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[ED STATES TMENT OF IMERCE

.ICATION

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NBS TECHNICAL NOTE 584

Analytical Coordination

Chemistry Section:

Summary of Activities

July 1970 to June 1971

U.S.

ARTMENT

OF

)MMERCE

I OO

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NATIONAL BUREAU OF STANDARDS

The National Bureau of Standards1 was established by an act of Congress March 3, 1901. The Bureau's overall goal is to strengthen and advance the Nation's science and

technology and facilitate their effective application for public benefit. To this end, the Bureau conducts research and provides: (1) a basis for the Nation's physical measure- ment system, (2) scientific and technological services for industry and government, (3) a technical basis for equity in trade, and (4) technical services to promote public safety.

The Bureau consists of the Institute for Basic Standards, the Institute for Materials

Research, the Institute for Applied Technology, the Center for Computer Sciences and

Technology, and the Office for Information Programs.

THE INSTITUTE FOR BASIC STANDARDS provides the central basis within the United States of a complete and consistent system of physical measurement; coordinates

that system with measurement systems of other nations; and furnishes essential services leading to accurate and uniform physical measurements throughout the Nation's scientific community, industry, and commerce. The Institute consists of a Center for Radiation Research, an Office of Measurement Services and the following divisions: Applied Mathematics—Electricity—Heat—Mechanics—Optical Physics—Linac

Radiation2—Nuclear Radiation2—Applied Radiation2—Quantum Electronics3—

Electromagnetics3—Time and Frequency3—Laboratory Astrophysics3—Cryogenics3

. THE INSTITUTE FOR MATERIALS RESEARCH conducts materials research lead- ing to improved methods of measurement, standards, and data on the properties of

well-characterized materials needed by industry, commerce, educational institutions, and

Government; provides advisory and research services to other Government agencies;

and develops, produces, and distributes standard reference materials. The Institute consists of the Office of Standard Reference Materials and the following divisions: Analytical Chemistry—Polymers—Metallurgy—Inorganic Materials—Reactor

Radiation—Physical Chemistry.

THE INSTITUTE FOR APPLIED TECHNOLOGY provides technical services to pro- mote the use of available technology and to facilitate technological innovation in industry and Government; cooperates with public and private organizations leading to the development of technological standards (including mandatory safety standards), codes

and methods of test; and provides technical advice and services to Government agencies

upon request. The Institute also monitors NBS engineering standards activities and

provides liaison between NBS and national and international engineering standards

bodies. The Institute consists of the following technical divisions and offices: Engineering Standards Services—Weights and Measures—Flammable Fabrics

Invention and Innovation—Vehicle Systems Research—Product Evaluation

Technology—Building Research—Electronic Technology—Technical Analysis

Measurement Engineering.

THE CENTER FOR COMPUTER SCIENCES AND TECHNOLOGY conducts re- search and provides technical services designed to aid Government agencies in improving cost effectiveness in the conduct of their programs through the selection, acquisition,

and effective utilization of automatic data processing equipment; and serves as the principal focus within the executive branch for the development of Federal standards for automatic data processing equipment, techniques, and computer languages. The Center

consists of the following offices and divisions: Information Processing Standards—Computer Information—Computer Services —Systems Development—Information Processing Technology.

THE OFFICE FOR INFORMATION PROGRAMS promotes optimum dissemination

and accessibility of scientific information generated within NBS and other agencies of

the Federal Government; promotes the development of the National Standard Reference

Data System and a system of information analysis centers dealing with the broader

aspects of the National Measurement System; provides appropriate services to ensure

that the NBS staff has optimum accessibility to the scientific information of the world,

and directs the public information activities of the Bureau. The Office consists of the following organizational units: Office of Standard Reference Data—Office of Technical Information and

Publications—Library—Office of Public Information—Office of International

Relations.

1 Headquarters and Laboratories at Gaithersburg, Maryland, unless otherwise noted; mailing address Washington, D.C. 20234.

2 Part of the Center for Radiation Research.

8 Located at Boulder, Colorado 80302.
PNAL BUREAU OF STANDARD

N 6 1972 UNITED STATES DEPARTMENT OF COMMERCE

^v aL^ Maurice H. Stans. Secretary

, ,^q NATIONAL BUREAU OF STANDARDS • Lewis M. Bra nscomb. Director

l<?HS*H

NBS TECHNICAL NOTE 584

ISSUED DECEMBER 1971

Nat. Bur. Stand. (U.S.), Tech. Note 584, 175 pages (December 1971)

CODEN: NBTNA

Analytical Coordination Chemistry

Section:

Summary of Activities

July 1970 to June 1971

0. Menis and J. I. Shultz, Editors

Analytical Coordination Chemistry Section

Analytical Chemistry Division

Institute for Materials Research

National Bureau of Standards

Washington, D.C. 20234

^orc

\ w /

NBS Technical Notes are designed to supplement the Bureau's regular publications program. They , vvide a means for making available scientific data that are of transient or limited interest. Technical Notes may be

listed or referred to in the open literature.

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

(Order by SD Catalog No. C 13.46:584). Price $1.50

FOREWORD

The Analytical Chemistry Division was established as a separate division at the National Bureau of Standards on

September 1, 19^3, and became part of the Institute for

Materials Research in the February 1, 1964, reorganization.

It consists at present of nine Sections and about 100 technical personnel encompassing some 45 different analytical

competences from activation analysis and atomic absorption to

vacuum fusion and x-ray spectroscopy. These competences, and

in turn the Sections which they comprise, are charged with

research at the forefront of analysis as well as awareness of

the practical sample, be it standard reference material or

service analysis. In addition it is their responsibility to

inform others of their efforts.

Formal publication in scientific periodicals is a highly

important output of our laboratories. In addition, however,

it has been our experience that informal, annual summaries of

progress describing efforts of the past year can be very valuable in disseminating information about our programs. A word

is perhaps in order about the philosophy of these yearly

progress reports. In any research program a large amount of

information is obtained and techniques developed which never

find their way into the literature. This includes the

"negative results" which are so disappointing and unspectacular but which can often save others considerable work. Of

importance also are the numerous small items which are often

explored in a few days and which are not important enough to

warrant publication — yet can be of great interest and use to

specialists in a given area. Finally there are the experimental techniques and procedures, the designs and modifications

of equipment, etc., which often require months to perfect and

yet all too often must be covered in only a line or two of a journal article.

in
Thus our progress reports endeavor to present this information which we have struggled to obtain and which we

feel might be of some help to others . Certain areas which

it appears will not be treated fully in regular publications

are considered in some detail here. Other results which are

being written up for publication in the journal literature

are covered in a much more abbreviated form.

At the National Bureau of Standards publications such

as these fit logically into the category of a Technical

Note. We plan to issue these summaries for all of our

sections. The following is the seventh annual report on

progress of the Analytical Coordination Chemistry Section.

W. Wayne Meinke, Chief

Analytical Chemistry Division

IV
PREFACE

The seventh annual progress report of the Analytical

Coordination Chemistry Section describes the research

and development activities related to the needs of the

Standard Reference Materials Program. The current report

reflects the continuation of high accuracy measurements in

the fields of spectrophotometry and spectrofluorometry

which are applicable to problems in clinical chemistry

and environmental studies . It provides background information

on the two new Standard Reference Materials, SRM-930,

solid filters, and SRM-931, liquid filters, issued for the

purpose of calibrating the photometric scales of spectrophotometers. As a progress report, it is fragmentary and

only indicates the areas of work undertaken by the Section.

Inquiries and suggestions regarding these efforts are

solicited

.

To adequately describe experimental procedures, it

is occasionally necessary to identify commercial products

and equipment by the manufacturer's name or label. In no

instances does such identification imply recommendation or

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.

