

Pb 2833 15.0 pg" 10.0 pg

Rb 4202 1.5 ng"

Sb 2311 16.0 pg" O.I ng

Se 1%1 1.5 ng

Sr 4607 8.2 pg 10.0 pg

Te 2259 290.0 pg"

Tl 2768 40.0 pg

Ti 4982 2.0 ng

Zn 2139 0.2 pg" 0.8 pg

" Analyses were made at a pressure between 3 and 6 atm. Note:

pg = picogram (10"^^ g). ng — nanogram (10"'g).

International Conference on Atomic Absorption Spectroscopy in Sheffield,

England. He used an electrically heated carbon filament or graphite rod

as the atomizer. In this device, 1 to 5 of the sample was placed on the

carbon filament mounted horizontally between two metal supports. To

prevent oxidation of the carbon filament, it was placed in a glass cell through which argon flowed. The carbon filament was heated slowly to vaporize the solvent, and a current of 100 A at 7 V was then passed

through the filament to vaporize the analyte and produce a cloud of

atomic vapor. Absorption was measured by passing the radiation from a hollow cathode lamp just above the filament.

At the present, two types of graphite rod or furnace devices are commercially available. These instruments have been redesigned from the

230
SOME FUNDAMENTALS OF ANALYTICAL CHEMISTRY

original designs of Massmann and West for compactness and ease of

operation. Both of these devices produce excellent sensitivity for a number

of elements. Regardless of the devices selected by the analyst, it is important that he observe certain precautions. The sample (5 to 100 /ol depending upon device) must be transferred to the graphite cell in a reproducible manner. Then the sample is dried with very gentle heating. If

the heating is too rapid, vigorous boiling and spattering will result. The

drying time will vary with different sample types, but it should be long

enough to dry the sample completely. If volatile organic and inorganic

compounds are present, the sample must be heated at a higher

temperature to remove these compounds. This step is frequently referred

to as the ashing stage. The selection of temperature and time parameters

for the ashing stage is vital for good precision. If the temperature is too

low or the time too short, some of the matrix will not be removed and will cause light scattering during the atomization step. On the other hand, if the ashing temperature is too high, some of the more volatile metals will

be lost. Usually a setting can be selected so that the matrix is removed but

the analyte remains. For some samples the ashing step may have to be

repeated several times to completely remove the volatile matrix. The final

stage is atomization. The temperature of furnace and time required for

complete vaporization will depend upon the analyte and matrix. As a rule,

the lowest possible temperature is advisable to increase the life of the

graphite rod or tube. To increase the life of these rods or tubes, 3 to 5

percent methane is introduced to the inert gas sweep during the

atomization state. Interferences encountered with graphite rods or tubes can be classified as physical, chemical, and interelement in nature. Physical interferences are more pronounced with graphite rods or tubes than in most flame systems.

Light scatter due to incomplete volatization of organic or inorganic

compounds is of major importance. To minimize this type of interference,

the time of ashing should be increased. However, if it persists, background

measurements should be made with a continuum light source or at a nearby nonabsorbing line and then substracted from the absorbance value

obtained for the analyte.

Three possible mechanisms account for chemical interferences. The

analyte may be lost by the formation of a volatile compound, by occlusion

in a nonvolatile matrix, and by the formation of carbides. As an example,

lead chloride which boils at 950° C has a considerable vapor pressure at lower temperatures and can be lost in the ashing step. Therefore, it is frequently necessary to add a reagent which will react with the analyte to produce a less volatile species. Hydrogen peroxide has been observed to minimize the losses of lead in the presence of chloride. Atomic arsenic is

volatilized readily from aqueous residues, but arsenic in a metal matrix is

volatilized slowly. This is an example of an element forming an occlusion

231
RAINS ON ATOMIC ABSORPTION SPECTROMETRY

to produce a less volatile compound. The net result is that the rate of

volatilization will vary, and this can lead to erroneous results, especially if peak heights are being measured. Another major interference is from the

reaction of the furnace material to form nonvolatile metal carbides.

Although carbide formation does occur, it can be controlled with the use

of a pyrolytic coating on the inner surface of the rod or tube. Also the

continued use of methane during the atomization stage should greatly help

to maintain the pyrolytic coating for many hours of operation.

The interelement effect is more pronounced in the flameless techniques

than in tlame systems. It occurs when the analyte reacts with another

element which may produce a suppression at certain atomic ratios and an enhancement under certain other conditions. This type of interference can sometimes be corrected by the standard addition method; however, the

best technique is to prepare the standards in a similar matrix as the

unknowns [23].

Feldman [24] described a slightly different system in which the carbon

filament is replaced by a tantalum strip or ribbon. The major advantage

of the tantalum strip over the graphite furnace or rod is the power

requirements; however, temperatures high enough to excite the more

refractory elements are not obtained with the tantalum strip. A

comparison of absolute detection limits for the three systems is given in

Table 2.

Cold Vapor Method

The earliest application of AAS extends many years before the first paper by Walsh [13]. Mercury has an appreciable vapor pressure at room

temperature, and the volatile mercury vapor is capable of absorbing at the resonance lines of mercury. This technique has been employed for years by

geologists and industrial hygienists for monitoring mercury levels in the at- mosphere. The first use of the technique was described by Poluektov et al

in 1964 [25]. In 1968, Hatch and Ott [26] described a similar method. It is well known that mercury and many of its compounds are highly toxic, but

the wide distribution of mercury in our environment was not fully realized

until our analytical capability was developed to detect mercury at trace

and subtrace levels. Since the early publication by Hatch and Ott, there

have been numerous publications. A review of the techniques has been

made by Manning [27]. Rains and Menis [28] modified the Hatch and Ott

method by wrapping the absorption cell with a heating tape and by

heating the absorption cell to 200° C. This modification eliminated the

need for a drying tube prior to the absorption cell and improved the

precision of analysis. Since losses of mercury at the nanogram level may

occur due to absorption on the wall of the vessel or to its high vapor

pressure, it is essential that an oxidant such as KjCrzOy be added to standards and unknowns when these solutions are prepared. Feldman [29]

232
SOME FUNDAMENTALS OF ANALYTICAL CHEMISTRY

TABLE 2

Detection limits for flameless atomic absorption spectrometry.

Detection Limit, g

Hlement (jrapnite Furnace L.arDon koo DUU C

1 antaJum Kibbon

Ag 2 X 10-" 2 X 10-" 4 X 10-"

Al 3 X 10-" 3 X 10-" 3 X 10-" As 1 X lO-" 2 X 10-9 Au 8 X 10"" 1 X 10-" 1 X 10-"

Be 9 X 10-" 3 X 10-"

Bi 1 X 10-" 7 X 10-"

Ca 3 X 10-"

Cd 1 X 10-" 1 X 10-" 2 X 10-"

Co 4 X 10-" 6 X 10-" 4 X 10-"

Cr 5 X 10-" 5 X 10-" 8 X 10-"

Cs 2 X 10-"

Cu 1 X 10-" 7 X 10-" 1 X 10-"

Eu 1 X lO-" Fe 3 X 10-" 3 X 10-"

Ga 2 X 10-"

Hg 1 X lO-i" 5 X 10-"

K 9 X 10-"

Li 5 X 10-"

Mg 6 X 10-" 1 X 10-"

Mn 1 X 10-" 5 X 10-" 5 X 10-"

Mo 4 X 10-"

Na 1 X 10-"

111 1 X 1 n-11 1 X in-ii Pb 6 X 10-" 5 X 10-" 7 X 10-"

Pd 2 X 10-"

Pt 2 X 10-"

Rb 6 X 10-"

Sb 2 X 10-" 3 X 10-" 9 X 10-"> Se 1 X lO-'o

Si 8 X 10-"

Sn 6 X 10-" 2 X 10-10

Sr 5 X 10-"

Tl 1 X 10-" 3 X I0-" V 1 X lO-'o 4 X 10-"

Zn 6 X 10-" 8 X 10-" 5 X 10-"

" Perkin-Elmer Corporation brochure RE 67210, 1972, "High-Sensitivity Sampling Systems

for Atomic Absorption."

b Varian Techtron Carbon Rod Atomizer, Model 63 brochure, April 1972. '^Instrumentation Laboratory, Inc., "Sensitivities and Detection Limits in I. L. Model 355

Flameless Sampler," June 1971.

Studied the effect of 5 percent (volume per volume) HNO3 plus 0.01

percent CrjOy^" on 0.1 to 10 ng Hg/ml in glass and found the solutions to

remain stable for as long as five months.

Optical Design

Commercial instruments for AAS are of two basic designs — single

beam and double beam. In a single-beam system the source (HCL) is

233
RAINS ON ATOMIC ABSORPTION SPECTROMETRY

focused by means of two lenses through the flame and onto the entrance

slit of the monochromator. In a double-beam system the light from the source is interrupted periodically, and diverted around the flame, and re- combined before being focused on the entrance slit by a series of mirrors,

lenses, and rotating mirrors. The chief advantage of a double-beam

system is that the intensity of the resonance line is periodically monitored

even while a sample is in the flame. Thus, any source noise, drift, or in- stability may be easily corrected electronically. However HCL's have been

improved to the extent that the flame is the limiting factor in precision in

AAS. In AAS, the monochromator is an essential part of the system. The

monochromator must isolate the analytical line from the various other

lines emitted by the source. Failure to resolve the analytical line from all spectral irradiation will result in a loss of sensitivity and nonlinear calibration curve. Another important component of an optical system is the slits to the

monochromator. It is desirable that the slit widths be variable as they

control the resolution of the monochromator. Normally, we operate at a

slit width which gives the desired resolution from any adjacent lines. For

some elements the minimum slit width fails to give the desired resolution

of the analytical line, and, in that case, a tradeoff is made usually between

spectral resolution and sensitivity of the analyte. The effect of slit width on

resolution (spectral bandwidth) of the nickel HCL is shown in Fig. 2. The

most sensitive AAS ground state resonant line of nickel is located at 2320

A. However, it is very close to a nonresonant line at 2321.4 A, and only by

means of a high resolving monochromator can these two lines be completely resolved. The effect of spectral bandwidth on the absorbance of

nickel is shown in Fig. 3.

Electronic and Readout

The basic electronics most often used in AAS consist of a ^jower supply

for the HCL, a photosensitive detector and its necessary power supply, an

a-c amplifier, and readout device. The choice of a HCL/power supply will

depend upon the limit and accuracy of the desired detectability. It will

also depend upon the type of instrument that is being used. For precise

single-beam measurement, a well-regulated current supply providing 0.1

percent or better current regulation is desirable. With a double-beam

system the requirements are less stringent, and a regulated power supply

of 1 percent is adequate for most purposes.

The multiplier phototube is by far the most widely used detector in

AAS. A list of the most commonly used multiplier phototubes is given by

Rains [30]. These devices require voltages ranging from under —500 to — 2000 V, most being between —500 and —1500 V. The voltages are

234
SOME FUNDAMENTALS OF ANALYTICAL CHEMISTRY
S ON ATOMIC ABSORPTION SPECTROMETRY

SPECTRAL BAND1.6 p WIDTH, X

0 10 20 30 40

Ni, fig/ml

FIG. 3

Effect of spectral bandwidth on absorbance of nickel at 2320A.

negative with respect to ground, and the positive terminal is usually at ground potential. The power supply for these tubes should provide outputs

of up to —2000 V at several milliamperes of current regulated to within

0.001 percent against line voltage and load current changes. Also, it

236
SOME FUNDAMENTALS OF ANALYTICAL CHEMISTRY

should provide a stability of better than 0.01 percent over periods of hours

with a-c ripple components of less than 1 mV.

The signal produced by the multiplier phototube is usually too weak to drive indicators and requires amplification. It is essential for the optimum

signal-to-noise ratio (S/N) that the amplifier have high gain stability and

low noise. The principles, type, and operation of amplifiers are discussed

in detail by Veillon [31].

For readout devices, meters and recorders have been popular. Digital

readout devices are gaining in popularity and may be considered essential

for highly precise work. Advantages are that operator bias in making

readings is eliminated, and since the digital device employs a decimal-tobinary or BCD converter on the output, the signal can be fed to a printer or tape punch for subsequent computation on a computer.

Preliminary Instrumental Operations

After the analyst has a basic understanding of the components of his

instrument, he is ready to perform the analysis. Prior to the actual

determination, there are several basic operations. The primary light source must be "turned on" with optimum operating current and allowed to "warm up." The time required for the HCL to stabilize will vary with

different lamps, but, as a general rule, the operator should allow 15 min.

Next, a resonance line of the analyte is selected. Usually the manufacturer

of the instrument will provide a listing of the most sensitive analytical

lines. Then the wavelength dial of the monochromator is set to that

wavelength and adjusted until maximum intensity is obtained on the

readout device. For this operation the prior discussion on slit widths

should be considered for optimum operating conditions.

The selection of oxidant-fuel and burner is made if a flame system is being used. There are several gas combinations available (see Table 3), but

TABLE 3

Temperatures and burning velocities of common premixed flames.

Gas Mixture

Burning Velocity,

cm/s

Temperature,

°C

Air-methane 70 1875 to 1950

Air-propane 82 1925

Air-acetylene 160 2125 to 2400

Air-hydrogen 320 2050 to 2115

Nitrous oxide-acetylene 160 to 680 2600 to 2955

greater than 90 percent of all AAS measurements are made using an

air-acetylene or nitrous oxide-acetylene flame. Selecting the optimum

conditions for a given analysis is somewhat difficult, and usually one must

237
RAINS ON ATOMIC ABSORPTION SPECTROMETRY

resort to experimentation with various gases and flow ratios or consult the

Hterature for data on a specific problem.

Sample Preparation

The samples suitable for analysis by AAS cover a wide range of

materials and concentrations. The analyte may be present as a trace or as a major constituent. The amount and nature of sample preparation

required will depend on a number of variables, such as the type of sample,

the elements to be determined, and the precision and accuracy desired.

Therefore, no general method can be described for the preparation of all samples. Solution samples are nebulized directly into the flame and the

measurement taken. Solid samples are dissolved in hydrochloric, nitric, or perchloric acids. Sulfuric acid may be substituted for perchloric acid with

certain samples; however, sulfates are undesirable and should be avoided,

if possible. For the more refractory materials, a fusion with lithium

metaborate [32] or with a high pressure "bomb" vessel [33] may be

required. Fluxes for fusions must be completely free of trace inorganic

ions. They may also add high concentrations of solids to the solution

which may result in clogging of the burner. The advantage of a high

pressure vessel is that the sample is dissolved in mineral acids which later can be removed by fuming. However, these devices have a tendency to leak, especially after long periods of use, and some of the volatile elements

may be lost. After the sample has been dissolved, a separation or preconcentration

may be required. As a rule, interferences can be corrected without a separation, but preconcentration steps may be required when the sensitivity

of the analyte is insufficient for the sample concentration. Preconcentration

is accomplished by liquid-liquid extraction, evaporation, ion-exchange, or coprecipitation. These methods are discussed in detail by Ellis [34].

Calibration and Data Evaluation

Before discussing methods used for preparing a standard curve for a

particular element, we should consider some factors which may offset the

accuracy of the results. Impurities in the reagents can decrease the

practical detection limit or if contamination or impurities occur at random

concentrations, correction is difficult and accuracy may falter. Therefore,

high-purity reagents and a clean environment are essential for the highest

precision and accuracy in AAS. Also, cleanliness of glassware is of the

utmost importance in trace analysis. Cleaning of glassware with dichromatesulfuric acid cleaning solution is a common means of obtaining grease-free

walls; however, this glassware should not be used for chromium analyses.

The boiling of glassware in aqua regia followed by several rinses with ultra

238
SOME FUNDAMENTALS OF ANALYTICAL CHEMISTRY

pure distilled water will result in essentially a metal-free surface. In spite of

precautions taken to prevent contamination, it is very difficult to completely

eliminate all traces of impurities. Therefore, it is essential that sample and

reagent blanks be processed along with the sample.

Preparation of Standard Solution

Standard stock solutions should be prepared from high-purity metals or

salts of known stoichiometry. Michaelis [35\ has compiled a comprehensive listing of suppliers of high purity materials for virtually every element.

A standard stock solution should contain at least 1.000 mg/ml of the

anal3rte in an appropriate acid to prevent hydrolysis. Working solutions

should be prepared daily from this stock solution. Procedures for the

preparation and storage of standard solutions are given by Dean and

Rains [36\.

Preparation of Calibration Curve

There are several ways to prepare a standard calibration curve. The

simplest method is to prepare a series of standard aqueous solutions of the

analyte to cover the desired concentration range and to measure their

absorbance at an appropriate wavelength. Failure to select the appropriate

concentration range could lead to a nonlinear calibration curve. Solutions

of the unknown sample are then measured, and their concentrations are determined from a calibration graph. This is often the least reliable way

because of matrix or inter-element effects. Therefore, possible interferen- ces must be considered in the preparation of a calibration curve.

Interferences

Interference effects which occur in flames and nonflame systems can be

classified as physical, chemical, inter-element, and ionization.

Physical interferences occur when components of the sample alter their

physical properties such as viscosity, surface tension, and density. To

minimize these effects it is essential that the physical properties of

standards and samples be similar. The standards should contain about the

same acid concentration as the samples. As a general rule, the acid concentration should not exceed 5 percent, but the preferred acid concentration

for most elements is 0.1 to 1 percent. High salt concentrations would change

the viscosity and density of the sample. To compensate for these effects, it

is advisable, whenever possible, to add an equal concentration of the major

ions of the sample to the standards.

Chemical interferences originate during the desolvation of the analyte in

the flame or nonflame cell forming a compound which volatilizes at a

different rate than the standard solutions. This type of interferent may be

239
RAINS ON ATOMIC ABSORPTION SPECTROMETRY

CTvercome with a releasing agent or protective chelate added to sample and

standards. Lanthanum or strontium are the most frequently used,

although calcium, magnesium, and rare earth elements have been used. It

is essential that the releasing agent be free of the analvie or high blanks

will result.

Inter-element interferences are not well understood. For example, the

absorption of nickel can be suppressed or enhanced by iron depending

upon the iron concentration and instrumental conditions. Also, titanium

has been obser\'ed to enhance the absorption of aluminum and vanadium,

and aluminum will enhance the absorption of vanadium. To prevent this

inter-element effect, the interfering element should be added to standard

solutions in approximately the same concentration as the sample.

Ionization interference occurs when the analne is ionized in the flame.

In the cooler flames, ionization is not a serious problem. With high

temperature flames such as nitrous oxide-acetylene, it is a serious

problem, especially for those elements which have a low ionization

potential. If ionization cannot be avoided by the choice of flame, an easily

ionizable element is added to standard and samples to suppress ionization. As a rule. 500 to 1000 ^ig/ml of potassium or cesium is sufficient to suppress the ionization of most analyies. Other elements have been used

but a higher concentration may be required.

Accuracy of Method

After optimization of all instrumental parameters and consideration of

possible interferences, the analyst must evaluate the reliability of the data

or accuracy of the method. Grant [37] has defined accuracy as "a

quantitative measure of the variability associated with the relation of

anahtical results with what is assumed to be the true value." The obvious

way of checking the accuracy of an AAS method is to compare the

anahlical results with those obtained by an "accepted method." This

must be done with discretion as an "accepted method" may have a bias.

.Another approach is to evaluate the method using Standard Reference

Materials which have been certified by two or more independent analytical

techniques.

If similar material of known composition is not available, a check of the

accuracy of the method can be made by the following techniques. (1) The

dilution method is useful for revealing interferences. In this method the

aliquots or dilution should cover as wide a range of concentrations as

practical. (2) The accuracy can be checked by recovery experiments. It is advisable to add approximately the same concentration of analyte as was

originally found in the test sample. A complete recovery of the added test ponion, however, does not always assure an accurate value. (3) The

method of standard addition can be used to check for possible chemical

interferences. (4) The presence of light scatter or molecular absorption

should be checked using a nonabsorbing line or a continuum source. (5)

240
SOME FUNDAMENTALS OF ANALYTICAL CHEMISTRY

And, finally, if in doubt, the analyte can be separated from the matrix by

liquid -liquid extraction, coprecipitation or ion-exchange techniques. While

this final approach will add considerable time to the analysis, it may be

essential to prove the accuracy of the method.

References

[/] Butler, L. R. P. and Brink, J. A. in Plane Emission and Atomic Absorption Spectrometry. Vol. 2, Dean, J. A. and Rains, T. C, eds., Dekker, New York, 1971. pp. 21-56.

[2] Batchelor, E. H., Spectrovision. Vol. 25, 1971, pp. 7-15.

[J] Jackson, D. A., Proceedings of the Royal Society (London), Vol. A121, 1928, pp. 432-

447.

[4] Meggers, W. F. and Westfall, F. O., Journal of Research of the National Bureau of

Standards, Vol. 44, 1950, pp. 447-455.

[5] Metters, W, F, and Stanley, R. ., Journal of Research of the National Bureau of

Standards. Vol. 61, 1958, pp. 95-103.

[6] Fehsenfeld, F. C, Evenson, K. M., and Broida, H. P., Review of Scientific Instru- ments. Vol. 36, 1%5, pp. 294-298.

[7] Tomkin, F. S. and Fred, M., Journal of the Optical Society of America, Vol. 47, 1957, pp. 1087-1091.

[8] Zacha, K. E., Bratzel, M. P., Winefordner, J. D., and Mansfield, J. M., Jr., Analytical

Chemistry. Vol. 40, 1968, pp. 1733-1736.

[9] Dagnall, R. M. and West, T. S., Applied Optics, Vol. 7, 1%8, pp. 1287-1294.

[10] Menis, O., ed., "Analytical Coordination Chemistry Section: Summary of Activities, July 1968 to June 1969," Technical Note 504, National Bureau of Standards, U.S. De- partment of Commerce, 1969, pp. 1-2.

[//] Browner, R. F., Rietta, M. E., and Winefordner, J. D., "Improvement in EDL Radiant

Flux and Stability by Precise Temperature Control," presented at Pittsburgh Conference

on Analytical Chemistry and Applied Spectroscopy, Cleveland, Ohio, 6-10 March 1972, paper number 136.

[12] Herrmann, R. in Flame Emission and Atomic Absorption Spectrometry, Vol- 2, Dean,

J. A. and Rains, T. C, eds., Dekker, New York, 1971, pp. 57-93.

[13] Walsh, A., Spectrochimica Acta, Vol. 7, 1955, pp. 108-117.

[14] Delves, H. T., Analyst, Vol. 95, 1970, pp. 431-438.

[15] Manning, D. C, Atomic Absorption Newsletter, Vol. 10, 1971, pp. 123-124.

[16] Yamamoto, Y., Kumamaru, T., Hayashi, Y., and Tsujino, R., Analytical Letters, Vol.

5, 1972, pp. 419-424.

[17] Gatehouse, B. M. and Walsh, A., Spectrochimica Acta. Vol. 16, 1960, pp. 602-604.

[18] Dogan, M., Laqua, K., and Massmann, H., Spectrochimica Acta, Vol. 26B, 1971, pp. 631-649.

[19] L'vov, B. v., Inzhenemo-Fizicheskii Zhumal. Vol. 2, 1959, pp. 44-52.

[20] Massmann, H., Spectrochimica Acta, Vol. 23B, 1%8, pp. 215-226.

[21] Woodriff, R. and Ramelow, G., Spectrochimica Acta, Vol. 23B, 1%8, pp. 665-671.

[22] West, T. S., TUPAC Atomic Absorption Spectroscopy, Dagnall, R. M. and Kirkbright,

G. F., eds., Butterworth, London, 1970, pp. 99-126.

[23] Schrenk, W. G. in Flame Emission and Atomic Absorption Spectrometry, Vol. 2, Dean, J. A. and Rains, T. C, eds., Dekker, New York, 1971, pp. 303-325.

[24] Feldman, F. J., "A Non-flame Sampling Technique for Atomic Absorption with Automatic Background Correction," presented at Pittsburgh Conference on Analytical

Chemistry and Applied Spectroscopy, Cleveland, Ohio, 1-6 March 1970, paper number 49.

[25] Poluektov, N. S., Vitkun, R. A., and Zelyukova, Yu. Y., Zhumal Analiticheskoi

Khimii. Vol. 19, 1964, pp. 937-942.

[26] Hatch, W. R. and Ott, W. L., Analytical Chemistry, Vol. 40, 1%8, pp. 2085-2087.

[27] Manning, D. C, Atomic Absorption Newsletter, Vol. 9, 1970, pp. 97-99.

[28] Rains, T. C. and Menis, O., Journal of the Association of Official Analytical Chemists,

Vol. 55, 1972, pp. 1339-1344.

241
APPENDIX XIII

REPRIOTED BY PERMISSION OF ANALYTICAL CHEMISTRY

Reprinted from ANALYTICAL CHEMISTRY, Vol. 42, Page 394, March 1970

Flame Emission Spectrometry with Repetitive

Optical Scanning in the Derivative Mode

W. Snelleman,! T. C. Rains, K. W. Yee,^ H. D. Cook,^ and O. Menis

Analytical Chemistry Division, National Bureau of Standards, Washington, D. C.

A flame emission spectrometer using a rapid repetitive scan of a narrow wavelength region has been developed. By this method of wavelength scanning the

second derivative of the output intensity is measured.

The use of this approach to minimizing spectral inter- ference in matrices and the use of microsamples

greatly enhance the potentialities for flame emission

spectrometry, and minimize the need for a monochromator of high resolving power. A quartz plate,

made to vibrate at 145 Hz, is mounted behind the

entrance slit of the monochromator. The ac amplifier

is synchronized with the oscillations of the quartz

plate. When the amplifier is tuned to twice the fre- quency of vibration, the second derivative of the

spectrum is obtained. This permits the measurement

of weak line spectra nested in or on a broad band or continuum. It is demonstrated that spectral inter- ference due to CaOH bands and/or a continuum are minimized in the measurement of barium. The elimination of interferences from bands and flame struc- ture ted to an improvement in detection limits of

alkali and alkaline earth elements in the presence of many matrix ions. An analysis can be performed with

50 mI of solution which makes it applicable to biochemical and air pollution studies.

The measurement of the radiant intensity of an atomic line which is located on a bandhead arising from the concomitant

or flame gases is of concern to the flame emission spectroscopist. Frequently, an atomic line cannot be used because of interferences from some overlapping band structure. Two

typical examples are the CaOH bands (5430-6220 A), which

interfere with the Na 5890 A and Ba 5536 A lines, and the MgOH bands (3600-4000 A) which contribute to high back- ground for the Fe 3720 A and Ru 3727 A lines. Also, the OH band system, covering the region of 2800 to 4000 A, limits the measurement of the radiant intensity of many atomic lines in flame emission spectrometry. These interferences have dis- couraged many workers in the field of flame emission. However, Buell (7) in using a high resolution monochromator has

shown that many atomic lines in this region can be used.

Present address, Fysisch Laboratorium, Rijks-Universiteit

Utrecht, The Netherlands.

' Measurement Engineering Division, National Bureau of Standards, Washington, D. C.

(1) B. E. Buell in "Flame Emission and Atomic Absorption Spectrometery," J. A. Dean and T. C. Rains, Eds., Dekker, New

York, N. Y., 1969.

A technique which permits the measurement of weak line spectra nested in or on a broad band or a continuum resulted

in an improvement in detection limits. In addition, microliter samples can conveniently be taken for analyses. The

newly designed optical system permits a rapid repetitive scan of a narrow wavelength region. This scan is synchronized

with the ac amplifier. With the ac amplifier tuned to the same frequency as the vibrating quartz plate, the signal pro- duced is the first derivative of the emission intensity with re- spect to wavelength. At twice the frequency, the second

derivative is obtained. This mode of operation permits the measurement of weak line spectra without the interference from background radiation or broad band spectra. When

operating at high frequencies (145 Hz), the apparent signal-to- noise ratio is improved by eliminating the low frequency

flicker noise of the flame. This technique was applied to the determination of lithium in 50 lA of solution containing high

concentrations of sodium, and to the determination of barium

in a calcium matrix. Derivative spectroscopy has been described by several authors (2-4). Giese and French (5) used it to detect low in- tensity bands by measuring the first derivative of the trans- mission curve with respect to wavelength. Under certain conditions, a recording of the first or second derivative of an

absorption spectrum can give increased resolution over a normal spectrum. In magnetic resonance spectroscopy the

first derivative and occasionally the second derivative are re- corded to increase resolution of the spectrum (6). There are various ways for obtaining the derivative of absorption lines or bands with respect to wavelength. Balslev

(7) vibrated the exit slit of his spectrometer and synchronously

detected the signal at the vibrating frequency while Gilgore

et al. used optical wobblers (S). Snelleman (9) was able to in-

(2) A. Perregaux and G. Ascarelli, Appl. Opt., 7, 2031 (1968).

(3) F. R. Stauffer and H. Sakai, ibid., 7, 61 (1968).

(4) E. C. Olson and C. D. Alway, Anal. Chem., 32, 370 (1960).

(5) A. T. Giese and C. S. French, Appl. Spectrosc, 9, 78 (1955).

(6) C. H. Townes and A. L. Schwawlow, "Microwave Spectros- copy," McGraw-Hill, New York, N. Y., 1955, Chapters 14 and

17. (7) I. Balslev, Phys. Rev., 143, 636 (1966).

(8) A. Gilgore, P. J. Stoller, and A. Fowler, Rev. Sci. Instrum., 38, 1535 (1967).

(9) W. Snelleman, Spectrochem. Acta, 23B, 403 (1968).

242
Tilt ongle = a

Figure 1. Displacement of an oblique ray laterally and parallel

to its initial direction

crease the sensitivity of the measurement of copper by atomic

absorption using a continuum source and optically scanning

part of the continuum. It was accomplished by placing in the optical path a vibrating mirror which was mounted on the movement of a milliammeter. While this arrangement clearly

demonstrated the feasibility of optical scanning in atomic absorption, it had certain limitations because of alignment

difficulties. The current investigation led to the development

of a system utilizing an oscillating quartz plate which pro- vided a stable and easy to operate apparatus. Also, it was

demonstrated that this technique had special applications in flame emission spectrometry.

EXPERIMENTAL

Apparatus. General Instrumentation. The instru- mental system used in this study consists of a 0.75m Czerny- Turner mount monochromator, f/6.5 aperture, curved slitS' exchangeable gratings blazed for 3000 and 7500 A with 1180

grooves/mm; an oxygen-hydrogen flame with a total con- sumption burner and an air-acetylene flame with a premixed

nebulizer burner; gas flow meters and pressure regulators; an end-on multiplier phototube, EMI 9558AQ, with 44-mm

diameter photocathode and S-20 spectral response; a 0 to 2100 V multiplier phototube power supply, 0-30 mA,

0.001% regulation (line and load), ±0.25% accuracy; a phase sensitive lock-in amplifier with full-scale sensitivity ranges of 100 nV to 500 mV, and a 10-mV strip-chart

recorder, 0.2 sec full-scale response time. Quartz Plate. A quartz plate 12 X 14 X 2 mm serving

as the displacement element was mounted on a rotating mechanical oscillator with a variable amplitude of up to 6° peak to peak. The light beam, in passing through the plate

(as shown in Figure 1), will be displaced laterally (perpendicular to the axis) and parallel to its initial direction when

measured normal to the ray, and is given by the following

relationship

'^-Lray = / " COS Of (tan a — tan a') ~ [/a(« - Din]

where / = the thickness of the plate, a = radians in air, a' = radians in the plate, and n = index of refraction of the

material. The image shift is approximately proportional

to the thickness of the quartz plate and the angle of rotation. In this experiment with a peak to peak amplitude of 5°, the perpendicular shift was 132 /x using amcan = 0° which cor- responded to about 1.5 A of the spectrum. A larger dis- placement can be obtained by using a thicker quartz plate

and by increasing the angle of rotation. However, to avoid

O

1

Till = 0

OdIIcoI Axis

s

Air Air

Figure 2. Displacement of an image along the optical axis

HaE * TOP DISC \U.

4 SLOTS 90'»[MT

0,010 ilDE X DEEP

ALHICO X PERMWOIT M»aCT1/2 X 5^2' X Vie'

'a,' X VCUT IH EACH EW

TOP OlSC,'4DIA,'/ljT~^r

COIL CORE

4 LAKERS SILICON -pi

STEEL 0.014" THIM ^

,

. STAND OFF- %OIA,xrL BRASS

CLEARANCE HOLE FOR N0.Z-S6 SCREW

4 STRIPS, 0.010 'I, PHOSPHOR BRONZE

SHIM STOCX %4XI^/|5

^BSASS WSC '/|5 DIA , '/u T ^ 4 SLOTS »• APART

0.010" x5/54

DRILL #42 HOLE IN CENTER

BRASS ROD

Figure 3. Mechanical oscillator for optical scan

an ofl'-set of the wavelength calibration of the monochromator and light losses due to increased reflection at large tilt angles, an angle of a = 0° was selected. The ray in addition to being displaced laterally and parallel to its initial direction will be shifted longitudinally. The

displacement of the image from the exit slit by the quartz

plate, as shown in Figure 2, is given by the following relation- ship (/O):

''I Us = t{n-\)ln ~ (?/3 for common crown glass quartz)

Because the image was defocused at the exit slit due to the change in optical path length, the quartz plate was ground

to a slight convex curvature corresponding to a focal length of 12 m.

(10) W. Meuller-Herget, Philips Laboratories, BriarcIifT Manor,

N. Y., personal communication, 1969.

243
ENTRANCE

I SLIT

I EXIT SLIT

Figure 4. Location of quartz plate within the

optical system

no tuais

Figure 5. Electrical circuit for mechanical oscillator

Design of High Frequency Wavelength Oscillator.

The mechanical oscillator shown in Figure 3 consists of two

brass disks connected by four narrow phosphor bronze

strips cut from 0.010" thick shim stock. The strips are soldered at each end into slots cut radially 90° apart at the edges of the disks. One disk is rigidly attached to the end

of a rod which provides a large mass for an inertial reference. The other disk will oscillate in a rotational mode about the

axis. The spring structure allows a large angular amplitude

while retaining a high stiffness for other vibrational modes.

The quartz plate is mounted on edge in a channel cut in this disk. The frequency of oscillation is determined by the

stiffness of the spring strips and the moment of inertia of the top disk, including any attachments. The Q of the device

is approximately 300. The torque required to overcome the

losses is produced by the force between current carrying coils and the ends of a permanent-magnet bar cemented to the underside of the moving disk. The angle between the magnet

and the drive core is 45°. The actual core is in the shape of

a cross. The two halves of the drive coil are wound on

opposite arms of the cross, and an identical pick-up coil is wound on the remaining two arms. The signal induced in the pick-up coil is amplified and fed back to the drive coil. The system oscillates at a frequency determined by the mechanical resonance. Operation at the mechanical reso- nance is necessary to obtain sufficient angular displacement

with a reasonable driving power, and to ensure a constant phase relation between the oscillator motion and its driving

voltage.

The lower disk holds the bottom ends of the spring strips

rigidly. This construction simplified the assembly of the

delicate parts. To assemble the springs without strain, a

jig was used to position the disks the desired distance apart

and perpendicular to the axis. The strips were soldered

into the slots using a minimum amount of heat and solder. The core was mounted on a standoff which also served as a nut for attaching the bottom disk to the rod. The magnet was cut to size with a thin abrasive-filled cutting disk. The

slots in each end provide clearance for the spring strips. The magnet and quartz plate must be assembled after the coil

H

Bo, 5336 *—•.

Bo, 533« X—

Figure 6. Flame emission spectrum of 10 pg/ml of barium

Mechanical disk chopper, (fi) Optical scan-first derivative.j (C) Optical scan-second derivative

assembly has been installed. The quartz plate is placed within

the optical path as shown in Figure 4. Associated Electronic Controls. The electrical circuit for the oscillator, shown in Figure 5, consists of the magnet,

pick-up coil, impedance step-up transformer Tl, limiting

amplifier circuit of ARl, variable gain amplifier circuit of

AR2, impedance stepdown transformer T2, and the drive coil. The drive and pick-up coils are oriented to eliminate any

direct magnetic coupling. The varying magnetic field of the moving permanent magnet induces a voltage in the pick-up

coil. This voltage is applied to the input of amplifier ARl

through transformer Tl. Amplifier ARl is connected as a

gain of 101 amplifier with output voltage limiting. For all useful oscillation amplitudes, the peak output voltage is approximately ±0.5 V. The limited voltage is amplified by

the adjustable gain circuit of AR2. Transistors Ql and Q2

form a complementary-emitter-follower output stage to in- crease the available output current. At maximum gain, the output voltage is nearly ±10 V peak. Reducing the gain lowers the output voltage and the oscillation amplitude.

The output voltage is applied to the drive coil through trans- former T2. The 100 resistor in series with the primary

increases the dc resistance seen by the amplifier. The offset voltages of ARl and AR2 should be adjusted for less than

±100 mV at the emitter of Ql. An auxiliary winding of T2

provides an output equal in amplitude and 180° out of phase

with the voltage from Ql and Q2. These two voltages are half-wave rectified and are added to provide sufficient signal

at twice the oscillation frequency to drive the ac-coupled

tuned amplifier in the reference channel of the lock-in amplifier used. A reference output at the frequency of oscillation

is obtained from the output of ARl. The capacitors across

the feedback resistors of ARl and AR2 are selected for zero phase shift around the loop at the mechanical resonant fre- quency. The loop may be opened between the two amplifiers

and AR2 driven by a frequency-slable audio oscillator at the resonant frequency. The feedback capacitors should then be adjusted for zero phase shift between the input voltage

and the output voltage from ARl. Alternatively, with the system oscillating, the feedback capacitors may be adjusted

for maximum voltage at the secondary of Tl. Reagents. All solutions used were prepared from high

purity metals or salts.

244
Operating Characteristics. The spectrum of the resonance

line of barium, obtained in an oxygen-hydrogen flame with a

total consumption burner, is shown in Figure 6A. The

wavelength region of the barium line at 5536 A was scanned

in the normal mode of a rate of 5 A/min with a monochromator having a spectral band width of 0.6 A. The light

signal was chopped at 87 Hz using a mechanical disk chopper.

The mechanical disk chopper was then removed from the

optical system and the quartz plate was placed behind the entrance slit within the optical path (Figure 4). By means

of the drive coil, the quartz plate was oscillated at 145 Hz. The synchronous lock-in amplifier was tuned to this fre- quenc)^. As shown in Figure 6B, a wavelength scan of the 5536 A barium resonance line was repeated. The resulting

spectrum was the first derivative with the peak corresponding

to the zero crossing of the X axis. While the first derivative has been used in spectroscopy (5) for identifying overlapping

band structures, a scan of the wavelength region is required

and off'ers no advantage in flame emission over the mechanical

disk chopper. The experiment was repeated, with the lock-in

amplifier turned to twice the frequency of the vibrating quartz

plate. The signal (Figure 6C) which is the second derivative with respect to wavelength is proportional to the concentration of the analyte. The effect of calcium on barium was

investigated to evaluate the use of the optical scan in the second derivative mode for the measurement of the intensity of a line nested on a broad continuum. Operating in the normal mode with a mechanical disk chopper the wavelength

region of 5500 to 5560 A was scanned at a rate of 25 A/min

with 4 /ig/ml of barium in the presence of 20 Mg/ml of calcium

in an oxygen-hydrogen flame. The spectrum is shown in Figure 7A. The spectrum produced with the vibrating quartz

plate within the optical path is shown in Figure IB. The re- corded data shown in Figure 8 were obtained by setting on

the peak intensity of the barium line and repetitively scanning a narrow wavelength region. By measuring the emission

intensity in the second derivative mode, the signal is proportional to the analyte concentration. However, to obtain the maximum signal, the amplitude of the optical scan is adjusted with 5K Q variable resistor, shown in Figure 5.