The assistance of Mrs. Rosemary Maddock is gratefully

acknowledged. Also, we acknowledge the invaluable assistance

of Mrs. Mary Pantazis, Section Secretary, who performs a wide variety of duties essential to the work of this section,

and for her tireless efforts in typing and assembling this

report

.

Oscar Menis, Chief

Analytical Coordination Chemistry

James I. Shultz, Assistant Chief

Analytical Coordination Chemistry
TABLE OF CONTENTS

PAGE

SPECTROPHOTOMETRY 1

A. Introduction 1

B. Solid Standard Reference Materials to Check

the Photometric Scale of Spectrophotometers - R. Mavrodineanu and J. R. Baldwin 2

1. Study of Surface Properties of Filters. . 6

2. Ultraviolet Region 7

3. Cooperative Study of Colored Neutral

Glass Filters 7

C. Instrument Development - R. Mavrodineanu. . . 9

Description of the NBS Instrument .... 10

a. The Light Source 11

b. The Monochromator 12

c . Sample Carrying Unit 13

d. System to Check the Accuracy of the

Photometric Reading 13

e. Integrating Sphere and Photomultiplier

Arrangement 16

D. Experimental Results 18

E. Studies of Liquid Filters as Potential

Absorbance Standards - R. W. Burke and

E. R. Deardorff 21

1. Solutions of Inorganic Salts 21

2. Composite Mixtures 22

3. Organic Dyes 24

F. The Use of Chromium(VI) Solutions as Spectrophotometry Absorbance Standards - R. W.

Burke, E. R. Deardorff and D. S. Bright . . 27

1. Introduction 27

2. Chromium (VI) Equilibria 29

3. Redetermination of K^ , the Dimerization

Constant for the Reaction: 2HCrO|.

_

= Cr

2

0" + H

2

30

Experimental 31

4. Absorption Spectra of HCrOn" and Cr^O "

. 36

5. Agreement Between Experimental and

Calculated Values of Apparent Molar

Absorptivity. 36

6. Using Acidic bichromate Solutions to

Check Linearity in the Ultraviolet. . . 39

7. Optimum pH and Choice of Acid for

Preparing Acidic Chromium(VI) Solutions 4l

8. Effect of Temperature on the Absorbance

of K

2

Cr

2

at pH 3 42

VI
TABLE OF CONTENTS (CONT'D)

PAGE

9. K

2

Cr

2

in 0.05M Na^PO^ and 0.05M KOH. . 43

10. Conclusion 44

G. The Absorptivity of Bilirubin, NBS-SRM 916 - R. W. Burke and E. R. Deardorff 45

1. Introduction 45

2. Experimental 45

a. Procedure 46

b. Results 46

3. Conclusions .• • 46

H. Spectrophotometric Study of the Dye Ratio in

Sample of Ballpoint Ink - R. W. Burke ... 47

1. Introduction 47

2. Experimental 47

a. Calibration 48

b. Analysis of Composite Dye Solutions . 49

c. Analysis of the Bulk Ink 50

d. Analysis of Bulk Ink After Application

to Bond Paper 50

e. Dye Composition of Manufacturer's

Scribble Chart Ink 51

f. Light Fastness 51

3. Conclusion

2. FLUORESCENCE 53

A. Introduction 53

B. Instrumentation Evaluation - R. A.

Velapoldi 54

C. Fluorescence - Inorganic Ions in Glass

Matrices - R. A. Velapoldi and R. Reisfeld. 6l

D. Inorganic Ions with Broad Spectra 61

1. General 61

2. Quantitative and Qualitative Analysis . . 62

3. Matrix Effects 63

4. Quantum Efficiencies 67

5. Future Research 71

E. Lanthanides 71

1. Analytical 71

2. Matrix Effects 72

3. Quantum Efficiency Determinations of

Eu(III) in Silicate Matrix 74

F. Energy Transfer 79

G. Data and Spectra Storage 8l

H. Computer Programs - R. A. Velapoldi ....... 83

Vll
TABLE OF CONTENTS (CONT'D)

PAGE

ANALYTICAL FLAME SPECTROSCOPY 84

A. Introduction 84

B. Evaluation of the Multichannel Direct

Reading Spectrometer - R. Mavrodineanu . 84

C. Long Path Flames . 87

D. A Triple Pass On-Axis Optical System for

Analytical Flame Spectroscopy - R. Mavrodineanu and W. Miiller-Herget . . 88

E. Premixed Acetylene-Oxygen Flame Supplied

Through an Ultrasonic Spraying System -

L. Owen ana R. Mavrodineanu 91

F. Flame Emission Spectrometry With Optical

Scanning - T. C. Rains 93

G. High-Precision Atomic Absorption

Spectrometry for Major Components - T. C. Rains 99

H. Effect of Spectral Bandwidth in Atomic

Absorption Spectrometry - T. C. Rains. . 102

I. Analytical Application to Standard

Reference Materials - T. A. Rush and

T. C. Rains 10 3

1. Determination of Calcium, Magnesium,

Silver and Zinc in Special Steel . . 103

2. Determination of Calcium, Potassium,

Magnesium, Sodium, Rubidium, Cadmium,

Zinc and Manganese in Orchard Leaves - T. C. Rains and T. A. Rush 104

a. Preparation of Samples 106

b. Determination of Constituents. . . 107

3. Determination of Mercury in Orchard

Leaves - T. C. Rains 108

a. Dissolution of Sample' 110

b. Reduction of Mercury Ill

c. Trapping of Volatile Vapors. . . . 112

d. Absorption Cells 112

e. Contamination Problems 112

f. Measurement 113

4. Service Analysis - T. A. Rush and

T. C. Rains 113

THERMAL ANALYSIS . . . 116

A. Introduction 116

B. Certification of SRM's for Calibrating the

Temperature Scale of DTA Apparatus - D. S. Bright and 0. Menis 117

C. Scanning Calorimetry - D. S. Bright,

E. Prosen and 0. Menis. 119

viii
TABLE OP CONTENTS (CONT'D)

PAGE

1. Introduction . 119

Certificates for NBS-ICTA

Standard Reference Materials 758,

759, and 760 120

2. Principle and Measurements 128

3. Apparatus 131

a. Sample Holder 131

b

.

Copper Block and Housing for

Heater and Thermocouples. . . . 132

4. Electrical Calibration 132

5. Observations 133

6. Conclusion 130

5. STANDARD REFERENCE MATERIALS. . . , 14

A. Introduction - J. I. Shultz 140

B. Standard Reference Materials Information

and Summary l4l

1. Stainless Steels 121d and 1171. . . . l4l

2. Stainless Steels 123c and 1172. ... 142

3. Special Steels 361, 1261; 362, 1262;

363, 1263; 1264 143

6. PERSONNEL AND ACTIVITIES 145

7. REFERENCES 152

IX
LIST OF TABLES

TABLE NO. PAGE

1. Comparative % T measurements performed by four

laboratories on three Schott NG-4 colored glass

filters 7

2. Transmittance measurements on a Schott NG-4

glass filter with the NBS high accuracy spectrophotometer 19

3. Repeatability of % T measurements performed

with the NBS high accuracy spectrophotometer

on three colored glass filters 20

4. Comparison of % T measurements performed on a

Schott NG-4 filter using the NBS high accuracy

instrument and a conventional spectrophotometer

provided with a quartz prism double monochromator in a double beam arrangement 20

5. Apparent molar absorptivity of nickel (II)

solutions at 25 °C 22

6. Format of the interlaboratory evaluation of two

liquid filters, designated T and NBS , as

potential absorbance standards 24

7- Absorbance of KpCr ? 7 solutions as measured in

1.023 mm cuvette at 25 °C; pH = 2.92 + 0.02

(HClcy " 34

8. Apparent molar absorptivities of KpCr p 7solutions

as a function of chromium(Vl) concentration at

25 °C; pH = 2.92 + 0.02 (HCIO^) 34

9. K~ at 25 °C as determined at various wavelengths 35

10. Provisional molar absorptivities of HCrOn" and

Cr

2

= at 25 °C 37

11. Experimental and calculated values (E, C) of

apparent molar absorptivity, e , at 25 °C . . . 3 8

12. a, 1-a, and apparent weight factors at 25 °C for

selected concentrations of KpCr p 7

-0

13. Tentative molar absorptivity values of KpCr„0 7

in 0.05M Na

2

HP0 1| and 0.05M KOH at 25 °C . . . . ^3

14

.

Apparent absorptivity of SRM 916, Bilirubin, in

chloroform at 25 °C 47

x
LIST OP TABLES (CONT'D)

TABLE NO. PAGE

15. Analysis of known mixtures of Methyl Violet DY

Base (MV) and Victoria Pure Blue BO Base (VB) . 50

16. Analysis of bulk ink 50

17. Analysis of bulk ink following application and

elution from bond paper 51

18. Analysis of ink on manufacturer's scribble

chart 51

19. Variation of relative dye concentration after

exposure to light 52

20. Reproducibility of preparation and measurement

of quinine sulfate solutions in 0.1N HpSOj. peakheight measurements 57

21. Statistical analysis of instrumental stabilities

using quinine sulfate and uranium glass as

standards 58

22. Statistical analysis for eleven fluorescence

determinations of quinine sulfate during an

eight hour span using a corrected spectrofluorometer 59

23. Emission and excitation wavelength dependence on

matrix for lead, cerium and thallium 62

24. Ionic field strengths of cations in glasses . . 65

25. Infrared stretching frequencies and bond lengths

for active ion-oxygen interactions in the matrix 67

26. Oscillator strengths for T1C1 in KC1 70

27. Excitation and emission wavelengths for

lanthanides in glass matrices yielding maximum

emission intensities 72

28. Shifts of major europium, samarium and thulium

emission peaks as a function of matrix 73

29- Average fluorescence lifetimes of rare earths

in various matrices 7^

30. Effects of glass matrix on fluorescence

intensities of the rare earth ions 7 4

31. Major emission peaks for europium(III) in water

and silicate matrices 75

32. Transition assignments and relative intensities

of major emission and excitation bands for

europium( III) in silicate and phosphate

matrices 82

xi
LIST OF TABLES (CONT'D)

TABLE NO. PAGE

33. Accuracy of analytical results for magnesium,

potassium, calcium and sodium determined

simultaneously 86

3^. Determination of aluminum in 0.4% carbon steel

by flame emission spectrometry 100

35. Determination of zinc in SRM 124d (ounce metal) 101

36. Determination of chromium in SRM 121d 102

37- Summary of SRM analyses by flame emission

and atomic absorption spectrometry 105

38. Determination of mercury in orchard leaves

by flameless atomic absorption 113

39 • Analytical conditions and concentration

ranges for trace elements in water effluents

with and without NTA 115

40. Mean values of onset heat temperatures of

ICTA substances 117

41. Calibration data - determination of

proportionality coefficient, k, for transition

(melting) on heating 134

42. Stainless Steels 121d and 1171 143

43. Stainless Steels 123c and 1172 143

Xll
LIST OF FIGURES

FIGURE NO . ^^

1. Set of three glass filters with the corresponding

provisional certificate constituting SRM-930 . . 3

2. Schematic description of the NBS high accuracy

spectrophotometer 10

3 NBS hie:h accuracy spectrophotometer. At right , the light source, at upper left, the lm monochromator. The optical bench at the exit slit

of the monochromator carries an achromate; the

sample platform; a second achromate followed by

the integrating sphere and photomultiplier

assembly

4. NBS high accuracy spectrophotometer. Same

arrangement as in figure 3 except for the

aperture system which is now placed after the

exit slit of the monochromator. This arrangement

is used when making linearity measurements .... 16

5. Ampouled filters now undergoing evaluation in

cooperating clinical laboratories 23

-4

6. Comparison of absorption spectra of 1.0 x 10 M

K

2

Cr 2°7 ln °- 001N HCIOk and 0.05N KOH 28

7- Difference spectrum of 391 mg KpCrpO„/liter

in 1.023 mm cuvette and 40.0 mg KpCr

? 7 /liter

in 10.00 mm cuvette; pH = 3 33

8. Least squares determination of K. 35

9. Absorption spectra of HCrOj." and Cr

p 0„ from

220-400 7 ( 37

10. Temperature dependence of a solution of K^Cr^O,,

at pH 3 • • • • ^2

11. Absorption spectra of acidified ethanolic

solutions of Methyl Violet DY Base (MV) andVictoria Pure Blue BO Base (VB) ^9

12. Corrected and uncorrected emission and excitation

spectra of quinine sulfate in . IN HpSOj. 55

13. Emission peak height is directly proportional

to emission peak area 56

14. Relative fluorescence intensity versus quinine

sulfate concentration showing "concentration

quenching" 57

jxiii
LIST OF FIGURES (CONT'D)

FIGURE NO PAGE

15. Instrumental stability and reproducibility as a

function of time using liquid and solid samples. . 58

16Typical warm-up times for a commercial instrument

in uncorrected mode using aluminum foil as

reflecting sample 60

17. Relative fluorescence intensity versus concentration in weight percent for cerium(III),

lead(II) and thallium(I) 63

18. Wavelength dependence of emission peaks on

matrix for cerium, lead and thallium 64

19 • Matrix dependent ligand field effects on

degenerate and ion-degenerate states 66

20. Fluorescence intensity dependence on sample

angle for thallium(I) in KC1 and quinine

sulfate in 0.1N H

2

SO i| .68

21. Relative fluorescence intensity versus rare earth

concentration in weight percent 71

22. Corrected excitation, emission and absorbance

spectra for europium(III) in a silicate matrix . . 73

23. Major emission peaks of europium(III) in silicate

matrix and water solution 76

24. Gaussian Analysis for absorbance spectra of

europium in silicate 7 Fo,i^ 5 D transitions .... 77

25. Fluorescence intensity enhancement of

eurooium( III) by energy transfer from samarium(III)

to europium(III) , X = 402.0 nm 79 GXC

26. Fluorescence intensity enhancement of terbium(III)

by energy transfer from gadolinium(III) to

terbium(III) ,3 X = 275.0 nm 80

exc

27- Excitation spectra obtained showing gadolinium( III)

spectra (275.0, 312.0 nm) when monitoring terbium(III)

emission at 490.0 nm,

5 Dit ^-7 F6 transition 81

28. Proposed system and basic dimensions for the

triple pass, on-axis optical arrangement

(dimensions in mm) 88

29. Schematic drawing of the on-axis triple pass

system 90

30. Premixed C

2

H

2

-0

2

burner and ultrasonic nebulizer 92

xiv
LIST OF FIGURES (CONT'D)

FIGURE NO. PAGE

31. The unit from figure 30 in operation. ..... 93

32. The unit from figures 30 and 31 placed on the

optical bench in front of a lm grating

spectrometer; shown here for measurements in

the atomic mode. At right, the hollow cathode

followed by a lens and a chopper. At lower

right, the generator, which drives the ultrasonic generator and the electrically operated

sample pump which supplies the ultrasonic

sprayer with the analytical sample solution . 9^

33. Flame emission spectra of aluminum over

wavelength region of 3925 to 3970 A from a nitrous oxide-acetylene flame 95

34

.