RESULTS AND DISCUSSION

In flame emission spectrometry, the barium resonance line at 5536 A suff"ers serious spectral interferences from calcium,

magnesium, and sodium (11, 12). Also in atomic absorption,

the CaOH band system with its peak at 5540 A will interfere

in the determination of barium in an air-acetylene flame

{13, 14). As shown in Figure lA, barium can be determined

in the presence of calcium by flame emission spectrometry

provided the calcium to barium ratio does not exceed 10:1. For this purpose the wavelength region of 5530 to 5540 A must

be scanned and the proper background correction applied.

By operating with a smaller spectral band width, the calcium

band structure is reduced; however, to obtain the desired

sensitivity for barium, a spectral band width of 1.0 to 1.5 A

is usually necessary. Under these conditions the high back- ground emission from CaOH band structure becomes a limit- ing factor and zero offset is required to shift the spectrum on

the recorder scale. Also, in making a wavelength scan of this region, two to five milliliters of solution are required.

(11) J. A. Dean, "Flame Photometry," McGraw-Hill, New York,

N. Y., 1960, p 211.

(12) J. A. Dean, J. C. Burger, T. C. Rains, and H. E. Zittel, Anal.

Chem., 33, 1722 (1961).

(13) S. R. Koirtyohann and E. E. Pickett, ibid., 38, 585 (1966).

(14) G. K. Billings, Ph.D. Thesis, Rice University, Houston, Texas,

1963.

100 r

_,oL I 1 1 1 1 1 1 5S00 S5J0 5540 5560 WAVELENGTH, A

Figure 7. Flame emission spectrum of 4 ii%lm\ of barium in the presence of 20 ii%lm\ of calcium in an oxygen-hydrogen

flame with (A) a mechanical disk chopper and (B) optical scan and derivative mode

Ba. Kg/ml 5 10 5 10 5 10

Ca. «g/ml 0 0 100 100 1000 1000

Ca blank

Figure 8. Effect of calcium on barium in flame emission spec- trometry using optical scanning and derivative mode

With the optical scan and taking the second derivative, the high background emission due to the CaOH band structure is eliminated as shown in Figure IB. The intensity of the barium line is measured from the dotted base line to the peak

height, and is approximately two-thirds the height obtained

with the disk chopper.

The continuum of unresolved band structure with a nominally zero curvature which lies under the analytical line is not

detected. The amount of band structure or continuum which

can be tolerated with the optical scanning technique will de- pend upon the optical system and the photodetector. The

major limiting factors are the cathode loading and fatigue of

photodetector. Under the optical conditions used the radiant

intensity of analytical hnes that were 0.1 % of the relative in- tensity of broad band structure have been measured without

interference. However, narrow band peaks still appear as a

slight deviation from the background.

The unique advantage of this technique is shown in the study

of the effect of various concentrations of calcium on the

245
Table I. Detection Limits by Flame Emission Spectrometry

with Optical Scanning in the Presence of Matrix Ions

Burner-difTuslon Oxidant-fuel-oxygen-hydrogen

Sample size-0.25 ml

Matrix, element Detection limit," ng/ml

1 mg/ml Li K Ca

Li 0.2 3.0 Na 0.02 0.08 0.5 K 0.02 3.0 Rb 0.02 l.O' 3.0 Mg 0.02 0.1 C Ca 0.02 0.03 Fe 0.02 0.2 3.6 ng = 10"' grams

" Detection limit is defined as 2a of background.

' Rubidium free of potassium not available.

' Magnesium free of calcium not available.

measurement of the barium emission line. As shown in Figure 8, by setting on the resonance line of barium and

measuring the peak height of the second derivative with re- spect to wavelength, barium was determined in the pres- ence of a 100-fold concentration of calcium with an air- acetylene flame. By scanning a narrow wavelength region of one A an appropriate background correction is made. While

in this case the peak height measurement was obtained with 2 ml of solution and a time constant of 1 sec, the analysis can

be performed with a fraction of this sample. With a higher

temperature flame, such as oxygen-hydrogen or nitrous oxide-acetylene with an appropriate burner, the calcium to barium ratio can be extended to 1000 :L A higher weight

ratio can be tolerated in the presence of magnesium or sodium.

The correction for background even in the presence of 1000

ng of calcium is easily applied, and the need for extensive zero

offset in the electrical circuit is eliminated. The detection limits determined for lithium, potassium,

and calcium in the presence of 1000 Mg/ml of various matrix

elements are given in Table \. The continuum or unresolved band structure produced by the matrix element did not show

a shift in the base line. The detection limit of potassium in rubidium could not be extended below 1 ng because potassiumfree rubidium was not available. The detection limits which are generally reported in the

literature are usually based on pure water solution. These

values can be very misleading when applied to samples containing other cations. Generally, for a complex matrix the

optical scan led to an improvement in the detection limits of an

analyte of 1 to 2 orders of magnitude over the normal wavelength scan with a mechanical chopper. However, in a pure

(15) V. A. Fassel and D. W. Golightly, Anal. Chem., 39, 466

(1967).

(16) E. E. Pickett and S. R. Koirtyohann, Sjjectrochim. Acta, 23B,

235 (1968).

(/3 a

>

w

05

UulJuU Jul

Li, 1^8 'ml

Figure 9. Lithium calibration results of flame emission spectrometry using optical scanning and derivative mode

water solution and with an oxygen-hydrogen flame, the detection limits of the two methods were identical and cor- responded to the values reported in the literature {15, 16). Another unique feature of the technique is the amount of

sample required for analysis. Normally with the mechanical

disk chopper, three to five milliliters of solution are required

for a scan of the wavelength region of interest and for the base line recording for measuring the peak height. By setting on the peak of a line with the optical scan and measuring the second derivative of the output signal, a fraction of a milliliter of solution is only required. In this manner, with a total consumption burner and a small bore capillary, lithium was

determined in a sodium matrix. The data in Figure 9 show

replicate determinations of lithium in 50 ^1 of solution. The

precision in this method was 2% of the full scale. In this case the response of the electronics was the limiting factor in determining the volume of the solution needed. With a faster electronic detection system or an integrating system, this could

be reduced further. In flame emission spectrometry this technique, in many

cases, will eliminate the need for a high resolution spectrometer to minimize spectral interferences. Because the optical scan is applicable to microliter samples, it should be of great

assistance in the analysis of biochemical materials and air pollution particulates. Also, it is being extended to atomic

absorption spectrometry with a continuum light source.

Received for review September 15, 1969.

9, 1970.

Accepted January

246
APPENDIX XIV

REPRINTED BY PERMISSION OF M4RCEL DEKKER, INC.

13 Standard Solutions for Flame Spectrometry

John A. Dean and Theodore C. Rains

DEPARTMENT OF CHEMISTRY ANALYTICAL CHEMISTRY DIVISION UNIVERSITY OF TENNESSEE NATIONAL BUREAU OF STANDARDS

KNOXVILLE, TENNESSEE WASHINGTON, D.C.

III. IV.

II.

I. Introduction ........ A. Sources of Standard Reference Materials . B. Types of Standard Reference Materials Available

Aqueous Stock Solutions ...... A. Relation of Concentration Units .... Specialized Stock Solutions ...... Standards for Trace Elements in Petroleum Products

Appendix 1 : Standard Stock Solutions .... Appendix 2: Special Aqueous Stock Solutions

Appendix 3: Standard Reference Materials in Lubricating Oil References . . . . .

327

328

328

330

331

331

331 332

336

337

338

I. Introduction

In all evaluation methods one must have available standard samples

and high-purity materials for each element being determined, often in the

same matrix as the sample. Quite often several standards must be prepared

for a given element, one for each matrix or system that contains the test element. To assist the individual chemist who must prepare these standard

solutions, this chapter will outline the preparation of various individual

stock solutions and suggest sources where high-purity materials may be

obtained or from whom standard samples may be purchased. An excellent

report on available standard reference materials and high-purity materials

has been compiled by Michaelis (i).

Copyright © 1971 by Marcel Dekker, Inc. NO PART of this work may be reproduced or utilized in any form by any means, electronic or mechanical, including xerography,

photocopying, microfilm, and recording, or by any information storage and retrieval system, without the written permission of the publisher.

247
JOHN A. DEAN AND THEODORE C. RAINS

A. Sources of Standard Reference Materials

The National Bureau of Standards (U.S.) offers a wide selection of

standard alloys, steels, ores, glasses, and miscellaneous types of materials

in the form of chips, turnings, powders, or granules. Full information

concerning the values that are certified, the weights, and prices is contained

in Ref. (2).

The Bureau of Analysed Samples, Ltd. (Newham Hall, Middlesborough,

Yorkshire, England) also offers a wide selection of standard samples.

Many of these standards may be obtained through the Jarrell-Ash Division

of Fisher Scientific Company (590 Lincoln Street, Waltham, Mass. 02154)

or from Spex Industries, Inc. (3880 Park Avenue, Metuchen, N.J. 08841).

A series of 20 ceramic powders are supplied by Applied Research

Laboratories (9545 Wentworth Street, Sunland, Calif. 91040) for the

analysis of barium, calcium, magnesium, and strontium titanates, and

titanium dioxide. The standards have a variation of composition to cover

the range of impurity constituents normally found in ceramic materials.

In addition to the various standard samples, reference samples are also

available. Generally compositions are less accurately known. Smith and

Underwood (1023 Troy Court, Troy, Mich. 48084) supply over 1000

different analyzed materials, primarily for student use in universities and

colleges.

B. Types of Standard Reference Materials Available

Three aluminum-base alloys prepared in the form of turnings are

available from the National Bureau of Standards (NBS), and seven chip

samples are issued by the Bureau of Analysed Samples (BAS). These

samples possess certificates of analysis for Cu, Fe, Mg, Mn, Pb, Si, Ti,

and Zn in all cases; individual samples may contain also Cr, Ga, Ni, Sb,

Sn, and V.

Two cobalt-base alloys are issued by NBS and contain Ni, Mn, Si, Fe,

Cr, Mo, W, Nb, and Cu.

Nine copper-base alloys are available from NBS and five from BAS,

all in chip form. In addition to Cu, Zn, Sn, Pb, Ni, and Fe, certain

standards also contain Al, As, Ag, Co, Mn, Sb, Si, S, P, and Mg. Six

standards in the form of turnings from high-purity copper are available

from Bundensanstalt fiir Materialpriifung (Berlin-Dahlem, Unter den

248
13 STANDARD SOLUTIONS FOR FLAME SPECTROMETRY

Eichen 87, Berlin, Germany). These standards contain less than 0.07%

of Pb, Sn, Bi, As, Sb, Fe, and Ni.

Over 120 standards for iron-base alloys are available from NBS and

BAS in the form of chips. These include cast iron, ingot iron, B.O.H. and

A.O.H. steels, plain-carbon steels, mild steels, and alloy steels. The mild

steels and alloy steels contain accurately determined traces of a number of

residual elements (W, Co, Ti, As, Sn, Al, Zr, Pb, Nb, Ta, B, and Ag) in

addition to the usual constituents: Mn, P, S, Si, Cu, Ni, Cr, V, and

Mo.

Two lead-base standards are issued by NBS and one by BAS; these

contain Sn. Sb, Bi, Cu, As, and traces of Ag and Ni.

One magnesium-base alloy is available from NBS and one from BAS.

The former contains Al, Zn, Mn, Si, Cu, Pb, Fe, and Ni; the latter

contains total rare earths, Zn, Zr, Cu plus traces of Mn, Ni, and Fe.

The NBS issues three primary standards of nickel oxide (powder form)

and four nickel-base alloys (Monel, nickel-base casting alloy, Ni-Cr alloy,

Waspaloy). The BAS supplies one standard (Nimonec 90).

One tin-base alloy is available from NBS and from BAS. The standard

contains Sb, Cu, As, Pb and traces of Bi, Ag, Ni, and Fe.

Only one zinc-base alloy is available from NBS. It contains Al, Cu,

Mg, and traces of Fe, Mn, Pb, Ni, Sn, and Cd.

One zirconium-base alloy, Zircaloy 2, is issued by NBS. It contains Sn,

Fe, Cr, and traces of Ni, Mn, and Cu.

Steel-making alloy standards from NBS and BAS include refined

silicon, ferrovanadium, ferrochromium, spiegeleisen, ferrophosphorus,

ferroboron, and calcium molybdate.

A set of six cement standards is available from the NBS with these

constituents certified: SiOj, AI2O3, FcgOs, TiOg, P2O5, CaO, SrO, MgO,

SO3, MnjOg, NagO and K2O.

Miscellaneous standard samples issued by the NBS include: three

alumina refractories, two silica refractories, one chrome refractory, a

bauxite, a plastic clay, a glass sand, four glasses, and argillaceous limestone, a silica brick, a burned magnesite, titanium dioxide, and silicon

carbide. These are supplemented by three basic slags, a silica brick, a

firebrick, and a sillimanite from the BAS.

Among ore samples available are six iron ores from the BAS. A phosphate rock, a Mesabi iron ore, three lithium ores (Spodumene, Petalite,

and Lepidolite), one manganese ore, two tin ores, and one zinc ore are

available from the NBS. Except for the phosphate rock and Mesabi

iron ore, the NBS ores are certified only for the component of interest.

249
JOHN A. DEAN AND THEODORE C. RAINS

A syenite rock sample and a sulfide ore are also available (Department of

Geological Sciences, McGill University, Montreal, P. Q., Canada).

Four horticultural standards of apple, cherry, citrus, and peach

leaves have been prepared (Horticultural Department, Michigan State

University, East Lansing, Mich.)-

n. Aqueous Stock Solutions

High-purity materials (99.9% or greater) can be obtained from several sources: Alpha Metals, Inc. (60 Water Street, Jersey City, N.J.), Johnson

Matthey & Co., Ltd. (Hatton Garden, London, E.C.I, England), Spex

Industries, Inc. (Section I. A.), and various chemical supply houses, for

example. Baker & Adamson, J. T. Baker & Co., Fisher Scientific Co.,

Mallinckrodt Chemical Works, and Merck and Co. A comprehensive

listing of suppliers of high-purity materials for virtually every element is available (/).

In selecting a suitable compound or material for use as a standard,

several criteria must be met: (1) It must be available commercially in a

high degree of purity and meet A.C.S. specifications or a similar "official"

certification. Established methods should be available for confirming its purity (i). (2) The compound should not be hygroscopic, nor should it be

efflorescent. It should be stable under the conditions specified for drying.

(3) The composition of the selected material must be known; if not, the

final stock solution will have to be standardized. (4) Metallic and other

contaminants present in these standard materials must not be in amounts

greater than that of the trace element to be determined in any particular

sample, otherwise mixed standards could not be prepared.

Procedures which provide standard stock solutions containing 1000

/^g/ml of the element of interest are listed in Appendix 1. Precautions

outlined in Chapters 10 and 1 1 (Section II. E) should be reviewed. Molarity

of the reagents used is as follows:

Reagent Molarity Reagent Molarity

Acetic acid

HCl (38%)

HCIO4 (70%)

HF(45%)

17.4

12.0

11.6 25.7

HNO3 (69%)

H3PO4 (85%)

H2SO4 (94%)

15.4

14.7 17.6 Ammonia (27 %) 14.3

250
13 STANDARD SOLUTIONS FOR FLAME SPECTROMETRY

Many stock solutions should be slightly acidic to prevent hydrolysis.

Unless stated otherwise, final dilution should be made to 1000 ml with

deionized water—ordinary distilled water which has been passed through

a mixed-bed ion-exchange resin such as Amberlite MB-3, then freshly

boiled and cooled to room temperature. Solutions are stored in polyethylene containers unless a special containment material is specified. A

crystal of thymol in each stock solution will inhibit bacterial growth.

A. Relation of Concentration Units

The interrelationship between various expressions for concentrations is:

1 //g/ml = 1 mg/liter = 1 ppm (w/v)

1 meq/liter = (weight in grams per equivalent/ 1000) in 1 liter

1 mg atom per liter = (atomic weight in grams/ 1000) in 1 liter

1 yMg/ml = (1 meq/liter)(1000/weight in grams per equivalent)

m. Specialized Stock Solutions

Solutions for clinical, agronomic, and certain other special situations are listed in Appendix 2. These solutions are prepared in the same manner

as described in Section II.

rv. Standards for Trace Elements in Petroleum Products

Stable, oil-soluble compounds are available from the National Bureau

of Standards (U.S.) as certified standards for 24 elements in petroleum

products. Appendix 3 lists the compounds; all are solids. Directions for

the preparation of individual compounds have been published (5) for

those who may wish to prepare their own materials. Some of the compounds are fairly hygroscopic. As a precautionary measure, bottles containing the standard material should be stored, tightly closed, in desiccators

containing P2O5, and that, before use, the required amount of a compound

(plus a slight excess) should be dried over P2O5 in an open container for

the length of time recommended in Appendix 3, column 4. To prepare an oil solution containing one or several elements of interest, a solubilizing agent is prepared by placing 5 ml of 2-ethylhexanoic acid,

4 ml of 6-methyl-2,4-heptanedione, and 2 ml of xylene in a weighed

200-ml flask. The calculated weight of standard sample (Appendix 3,

251
JOHN A. DEAN AND THEODORE C. RAINS

column 5) is carefully transferred to the flask. The suspension is gently

heated on a hot plate, with swirling, until the salt has dissolved to a clear

solution.

The salts are added, one at a time, and completely dissolved before the

next compound is added. After each addition, the mixture is gently heated,

tilted, and rotated in order to wash down any particles adhering to the

inside of the flask. After the last compound has been added and dissolved,

2 ml of bis(2-ethylhexyl)amine is added. The beaker is now washed with

six 15-ml portions of lubricating oil, and these rinsings are all transferred

to the flask. This last step is omitted if the sample is transferred

directly to the flask from the sample container, and not weighed into an

intermediate beaker.

The oil solution is now allowed to cool to room temperature, and cool

lubicating oil is added to the flask until the weight of the solution is 100 ± 0.5 g. The solution is reheated to 85°C, allowed to cool to room

temperature, transferred to a storage bottle, and tightly stoppered.

Compounds of silver and mercury are sensitive to prolonged heating and

so, they should be added last to any mixture. For the other compounds,

the order of addition is immaterial.

Appendix 1: Standard Stock Solutions

Element Procedure

Aluminum Dissolve 1.000 g Al wire in minimum amount of 2 M HCi; dilute

to volume.

Antimony Dissolve 1 .000 g Sb in (1) 10 ml HNO3 plus 5 ml HCI, and dilute to volume when dissolution is complete; or (2) 18 ml HBr plus 2 ml

.

liquid Bra, when dissolution is complete add 10 ml HCIO,, heat in a

well-ventilated hood while swirling until white fumes appear and

continue for several minutes to expel all HBr, then cool and dilute to volume.

Arsenic Dissolve 1.3203 g of AS2O3 in 3 ml 8 M HCI and dilute to volume;

or treat the oxide with 2 g NaOH and 20 ml water, after dissolution

dilute to 200 ml, neutralize with HCI (pH meter), and dilute to volume.

Barium (1) Dissolve 1.7787 g BaCl2-2H20 (fresh crystals) in water and

dilute to volume. (2) Dissolve 1.516 g BaClj (dried at 250°C for 2 hr) in water and dilute to volume. (3) Treat 1 .4367 g BaCOj with

300 ml water, slowly add 10 ml of HCI and, after the CO2 is released by swirling, dilute to volume.

* 1000 ^g/ml as the element in a final volume of 1 liter unless stated otherwise.

252
13 STANDARD SOLUTIONS FOR FLAME SPECTROMETRY

Appendix 1: (Cont.)

Element Procedure

Beryllium

Bismuth

Boron

Bromine

Cadmium

Calcium

Cerium

Cesium

Chlorine

Chromium

Cobalt

Copper

Dysprosium

Erbium

Europium

Fluorine

Gadolinium

Gallium

Germanium'

(1) Dissolve 19.655 g BeSO,,-4H20 in water, add 5 ml HCl (or

HNO3), and dilute to volume. (2) Dissolve 1.000 g Be in 25 ml 2 M

HCl, then dilute to volume.