Calibration curve for aluminum using

repetitive optical scanning 96

35. Comparison of flame emission spectrum of 0.1 yg/

ml of aluminum and H

2 using a nitrous oxideacetylene flame. Conditions: Time constant,

1 sec; voltage applied to photomultiplier

tube, 1000 V; amplifier gain, 1 mV; flame,

fuel rich (1-1/2 inch pink plume) 97

36. Relative intensity of aluminum using

repetitive optical scanning.

(A) Optical conditions the same as in figure 35.

(B) Voltage applied to photomultiplier tube

increased to 1200 V 98

37- Relative intensity of 0.1 and 0.2 yg of

aluminum with repetitive optical scanning

using a nitrous oxide-acetylene flame.

Conditions: Same as in figure 35, except

voltage applied to photomultiplier tube,

1300 V 99

38. Effect of flame cell path-l%ngth on the

absorbance of zinc at 2138 A 100

39. Effect of spectral bandwidth on absorbance

of iron at 2483 A 103

40. Schematic diagram of flameless atomic

absorption apparatus for mercury

determination 109

41. Data from 34 cooperating investigators for

each of the 10 substances (submitted to the

Standards Committee of ICTA) 118

xv
LIST OF FIGURES (CONT'D)

FIGURE NO. PAGE

42. Deviations from the mean value as reported

by the 34 investigators (submitted to the

Standards Committee of ICTA) 118

43. Cross section of calorimeter chamber 129

44. Cross section of copper block 130

45. Proportionality coefficient, k, vs

temperature 136

46. Electrical calibration curve 137

47. Tin heating curve 138

48. Benzoic acid heating curve 139

49. Types of Standard Reference Materials

characterized 142

xvi
ABSTRACT

Progress In research and development in spectrophotometry, spectrofluorometry , flame emission, atomic absorption,

thermal analysis and the related program for the certification of Standard Reference Materials is described. Efforts,

to improve the accuracy In spectrophotometric measurements

led to the construction of a NBS high accuracy spectrophotometer, the certification of glass filters (SRM 930) in the

visible region of the spectrum, establishment of a more

accurate value or molar absorptivity of the acid dichromate

system, and, the evaluation of spectrally neutral filters

of Interest in automation for clinical studies. A single

beam high accuracy spectrophotometer was constructed and

its linearity was calibrated by the light addition technique

.

The glass filters, Schott NG-4 type were then calibrated at

four wavelengths with an uncertainty of . 5 percent relative,

based on the sum of random errors and biases between instruments, as well as material stability. New data for the

dichromate system In diluted perchloric acid led to new

values for the dimerization equilibrium constant of 32.8.

Also, programs for evaluating neutral gray solutions of

inorganic salts are described. Other spectrophotometric

studies included the measurement of molar absorptivity of

bilirubin and the dye ratio of a commercial Ink. Fluorescence

studies Involved determination of criteria for the evaluation

of instrumental performance of proposed SRM's.

Studies were made of new solid fluorescence filters

incorporating Tl(l), Ce(lll) and Pb(II) and rare earths in

borate, phosphate and silicate glass matrices. Quantum

efficiencies of thallium in potassium chloride and europium

in a silicate matrix were determined. The latter, measured

by two independent methods, was 0.9 3 + .05. Also, computer

handling of the data is briefly described.

XV" 11
In analytical flame spectroscopy, developments in

instrumentation and methodology led to increased sensitivity,

improved precision and accuracy in methodology and certification for several important trace elements in SRM's. Data

were obtained with the multichannel direct reading spectrometer for elements: magnesium, calcium, potassium and sodium,

with an accuracy of one to three percent relative standard

deviation. A triple pass on-axis optical system offered

an increase in signal of over 150 percent. An ultrasonic

spraying system provided a 50 percent efficiency in the

nebulization of sample. The repetitive optical scanning

of flame emission overcame matrix and flame background

problems for the determination of 0.04 percent aluminum in

carbon steel with a relative standard deviation of 1.6

percent. Calcium, magnesium, silver and zinc were determined

in special steels. Nine trace elements, including mercury,

were determined in SRM botanical, orchard leaves. The

studies of the variables in the determination of mercury by

the flameless atomic absorption method led to an improved

accuracy in the certification of a mercury value at 160

nanograms level in orchard leaves . In the area of thermal

analysis, the final certification of eight new materials

in cooperation with the International Confederation on

Thermal Analysis was carried out. A scanning calorimeter

was constructed and tested for enthalpy measurements on a number of DTA standards up to 300 °C.

Finally, a brief discussion is presented of a variety

of materials that have been characterized as Standard

Reference Materials . Part of the activities resulted in

the certification of carbon, stainless and high-silicon

steels, high temperature alloys, orchard leaves, cholesterol,

bilirubin, sodium chloride, and DTA standards.

xvm
1. SPECTROPHOTOMETRY

A. Introduction

During the past year considerable progress was made in

the three-pronged program outlined in last year's progress

report. These efforts included the construction and measurements with the high accuracy spectrophotometer , development

and certification of glass filters for use in the calibration

of the photometric scale in the visible region of the spectrum, and studies with liquid filters for checking the

accuracy of spectrophotometric measurements which will be

useful in automated clinical measurements. Last year's

report discussed extensively the overall philosophy, need

and the justification for the various approaches that were

contemplated. The current report covers the details of the

construction of the new high accuracy spectrophotometer,

its calibration, and its application to the certification of

Standard Reference Material 930, Glass Filters. These

filters are suitable for calibrating the photometric scale

in the region of the spectrum from 400 nm to 700 nm. The

report also describes new studies leading to more accurate

molar absorptivity values of the acid dichromate system and

further progress in the development of spectrally neutral

liquid filters

Significant advances have been made in providing the

basis for more accurate spectrophotometric measurements

The NBS single beam spectrophotometer now provides transmittance accuracy of at least 3 parts per thousand based

on the stability of its components and an independent physical

measurement of the linearity of transmittance . The accuracy

value of 0.5 percent relative standard deviation assigned

to the SRM 930, Glass Filter, is limited at present by the

quality of the material.

A restudy of the acid dichromate systems led to a tenfold improvement In the values for the equilibrium

1
formation of the dichromate dimer and provides a basis for

measurement and recalculating of the molar absorptivity of

the complex in the visible and UV regions of the spectrum and

to provide flexible systems for calibrating multielement

analysis in automated systems

.

A major need is to improve the accuracy of the data

being collected in the field of clinical chemistry. In

addition to standards, improved methodology is needed to

attain higher accuracy in this field. The present efforts in

the area of standardization need to be extended to include

stray light, bandpass, cell path calibrations, and ultimately

a definite Standard Reference Material incorporating the

multicomponent material of interest in health or pollution

problems. To focus on the overall problem, a conference on

Spectrophotometry and Luminescence Analysis has been planned

for March 22-24, 1972 at NBS . Papers will be presented by

ex:perts in these measurement areas and also by panelists whose

concern is primarily with improvement of health and alleviation of pollution.

B. Solid Standard Reference Materials to Check the

Photometric Scale of Spectrophotometers - R. Mavrodineanu

and J. R. Baldwin

The need for providing means and materials to check the

proper functioning of a spectrophotometer was discussed in

some detail in a previous publication [1]. At that time it

was established that the accuracy of the photometric scale

is critical, and the most demanding parameter in spectrophotometry measurements. Hence, particular attention was

given to a number of ways for checking this parameter.

Investigations have indicated that solid colored glass

filters, exhibiting optical neutrality over the spectral

range from 400.0 nm to 700.0 nm, would constitute an

acceptable Standard Reference Material. From the various

colored glass filters available, Schott NG 4 "neutral glass"

was selected, prepared and characterized and is now being

2
issued as SRM 930. This standard consists of three filters

each 25 mm long and 10 mm wide, They are about 1.0 mm,

1 5 mm and 2 mm thick and have nominal transmittances of

10, 20 and 30 percents, respectively. Each filter is mounted

in'a metal holder which fits the cell compartment of most

conventional spectrophotometers; a set of three such filters

is illustrated in figure 1. The provisional certificate

Figure 1. Set of three glass filters with the corresponding

provisional certificate constituting SRM-930.

3
U. S. Departfaeirt' of Commerce

Mauriceifi. Stans Secre

National Bureaft of! Standards

L. M. Branscprnb,' Director (Certificate

Standard Reference Material 930

Glass Filters for Spectrophotometry

R. Mavrodineanu and J. R. Baldwin

This standard reference material is intended as a reference source for the calibration of the photometric scale of spectrophotometers. It consists of three individual filters. Each filter bears an

identification number, and the upper left corner has been removed to indicate correct placing in the metal holder.

Filter and Set

Identification Number

ABSORBANCE (A) TRANSMITTANCE (%T)

Wavelength and (Bandpass) nm

Wavelength and (Bandpass) nm

440.0

(2.2)

465.0

(2.7)

590.0

(5.4)

635.0

(6.0)

440.0

(2.2)

465.0

(2.7)

590.0

(5.4)

635.0

(6.0)

The transmittance values given are certified with a relative uncertainty of ± 0.5 percent

(example: a nominal value of absorbance of 0.500 ± 0.0022). This uncertainty is the sum of the random error of ± 0.1 percent (2SD limit) and of estimated biases which are ± 0.4 percent. These

biases are due to possible systematic errors. Measurements were made at 24 °C. Room temperature

variations within several degrees Celsius of this temperature will not significantly affect the calibra- tion of these filters.

It is recommended that the filters be handled only with soft plastic (polyethylene) gloves and

optical lens tissue. When not in use they should be stored in their holders and in the box provided

for this purpose. Extended exposure to laboratory atmosphere and dusty surroundings should be

avoided.

The overall direction and coordination of the technical measurements leading to certification were performed under the chairmanship of 0. Menis and J. I. Shultz.

The technical and support aspects involved in the preparation, certification and issuance of this standard reference material were coordinated through the Office of Standard Reference Materials by T. W. Mears.

Washington, D.C. 20234

February 24, 1971

J. Paul Cali, Chief

Office of Standard Reference Materials
The transmittance measurements were made with a double-beam spectrophotometer containing

a quartz double monochromator. The photometric scale of the instrument is divided into 1000

divisions, each division representing 0.1 percent transmittance. A corresponding absorbance scale is also provided on the instrument. The accuracy of the photometric scale was confirmed by the high-accuracy spectrophotometer designed and constructed at the National Bureau of Standards. The accuracy of this instrument was established by light-addition measurements.

The neutral NG-4 glass for the filters was provided by Schott of Mainz, Germany and is desig- nated as "Jena Colored and Filter Glass." Nominal transmittance for a filter 1.5 mm thick is 20 percent at 400.0 nm wavelength and 32 percent at 700.0 nm wavelength. Between these limits the transmittance varies in a monotonous manner [1]

.

The filter holder and the size and shape of the filters were selected to conform to the dimensions

of the sample compartment of most conventional spectrophotometers. The filters are approximately 1.0, 1.5, and 2.0 mm thick. Corresponding to these thicknesses are nominal transmittances of 30, 20, and 10 percent, respectively. These thicknesses were selected to provide a means for

calibrating the photometric scale at three different levels.

The transmittance of filters depends on the intrinsic properties of the material. Spectral bandpass, wavelength [1,2], geometry of the optical beam, surface conditions, and positioning of the

filter also affect the transmittance values, and can lead to further biases. The certified data will be

reproduced when transmittance measurements are made under similar conditions. The effective

spectral band passes used to determine the certified values are given on the face of the certificate.

Prior to the certification measurements, each filter was examined for surface defects and

thoroughly cleaned. If, through handling, the surface of the filter becomes contaminated with dust,

it may be cleaned with a small soft brush attached to a rubber tube connected to a vacuum

source [3] . If the surface becomes contaminated with fingerprints, they must be eliminated before making measurements. This may be accomplished by removing the filter from its holder, breathing

lightly on it, and rubbing the surface gently with optical lens tissue. The clean filter is then replaced

in its proper position in its holder. To remove and replace the filter in the metal holder, the spring-loaded plate should be lifted with care to prevent damage to the filter. As little handling as possible is recommended.

NOTE: The check of the calibration of photometric scales defines only one of the parameters

required for obtaining accurate transmittance values and molar absorptivities. Other factors that

also must be established are wavelength accuracy, stray light, cell parameters, fluorescence, polarization, reflection, and temperature coefficient. Some of these variables are discussed in NBS Technical Notes [lj. It is planned to summarize various aspects of accurate spectrophotometric measurements

in an NBS-260 Special Publication which would provide additional data on specific standard refer- ence materials. In the interim, SRM 930, should be used as described in the certificate. Consult the manufacturer of the instrument if differences are obtained that exceed those specified by the manufacturer. We wish to acknowledge the cooperation of George N. Bowers, Jr., M.D., of Hartford Hospital,

Hartford, Connecticut; Royden N. Rand, Ph.D., of the Hospital of the University of Pennsylvania,

Philadelphia, Pennsylvania; and Donald S. Young, M.B., Ph.D., of the National Institutes of Health,

Bethesda, Maryland.

1. R. Mavrodineanu, Solid Materials to Check the Photometric Scale of Spectrophotometers, NBS

Tech. Note 544, 0. Menis and J.I. Shultz, ed., pp. 6-17, U. S. Government Printing Office,

Washington, D. C. 20402 (Sept. 1970), ibid.NBS Tech. Note 584, 1971 (to be issued).

2. K. S. Gibson, Spectrophotometry, NBS Circ. 484, (Sept. 1949).

3. J. R. Edisbury, Practical Hints on Absorption Spectrophotometry, Plenum Press, New York

(1967).
which accompanies these filters and which gives the absorbance

and transmittance values together with certain information

concerning the handling of the filters, is reproduced on

pages H and 5.

1 . Study of Surface Properties of Filters

The transmittance data given in the certificate depend

not only on the intrinsic properties of the glass, but

also on the surface condition of the glass. This parameter

varies with time and exposure conditions. When glass is

exposed to normal room atmosphere and temperature, its surface

is corroded to an extent depending on the composition, time of

exposure, concentration, temperature and nature of the glass

surface and acting agents. This action produces a change in

the reflecting and transmitting properties of the material

[2, 3, 4]. For instance, one of the factors which can produce such changes is called "blooming" of the glass which is

due to the formation of an SiO layer at the surface of the

glass. This layer, which increases the transmittance, acts

as an antiref lection coating. The speed with which such a layer is formed varies with the composition of the glass, the

atmosphere and time. Generally speaking, several years are

needed for a fresh surface to reach an equilibrium. This and

similar phenomena are presently being studied, along with

means to stabilize the surface state of glass filters. Until

more information is acquired in this field, we recommend that

the colored glass filters be rechecked annually to determine

whether any physico-chemical changes, which might affect the

transmittance values, have occurred.