Dissolve 1.000 g Bi in 8 ml of 10 M HNO3, boil gently to expel

brown fumes, and dilute to volume.

Dissolve 5.720 g fresh crystals of H3BO3 and dilute to volume.

Dissolve 1.489 g KBr (or 1.288 g NaBr) in water and dilute to volume.

(1) Dissolve 1.000 g Cd in 10 ml of 2 M HCl; dilute to volume.

(2) Dissolve 2.282 g SCdSOi-SHzO in water; dilute to volume.

Place 2.4973 g CaC03 in volumetric flask with 300 ml water,

carefully add 10 ml HCl, after CO2 is released by swirling, dilute to volume.

(1) Dissolve 4.515 g (NHi)4Ce(S04)4-2H20 in 500 ml water to which 30 ml H2SO4 had been added, cool, and dilute to volume.

Advisable to standardize against As203.

(2) Dissolve 3.913 g (NH4)2Ce(N03)6 in 10 ml H2SO4, stir 2min,

cautiously introduce 15 ml water and again stir 2 min. Repeat

addition of water and stirring until all the salt has dissolved, then

dilute to volume.

Dissolve 1.267 g CsCI and dilute to volume. Standardize: Pipette

25 ml of final solution to Pt dish, add 1 drop H2SO4, evaporate to dryness, and heat to constant weight at >800°C. Cs (in ^g/ml) =

(40)(0.734)(wt of residue)

Dissolve 1.648 g NaCl and dilute to volume.

(1) Dissolve 2.829 g K2Cr207 in water and dilute to volume. (2) Dissolve 1.000 g Cr in 10 ml HCl, and dilute to volume.

Dissolve 1.000 g Co in 10 ml of 2 M HCl, and dilute to volume.

(1) Dissolve 3.929 g fresh crystals of CuS04-5H20, and dilute to volume. (2) Dissolve 1.000 g Cu in 10 ml HCl plus 5 ml water to which HNO3 (or 30% H2O2) is added dropwise until dissolution is complete. Boil to expel oxides of nitrogen and chlorine, then dilute to volume.

Dissolve 1.1477 g DyjOj in 50 ml of 2 M HCl; dilute to volume.

Dissolve 1.1436 g ErjOj in 50 ml of 2 M HCl; dilute to volume.

Dissolve 1.1579 g EU2O3 in 50 ml of 2 M HCl; dilute to volume.

Dissolve 2.210 g NaF in water and dilute to volume.

Dissolve 1.152 g Gd203 in 50 ml of 2 M HCl; dilute to volume.

Dissolve 1.000 g Ga in 50 ml of 2 M HCl; dilute to volume.

Dissolve 1.4408 g Ge02 with 50 g oxalic acid in 100 ml of water;

dilute to volume.

253
JOHN A. DEAN /\ND THEODORE C. RAINS

Appendix 1: (Cont.)

Element Procedure

Gold

Hafnium

Holmium

Indium

Iodine

Iridium

Iron Lanthanum

Lead

Lithium

Lutetium

Magnesium

Manganese

Mercury

Molybdenum

Neodymium

Nickel

Niobium

Dissolve 1.000 g Au in 10 ml of hot HNO3 by dropwise addition of

HCI, boil to expel oxides of nitrogen and chlorine, and dilute to volume. Store in amber container away from light.

Transfer 1.000 g Hf to Pt dish, add 10 ml of 9 H2SO4, and then

slowly add HF dropwise until dissolution is complete. Dilute to volume with 10% H2SO,,.

Dissolve 1.1455 g H02O3 in 50 ml of 2 Af HCI; dilute to volume.

Dissolve 1 .000 g In in 50 ml of 2 M HCI ; dilute to volume.

Dissolve 1.308 g KI in water and dilute to volume.

(1) Dissolve 2.465 g NajlrCle in water and dilute to volume. (2) Transfer 1 .000 g Ir sponge to a glass tube, add 20 ml of HCI and

1 ml of HCIO,. Seal the tube and place in an oven at 300°C for 24 hr. Cool, break open the tube, transfer the solution to a volumetric flask, and dilute to volume. Consult (4) and observe all safety precautions in opening the glass tube. Dissolve 1.000 g Fe wire in 20 ml of 5 M HCI; dilute to volume.

Dissolve 1 . 1 7 1 7 g La,.03 (dried at 1 1 0°C) in 50 ml of 5 MHCI , and

dilute to volume.

(1) Dissolve 1.5985 g Pb(N03)2 in water plus 10 ml HNO3, and

dilute to volume. (2) Dissolve 1.000 g Pb in 10 ml HNO3, and dilute to volume.

Dissolve a slurry of 5.3228 g LiaCOj in 300 ml of water by addition

of 15 ml HCI, after release of CO2 by swirling, dilute to volume.

Dissolve 1.6079 g LuCIa in water and dilute to volume.

Dissolve 1.000 g Mg in 50 ml of 1 M HCI and dilute to volume.

(1) Dissolve 1.000 g Mn in 10 ml HCI plus 1 ml HNO3, and dilute to volume. (2) Dissolve 3.0764 g MnSOj HaO (dried at 105°C for 4 hr) in water and dilute to volume. (3) Dissolve 1.5824 g MnOa

in 10 HCI in a good hood, evaporate to gentle dryness, dissolve

residue in water and dilute to volume.

Dissolve 1.000 g Hg in 10 ml of 5 M HNO3 and dilute to volume.

(1) Dissolve 2.0425 g (NH,)2MoO, in water and dilute to volume.

(2) Dissolve 1 .5003 g M0O3 in 100 ml of 2 M ammonia, and dilute

to volume.

Dissolve 1.7373 g NdCU in 100 ml 1 M HCI and dilute to volume.

Dissolve 1 .000 g Ni in 10 ml hot HNO3, cool, and dilute to volume.

Transfer 1 .000 g Nb (or 1 .4305 g NbaOs) to Pt dish, add 20 ml HF,

and heat gently to complete dissolution. Cool, add 40 ml H2SO4,

and evaporate to fumes of SO3. Cool and dilute to volume with

8 M H2SO1.

254
13 STANDARD SOLUTIONS FOR FLAME SPECTROMETRY

Appendix 1: (Cont.)

Element Procedure

Osmium Dissolve 1 .3360 g OsOi in water and dilute to 100 ml. Prepare only

as needed as solution loses strength on standing unless Os is reduced by SO2 and water is replaced by 100 ml 0.1 M HGl.

Palladium Dissolve 1.000 g Pd in 10 ml of HNO3 by dropwise addition of

HCl to hot solution; dilute to volume.

Phosphorus Dissolve 4.260 g (NH4)2HP04 in water and dilute to volume.

Platinum Dissolve 1.000 g Pt in 40 ml of hot aqua regia, evaporate to

incipient dryness, add 10 ml HCl and again evaporate to moist

residue. Add 10 ml HCl and dilute to volume.

Potassium Dissolve 1.9067 g KCl (or 2.8415 g KNO3) in water and dilute to volume.

Praseodymium Dissolve 1.1703 g PraOj in 50 mi of 2 A/ HCl; dilute to volume.

Rhenium Dissolve 1.000 g Re in 10 ml of 8 M HNO3 in an ice bath until

initial reaction subsides, then dilute to volume.

Rhodium Dissolve 1 .000 g Rh by the sealed-tube method described under

iridium (4). Rubidium Dissolve 1.4148 g RbCl in water. Standardize as described under

cesium. Rb (in /Mg/ml) = (40)(0.320)(wt of residue).

Ruthenium Dissolve 1 .317 g RuOz in 15 ml of HCl; dilute to volume.

Samarium Dissolve 1.1596 g SmjOj in 50 ml of 2 M HCl; dilute to volume.

Scandium Dissolve 1.5338 g SC2O3 in 50 ml of 2 M HCl; dilute to volume.

Selenium Dissolve 1 .4050 g SeOz in water and dilute to volume or dissolve 1.000 g Se in 5 ml .of HNO3, then dilute to volume.

Silicon Fuse 2.1393 g SiOa with 4.60 g NaaCOj, maintaining melt for 15 min in Pt crucible. Cool, dissolve in warm water, and dilute to volume. Solution contains also 2000 /^g/ml sodium.

Silver (1) Dissolve 1.5748 g AgNOa in water and dilute to volume. (2) Dissolve 1.000 g Ag in 10 ml of HNO3; dilute to volume. Store in amber glass container away from light.

Sodium Dissolve 2.5421 g NaCl in water and dilute to volume.

Strontium Dissolve a slurry of 1.6849 g SrCOj in 300 ml of water by careful addition of 10 ml of HCl, after release of COj by swirling, dilute to volume.

Sulfur Dissolve 4.122 g (NH4)2S04 in water and dilute to volume.

Tantalum Transfer 1.000 g Ta (or 1 .2210 g Ta^O^) to Pt dish, add 20 ml of

HF, and heat gently to complete the dissolution. Cool, add 40 ml

of H2SO4 and evaporate to heavy fumes of SO3. Cool and dilute to volume with 50% H2S04.

Tellurium (1) Dissolve 1 .2508 g TeOa in 10 ml of HCl; dilute to volume. (2) Dissolve 1.000 g Te in 10 ml of warm HCl with dropwise addition

of HNO3, then dilute to volume.

255
JOHN A. DEAN AND THEODORE C. RAINS

Appendix 1: (Cont.)

Element Procedure

Terbium Dissolve 1 .6692 g of TbCls in water, add 1 ml of HCl, and dilute to volume.

Thallium Dissolve 1 .3034 g TINO3 in water and dilute to volume.

Thorium Dissolve 2.3794 g Th(N03)4-4H20 in water, add 5 ml HNO3, and

dilute to volume.

Thulium Dissolve 1.142 g TmaOj in 50 ml of 2 M HCl; dilute to volume.

Tin Dissolve 1.000 g Sn in 15 ml of warm HCl; dilute to volume.

Titanium Dissolve 1.000 g Ti in 10 ml of H2SO4 with dropwise addition of

HNO3; dilute to volume with 5% HSOf

Tungsten Dissolve 1.7941 g of Na2W04-2H20 in water and dilute to volume.

Uranium Dissolve 2.1095 g U02(N03)2-6H20 (or 1.7734 g uranyl acetate dihydrate) in water and dilute to volume.

Vanadium Dissolve 2.2963 g NH4VO3 in 100 ml of water plus lOmlofHNOj;

dilute to volume.

Ytterbium Dissolve 1.6147 g YbCl3 in water and dilute to volume.

Yttrium Dissolve 1.2692 g Y2O3 in 50 ml of 2 M HCl and dilute to volume.

Zinc Dissolve 1 .000 g Zn in 10 ml of HCl ; dilute to volume.

Zirconium Dissolve 3.533 g ZrOCla SHaO in 50 ml of 2 M HCl, and dilute to volume. Solution should be standardized.

Appendix 2: Special Aqueous Stock Solutions

Concentration Procedure

Element

Barium 0.0625 Dissolve 6.506 g BaCla (dried at 250°C for 2 hr), dilute to volume. One ml is equivalent

to 1 .00 mg of sulfate sulfur. Calcium 100 meq per liter Dissolve a slurry of 5.006 g CaC03 (dried

at 105°C) in 300 ml of water by careful addition of 10 ml HCl; after CO2 is re- leased, dilute to volume.

Lithium 200 meq per liter Dissolve a slurry of 7.392 g U2CO3 in 300

(as Li) ml of water by addition of 15 ml HCl; after release of CO2, dilute to volume.

2000 /^g/ml (as Dissolve 4.945 g Li2C03 as above.

LUO)

256
13 STANDARD SOLUTIONS FOR FLAME SPECTROMETRY

Appendix 2: (Cont.)

Concentration Procedure

Magnesium 100 meq per liter Dissolve J.2156g Mg in 50 ml of 1 MHCl;

(as Mg) dilute to volume.

Manganese 1000/«g/ml (as Dissolve 2.1412 g MnSOi HjO (dried at MnaOj) 105°C for 4 hr) and dilute to volume.

Potassium 100 meq per liter Dissolve 7.456 g KCl and dilute to volume.

(as K)

1000 ngim\ (as Dissolve 1 .5830 g KCl and dilute to volume.

K2O)

Sodium 100 meq per liter (as Na)

Dissolve 5.846 g NaCl and dilute to volume.

1000 ^g/ml (as Dissolve 1 .886 g NaCl and dilute to volume.

Na^O)

Strontium 1000 /(g/ml (as Dissolve a slurry of 1 .6848 g SrCOs as SrO) described for calcium.

Mixture

Alkali- 1000 //g/ml each Dissolve in water 1 .886 g NaCl, 1 .583 g

manganese Na^O, K2O, Mn^Os KCl, and 2.1412 g MnSOi HaO (dried at solution 105°C for 4 hr) and dilute to volume.

Lime-acid 6300 //g/ml CaO in Add 100 ml HCI to 11.244 g CaCOa

solution 1:9 HCI suspended in 300 ml of water ; after CO2 is released, dilute to volume.

Sea water Chlorinity 38% Dissolve 47.0 g NaCl, 21.3 g MgCl2-6H20,

17.8 g Na^SOi-lOHaO, 2.20 g CaClj, 1.46 g

KCl, 0.38 g NaHCOa, and 0.015 g SrCU;

and dilute to volume.

Appendix 3: Standard Reference Materials in Lubricating Oil

(Element of Interest, 500 (xg/g)

NBS No."

Element of

interest Compound

Drying

time, hr"

Weight

used,^''

1075a Al 2-Ethylhexanoate 48 0.620

1051b Ba Cyclohexanebutyrate 24 0.174

1063a B Menthyl borate 2 2.08 1053 Cd Cyclohexanebutyrate 48 0.202

1074a Ca 2-Ethylhexanoate 48 0.373

257
JOHN A. DEAN AND THEODORE C. RAINS

Appendix 3: (Cont.)

Element of Drying Weight

NBS No." interest Compound time, hr" used, g"

1078 Cr Tris(l-phenyl-l ,3-butanediono)- 1 0.515

chromium(III)

1055b Co Cyclohexanebutyrate 24 0.337

1080 Cu Bis(l-phenyl-l ,3-butanediono)- 0.5 0.303

copper(II)

lQ79b Fe Tris(l -phenyl- 1 ,3-butanediono)- 1 0.485"

iron(lll)

1059b Pb Cyclohexanebutyrate 48 0.136

1060a Li Cyclohexanebutyrate 24 1.22 1061b Mg Cyclohexanebutyrate 48 0.765

1062a Mn Manganese(II) 48 0.362

cyclohexanebutyrate

1064 Hg Mercury(II) 2 0.138

cyclohexanebutyrate

1065b Ni Cyclohexanebutyrate 48 0.360

1071a P Triphenyl phosphate 2 0.526

1076 K Erucate 2 0.495

1066 Si Octaphenylcyclotetrasiloxane 0 0.141'

1077a Ag 2-Ethylhexanoate 24 0.117

1069b Na Cyclohexanebutyrate 48 0.417

1070a Sr Cyclohexanebutyrate 24 0.242

1057b Sn Dibutyltin bis(2-ethylhexanoate) 2 0.218

1052b V Bis(l -phenyl- 1 ,3-butanediono)- 2 0.348

oxovanadium(IV)

1073b Zn Cyclohexanebutyrate 48 0.299

" National Bureau of Standards (U.S.) standard reference material.

" Dried over PjOs in desiccator.

" Weight of dried material which contains 50 mg of element of interest. "Dried in oven at 110°C.

' Weight of element, 20 mg, and gives a final concentration of 200 /(g/g.

REFERENCES

1. R. E. Michaelis (ed.). Report on Available Standard Samples, Reference Samples,

and High-Purity Materials for Spectrochemical Analysis, ASTM Data Series

DS-2, American Society for Testing Materials, Philadelphia, Pa., 1963.

2. Catalog and Price List of Standard Materials Issued by The National Bureau of

Standards, NBS Spec. Publ. 260, U.S. Government Printing Office, Washington,

D.C.

258
13 STANDARD SOLUTIONS FOR FLAME SPECTROMETRY

3. I. M. Kolthoff and P. J. Elving (eds.), Treatise on Analytical Chemistry, Part II (12 vols), Interscience-Wiley, New York.

4. W. F. Hillebrand, G. E. F. Lundell, H. A. Bright, and J. I. Hoffman, Applied

Inorganic Analysis, 2nd ed., Wiley, New York, 1950, pp. 343-351.

5. H. S. Isbell, R. S. Tipson, J. L. Hague, B. F. Scribner, W. H. Smith, C. W. R. Wade, and A. Cohen, Analytical Standards for Trace Elements in Petroleum

Products, National Bureau of Standards Monograph 54, U.S. Government Printing

Office, Washington, D.C., 1962.

259

1
APPENDIX XV

Reprinted from JOURNAL OF THE AOAC, Vol. 55, No. 6, 1972, pp. 1339-1344.

Determination of Submicrogram Amounts of Mercury in

Standard Reference Materials by Flameless Atomic Absorption

Spectrometry

By THEODORE C. RAINS and OSCAR MENIS

Bureau of Standards, Washington, D.C. 20234)

In a study of the flameless atomic absorption

method for the determination of nanogram

amounts of mercury in organic materials, the

technique was improved to provide greater

precision and accuracy. The loss of mercury

during the digestion of organic materials with

nitric, sulfuric, and perchloric acids is prevented by controlled heating and a packed

refluxing column. The reduction and absorption system was simplified by using a heated

absorption cell which alleviates the interfer- ence of volatile vapors. This method has been

applied to the determination of mercury in

NBS Standard Reference Materials of orchard

leaves, liver, and coal, and the results were compared with results obtained with 2 other

analytical methods.

As part of the program for characterizing

botanical and related materials for trace elements,

flameless atomic absorption was evaluated for the

determination of mercury in orchard leaves, liver,

and coal. Flameless atomic absorption yielded

results comparable in accuracy and precision to

results obtained by all other methods compared

for the determination of nanogram quantities of

mercury.

As early as 1930 Miiller (1) took advantage of

the high vapor pressure of mercury for its determination in air and in 1939 Woodson (2) described an instrument used for the same purpose. Since

this early work, many workers have used the same general approach. A review of flameless

methods for the determination of mercury by

atomic absorption spectrometry has been pre- sented by Manning (3). Hatch and Ott (4) used the cold vapor technique for determining mercury in nickel and co- balt metals and in rock samples. The cloud sys- tem was used in which the mercury vai)or was

continuously recirculated until a maximum signal was attained. The i)rocedure included a cumber- some technique for the removal of water vai)or

(Analytical Chemistry Division, National

prior to final measurement and frequent changing

of the drying agent was essential to obtain reproducible values. In addition, the precision and

accuracy necessary for the analysis of standard

reference materials were not attainable with the

method of Hatch and Ott. Further studies in our

laboratory led to modifications of the general procedure. These included selection of the dissolution step, elimination of the drying agents, and

provision for the heating of the absorption cell. Other parameters, such as techniques to minimize

the contamination of the environment, selection

of a reductant, control of reaction volume, choice

of purge gas, and the role of interfering substances, were studied. Also, the use of a double

beam (ratio method) was investigated.

METHOD

Reagents

(a) Mercury standard solutions.—(/) Stock solution. —1.000 mg/ml. Dissolve 1.000 g ultra high

purity mercury (99.99%) in O.liV HNO3 and dilute

to 1 L in same solvent. (2) Working solution.—Pre- pare fresh daily by transferring 10.0 ml stock solution

to 1 L volumetric flask and dilute to volume with

O.ITV^ HNO3. Mix and transfer 1.0 ml of this solution

to 1 L volumetric flask and dilute to volume with

O.IA' HNO3.