Another important factor is the need for a clean

surface. Until now the final cleaning of the NG-4 filters was

made with redistilled ethyl alcohol and pure water (thermally

distilled and deionized). Other cleaning procedures are

under consideration and the use of isopropyl alcohol in vapor

or liquid form is being investigated [5].
2

Ultraviolet Region

The Schott NG-4 type glass now in use has a transmittance which is limited to the visible region of the

spectrum. Since the ultraviolet region, from about 200 nm,

is also important to the analyst who uses spectrophotometric methods, exploratory work is underway to select and

certify solid materials for checking the photometric scale

in this spectral region.

3

Cooperative Study of Colored Neutral Glass Filters

A cooperative study was carried out in collaboration

with three clinical laboratories to determine the reproducibility of transmittance measurements on the Schott NG-4

neutral glass filters. Three filters having nominal transmittances of 10, 20 and 30 percent were measured at

four wavelengths on a conventional spectrophotometer at

the NBS . The same filters were then sent to laboratories A,

B and C describing the technique to be used in measuring their

transmittances . The results obtained are summarized in

table 1 and include the percent transmittance (% T) values,

Table 1. Comparative % T measurements performed by four

laboratories on three Schott NG-4 colored glass

filters

Laboratory A_

Measurement % T

No. 440 nm 465 nm 590 nm 635 nm

Ave . 12.91 14.99 11.68 12.71

1 S.D. 0.0063 0.004 0.008 0.005

% S.D, .049 .027 .070 .041

Diff

% - .4 - .07 - .6 - .5

Ave . 19.61 22.37 19.06 20.50

2 S.D. 0.004 0.010 0.004 -

% S.D, .021 .046 .022 — Diff. % - .5 - .3 - .3 -0.1

Ave. 32.81 35.44 31.12 32.58

3 S.D 0.0080 0.0080 0.0050 0.0050

% S.D, .023 .023 .017 .016

Diff. % + .3 + .2 + .1 + .2
Laboratory

Measurement % T

No. 440 nm 465 nm 590 nm 635 nm

Ave. 12.91 14.87 11.56 12.59

1 S.D. -r 0.041 0.041

% S.D. - .274 .36

Diff. % -0.4 - .9 -1.6 -1.4

Ave . 19.59 22.39 18.90 20.51

2 S.D. - — 0.053 -

% S.D. - — .281 - Diff. % -0.6 -0.2 -1.1 -0.05

Ave. 32.58 35.23 30.90 32.36

3 S.D. - 0.093 - -

% S.D. - .264 — — Diff. $ -0.4 - .3 -0.6 -0.4

Lab oratory C

Measurement % T

No. 440 nm 465 nm 590 ran 635 nm

Ave. 12.88 14.96 11.66 12.65

1 S.D. - 0.014 0.011 0.018

% S.D. - .09 .09 .15

Diff. % -0.6 - .3 - .8 - .9

Ave. 19.54 22.34 19.12 20.37

2 S.D. - - 0.024 -

% S.D, - - .13 - Diff. % -0.9 -0.4 + .5 -0.7

Ave. 32.88 35.40 30.97 32.43

3 S.D. - - - -

% S.D, - - - - Diff. % +0.5 +0.1 -0.4 -0.2
Laboratory NBS

Me a surement % T

No. 440 nm 465 nm 590 nm 635 nm

1

Ave.

S.D.

% S.D,

Diff

'%

12.96

0.0053

.041

15.00

0.0037

.024

11.75

0.0048

.041

12.77

0.0057

.045

2

Ave.

S.D.

% S.D,

Diff. '%

19-71

0.0049

.025

22.44

0.0052

.023

19.11

0.0038

.020

20.52

0.0049

.024

3

Ave . S.D.

% S.D,

Diff. '%

32.72

0,0045

.014

35.35

0.0056

.016

31.09

0.0040

.013

32.50

0.0043

.013

the standard deviation (S. D.) a the percent standard deviation

(% S. D.), and the percent difference (% difference). This

difference was calculated using the NBS data as a reference.

In this study, the cooperation of George N. Bowers, Jr.,

M. D., of Hartford Hospital, Hartford, Connecticut, Royden

N. Rand, Ph.D., of the Hospital of the University of

Pennsylvania, Philadelphia, Pennsylvania, and Donald S. Young,

M.D., Ph.D., of the National Institutes of Health, Bethesda,

Maryland is gratefully acknowledged.

C. Instrument Development - R. Mavrodineanu

The decision to provide solid and liquid filters to

check the photometric scale of spectrophotometers implicitly

required that the certified transmittance data assigned to

these filters be given with a known accuracy.

Since conventional spectrophotometers cannot readily

provide such information, the design and construction

of a research instrument on which transmittance data

could be measured with known accuracy was Initiated. After

a comprehensive examination of the existing literature

9
in this field, (Ref. 6 to 3^ arranged in chronological

order) a decision was made to construct an instrument

similar in design to the instrument at the National Physical

Laboratories (NPL), Teddington, England, where a long

tradition in high accuracy spectrophotometry exists. A visit

was arranged with Dr. Frank J. J. Clarke, Head of Colorimetry

and Photometry Section, Metrology Centre at NPL. During this

visit, the NPL high accuracy spectrophotometer was examined

in detail and work was performed with the instrument

.

Furthermore, detailed discussions with Dr. Clarke and his

associates, Mr. G. Lambert and Anne Compton, provided valuable

scientific information for use in our work.

1. Description of the NBS Instrument

The NBS high accuracy spectrophotometer is a single

beam instrument which contains the following elements

(A) a constant light source, (B) a monochromator , (C) a sample

holder, (D) a system to check the accuracy of the photometric

data and (E) an integrating sphere attached to a photomultiplier - digital voltmeter unit. Figure 2 illustrates

Precision Optic*! Bench

iii iiiiu mi

llll-.ttllltT iritTIIHITIIITII

Mirror

CircularMNeutral Weds*

1m Grating Monochromator (Czemy-Tumer)

NPL Aperture for Linearity Control

(Alter F.J.J. Clarke)

(A)+(B)=(A+B)

Lens

Light J

Source!

Lent t Carriage

| , | Shutter]

Precision Optical Bench In Lightproof Enclosure B[ank

Sample

Constant Power

Supply _L I Monitor I

Integrating Sphere

Figure 2. Schematic description of the NBS high accuracy

spectrophotometer.

10
schematically the arrangement of these various components.

A neutral wedge is placed after the light source to select

various levels of radiation intensities required for measurements. A description of the components is presented in the

following sections.

a. The Light Source . Since the instrument is a

single beam type it is essential that the radiation source

be constant and homogeneous. Additional desirable conditions

are: (1) capability of monitoring the current supplied to

the source and (2) black-body type radiation. The source

which was selected was designed and used at NBS by

H. J. Kostkowski and R. D. Lee of the Institute for Basic

Standards in their radiation studies. This source was

duplicated in our instrument with considerable assistance

from its developers.

The source consists of a tungsten incandescent

filament lamp and includes a tungsten ribbon 8 mm x 2 mm.

The current used to operate this lamp at approximately

3000 °K is 6 V and 18 amp; our source is operated at 5 V and

13 amp. The power supply is capable of delivering 15 V and

50 amp, when operated in the constant current mode. To

achieve this mode of operation, an external sensing resistor

of 0.1 ft and 50 amp and a current control circuit is placed

in series with the supply. A feedback across this resistor

is connected to the power supply sensing system. The

characteristic of this operation is the ability to automatically change its output voltage so as to maintain a

constant current to the load resistor, which, in our case,

is the lamp source. The nominal current regulation obtained

is better than 0.01 percent and the stability over an 8 hour

period, at constant load temperature, is better than 0.02

percent. The stability of the current delivered to the lamp

is monitored with a high accuracy potentiometer having a range

from to 1.6110 V to to 0.16110 V and a corresponding

limit of error of + 0.01 percent + 20 yV to 0.015 percent

11
+ 0.5 y V. This potentiometer is used in conjunction with a null meter which is sensitive to variations in the current

supplied to the lamp from 1 part in 1000 to 1 part in

1,000,000 per division. The potentiometer is connected to

the current source across a resistor of 0.01 Q and 100 amp,

and placed in series with the lamp.

The demagnified (2 to 1) image of the ribbon filament

is projected on the entrance slit of the predisperser by a

glass achromate whose focal distance is 254 mm and diameter

is 44 mm. This achromate, as well as the other two achromates

used in the optical system, was calculated and selected by

K. Mielenz, of the Institute for Basic Standards. A circular

neutral wedge is placed between the light source and the

predisperser. This wedge, evaporated inconel on a glass

disk (150 mm diam.), is linear in density and provides a

light attenuation of 100 to 1. The wedge can be rotated by

an electrical motor (1 rev. per sec) to select proper

radiation intensity levels as required by the measurements.

b . The Monochromator . The monochromator is a 1-m Czerny -Turner type grating instrument. The flat grating

has 1200 grooves per mm over a surface of 100 x 100 mm. The

monochromator is provided with a predispersing attachment to

reduce the stray light. This predisperser is a small quartz

prism monochromator connected to the scanning system of the

1-m instrument. According to the data provided by the manufacturer, the stray light of the predisperser-monochromator

unit is less than 1 part in 10,000,000. A wavelength

o counter permits readings to 1 A and the scanning speed can

o be varied from 0.5 to 2000 A/min through a 12 speed synchronous electric motor.

The optical components are placed on precision lathebed type optical benches, 120 and 160 cm long, and are

equipped with appropriate carriers provided with x-y-z

adjustments

.

12
c

Sample Carrying Unit . The sample carrying unit

consists of a platform provided with two holders which can

accept 3/4 "in. rods and a variety of sample supports. These

holders can be moved laterally through a rack and pinion

arrangement. The platform is mounted on 4 ball bushings

which run on two horizontal rods and can be moved pneumatically

across the optical axis. The use of a pneumatically operated

motion was recommended by G. E. Moore and J. T. Sterling of

the Institute for Applied Technology and by L. Owen. The

total travel is 8 in. and the linear movement is smooth. The

position of the platform can be reproduced within 0.025 mm.

This unit is illustrated in figure 3 and is located between

the two achromates. The sample holder is designed to accept

conventional solid or liquid filter holders which fit most

spectrophotometers. The holders are provided with a thermostated jacket.

d. System to Check the Accuracy of the Photometric

Reading. Since the high accuracy spectrophotometer is single

beam, accurate photometric data are obtained when there is

a linear relation between the radiation flux and the corresponding response of the photodetector

Linearity of photodetectors can be measured by several

means: the inverse square law [8, 16], the use of optical

elements having a known transmittance which can be determined

by other means [18] and the light addition principle using a plurality of light sources [6, 7, 9, 10, 11, 14, 19, 20, 21,

29, 32, 33, 34] or multiple apertures [12, 13, 15, 17, 22,

24, 26, 27, 28, 31]. A novel approach to the problem of

accurate photometric measurements was described by 0. C. Jone^

and F. J. J. Clarke [25, 30]. A critical discussion of some

aspects of accurate spectrophotometry will be found in an NBS

manuscript by Gibson and Associates [23], The light addition

principle, using two apertures with one source of radiation,

was chosen in our work. The aperture method for checking

the linearity of photometric data was used in 1936 at the

13
Figure 3. NBS high accuracy spectrophotometer. At right, the

light source, at upper left, the lm monochromator

The optical bench at the exit slit of the monochromator carries an achromate; the sample platform;

a second achromate followed by the integrating

sphere and photomultiplier assembly.

14
NPL by Preston and Cuckow [12] in conjunction with a

single beam spectrophotometer, employing a five-aperture

screen. One year later, Buchmuller and fConig [13] described and used, among others, a two-aperture unit. At

NBS, Barbrow [15] used a ten-aperture arrangement, while

Harding [17] and Cordle and Habell [26] at the NPL described

a two -aperture system. Multiapertures were used by Hoppman

[22], Bischoff [24], Sanders [27] and Nonaka and Kashima [28].

Finally, Clarke [31] discussed in detail the use of a twoaperture system to check the accuracy of photometric data

obtained on his spectrophotometer at NPL. It is this system

which was reproduced and used at NBS.

The two-aperture unit consists of a metal plate (130

mm by 100 mm) containing 2 rectangular windows, A and B,

(20 mm by 8 mm) located one above the other. Each aperture

can be closed by a light-tight shutter which is operated

pneumatically by remote control. The aperture plate is placed

in the optical path after the exit slit of the monochromator

and within the optical solid angle of the instrument. The

image of the apertures is then projected on the target of

the integrating sphere. A glass achromate with a focal

distance of 190 mm and a diameter of 60 mm is used for this

purpose. The arrangement is illustrated in figure 4. No

optical element should be placed between the aperture plate

and the monochromator. The linearity check consists of

measuring the photocurrent produced when aperture A is open,

then closed and then aperture B is open and then closed. The

value of A + B is then compared with the values obtained with

both apertures (A + B) open. If the system is linear (or

accurate) these two values should be identical:

(A) + (B) e (A + B)

If this is not the case, the system shows nonlinearity

which is proportional to the amount by which the sum of

15
Figure 4. NBS high accuracy spectrophotometer. Same arrange-,

ment as in figure 3 except for the aperture system

which is now placed after the exit slit of the

monochromator . This arrangement is used when

making linearity measurements.

(A) + (B) differs from (A + B) . This difference is then

used to correct the transmittance values measured on the

solid or liquid filters.

e . Integrating Sphere and Photomultiplier Arrangement . Radiations are passed through the aperture or the

filter and are then received on the target of the integrating

sphere. This sphere is illustrated in figures 3 and 4. A

rectangular block of aluminum made from identical halves was

cut to produce a half sphere in each half block. Then the

halves were joined together to form the hollow sphere. Its

16
diameter is 125 mm. A target made from a circular plate 35 mm

in diameter, is located at the center of the sphere. The

front surface of the sphere has a 20 mm diameter entrance

opening which can be closed by a shutter that is operated

remotely by a pneumatic system. A 50 mm diameter opening is

at the opposite end to which the housing of the photomultiplier is attached by an "0" ring to provide a lighttight joint. The inside of the sphere is coated with a

suspension of BaSOj, ; the outside is painted black (1 percent

reflection)

The photomultiplier is a 50 mm flat-faced, end quartz

window tube with a 44 mm cathode and 11 Venetian blind

dynodes having CsSb secondary emitting surfaces . The

cathode is identified as S-20 or tri-alkali type. The

spectral range of this tube is from below 200.0 nm to

850.0 nm. The operating voltage is 850 V. The photomultiplier output is supplied to an operational amplifier and a series of high precision resistors rated at 10 , 3 x 10 , 7 7 Pi 10 , 3 x 10 and 10 ft. A dark current compensation is

also available. The output from the amplifier-resistors

unit is supplied to a 10 V digital voltmeter. This

electronic system was designed and assembled at NBS by

R. J. Carpenter and K. W. Yee.

The optical components located after the exit slit of

the monochromator , including the photomultiplier tube, are

enclosed in a light-tight box 200 cm long, 70 cm wide and

76 cm deep. The front panel, which is removable, is

provided with a sliding door to permit rapid access to the

filter-holder system. The box contains outlets for the

compressed air which operates the apertures, sample carriage

and integrating sphere shutter, and the electrical

connection from the photomultiplier. The inside walls are

lined with thermal insulation painted black. When in use,

all non-black metal parts are covered with a black cloth to

reduce stray light

17
D . Experimental Results

The stability of the electronic system was determined

with a tritium-activated phosphor. Because tritium has a

long half-life (12.5 years), the light output is considered

stable. This source was placed in front of the integrating

sphere and four series of 20 measurements each were made.