(b) Reducing solution, —To 500 ml volumetric

flask, add 300 ml water and 50 ml H2SO4. Cool to ambient temperature, add 15 g NaCl, and shake to

dissolve. After NaCi is completely dissolved, add 25

g SnCla, shake until .solution occurs, and dilute to volume.

(c) Diluting solution.—Aqueous acid solution lA^

in HNO3 and 2.4iV in H2SO4. U.se ultra high purity

acids and check for mercury content before u.se. (Use

only acids which contain <0.5 ng Hg/ml.) Use distilled water pas.sed through mixed ion exchange

column resin bed prior to use.

Apparatus

(a) Atomic absorption spectromeler. —Dcnible beam

(ratio method) instrument, consisting of 0.5 and

260
JOURNAL OF THE AOAC (Vol. 55, No. 6, 1972)

0.25 m Ebert mount monochromators (Jarrell-Ash)

with gratings blazed for 3000 A, beam splitter, mechanical chopper, electronic circuitry comprised of 4 units (0-2100 v multiplier phototube power supply, selective amplifier and syi\chronou.s detector,

ratio converter, and l)CK-2 digital readout), and

chart recorder. Schematic diagram of system is shown in Fig. 1. In ratio mode, use 2 identical channels, one for signal A (Hg line at 2.");{7 .\) and the other for signal

B (Al line at 26.')2 .\). Use signal B as reference for obtaining A/B ratio. Input of each channel is derived from multiplier phototube powered by single

dc power .source. Output of photodetector is amplified and fed into digital readout. In absorption mode,

outputs of 2 filter-amplifier stages provide 2 potentials for ratio mea-surement.

(b) Mercury hollow cathode lamp.—Powered by

stable power supply and operated at 8 ma. Aluminum signal is obtained from aluminum-mercury

amalgam on aluminum sleeve in lamp. Any commercially available hollow cathode lamp capable of providing both mercury and aluminum signals is acceptable.

(c) Absorption cell. —Construct from 30 cm X 0.8 cm id Vycor tubing with ga.s inlet and outlet ports

and quartz windows. Secure quartz windows to Vycor tubing with epoxy cement. Wrap cell with

heating tape to maintain surface temperature at 200°C with aid of variable autotransformer.

(d) Reduction cell. —12.') ml gas-washing bottle with coarse fritted bubbler. Introduce purge gas into

reduction cell through flowmeter (0.3 L/min) and

Vfe" Teflon tube connecting reduction cell to absorption cell (length of connecting tube is not critical).

(e) Digestion flask. —250 ml flat-bottom boiling

flask with T 24/40 joint, 300 mm Allihn condenser

filled to .50 mm with Raschig rings and glass beads,

and heating mantle.

Calibration

(Note: Use ratio mode for all samples in which an

interferent is suspected. If no interferent is present, use single beam system.) In the ratio mode, adjust 0.5 and 0.25 m monochromators to read 100% transmission at Hg 2537

and Al 2652 lines, respectively. For the single beam

system, adjust only 0.5 m monochromator to read

100% transmission at Hg 2537 A. Heat absorption

cell to 200°C and adjust argon flow through reduction cell and absorption cell to 0.25 L/min. Transfer

aliquots of standard mercury working solution containing 0.01-0.2 Mg Hg to reduction cell and dilute

to 25 ml with diluting solution. Add 20 ml reducing

solution and immediately close system. Record max- imum absorbance and minimum ratio value of Hg/

TO

EXHAUST

Hg

H.C.L.

1

H C.L POWER SUPPLY

CHOPPER

MONOCHROMATORS

-0.25m

HEATINC TAPE

30 cm

ABSORPTIONCELL

f M TUBE POWER SUPPLY REFERENCE

SIGNAL = DUAL AMPLIFIER

VOLTAGE

DIVIDER

RATIO

I JOINT

-ARGON

REDUCTION CELL

LOGARITHMIC

DIGITAL VOLTMETER

FIG. 1—Schematic diagram of instrumentation for flametess atomic absorption spectrometry.

261
RAINS 4 MENIS: MERCURY IN STANDARD REFERENCE MATERIALS

Al line from digital readouts. Plot absorbance and/or

ratio values obtained independently against /ig Hg

to give linear calibration curve. Absorbance signal appears immediately, reaches

maximum within 20-30 sec, remains constant ca 5

sec, and then slowly decreases to zero. Time required

for signal to return to background under these conditions is ca 2 min. To speed up return of signal to

baseline, remove aeration tube from sample solution

in reduction cell after peak absorbance has been re- corded and insert tube into clean flask containing

dilute HNO3. Analysis is completed within 1 min if this technique is used.

Determination

Transfer 2-3 g sample to digestion flask. Add 25

ml 18iV IhSOi and 20 ml 7N HNO3 and connect

Allihn condenser with cooling water. Using heating

mantle, gently heat flask 1 hr at 150°C. (See Note

for digestion of coal samples.) Cool flask and add 20

ml HNO3-HCIO4 (1 + 1) through condenser. Drain

water from condenser and heat flask until dense

fumes of perchlorate appear. Cool and rinse con- denser with two 5 ml portions of water. Reheat flask to dense fumes of perchlorate. Cool and transfer solution to volumetric flask and dilute to volume.

Note: For coal samples, digestion time will vary

with particle size. Carry out HCIO4 digestion of coal

behind safety shield.

Results and Discussion

Digestion.—The digestion step is critical, since

mercury is easily reduced and subsequently lost

in the vapor phase during the destruction of or- ganic material. Therefore, several methods for digestion were screened. Combustion by the

Schoninger method was examined; however, this

technique is governed by flask size. For a 1 L

flask, the amount of organic material was limited

to 0.2 g. The sealed tube method (5) employing

nitric and sulfuric acids was also investigated for digesting organic material (orchard leaves) ; however, 2 difficulties arose. The maximum sample

size was again limited to 0.2 g and the oxides of

nitrogen which were liberated during the digestion caused excessive frothing in the subsequent

reduction step. To overcome this difficulty, the

solution was removed from the sealed tube and

gently heated. Under these conditions, and even though the mercury is present as Hg^"*", there is danger of loss of trace quantities of mercury (6). The most successful digestion procedure was a modification of the one described by Munns and

Holland (7). Initially, frothing and physical losses were difficult to control when refluxing was done

by flame. By carefully controlling the heating

rate with a heating mantle, foaming was pre- vented and antifoaming agents were unnecessary.

Organic materials such as orchard leaves, liver,

and coal were successfully digested without loss of mercury, using a mixture of nitric, sulfuric, and

perchloric acids.

The recovery of mercury was verified in a study

using orchard leaves and ^^^Hg tracer. The tracer was counted with a single channel scintillation

gamma counter. As shown in Table 1, loss of free mercury will occur unless the water condenser

contains Raschig rings. A 20-30% loss of activity was observed when a sample of orchard leaves

with ^'''Hg added was digested under reflux conditions to oxidize the mercury and then transferred to a beaker to reduce the volume of acid

with heat. Only if mercury carrier with ^"^Hg was

added to the sample solution which was subsequently evaporated did the loss of ^"^Hg become

negligible.

It was observed that carrier-free mercury had a tendency to escape. Experiments with 0.1 ml

carrier-free ^osjjg in a 10 X 100 cm counting tube

led to a reduction of 20%/day in the counting

rate. Upon investigation, it was found that the

203Hg activity collected on the cork stopper. This

occurred regardless of the acidity (nitric or sulfuric acid) of the tracer. However, in the presence

of 100 ^ig mercury carrier no loss of ^^^Hg to the

cork stopper of the counting tube was noted.

Interferences of Vapors. —Several methods are reported for trapping vapors which enter the absorption cell and lead to light scattering or molecular absorption. Either a cold trap with a mixture of salt and ice ( — lO^C) or drying agents

such as magnesium perchlorate or anhydrous cal- cium sulfate are used (3). Each method was tested

and was found satisfactory for only a short

operating period. Even a vapor trap containing

glass beads or glass wool was found to be unsatisfactory. The most successful method was to pass

the mercury vapor from the reduction cell directly into the absorption cell by means of a

Teflon tube. Since moisture condenses on the window of the absorption cell, the cell was heated

with heating tape to 200°C. At this temperature,

as shown in Table 2, there was no shift in the

baseline over a 2 hr period while the baseline was

constantly changing at the other absorption cell

Received April 7, 1972.

262
JOURNAL OF THE AOAC (Vol. 55, No. 6, 1972)

TabI* 1. Loss of '°'Hg in orchard loavos

during digestion"

Detn Loss of No. Conditions of Digestion ^°'Hg, %

1 digestion flasit, water condenser, and 0.2 Raschig rings

2 digestion flask, water condenser 4.1

3 250 ml Erienmeyer flask, cover lid 12.3

4 250 ml beaker, cover lid 61.7

5 50 ml solution from No. 1 evaporated 23.3

in beaker to 2 ml

" Digestion flask contained 1 g orchard leaves, 25 ml

18/V H2SO4, 20 ml IN HNO3, and 10 ml HCIO4. "'Hg

present as tracer gave 1 X 10* counts/min before di- gestion was started.

temperatures. With this arrangement (Fig. 1) the

flow rate of sweep gas was maintained at 0.2-

0.3 L/min. Flow rates above 0.5 L/miu introduced excessive amounts of moisture into the

absorption cell and caused a continuous baseline

drift. Reduction of Mercury.—Stannous chloride,

stannous chloride with hydroxylamine hydrochloride, and hypophosphorous acid were investigated as reducing agents for mercury. Hydroxylamine hydrochloride is normally added with the

stannous chloride as an aid in reducing excess oxidants; however, the hydroxylamine hydrochloride was a constant source of mercury con- tamination, resulting in a high reagent blank. To

overcome this difficulty, the hydroxj'laniine hydrochloride was eliminated from the i)rocedure

with no adverse effects. Hypophosphorous acid was about as effective as stannous chloride as a reductant for .solutions of mercury in dilute nitric acid. However, in solutions from samples of orchard leaves and coal, the

recoveries ranged from 0 to 90%. Also, excessive

foaming was ob.served in the reduction cell in the

presence of hypophosphorous acid. In some tests, foaming also occurred with stan- nous chloride as the reductant. To minimize this problem in the reduction cell, all traces of organic

material must be removed. To aid in the removal

of organic material during the digestion step, the

sample is taken to strong fumes of perchlorate.

After the sample has cooled 5 min, the reflux con- denser is rinsed with water and the sample is re- heated to strong fumes of perchlorate.

Effect of Volume in the Reduction Cell. —When

the reduction is carried out in a 125 ml vessel, the

absorbance reading is constant for sample vol- umes of 5-30 ml. Sample volumes greater f^han

Table 2. Effect of temperature of absorption ceii on

absorption of mercury and shift in baseline

from zero absorbance over a 2 hr period

Temp, of Absorption Shift in Absorbance of

Cell, "C Baseline 0.3^8 Hg

200 0.000 1.113

170 0.010 1.100

155 0.150 1.045

145 0.224 1.085

135 0.260 1.140

125 0.306 1.197

105 0.432 1.027

30 ml give a slightly lower absorbance reading.

For a sample volume of 75 ml, a 30% lo.ss in absorbance is ob.served.

Effect of Sweep Gas.—In the described flowthrough system for mercury, argon and air were

investigated as sweep ga.ses. In all tests, argon was found to be .superior with a 25% increase in ab.sorbance for mercury.

Since foaming in the reduction reaction ve.s.sel

is a problem a.s.sociated with some types of materials, the effect of a i)urge with argon or air,

prior to the reduction of mercury, was studied.

With a prior jmrge of the .sami)le solution, a 25-

75% loss of mercury was observed. The loss of mercury without a reducing agent has j)eeri attrilnited to the reduction of Hg^* to Hg+, which

subsequently disproportionates to Hg^"*" and Hg°

(6). Trace quantities of organic material which are known to be ])resent in water-])umi)e(l air or argon

gas are sufficient to reduce nanogram quantities

of mercury. If a prior purge of the .sami)le is needed, an excess of oxidant such as i)ota.ssium

permanganate must be ])re.sent to prevent mercury loss.

Interferences.—The role of acidity and anion

interference was studied. The effect of a number

of anions on the release of atomic mercury is shown in Table 3. In these studies, solutions containing 0.1 mercury in the presence of various

anions as the acid, excei)t for the iodide and

bromide, were carried through the i)rocedure.

The reference solution contained 0.1 ^g mercurj'

in O.OOLV HNO3. It is evident from this study

that the anions of all acids do affect the relea.se of mercury in varying degrees, es])ecially iodide,

bromide, sulfate, and jihosphate. In the i)resence of minor quantities of cations,

Mention of firm names or trade products does not imply endorsement or recommendation by tlie National Bureau of Standards over other firms or similar products not mentioned.

263
RAINS A MENIS: MERCURY IN STANDARD REFERENCE MATERIALS

Table 3. Effect of extraneous anions on recovery

Af 1

Anion Concn, N Rec, %

NOa"' 0.06 95 0.6 88 3.0 88 6.0 81 9.0 86

CI ' 0.4 95 1.6 97 3.6 94 5.4 82 7.2 73

CIO4-' 0.5 97 1.0 88 2.4 82 4.8 66

Br-"" 0.01 96 0.02 75

I-'" 0.002 97 0.02 3 504"^ 0.4 99 0.8 95 3.6 78 7.2 50

Cj04-» 0.2 92 0.4 90 0.6 77

PO4-' 0.18 98 1.8 82 9.0 65

" Total mercury present = 0.1 ^tg.

* Added as the sodium salt.

no adverse effect was observed. However, when

an e.xcess of reducible ion was present, e.g., CraOy"^, the mercury was not released, owing to an insufficient quantity of reducing agent.

Contamination Problems.—Contamination from

reagents, glassware, and the environment were

encountered with mercury. To minimize this

contamination, all glassware was rinsed with 10%

nitric acid. Samples stored in glass for over one week showed that values for mercury increased

by 50-100%. Therefore, polypropylene flasks were used for storage. To illustrate the degree of

contamination of reagents by mercury, 3 bottles

of the same lot of hydro.xylamine hydrochloride were analyzed and were found to contain 0.008,

0.027, and 0.031 ng Hg/g, respectively. In an- other lot, the mercury content was 0.001

Hg/g. Ultra high purity acids were also tested. Some lots of ACS reagent grade nitric acid contained as much as 30 ng Hg/ml and had to be

redistilled by sub-boiling distillation from an all- quartz still.

Table 4. Comparison of results obtained with

different methods for the determination of mercury in NBS Standard Reference

Materials"

Rel. Std

No. of Hg, Std Dev., Method Determinations |ig/g Dev.* %

Orchard Leaves (NBS-SRM-1571)

AAS-^ (single beam) 7 0.160 0.006 4.0 AAS (ratio) 6 0.160 0.006 4.0 Neutron activation 11 0.155 0.003 1.8 Isotopic dilution mass spectrometry 7 0.14 0.02 16

Liver (NBS-SRM-1577)

AAS (single beam) 6 0.0165 0.0008 5 Neutron activation 9 0.0145 0.0017 12

Coal (NBS-SRM-1630)

AAS (single beam) 7 0.139 0.007 4.7 Neutron activation 55 0.127 0.006 4.8

" NBS Standard Reference Materials are available

from the Office of Standard Reference Materials, National Bureau of Standards, Washington, D.C. 20234.

' For a single determination.

AAS = atomic absorption spectrometry.

Another possible source of contamination is the vacuum oven and/or desiccant. Samples of or- chard leaves, which were dried at different intervals over a period of 3 months in 2 different vacuum ovens at 90°C for 24 hr and then cooled

in a desiccator over anhydrous calcium sulfate,

produced mercury values which were 25-50%

higher than the undried samples. The anhydrous

calcium sulfate was tested and was found to contain traces of mercury. These studies reinforce

the conclusion for the need of careful checks on

all sources of possible contamination.

Comparison of Single Beam and Double Beam

{Ratio) Systems.—This study was designed to evaluate the accuracy of the flameless atomic

absorption method for determining mercury. By

comparing data in the literature by the flameless

method with other analytical techniques, we observed that the flameless method produced higher

values. The higher values may have resulted from

improper background correction due to molecular

absorption or from scattering of light. To correct these 2 .sources of error, mercury was determined

in orchard leaves by both the single beam and

double beam (ratio) system. As shown in Table 4,

the 2 methods gave the same values when the

264
JOURNAL OF THE AOAC (Vol. 55, No. 6, 1972)

proposed method was used. However, when the

temperature of the absorption cell was less than

145*'C, the absorption values for mercury were

erratic and higher in the single beam system. In

this case, by taking a ratio of the Hg line at 2537

A to the Al line at 2652 A in the double beam

system, a correction for light scattering and/or

molecular absorption, due to water and other

vapors, was made.

Application of the Method.—The method was

tested on 3 types of NBS Standard Reference

Materials as shown in Table 4. This technique has

also been used for the determination of nanogram

quantities of mercury in samples of aluminum

alloys, reactor cooling water, and water effluents. In summary, this study reveals that flameless

atomic absorption analysis of mercury can be

made to be precise and accurate in comparison

with 2 independent techniques.

Acknowledgments

The authors would like to thank E. C. Kuehner,

T. J. Murphy, and associates for making available

the special purity reagents.

References

(1) Muller, K. (1930) Z. Physik 65, 739-758

(2) Woodson, T. T. (1939) Rev. Sci. Instr. 10, 308

(3) Manning, D. C. (1970). At. Absorption Newsletter 9, 97-99

(4) Hatch, W. R., & Ott, W. L. (1968) Anal. Chem.

40, 2085-2087

(5) Hillebrand, W. F., Lundell, G. E. F., Bright,

H. A., & Hoffman, J. I. (1953) Applied Inor- ganic Analysis, 2nd Ed., Wiley, New York,

pp. 347-351

(6) Toribara, T. Y., Shields, C. P., & Koval, L. (1970) Talanta 17, 1025-1028

(7) Munns, R. K., & Holland, D. C. (1971) JAOAC

54, 202-205

265
Exlrait de la revne du GAMS, 1971, n" 3, p. 39. APPENDIX XVI

DISCUSSION OF SOME EXPERIMENTAL

AND FUNDAMENTAL CONDITIONS

IN ANALYTICAL FLAME SPECTROSCOPY

by R. MAVRODINEANU

Institute for Materials Research, National Bureau of Standards, Washington, B.C. 20234, U.S.A.

« Do not all fix'd Bodies, when heated beyond a certain degree,

emit Light and shine; and is not this Emission perform'd

by the vibrating motions of their parts? » Sir Isaac Newton.

Opticks, 1730.

The first part of this paper will be devoted to a more detailed

discription of this measuring tool which was designed and

built at NBS and is now used for analytical work and for studies

of various flame parameters.

In the second part the reader will find a general discussion of

nonthermal excitation phenomena occurring in combustion

flames. The particular case of chilled hydrogen flames will be

examined and illustrated with two examples involving the exci- tation of sulfur and phosphorus contained in air as inorganic

and organic molecules.

Discussion de quelques conditions experimentales

et fondamentales en spectroscopie analytique de flamme.

La premidre partie de ce travail est consacr6e a une description

d^taillde des procdd^s de mesure qui ont 6t6 congus et r6alis6s au National Bureau of Standards, et maintenant utilises dans

les travaux analytiques et pour I'^tude des divers param^tres

de flamme.

Dans la seconde partie, le lecteur trouvera une discussion g6nerale des ph6nom6nes d'excitation non thermique se produisant

dans les flammes de combustion. Le cas particulier de la flamme

hydrogdne refroidie, est examine et illustr6 de deux exemples

sur I'excitation du soufre et du phosphore contenus dans I'air, sous forme de molecules min^rales et organiques.

Diskussion iiber einige experimentelle

und grundlegende Bedingungen

in der analytischen Flamm-Spektroskopie.

Der erste Teil dieser Arbeiten wird einer ins Einzelne gehenden

Beschreibung der Messverfahren gewidmet, welche im National

Bureau of Standards ausgearbeitet und durchgefiihrt wurden,

und jetzt fiir die Analysen und die Studie der diversen Flammparameter zur Anwendung kommen.

Im zweiten Teil findet der Leser eine allgemeine Beschreibung

uber nicht thermische Erregungsvorgange, die in der Verbrennungsflamme auftreten. Der Sonderfall der gekuhlten Wasserstofflamme wird gepruft und an zwei Beispielen iiber die Erre- gung des in der Luft enthaltenen Schwefels und L-Phosphors

erlautert, in Form von mineralischen und organischen Molekiilen.

1. — Introduction.

This discussion will cover certain aspects of the

experimental and fundamental nature of flame

spectroscopy when used as a measuring tool in analytical chemistry. The underlying principles of this

measuring tool have been known for a long time,

and are based on the phenomenon that matter,

when properly energized, may emit or absorb the

exciting energy as electromagnetic radiations having

frequencies characteristic of the excited species and

whose energy i? proportional to the number of

particles involved in the process. Thus the frequencies

radiated or absorbed serve to identify the excited

species, whereas the amount of radiation gives a measure of the quantity of matter involved in the

phenomenon. These properties constitute the basis

of qualitative and quantitative analytical flame

spectroscopy.

Instruments for making measurements in the emission mode have been known for a long time and

include an exciting flame with its analytical sample

supplying system, a radiation-dispersing instrument,

and a photosensitive detector. These basic components are common to all emission spectrophotometers and can be found in the earliest instruments.

Figure 1 describes the arrangement used in 1860

by KmcHHOFF and Bunsen for their spectroscopic

investigations (^). The same basic elements were

applied 13 years later by CHAMPioNr, Pellet and

Grenier who produced in 1873 the first quantitative

emission spectrophotometer (^). The instrument,

named by its developers the « spectronatrometre », was based on the principle enunciated by Janssen

Fig. 1. — Components used by Bunsen and Kirchhoff

in their spectroscopic investigations. At the right, the flame

excitation source with its sample supplying system; in the middle, the spectroscope; the photosensitive device

used to observe the radiations was, in this case, the human

eye.

266
in 1870 The same principle and instrumental

elements have been used by Lundegardh in 1929

and 1934 in his pioneering work on analytical flame

spectroscopy, and are found today in every emission

flame spectrophotometer (*). The instrumental means for performing measure- ments in the atomic absorption mode include, in

addition to those mentioned for the emission mode

of operation, a source emitting those radiations

which originate from the chemical species to

be determined. Again, as was true for the flame

emission mode, the principle and the instrumental

elements of flame atomic absorption have been

known for a long time.

The optical arrangement described in 1872 and

1873 by John Tyndall and shown in figure 2 (^) can be used to illustrate the point. One can see the emission source is an electric arc, followed by

the absorbing flame with its sample-supplying system, and finally a means to observe the absorption

phenomenon. The same arrangement was used in

1953 by Alan Walsh and described in his pioneering

works on the application of the atomic absorption

principle to chemical analysis (®). These same components are found today in all analytical instruments

using the atomic absorption principle.

Fig. 2. — Optical arrangement used by Tyndall to demonstrate tlie principle of atomic absorption. From left to right,

emission source followed by the absorbing flame with its sample supplying system and the means to produce and

observe the spectrum.

The instrumentation which will be described now,

contains the basic components described above.

It was designed to utihze the fundamental property

of spectral sources to emit or absorb simultaneously

at all transitions originating from all species which

are excited in the particular source considered. As

a consequence of this property, the measuring system

was designed and built to detect and measure simultaneously a number of individual radiations emitted

or absorbed in the wavelength region of about

2 000 A to 10.000 A. The radiations are detected

and their intensities are measured in a conventional

manner using appropriate photomultipher tubes and

integration of the optical signal over a predetermined

time interval. The current developed in the photomultipliers, which is related to the radiation intensity originating from the excitation source and to

the concentration of the excited species, is displayed

both in analog and digital form. The digital information is printed and punched simultaneously on

appropriate tapes. The punched tape is compatible

for direct use with a computer programmed to

produce meaningful data to the analytical chemist.

While this instrumentation does not include any

original or new element, it is believed, nevertheless,

that the combination of principles and instrumental

elements will provide a final measuring tool showing

improvements and advantages over the more conventional flame spectrometers of the past.

The first part of this paper will be devoted to a more detailed description of this measuring tool

which was designed and built at NBS and is now

used for analytical work and for studies of various

flame parameters.

In the second part the reader will find a general

discussion of nonthermal excitation phenomena

occuring in combustion flames. The particular case

of chilled hydrogen flames will be examined and

illustrated with two examples involving the excitation of sulfur and phosphorus contained in air as inorganic and organic molecules.

2. — Instrumentation.

2.1. The Spectrometer.

The spectrometer used to detect the radiations

originating from the exciting flame is a conventional 1 m grating instrument using the CzernyTurner optical arrangement illustrated in figure 3 ('). The bilateral variable entrance slit is seen at the

left of the instrument. It is followed by a colli- mating concave mirror which projects a parallel

radiation beam on an interchangeable flat grating

(100 mm by 100 mm carrying 600 lines/mm or

1 200 lines /mm).

The difltracted radiations are focused as monochromatic images of the entrance slit at the focal

plane, contained in the rectangular light-tight enclo- sure at the right of the spectrometer. A vertical

frame is located at the focal plane where it can

be moved up and down through a mechanism

operated from the outside. It is provided with a counter to indicate its position. The frame can

accept interchangeably a bilateral variable exit slit or a cassette capable of holding a photographic

plate or film 20 cm by 5 cm. An adapter for Polaroid

267
Fig. 3. — NBS flame spectrometer for simultaneous multielement emission and atomic absorption measurements. At the

left, console with the electronics used in the scanning mode with digital and analog data presentation. In the

center, 1 m Czerny-Turner type scanning monochromator-spectrograph-multielement instrument. At the right, lathe- bed type Optical bench carrying the all-alumina sprayer and burner unit and the dual sample solution supplying

system. The exponential water column manometer used to monitor the pressure inside the sprayer and burner unit

is seen at the extreme right (').

film is also available. The vertical frame is provided

at its lower end with a longitudinal opening 20 cm

by 2.5 cm which can accept a number of fixed exit

slits or a metal plate into which can be cut a number

of slits. When the variable exit slit is placed in the proper

position in the vertical frame, the spectrometer

operates as a scanning monochromator. The scanning

is performed by rotating the grating through a synchronous electric motor provided with 12 scan- ning speeds (0.5 to 2 000A/mn). A wavelength

counter indicates the wavelength directly in 0.1 A

units. When the exit slit is exchanged for the cassette,

the instrument can be used as a spectrograph.

Finally, when the vertical frame is lifted in its upper

position, the multislit arrangement is placed at the

focal plane and permits the isolation of a number

of individual monochromatic radiations. These radiations are received by individual photomultiplier

tubes located on a horizontal support. In this case.

the spectrometer operates as a multichannel instrument; this latter arrangement is illustrated in

figure 4. In this case, six individual slits are cut

in the multislit plate at wavelengths selected to

isolate the radiations from Na (X 5 893 A) ; K

(X 7 667 A); Li (X 6 707 A); Ca (X 4 227 A); CaOH

(X 6 230 A); and Mg (X 2 852 A). Behind every slit

is a side-on photomultipher provided with the

corresponding connections, and a seventh tube is used for the scanning mode.

As can be seen in figure 3, a precision lathe-bed

type optical bench 160 cm long is placed along the

optical axis in front of the entrance slit of the spectrometer. The following equipment is placed on this

bench using appropriate carriers : a quartz lens

which produces a demagnified image of the analytical flame on the entrance slit, and the burner and

sprayer unit with its dual analytical sample solution

supplying system. A second quartz lens, a mirror

chopper and a dual hollow cathode arrangement

268
Fig. 4. —• Light-tight enclosure located at the focal plane of the spectrometer. It contains seven photomultiplier tubes

placed behind the exit slits carrying plate (').

Fig. 5. — The opposite extremity of the optical bench from

figure 3 is seen here with the sprayer and burner unit together

with the dual sample supplying system, and with the additional equipment. This consists of a quartz lens, the mirror

chopper, and the dual hollow cathode arrangement. At

upper left, console containing the electronics used in conjunction with the multichannel mode of operation. At the right,

console carrying the equipment used to regulate and monitor

the flow of oxidant and fuel gases supplied to the sprayer

and burner unit (').

269
follows and is illustrated in figure 5. The mirror

chopper directs successive radiations from the two

hollow cathodes through the optical axis of the

instrument, and at the same time generates an ac

signal. Such a dual hollow cathode system was

proposed previously by other workers.

The lenses, burner and sprayer unit, chopper, and

hollow cathodes can be positioned along the optical

axis in the x-y-z directions and can be tightly secured

in the correct position.

2.2. — Electronic Circuitry.

The electronic equipment used with the scanning

mode is shown in figure 3. It consists of conventional

solid state elements, such as a wide band pretaneous flame emission and atomic absorption

measurements uses a less conventional electronic

arrangement which will be discussed briefly. This

arrangement was designed and built in 1965 by

P. Cath and G. Ziedens. Its principle is described

in the block diagram shown in figure 6

(s). It consists

of four major steps

1) conversion of light intensity to electrical

current;

2) measurement of this current;

3) conversion of the measurement from analog to

digital form;

and 4) print-out and simultaneous punch-out of

the data.

-

I

INTEOR. OH-OFF

tWITCMES

SERIAL CHANNEL WITCHIN6

AHALO«

TO

OifilTAL convehtem

Fig. 6. — Block diagram of the electronic system

used for the multichannel mode of operation (*).

amplifier, providing a gain of 100, a selective amplifier providing a gain of 10, and a lock-in amplifier

providing a gain of 10. The output from the lock-in

amplifier is supphed to a 4 digit digital voltmeter

and to an analog recorder. The power supply for

the scanning photomultiplier is seen at the top of

the console and it has a dc output of 2 100 volts

and 30 milliamperes with a line and load regulation

of 0.001 percent and a stability of 0.005 percent

per hour. At the bottom of the same console, one

can see the power supply for the hollow cathode

tubes which has the following general characteristics : current output range 2.2 [xA to 220 mA ; voltage

730V; current regulation 0.1 percent; stability:

0.2 percent. The operation of the spectrometer in

the scanning mode is conventional and does not

require further discussion (since the electronic system

uses a lock-in amplifier the lock-in chopper seen

in figure 5 is moved between the flame source and

the entrance slit of the spectrometer).

The multichannel mode of operation for simulThe first step is achieved by a photomultiplier

tube, and the system can accept up to eight such

tubes. Only two tubes are described in figure 6

since the remaining six are of similar design. The

measurement of the current generated by the photomultiplier is performed by individual integrating

capacitors where it is stored and then sequentially

converted to measurement in digital form. This

information is then printed and punched simultaneously for all channels on appropriate tapes, and

the punched tape is compatible for direct input

to a computer. The eight channels can accept either ac or dc inputs in any combination. The ac (290 H^)

mode is used in conjunction with the atomic absorption measurements while the dc input is used for

flame emission measurements. Each channel can be

switched on or off and has a sensitivity adjustment

and a dark current compensation capability. The

integration time can be selected step-wise, as needed,

for 10, 20, 40, 100, 200, 400 and 1000 seconds.

The measurements can be made with 1 to 4 repli270
^ '% • H ^ :^

i 5 4 7 e

# r IT^

Fig. 7. — Front panel of the console containing the electronic circuitry used m the multichannel mode of operation (').

SAMPLE IDENTIFICATION

NUMBER OF RUNS = 3 INTEGRATION TIME CHANNEL SENSI- NUMBER TIUITY

20 SECONDS AVERAGE ESTIMATED CONCENTRATION STD DEV

(PPM)

1 - MG 3 ^.0 1939 .215

A - K 5 59.1101

1

1 .180

6 - CA 6 15.26171 2.692

B - NA 6 8.06193 2.010

SAMPLE IDENTIFICATION NUMBER OF RUNS = 5 I/NTEGRATION TIME CHANNEL SENSI10

20 SECONDS AVERAGE ESTIMATED NUMBER TIVITY

- MG

- K

- CA

- NA

CONCENTRATION

(PPM)

3.98570

58.31750

11.81140

7.89211

STD

.153

3 .900

2. 128

1 .560

SAMPLE IDENTIFICATION I 1 NUMBER OF RUNS = INTEGRATION TIME CHANNEL SENS I

8 20 SECONDS AVERAGE ESTIMATED NUMBER TIVITY CONCENTRATION

(PPM) STD DEV

1 - MG 3 3.89575 .131

4 - K 5 57.80517 3.910

6 - CA 6 14 .88100 1 .950

8 - NA 6 7.99028 1 .302 END OF COMPUTATION

CPU TIME: 27 SECS. TERMINAL TIME! 013:38

PLEASE LOG IN: 05

Km

1

1 20

1 3 1597

2 1 1 2 4 5 1 2

6 6 1 7

8 6 41

1 20

1 3 3 7 2

2 1 260

4 5 1578

6 6 32 5

8 6 4442

Fig. 8. — Digital and computer data presentation obtained from the unit from figure 6 and 7. At the right, printed

tape; center, punched tape; at the left, computer data presentation.

271
cations or continuously, the sequence in this case being interrupted manually. The console which

contains the electronic circuitry is seen in figure 5,

while the panel which carries the corresponding

command switches is illustrated in figure 7. The

simultaneous digital print-out and punch-out display

contains Ihe following information : channel number,

sensitivity setting, integration time, and the arbitrary counts corresponding to the charge of the

individual capacitors. The arbitrary counts are converted to meaningful information through calibration with analytical reference solutions containing

known amounts of elements. An actual example of

data presentation is illustrated in figure 8 which

shows a portion of the printed tape at right, followed

by the corresponding punched-out tape and the

computer information. The printed tape contains,

from left to right, in the first column the channel

number, the sensitivity setting, and the arbitrary

counts corresponding to the charge of the capacitors.

The first row lists an identifying number permitting

the computer to identify the reference solution (0)

and the unknown samples (1), followed by the integration time which, in this case, is 20 s. The computer program which handles the analytical data

obtained with the multichannel spectrometer was

written by Dr. J. Wing.

In the present work, the electronic system was

set to accept four signals in the dc mode for the

flame emission determination of sodium, potassium,

calcium, with lithium as an internal standard and

one signal in the ac mode for the determination

of magnesium by flame atomic absorption. Since

only one magnesium hollow cathode tube was

needed, the chopper was placed on the optical axis

in a perpendicular position and a conventional blade was substituted for the mirror blade illustrated in

figure 5. Since the amplifier system used in conjunction

with the scanning mode is the lock-in type, it was

necessary to select a suitable chopping frequency

which would be different from that of the ac line

current. A major source of interference in the operation of the lock-in amplifier could be the analytical

flame itself and its supply system. A series of measure- ments was designed and performed to determine

whether any periodicity originated in the flame.

These measurements were performed by setting

the monochromator on the sodium emission and then

connecting the output from the scanning photomultiplier to an oscilloscope. The time scan obtained

with the flame alone (A), the flame supplied with

distilled water (B), and the flame supplied with a sodium salt (C) is illustrated in figure 9. As can be

seen, the unsupplied flame in A and B exhibits a random noise of low amplitude. When sodium was

supplied to the flame through the pneumatic sprayer,

Fig. 9. — Illustration of premixed acetylene-air flame periodicity. A, oscilloscope tracing of the signal from the unsupplied flame, B, same tracing from the flame supplied with

distilled water. C, same tracing with the flame supplied

with a sodium salt. In all three cases, the monochromator

was set on the sodium emission at X 5 890 ('). The abscissa

represents the time; the ordinate represents relative intensities.

the resulting signal indicated a strong periodicity

with a frequency of 25 H. as illustrated in C. This

periodicity is probably due to pulsations generated

in the pneumatic sprayer and their frequency should

depend on the geometry and on the operating conditions of the particular unit used. As a result of

these measurements, the chopping frequency in the

ac mode was selected to avoid interferences with

the fundamental frequency of the flame or any of

its harmonics.

The stability of the electronic circuitry, including

the photomultiplier tubes, was measured by radiations provided by a fluorescent screen activated

with 2 Ci of tritium (12.5 years half-life) f). This

source, having an output of 56 yJm, was placed at

the focal plane preceding the photomultipher tubes.

An integration time of 10 s was selected for each

channel, and a series of measurements was obtained

for each photomultiplier tube. The standard deviation in response to this constant illumination was

0.1 to 0.3 percent.

272
2.3. — Burner and Sprayer Systems.

The burner and sprayer system used with the

spectrometer is illustrated in figure 10 (i"). It consists

of an indirect external pneumatic sprayer, a spraying

vessel, and a burner body, all made of pure and

Fig. 10. — Pure alumina (99 % AI2O3) pneumatic sprayer

and premixed burner system. At tlie right, spraying vessel with the external pneumatic sprayer at the bottom (only

the sample supplying tube is visible). In the middle, burner

body with the fuel nozzle made of tantalum. At the left, burner head with water cooled tantalum flame plate. The

thin Teflon flash back safety membrane is seen at extre- mity of the horizontal tube ('"). At the bottom, inter- changeable burner head. Courtesy Plenum Press.

impervious alumina (99%Al203); the interchangeable burner plate, made of tantalum, is water

cooled. This unit is corrosion and erosion proof and

non-combustible. The gas mixtures which can be

used to produce a premixed flame are air-acetylene

or nitrous oxide-propane with the same burner plate

and nitrous oxide-acetylene with an appropriate

burner plate. Details of the unit are given in the

isometric drawing from figure 11, and various parameters are given in table I and II (i"). The spraying

vessel is connected to a polyethylene drain bottle

through a glass hydraulic trap as illustrated in

figure 12. The side tube at the top of the trap is connected to an exponential water manometer which

indicates the pressure inside the burner and sprayer

unit. When the air-acetylene or nitrous oxidepropane mixture is used, this pressure is of about

1 cm of water; with the nitrous oxide-acetylene the

pressure should be 7 cm. Since this burner and

sprayer unit has been described in previous work, no further details will be given here The system

for regulating and monitoring the flow of oxidant and

fuel gases is illustrated in figure 5 f). The oxidant

gas is supplied to the panel first after passing through

a two-stage pressure regulator provided with a stainless steel check valve and a flash-back safety consisting of a porous metal plate. A second pressure

regulator controls the oxidant flow and its pressure

is measured with a two-turn 8 1/2 in-(21.59 cm)

diameter precision gauge which is capable of measuring 0.1 (0.0068 atm) psi per division. The flow is controlled with a 250 mm long glass flowmeter;

copper tubing and appropriate fittings are used with

air, oxygen, and nitrous oxide. The fuel is supplied

to the panel from a corresponding container through

a two-stage pressure regulator provided with a stainTABLE I

Functioning Parameters of the Sprayer.

Air to sprayer

"Vacuum

at sprayer

inlet,

cm Hg

Capillary,

X 1/1 000 in.

Analytical solution

Sprayer

efTiciency,

/o

Qv, air

Pressure Qv, sol psi

Flow,

liter /mn

Consumed

by the

sprayer

g/mn

Suppl

the I

g/mn

ed to )urner

g /liter

20 9.8 30.0 9 5.0 0.25 0.026 5.0 2 000

6 1.7 0.21 0.017 12.4 7 353

30 12.5 42.0 9 5.7 0.32 0.026 5.6 2 200

12 11.0 0.36 0.029 3.3 1 140

14 14.0 0.37 0.030 2.6 890

40 15.0 38.0 9 5.5 0.33 0.022 6.0 2 730

273
TABLE II

Particle Size Distribution at 30 psi.

Capillary,

X 1/1 000

in.

Particle diameter,

0.5 ^ 1 > 1 < 2 > 2 < 4>4<6>6^8> 8 ^ 10 > 10 ^ 12 > 12 < 14

6 61.4 23.7 11.8 2.7 0.2 0.05 0.05

9 62.6 22.9 11.2 2.6 0.5 0.2

1 A 14 62.3 21.8 11.4 3.2 0.7 0.3 0.2 0.2

Total weight, %

6 2.8 8.5 33.8 30.6 8.5 3.7 6.7 9 2.5 7.4 29.4 31.2 17.9 11.6 14 1.5 4.3 17.7 22.9 13.6 10.5 11.4 18.1

less steel check valve and flash-back safety consisting

of a porous metal plate; the acetylene purifying

cylinder is placed after this plate (see paragraph 2.4.1). The flow of fuel is further regulated

at the panel and its pressure is monitored by a

8 1/2 in (21.59 cm) diameter two-turn precision gauge

capable of measuring 0.01 psi (0.000 68 atm) per

division. The flow is controlled by a 250 mm long

glass flow-meter. Stainless steel tubing and appropriate fittings are used with acetylene.

274
JOINT CLftMP AND I 19/22 JOINT

RUBBER PAD

Fig. 12. — Hydraulic trap made of glass

Courtesy Plenum Press.

2.3.1. Operation of Ihe Burner and Sprayer Unit.

We have mentioned that the sprayer can be

supphed through a two-way Teflon (*) stopcock in

succession with the analytical solution or with the

sprayer with the stopcock and the constant level

flask on one side, and the analytical sample container on the other side. This dual arrangement simulates

to some extent a double-beam operation. Indeed,

if one assumes that the emission source and the

flame, with its supply system, are stable during the

interval required to change from the sample to the

blank, the system is equivalent to a double beam

operation. This interval of 15 s which is required

to change from sample to blank is determined by

the memory of the sprayer and burner units. Since

we use 20 s integrations of the sample and blank

signals, the total time required to perform a sample

measurement is 35 s; a similar interval of 35 s is required for the blank. Hence, one sample-blank

measurement cycle requires 70 s.

2.3.2. Analytical Data.

The technique described was used to obtain the

analytical data reported in table III and to estabhsh

the accuracy of the measurements. The elements

selected in this case were Mg, K, Ca, and Na because

of their significance in biochemistry. It is obvious

that any combination of elements which can be

determined by flame emission and atomic absorption

TABLE III

Accuracy of Analytical Results for Mg, K, Ca, Na Determined Simulianeously.

Element determined, |jig/ml

Number

of Mg (A) = 4.00 K(*) == 60.00 Ca(*) == 15.00 Na(*) = 8.00 Runs

Found % Diff Found % Diff Found % Diff Found % Diff

3 4.02 + 0.50 59.14 — 1.43 15.26 + 1.73 8.06 + 0.75

5 3.99 — 0.25 58.32 — 2.80 14.81 — 1.27 7.89 — 1.38

8 3.90 — 2.5 57.81 — 3.65 14.88 — 0.80 7.99 — 0.13 12 3.97 — 0.75 58.66 — 2.23 15.01 + 0.07 8.00 0.0 15 3.95 - 1.25 58.32 — 2.80 14.92 — 0.53 7.97 — 0.38 20 3.92 — 2.0 58.04 — 3.27 14.88 — 0.80 7.96 — 0.50 25 3.90 1

1 — 2.5 57.68 — 3.87 14.84 — 1.07 7.93 — 0.88

(A) By flame atomic absorption.

(*) By flame emission.

distilled water blank solution. This dual supplying

system is illustrated in figure 5 C). One can see the

stopcock, the constant level polyethylene flask, and

the polyethylene capillary tubing which connects the

(*) The name of manufactured products are given for information only and do not constitute an endorsement of the product.

techniques can be chosen. Magnesium was determined

in the atomic absorption mode while K, Ca and Na

were determined in the emission mode. The calibration was made on eight solutions containing all four elements of known concentrations; Li was used

as an internal standard for emission measurements.

Each standard solution was run in six replications.

275
each measurement consisting of the blank and sample

run. A solution was then selected from these standards and treated as the « unknown » sample. This

solution was run in 3 to 25 replications as indicated

in column one from table III, each run consisting

of the blank and sample measurements. Columns 2

to 5 show the analytical results obtained for Mg,

K, Ca, and Na as well as the percent difference

between the concentration found and the concentration taken. In all atomic absorption measurements

the computer was programmed to interpolate from

a log versus concentration system, while the

sample

interpolation for the emission measurements was

made from a sample and blank versus concentration

system. In the case described in table III, the internal

standard values obtained for lithium were not used

to compute and correct the emission measurements.

The analytical results obtained demonstrate the

accuracy which can be expected from the multichannel spectrometer when simultaneous measure- ments are made by emission and atomic absorption on four elements.

2.4. Increasing the Sensiiiviiy of Measurements.

Various means can be used to increase the sensitivity of measurements in analytical flame spectroscopy, and three possibilities will be discussed.

2.4.1. Decreasing the Flame Background.

2.4.1.1. It has been known for a long time that

phosphine (HgP) is one of the impurities of acetylene

which is difficult to eliminate. This compound imparts

a noxious odor and toxic properties to acetylene.

When introduced into the air-acetylene and especially oxygen-acetylene flames, it radiates a strong

visible continuous or non-resolved spectrum as illustrated in figure 13 a and b. This emission can interfeie with the determination of the elements, and it

is essential to eliminate this contaminant. Since the

purification of acetylene was discussed in detail in

some of our earlier works (^i- ^2), only the practical

aspect will be discussed here. The HgP can be eliminated from acetylene to an extent that no radiation can be detected in the flames with the instrumentation described in this work. This is done by oxidation to H3PO4 using a ferric chloride solution

containing an oxidation accelerator, adsorbed on an

inert support such as diatomaceous earth. The materials are packed in a stainless steel cylinder 15 in

(38.1 cm) long and 2 in (5.08 cm) in diameter which

is provided with side tubes. The cylinder is placed

between the acetylene tank and the gas monitoring

console illustrated in figure 5. Acetylene thus

obtained produces a faint blue flame when burned

with air, as illustrated in figure 136. Before purification, the flame is seen in figure 13a and the

corresponding spectrophotometer scannings are given

in figure 14.

2.4.1.2. Impurities which can interfere with the

measurements are those introduced into the flame

RADIATION FfiW «( AC£IYliW-AIR FUW PRODUCED WITH COfWRCIAl ORAK AMD PURIFIED FUEL, IKE RADIATItw OHIGIKAIES

FROM HjP PRESEHl III IMF contfiCUl GRAtt FUEL

Fig. 14. — Monochromator scanning of the" radiations

excited in tlie flame illustrated in figure 13 a and b.

Na COMTAIHHAIION of UBORATOI^V air - IIEnSURED BV m C2H2-flIR FLAIE

Fume

Background

Electronic Dark Current and IIoihseI

Fig. 15. — Monochromator scanning illustrating the emission

of sodium present in the laboratory air surrounding the

flame. In this case, the monochromator was set on the

sodium emission and the scanning was done in time.

276
from the surrounding air. Such impurities are sodium

and calcium salts which are always present in the

laboratory atmosphere. The presence of sodium in our laboratory was determined by monitoring its radiations, which were excited in an acetylene

(purified) — air (filtered) flame, using our spectrometer set on the Na wavelength. The results are given in figure 15 and show the recording of Na

radiation intensities on the ordinate versus time on

the abscissa. The full scale deflection corresponds

to 2 ppb Na. As can be seen, the Na content of the

air surrounding the flame can be more than 1 ppb.

If this element must be determined at the ppb level,

it is obvious that the air entering the flame from

the atmosphere must be free of sodium contamination. This can be done by working in a clean

room or by supplying purified air to the flame.

2,4.2. Increasing the Optical Pathlenglh Through

the Flame.

It was demonstrated by Gouy (}^) and later by

LuNDEGARDH (*) that the emission intensity of the

radiations from a flame can be increased by lengthening the optical path; similarly, Walsh (i*) has

shown that the absorption of a flame can be augmented by increasing its length. This increase can be achieved by producing a longer flame or by

increasing the optical path through the flame using

multiple reflection arrangements. Both have been

used in our work and will be described.

2.4.2.1. Long Path Flames.

Figure 16 shows a long path burner head capable

of producing a 30 cm long and 1 cm wide flame.

Figure 17 gives details on the construction of this

burner C). It is made of stainless steel type 316,

tubular sections assembled with appropriate fitting

of the same metal welded in a hydrogen furnace.

The premixed acetylene-air or propane-nitrous oxide,

together with the fine spray of analytical sample,

are supplied to the burner head from both extremities to insure the production of an even flame.

The air is supplied to the external pneumatic sprayer

under a pressure of 39 psi (2.65 atm) and at a rate

277
a b

Fig. 13. — a. piemixed acetylene-air flame produced witli purified fuel, b, same flame produced with unpurified

commercial grade acetylene. Exposure time same as that used for a.
i

DRAIIt

Fig. 17. — Drawing of the unit shown in figure 16. The burner

head carries 112 holes of 1mm in diameter distributed over three parallel rows. The holes from the center row

are staggered between those from the two lateral rows. Length of rows, 2.50 mm. Total width, 5 mm (center to

center). Distance between rows, 2.5 mm (center to center).

Distance between individual holes, 8 mm (center to center).

The stainless steel tubing has an inside diameter of 13 mm

and an outside diameter of 16 mm.

of 2 200 1 /hr. The acetylene was supplied to the

burner body through a tantalum nozzle to produce

a stoichiometric flame. The consumption of analytical solution in the sprayer 25 ml/mn and the

amount of analytical sample solution entering the

flame was 1.3g/mn. The optical system used with

this 30 cm flame consisted of two quartz lenses which

produce an optical beam having an average cross section of about 10 mm. Under these circumstances,

the analytical results obtained when Mg was determined by atomic absorption in conjunction with the

scanning electronics described in paragraph 2.2

showed an increase in sensitivity by a factor 3, as compared with a 10 cm path burner.

The optical system using the quartz lenses could

be improved by using a two Cassegrain mirror onaxis arrangement as described by Muller-Herget.

This would eliminate the chromatic aberrations

characteristic to all lens systems and produce a more adequate optical beam geometry (^^).

2.4.2.2. Triple Pass On-Axis Optical Arrangement.

This on-axis triple arrangement system (*) can be

used for atomic absorption and flame emission measurements and is based on the use of two Mangin-

(*) Designed by W. Muller-Herget. Philips Laboratories,

Briarclif! Manor, New York, 10510, U.S.A.

type mirrors with an asymmetric biconvex quartz

lens. The principle of the proposed system and the

specific dimensions are shown in figures 18 and 19.

HOLLOW TRANSFER IHTERHFDI ATE TRIPLE PASS CELL CATHODE LEIlS IMAGE

Fig. 18. — Principle of the on-axis triple pass system.

Fig. 19. — Drawing of the triple-pass, center bore,

Mangin mirrors assembly.

A view of the actual arrangement in working conditions is illustrated in figure 20.

The flame cavity is designed as a concentric triplepass system with parallel radiations in the second

pass. Because of heat and corrosion, it is advantageous to use rear surface mirrors made of fused

quartz. The optical elements are Mangin-type mirrors

with a center bore. The cavity has been designed

to minimize spherical and primary color aberrations

in the spectral range 240 nm to 436 nm with the

design center at 330 nm. The object at P- is imaged

through the cavity with unit magnification. The

conjugated points are both outside, providing

6.48 mm clearance beyond the mirror's back surface.

The minimum optical diameters are 6.70 mm for

the bore and 35.0 mm for the outside diameter of

the mirrors.

The hollow cathode is imaged by an asymmetric

biconvex quartz lens at the intermediate image

position P_-. It is designed for the same spectral

range with the design center at 330 nm. The spherochromatic aberrations have been minimized but are not suppressed in a simple lens. The aberrations of

the Mangin cell are only 2 percent of the transfer

lens blur. The hollow cathode image is demagnified

(1 : 0.635) and is inverse at position P- and upright

at position P.

279
Fig. 20. — On-axis triple-pass system from figures 18 and 19 shown in actual use.

If five passes through the flame are required, both

Mangin mirrors may be exchanged for two others

of corresponding design, while retaining all previous

dimensions. The focal length, f, of the mirrors for

the five-pass cell will be, in first order approximation : f = 2.618 X d = 398.98 mm for required

clearance of d = 152.4 mm between the mirrors. The

required diameter will be approximately 30 percent

larger.

The triple-pass Mangin mirror assembly is placed on the optical bench as illustrated in figure 20, with

the analytical flame located symmetrically between

the mirrors and at a height selected for best sensitivity. The system was tested in the atomic absorption mode using Mg. An increased sensitivity of 2.7

was observed when measurements were compared

with the sensitivity obtained under the same conditions, but without the triple-pass unit.

The triple-pass unit was also tested in emission

using sodium. When the rear Mangin mirror was

blocked out, an increase in the emission signal of

3.3 percent was observed when compared with the

signal obtained without the triple-pass unit. When

the front Mangin mirror was blocked out, the

increase was 43 percent; when both Mangin mirrors were unobstructed, the increase was 70 percent.

Finally, when a flat mirror was placed at the opening

of the rear Mangin mirror, the increase in the sodium

signal was 158 percent.

The increased sensitivity of flame measurements,

using the on-axis Mangin triple-pass mirror system,

recommends its use. This system requires careful

optical alignment and is somewhat wavelength dependent. This dependence can be eliminated by using

reflecting rather than refractive optics in place of

the 2 lenses mentioned above.

3. — Discussion of some fundamental excitation

conditions in combustion flames.

3.1. Introduction.

As a consequence of exothermic combustion reactions, the gases which constitute a flame attain

different temperatures, according to combustion

conditions, flame structure, and the geometry of the

280
system. The thermal energy in a flame is distributed

between particles activated to the various translational energies, and rotational, vibrational, electronic

and ionization states. Each state can be defined

separately by its own temperature. When this value

is identical for all states, the radiation from the

flame is in thermal equiUbrium. Boltzmann's wellknown equation ("), which relates the intensity

radiated by a given number of activated particles

to the temperature of the excited source, can be

applied only in this state of equihbrium. A further

limitation to its use is presented by chemical reac- tions and ionization and self-absorption phenomena

occuring in the excitation source. When the temperatures of translation, rotation and vibration are not equal and differ from the electronic and ionization

temperatures, the system is not in thermal equilibrium. In a flame, various reactions produce radiations which deviate from thermal equilibrium. We

call this process overexcitation. The Boltzmann

equation cannot be applied to interpret such emissions, and the notion of temperature loses its meaning. Thermal excitation and overexcitation are thus two widely different types of excitation mechanisms which are both found in flames used in analytical flame spectroscopy, and a study of the temperature distribution related to the flame structure will

help to identify the origin of the resulting radiations.

Temperature measurements performed in the outer cone of stoichiometric premixed combustion flames

using various procedures and states of excitation

(vibration, rotation, and electronic), have produced

results in agreement among themselves within the

experimental errors and agree also with the thermodynamically calculated value. When similar measure- ments were made in the inner cone of the same

flames, the temperatures difl'ered widely depending

to the measuring conditions. From the results, it can be concluded that thermal equilibrium is achieved

in the outer cone of premixed flames and the radiation from this zone is of thermal nature (perhaps

with the exception of ionization). On the other hand,

thermal equihbrium is not reached in the inner cone;

the radiations of excited particles in this zone result

from various reactions.

It is necessary to mention, however, that whether

the radiation of a chemical species excited in a flame is or is not in thermal equilibrium does not

preclude its use for analytical purposes. The only

condition to be satisfied is that the radiation must

be constant and reproducible.

The mechanism responsible for overexcitation was

discussed in 1951 by Laidler and Shuler (see

ref. 12) who studied the elementary reactions and

collision processes involving excited electronic states

and then applied the results to various reactions

occuring in combustion flames. Some of the reactions

listed by Shuler are

Collisional activation

A4-B-^A+B* I

Energy transfer

A* + B -> A + B* II

True chemiluminescence

A + B -> AB* III A + B + M -> AB* + M Iir

Induced chemiluminescence

A -f B -I- C -> AB + C* IV

True chemiluminescence

AB + C -> A + BC* V

True chemiluminescence

AB + C^ A* + BC VI

True chemiluminescence

AB + M -> A* + B + M VII

Induced chemiluminescence

AB + C -> A+B + C* VIII

The A, =B, C, and M may be radicals or molecules

as well as atoms. Equations I and II are energytransfer processes; in the first, the excitation energy

of one of the colliding particles comes from translational energy of the other particle. Equations III,

Iir, and IV are associated reactions; for III' and

IV, a three-body colhsion occurs; in III' the third

body, M, acts only in stabilizing the product,

whereas in IV, it absorbs the energy released by

the formation AB. Equations V and VI are metathetical reactions; part of a molecule or a radical

is transferred to the colliding particle, and the

released energy may be absorbed by the new species

(V) or by the remaining part of the dissociated

molecule (VI). Equations VII and VIII are dissociation reactions in which the colliding particle splits

the molecule AB into two parts ; the energy released

by the process, if any, is absorbed by a part of the

dissociated molecule (VII) or by the colliding particle (VIII) — this latter reaction being very unlikely, due to energy requirements.

Equations III, III', V, VI, and VII may be termed

chemiluminescent since the excited product results

from the reaction. Equations I and II enter the

category of energy transfers. In equations IV and

VIII, the excited partner is a product of the coUi281
sion, but is not chemically modified; it is acting

only to dissipate the energy made available by the

process. Such processes may be termed « induced

chemiluminescence ». Distinctions between true chemiluminescence,

induced chemiluminescence, and energy transfer

processes can be made only when the process is actually known. To distinguish nonthermal emissions from thermal emissions, but without emphasizing the actual process, it is proposed to designate

nonthermal emissions as « overemission » and their

excitation as « overexcitation » {^^).

The preferred area of overexcitation is the primary

reaction zone or blue cone of premixed flames.

Spectroscopic studies have shown that a number

of chemical species can be excited to emit specific

radiations in this particular zone. This is illustrated

in table IV where it can be seen that elements such

as Sn, Cd, Ni, Sb, Be, As, Pt, Co, Au, Pd, C, Fe,

Ir, Hg, In, Ru, Pb, Bi, Ga. Tl, Mg, are excited

preferentially in the blue cone of the premixed

acetylene-air or acetylene-oxygen flames with intensities varying from very, very weak (vvV) to strong

(S) (1^). Most of these emissions originate from an

overexcitation reaction and can be used for analytical measurements to detect the particular species

and to determine its concentration {^'').

The phenomenon of overexcitation was observed

many years ago. George-Louis Gouy was the first

to describe the dilTerences in radiations which originate from the blue cone and the outer cone of a premixed city gas-air flame when various chemical

species were supplied to the flame {^^). Detailed

studies of this phenomenon were carried out by

De Watteville and Hemsalech who, at an early

date, made a clear discrimination between thermal

and nonthermal excitation of additives in combustion

flames. In 1917 Hemsalech wrote:

« The light radiations from vapors in outer mantles

of the various flames behave as if governed by the

temperatures of these flames.

The spectrum emitted by metal vapours in the

region of the blue cone of the air-coal gas flame

cannot be accounted for by simple thermal actions,

but is probably due to some special chemical process

prevailing in the explosion region.

In the case of iron, we estabhshed the existence of

two distinct groups of lines. One group, some 200

lines, is emitted by the outer mantle of the aircoal gas flame, and the lines merely increase in

intensity on passing to flames of higher temperature.

We considered them as the fundamental lines of

the spectrum, which are easily excited by thermal

actions. The other group of over 500 lines, is emitted

by the region of the blue cone only, and even in

the oxy-acetylene flame we could detect only traces

of some of the lines (^®) ».

TABLE IV

Intensities of Spectrum Lines

Eradiated by the Atoms of Various Elements

Excited in the Inner Cone (IC) and Outer Cone (OC)

of the Premixed Acetylene-Air

and Premixed Acetylene-Oxygen Flames.

Excitation

Energy

(eV)

Wavelength

(A)

Emission o

C^H, — O2

bserved in

C2H2 — Air

IC OC IC OC

5.84 2 199.3 Sn vW

6.03 2 209.7 Sn vW

5.18 2 246.1 Sn wM

5.89 2 268.9 Sn -wM

5.84 2 286.7 Sn vW

5.42 2 288.0 Cd vW

6.77 2 288.1 As W

5.41 2 290.0 Ni vvW

5.36 2 311.0 Ni W

5.36 2 311.5 Sb wM M

5.52 2 312.3 Ni W

5.63 2 314.0 Ni W

6.42 2 317.2 Sn vW

5.51 2 317.2 Ni W

5.34 2 320.0 Ni W

5.61 2 321.4 Ni W

5.49 2 325.8 Ni W

5.59 .... 2 330.0 Ni vW

5.52 2 334.8 Sn wM

5.28 .... 2 345.5 Ni vW

5.28 2 348.6 Be vvW

6.59 2 349.8 As wM

5.47 .... 2 354.8 Sn W M

5.35 .... 2 357.1 Pt W

5.15 .... 2 407.3 Co vvW

5.24 .... 2 411.6 Co vvW

5.31 .... 2 414.5 Co vvW

5.36 .... 2 415.3 Co vvW

6.19 .... 2 421.7 Sn wM

5.11 2 424.9 Co vvW

5.10 .... 2 428.0 Au vW vvW W

5.23 .... 2 429.5 Sn W M

5.20 2 432.2 Co vvW

5.26 2 436.7 Co vvW

5.18 2 436.7 Pt vW

5.31 .... 2 439.0 Co vvW

5.08 2 440.1 Pt wM

5.06 2 447.9 Pd vvW vvW

5.02 2 467.4 Pt W

5.00 2 476.4 Pd vvW vvW

7.68 2 478.6 C M

4.99 2 483.3 Fe vvW vW vvW

5.42 2 483.4 Sn wM

4.98 2 487.2 Pt M

5.03 2 488.1 Fe vvW vW vvW

5.10 2 489.8 Fe vvW

5.08 2 490.1 Pt vW

5.06 2 490.6 Fe vW

6.28 2 492.9 As vvW

6.03 2 495.7 Sn W

5.78 2 495.8 Pt vvW

282
Excitation

Energy

(eV)

Wavelengtli

! (A)

i

1

!

1 Element

1

Emission (

C2H2 — O2

observed in

C2H2 — Air

IC OC

1

IC OC

5.06 2 498.5

i

1 Pt

1

1 VV 6.18 2 502.6 ; Ir V VV 4.95 2 503.0 i Ir V w

5.37 2 511.0 ' Co VV w

5.02 2 515.6 Pt w

4.92 2 521.4 Co VV w \tWT V w

4.91 2 522.8 ; Fe VV W V w -tTtrWT VV w

4.95 2 527.4 Fe VV w VV \v VV w

6.12 2 528.5 Sb c Vo

5.00 2 529.0 Co \T\rW7 VV w

5.06 2 536.0 Co VV w

4.89 2 536.5 Hg V v\ \t\tWT VV VV c

4.98 2 539.2 Pt V w

5.22 2 544.0 Ir WTW

5.10 2 544.3 Co VV VV 5.38 .... 2 544.9 Co VV w

4.87 2 546.6 Sn WTW ivr 4.84 2 560.2 In \t\tWT VV w

5.89 2 571.6 Sn wM

5.84 2 594.4 Sn WTW

5.99 2 598.1 Sb c0 Vo

5.10 2 611.3 Ir V V V 5.67 2 612.1 Ru VV w

5.71 2 613.7 Pb IVl 5.71 2 614.2 Pb IVl 6.13 2 627.9 Bi WTVV 4.81 2 628.0 Pt vv\\ V V V ITlo 5.64 2 631.3 Ru \t\tWT VV w

5.51 2 639.3 Pt V w

4.69 2 639.7 Ir wM

5.83 2 641.5 Au VV VV 5.75 2 643.0 Ru \t\tWT VV w

4.68 2 646.9 Pt mo

4.78 2 650.9 Pt mo

5.68 2 651.3 Ru VV w

5.48 2 651.8 Ru VV w

6.96 2 652.6 Sb VV w

4.85 2 659.0 Rli VV w

4.66 2 659.4 Pt •tWT V W wM c

4.66 2 659.9 Ga VV w

4.87 2 661.2 Sn WTW IVi 5.01 2 662.0 Ir V w

5.97 2 663.2 Pb WTw

4.65 2 664.8 Ir wM

5.70 2 670.6 Sb WTW

4.63 2 676.0 Au M mo WTW

4.63 2 677.1 Pt wM

4.95 2 694.2 Ir wM

6.01 2 696.8 Bi vvW

5.35 2 698.4 Pt W

4.68 2 702.4 Pt vvW W mS

4.68 2 705.9 Pt vvW W mS

4.79 2 706.5 Sn M w S

5.64 2 709.2 Ru W

4.85 2 710.3 In vW

5.26 2 718.5 Rh vvW

6.85 2 718.9 Sb W

4.66 2 719.0 Pt mS

4.56 2 719.0 Fe vW M vW

1

Excitation

Energy

(eV)

Wavelengtli

(A) Element

Emission c

CH, — 0.,

)bserved in

C,H., —Air

IC OC IC OC

4.61 .... 2 720.9 Fe vvW \V vvW

4.64 2 723.6 Fe vvW v\V

5.35 2 729.9 Pt vW

'

4.63 2 734.0 Pt vW W mS

4.53 2 735.7 Ru M

4.64 2 737.3 Fe

4.61 2 742.4 Fe \v vvW

6.77 2 745.0 As vW

5.65 2 748.3 Au vvW S ' 4.50 2 753.9 In vvW

5.76 2 753.9 Pt vvW

5.75 2 754.9 Pt \V

4.91 2 761.8 Sn vvW 1 4.49 2 763.1 Pd \v vW

4.63 2 763.4 Ru vvW

4.48 2 767.9 Tl wM M wM vvW

5.70 2 769.9 Sb W mS

4.57 2 771.7 Pt vvW

7.17 2 779.8 Mg vvW

5.53 .... 2 779.8 Sn wM

6.77 2 780.2 As wM

0.0 / . , . . Z /6U.0 VW

5.52 2 785.0 Sn W

5.30 2 788.1 Fe vvW vvW

A medium which is suitable to the production

and observation of overexcitation phenomena is hydrogen burning in air or oxygen under various

conditions and especially with an excess of fuel. The

overexcitation is greatly enhanced when the hydrogen

flame is cooled by water. This observation was made

more than 100 years ago by Salet who studied the

spectra produced by nonmetals in a hydrogen-air

flame chilled with water (^^). Salet studied phosphorus, sulfur, halogens, tin, selenium, tellurium and

silicon. Sulfur and phosphorus seem to have retained

his particular interest.

The pioneering works of Salet were forgotten for a long time, and it was only during the last 15 years

that spectroscopists have shown interest in the use

of hydrogen flames to excite nonmetals. At the

present time, the literature cites new instrumentation and many applications of this flame to solve

particular analytical problems. A recent comprehensive and authoritative review of these works,

covering over 200 pages and containing 602 refe- rences, together with a complete discussion of the

original works, was made by Gilbert (2"). The nonmetals examined in his review are H, B, C, Si, N,

P, As, 0, S, Se, Te, F, CI, Br, and I. We have selected

to discuss phosphorus and sulfur due to the parti283
cular properties of the two elements. They are easily

excited in chilled hydrogen flame and radiate beautiful characteristic molecular spectra; they are also

important to environmental pollution problems.

Since the excitation of P and S in various hydrogen

flames was discussed by Gilbert in the above

review we will mention here the original works

of Salet (^^) together with only a very few essential

and more recent contributions. A complete survey

of the field is given in Gilbert's review.

3.2. Experimental Arrangemenls

lo Excite Phosphorus and Sulfur, and Discussion.

The experimental arrangement used by Salet in

1869 and 1871 to excite P and S is illustrated in

figure 21. It consists of a hydrogen flame which is

Fig. 21. — Arrangement used by Salet

to produce a water chilled hydrogen-air flame.

produced at the extremity of a glass tube. The flame

is chilled by directing it against a water curtain.

The hydrogen is supplied with the vapor by passing

the gas over the surface of a P or S compound

contained in a closed vessel which can be warmed

to produce the necessary vapor. Salet describes the

blue and green radiations obtained from S and P

compounds, and mentions the high sensitivities

obtained in detecting these two elements.

These observations were used in a patent involving

the detection and determination of P and S compounds in gaseous matrices, by the Dragerwerk

Company in 1961 (^i). The hydrogen flame burns

with an excess of fuel and is produced inside a quartz tube cooled with water. The excess hydrogen

burns at the extremity of this tube where it comes

in contact with the surrounding air. This hydrogen

burner is illustrated in figure 22. The detection

OUTER HYDROGEN

FLAME

NOZZLE

RADIATION FROM

P or S

RADIATION FROM

P or S

WATER

INNER HYDROGEN

FLAME

DRAIN

SILICA TUBE

WATER CONDENSER

HYDROGEN

0.1-0.4 l/mln.

AIR CONTAINING

S or P VAPORS

0.3-0.6 l/mln.

Fig. 22. — Drager burner designed to produce a dual hydrogen-air flame. The inner diffusion flame is water chilled and burns with an excess of hydrogen; the outer flame

burns by diffusion with the surrounding air (^^). Courtesy

Plenum Press.

limits of P or S obtained with this burner were

10~* g of the compound per liter of air (< 0.01 ppm).

The water-cooled hydrogen flame of Salet was

used in our exploratory work combining the Dragerwerk burner with a sample supplying system similar

to that described long ago by Gouy (^^). The chilled

hydrogen flame can be supplied with solution samples

by pneumatic spraying. Argon was used for this

purpose. The experimental arrangement is described

in figure 23 and a corresponding drawing showing

some construction details is illustrated in figure 24.

The horizontal spraying vessel of Teflon is provided

at left with an external concentric pneumatic sprayer

made of tantalum. The sample solution enters the

sprayer through the polyethylene capillary tube.

Its supply rate can be varied between 0 and 5 ml/mn

through the needle valve and its graduated dial.

The argon is introduced into the sprayer through

the side tube.

284
Fig. 23. — Experimental arrangement used to produce a dual hydrogen flame supplied through pneumatic spraying

of the analytical solution. The horizontal Teflon spraying

vessel with the tantalum external variable pneumatic

sprayer is located on the optical bench. The vertical all quartz burner enclosure is adapted to the spraying vessel through an air tight rubber seal.

Fig. 24. — Drawing of the unit illustrated in figure 23;

dimensions in mm.

The sprayer vessel has a draining tube connected

to a hydraulic trap (see fig. 12), and at the right side a rupture disk made of a thin sheet of Teflon. The

polyethylene tube, seen at the right side of the

spraying vessel, is used to provide oxygen to the

argon-sample spray. It is adapted to the tantalum

nozzle which extends inside the vessel through a

fitting. The oxygen nozzle has a circular opening

0.5 mm in diameter at its extremity. On top of the

spraying vessel is a slot-shaped tantalum tube

through which the argon, oxygen, and sample spray

flows. This tube is surrounded by an all-quartz

cylindrical vessel open at both ends and provided

with a water cooling jacket. This quartz vessel is adapted to the spraying vessel through a rubber

washer which provides an air-tight assembly. The

lower side tube at right is used to drain intermittently

the water which condenses on the inside walls when

the flame burns inside the quartz vessel. The side

tube on the left is used to supply the hydrogen

which is metered by the flowmeter at left. The

entire unit is placed on the optical bench in front of

the spectrometer described in Section 2 of this paper,

and can be moved in the x-y-z direction as required

and secured in the chosen position. Discussion of

of similar works will be found in reference {^°). The

hydrogen-argon-oxygen flame is produced in the

following manner

The cooling water is turned on and the argon is supplied first to the sprayer at a pressure of 40 psi

and at a rate of 300 1 /h. Hydrogen is then introduced

into the cylindrical quartz vessel and ignited at the

upper end of the vessel where it burns by diffusing

with the surrounding air. Oxygen is added progressively until the upper hydrogen-air flame flashes back

inside the cylindrical quartz vessel and the hydrogen

burns with oxygen by diffusion. Sufficient hydrogen

is added to supply and maintain both the inner

flame and the outer flame. Under these conditions

two flames are produced : an inner chilled

H2 — O2 — Ar diffusion flame which burns with an excess of fuel, hence a reducing atmosphere,

and an outer diffusion hydrogen flame which burns

with the surrounding air, and hence with an excess

of oxidant. The sample solution is supplied as a

fine spray to both flames through the pneumatic

sprayer. Argon was chosen to operate the sprayer

since it has a low quenching cross section. This

property should emphasize overexcitation reactions.

Sulphur and phosphorus were supplied to the

flame as sulfuric and phosphoric acids, ammonium

sulfate, and ammonium phosphate, as well as organic

aqueous solutions of dimethyl sulfoxide and hexmethyl phosphoramide.

The hydrogen flames produced under these circumstances are illustrated in figure 25 a, b, and c. Figure 25a shows the appearance of the two flames

285
Fig. 26.

WAVELENGTH. A Monochromator recording of the inner hydrogen flame supplied with diluted sulfuric acid.

when only distilled water is supplied to the sprayer;

figure 25b shows the flames supplied with the sulfur

containing solution, while figure 25c shows the

flames supplied with the phosphorus containing solutions. The same types of radiations are obtained

with the inorganic or organic bound sulfur or phosphorus. Concentrations of these elements was about

1 percent. The detection limits for both elements

are below 0.1 ppm.

The radiations from the inner chilled hydrogen

flame supplied with sulphur and phosphorus compounds were examined with the spectrometer in the

scanning mode using a grating with 1 200 lines /mm,

a scanning speed of 200 A/mn, and a slit of 0.200 mm

(1.6 A). The recordings of the spectra obtained are given in figure 26 for the sulphur flame and figure 27

for phosphorus flame. As can be recognized from

data available in the literature (^''), these radiations

originate from the and HPO molecules.

The types and intensities of the radiations excited

in the chihed. hydrogen flame with an excess of

hydrogen depend strongly on the experimental

conditions used to produce them. The burner system

described in this work is only one of many possible

arrangements which can be used to produce these

kinds of excitation conditions. Complete and up-todate information on this subject can be found in

reference (^'^).

500 4750 5000 5250 5500 5750 60i: WAVELENGTH. &

Fig. 27. — Monochromator recordi: of the inner

hydrogen flame supplied with diluted hosphoric acid.

Both S2 and HPO emissions are characteristic

examples of the nonthermal excitation process

which we have called overexcitation. Indeed, the

thermal energy available in the chilled hydrogen

flame is certainly not capable of exciting species

such as HPO and which require about 2.4 and

3.9 eV respectively (*). Various potential reactions

have been postulated to account for these non- thermal emissions. Thus, for HPO, Lam Thanh and

Peyron considered both reaction (^^). H

and

PO + M

H + PO*

HPO* - HPO*,M, (III')

(HI)

while Gilbert proposed the chemiluminescent reac- tion (20),PO + H2 + OH HPO* + H2O. (UV)

For S2, Crone had suggested the chemiluminescent

reaction (^o),H + H S2* + H.„ (IV)

which was also postulated by Peyron. Sugden and

Demerdache proposed another chemiluminescent

reaction {^^),

H + OH + Ss S2* + H2O. (IV)

At this stage it seems that further work is needed

to elucidate these flame reactions and to arrive at an accurate interpretation of the excitation phenomena occuring in chilled hydrogen flames burning

in an excess of fuel.

Acknowledgment.

The author is particularly grateful to Dr. 0. Menis,

Section Chief of the Coordination Chemistry Section,

(*) Communication from Dr. H. Okate, Physical Chemistry

n'ii'ision, National Bureau of Standards.

286
Analytical Chemistry Division, for approving and

encouraging this work, and to Mr. J. I. Shultz,

Assistant Chief of the Coordination Chemistry

Section, Analytical Chemistry Division, for critically

reviewing the manuscript.

BIBLIOGRAPHY

(1) G. KiRCHHOFF and R. Bunsen, J. Prakt. Chem., 1860,

80, 449-477; Ann. Physik, 1860, 110, 161-189; ibid., 1861, 113, 337-381; Phil. Mag., 1860, 20, 89-109; ibid., 1861, 22, 329-349; Ann. CInm. et Phys., 1861, 113, 337-

381.

(2) P. Champion, H. Pellett and M. Grenier, Compl.

Rend., 1873, 76, 707-711.

(3) J. Janssex, Compt. Rend., 1870, 71, 626-629.

(4) H. LuNDEGARDH, The Quantitative Spectral Analysis of

the Elements, vol. 1 and 2, G. Fischer, Jena, 1929 and

1934.

(5) J. Tyxdall, Six Lectures on Light, 1872-1873, Appletoh,

New York, 1883.

(6) A. Walsh, Australian Patent, 163, 586, Application,

1953, Issued, 1955.

(7) R. Mavrodineanu, National Bureau of Standards Technical, Note 544, 1970, 58-70, O. Menis, J. Shultz, ed.

(8) G. ZiEDENS, Electronic Readout System for Multichannel

Flame Spectroscopy, Technical Report 228, Oct. 31, 1968,

Philips Laboratories, Briarcliff Manor, New York, 10510.

(9) R. Mavrodineanu and R. C. Hughes, Applied Optics,

1968, 7, 1281-1285.

(10) R. Mavrodineanu, Developments in Applied Spectros- copy, 1970, 8, 18-32.

(11) R. Mavrodineanu and H.Boiteux. L' Analyse Spectrale

Quantitative par la Flamme, Masson, Paris, 1964.

(12) R. Mavrodineanu and H. Boiteaux, Flame Spectros- copy, Wiley, New York, 1965.

(13) G. L. GouY, Ann. Chim. et Phys.. 1879. 18, 5-101.

(14) B. J. Russell, J. P. Shelton and .\. Walsh, Spectrochim. Acta, 1957, 8, 317-328.

(15) W'. MuLLER Herget, Some Considerations on Optical

Design and Selection of Spectroscopic Instruments in

« Analytical Flame Spectroscopy, Selected Topics >>, R. Mavrodineanu, ed. McMillan, London, Springer,

New York, 1970.

(16) R. Mavrodineanu, Developments in Applied Spectros- copy, 1965, 5, 371-401.

(17) P. T.- GiLHERT, Jr. Proc. Colloq. Spectros. Intern. 10th,

Univ. Maryland, 1963, 1962, 171-215.

(18) G. A. Hemsalech, Phil. Mag., 1917, 33, 1-18. (19) G. Salet, Compt. Rend., 1869, 68, 404-406; ibid., 1871,

73, 742-745; ibid., 1871, 73, 862-863; ibid., 1871, 73,

1056-1059; ibid., 1890, 110, 282-283; Bull. Soc. chim.,

France, 1869, 11, 302-304; ibid., 1890, 3, 328-329; Ann.

Physic, 1869, 137, 171-174; Ann. Chim., et Phys. 1873,

28, 5-71.

(20) P. T. Gilbert, Jr. Nonmetals, in <• Analytical Flame

Spectroscopy, Selected Topics », R. Mavrodineanu, ed. McMillan, London; Springer, New York, 1970.

(21) Dragerwerk, German Pat., 1962, 1, 133, 918; Recorded,

1961; and U.S. Patent, 1965, 3, 213, 747 to C. E. van

der Smissen.

(22) M. Lam Thanh and M. Peyron, J. Chim. Pliys., 1964,

61, 1531-1546.

(23) T. M. Sugden and A. Demerdache, Nature, 1962, 195, 516.

287

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4. TITLE AND SUBTITLE

PROCEDURES USED AT THE NATIOML BUREAU OF STANDARDS

TO DETERMINE SELECTED TRACE ELEMENTS IN BIOLOGICAL

AND BOTANICAL MATERIALS

5. Publication Date November 1977

6. Performing Organization Code

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emission spectrometry; fluorescence spectrometry; molecular absorption

spectrometry; neutron activation analysis; polarography; pure reagents;

saniple preparation; spark source mass spectrometric isotope dilution; tissues.

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