The average values obtained over a 10 min period were:

1.1340

2

V; 1.1342., V; 1.1342

Q

V; 1.1342

p

V. From these

values it can be concluded that the stability of the

photomultiplier tube, with its electronics, is better than

1 part in 10,000. When the ribbon filament lamp was used

after a 2 hour warm-up, the measurements indicated a

stability of several parts in 10,000.

The linearity of photometric data is a function of the

photomultiplier, the voltage supplied, the load resistor

and the amplifier. The wavelength at which the measurements are made is not critical.

A series of linearity measurements were made using 850 V

6

at the photomultiplier and the 3 x 10 Q resistor. The

results, expressed as a percent correction of the transmittance percent, indicate that a correction of -0.15 should

be made at 50 percent transmittance , -0.22 at 25 percent

transmittance , and -0.26 at 12.5 and 6.25 percent transmittance values. Interpolation between these values provides

the correction at other transmittance values. In our work,

linearity measurements were made at a wavelength of 560.0 nm.

A series of transmittance measurements were made on three

Schott NG-4 filters having nominal transmittances of 10,

20 and 30 percent. The optical arrangement used for the

measurements is described in figure 3. The image of the

e-xit slit (about 10 mm by 1.5 mm) was projected at the

surface of the filter and in its center. After passing

through the filter, the radiations are projected on the

target of the integrating sphere. The other parameters,

18
such as photomultiplier voltage , resistor value, amplifier

gain, and light source output, were the same as those used

in the linearity measurements.

The transmittance measurements are made by determinini

the average voltage reading by the digital voltmeter

which corresponds to the unattenuated radiation intensity,

I , and corresponding to the attenuated radiation after

°

I passing through the filter, I. The ratio =— is the uncorrected transmittance of the filter at the selected wavelength. To minimize the inevitable variations during the

measurements, a successive series of Io and I data are

collected, averaged and ratioed. Such a series of data

is illustrated in table 2. These transmittance measureTable 2. Transmittance measurements on a Schott

NG-4 glass filter with the NBS high

accuracy spectrophotometer.

Temperature = 23.50 °C

Wavelength

nm

I o

I I I I

440.0 2.0030

2.0015

0.6620

.6625

2.0010

2.0028

.6620

.6630

2.0010

2.0045

465.0 2.0040

2.0050

.7156

• 7150

2.0022

2.0015

.7150

.7160

2.0015

2.0045

590.0 2.0028

2.0048

.6274

.6275

2.0047

2.0028

.6282

.6270

2.0067

2.0050

635.0 2.0060

2.0055

.6570

.6568

2.0070

2.0068

.6573

.6570

2.0080

2.0080

Average

I

Average

I

%-T % T

uncorrected corrected

440.0

o 2.0023 0.6624 33. 08 32.99

465.0 2.0031 .7154 35. 71 35.62

590.0 2.0045 .6275 31. 30 31.21

635.0 2.0069 .6570 32. 74 32.65

19
ments were repeated over a period of several weeks, using the

same three filters. The summary of the results Is given in

table 3- Another series of transmittance measurements was

made using another NG-4 filter whose nominal transmittance

is 30 percent. The results were compared with similar

measurements made on a conventional spectrophotometer

(see table 4). An average bias of + . 2 3

1- relative percent

was found.

Table 3. Repeatability of % T measurements performed with

the NBS high accuracy spectrophotometer on three

colored glass filters.

Wavelength Filter 10 Filter 20 Filter 30

nm % T % T % T

440.0 11.62 11.62 11.62 19.83 19.82 19-91 32.98 32.97 32.99

465.0 13-57 13.58 11.60 22.63 22.59 22.69 35-66 35.67 35.62

590.0 10.38 10.38 10.39 19.16 19.16 19.27 31.19 31.18 31.21

635.0 11.37 11.31 11.40 20.58 20.60 20.71 32.61 32.61 32.65

Averages Averages

19.85 32.98

22.63 35.65

19.20 31.19

20.63 32.62

Table 4. Comparison of % T measurements performed on a

Schott NG-4 filter using the NBS high accuracy

instrument and a conventional spectrophotometer

provided with a quartz prism double monochromator

in a double beam arrangement.

Averages

440.0 11.62

465.0 13.58

590.0 10.38

635.0 11.36

Average

Wave- NBS Conventional Relative Relative

length

nm

Instrume nt Instrume nt Diff

.

% Diff. % Diff.

440.0 32.82 32.75 +0 .07 + 0.20

465.0 35.51 35.40 + .11 + .30 +0.23

5 590.0 30.98 30.93 + .05 + .16

635.0 32.38 32.32 +• .06 + .18

20
The data on the high accuracy spectrophotometer at NPL,

as well as at NBS, are made by visual averaging and manual

transfer. Such a method has bias which depends, among others,

on the experience of the operator. This type of bias can

be eliminated if the data acquisition is made through a computer interfaced with the digital voltmeter. In this

manner many readings can be taken over a predetermined time

interval and then averaged. A statistical evaluation of

each set of results can also be obtained simultaneously. The

design of such a computer data system was completed and the

hardware procured. The use of this non-bias procedure to

acquire and process the information given by the NBS high

accuracy spectrophotometer was started during the early

part of 1971.

The optical components used in the present instrument

are made of glass. This initial choice limits the use of

the instrument to the visible part of the spectrum (380.0 nm

to 800.0 nm) . This spectral range will be extended to the

ultraviolet region by providing a quartz optical system.

E . Studies of Liquid Filters as Potential Absorbance

Standards - R. W. Burke and E. R. Deardorff

A program involving the investigation of various liquid

filters as potential absorbance standards was described in

NBS Technical Note 5^4 [1]. Three types of filters were

identified: (1) individual solutions of inorganic salts,

(2) composite mixtures and (3) solutions of inorganic dyes.

During the past year, additional studies were undertaken on

each type of filter.

1 . Solutions of Inorganic Salts

The need for ultraviolet absorbance standards, especially by clinical chemists, resulted in a critical investigation of the acidic potassium dichromate system. The

results of this study are described in a subsequent section

of this report

21
Additional studies on cobalt (II) solutions In sulfate

and perchlorate media verified our earlier finding that

the apparent molar absorptivity obtained in . 1M HpSCv

is higher than that observed for . 1M HClOj, solutions

—1 —1

( £ ir-jo = 4.88 and 4.82 liter mole cm , respectively).

Similar behavior was observed for nickel(II) solutions

(table 5) . This enhancement is probably produced by the

Table 5. Apparent molar absorptivity of

nickel(II) solutions at 25 °C

-1 -1

Solution pH A v , nm e, liter mole cm

mdA . ^

NiSO^-HpSO^ 1 394 5.17

Ni(C10

4

) 2

-HC10

4

1 394 5.H

formation of aquo-metal-sulfate complexes . The cobalt and

nickel solutions were prepared from metal shot (spectrographic

purity > 99-9 percent).

Absorbance studies on potassium nitrate solutions showed

that this system exhibits negative deviations from Beer's

Law with increasing potassium nitrate concentrations. At

302 nm, the apparent molar absorptivity of a solution

containing approximately 2.8g KNO^/liter was 7.20 liter

-1 -1 mole cm while, at a concentration of l4.0g KN0„/liter

a value of 7.11 liter mole" cm was obtained. Edisbury [35]

has given 7.04-7.20 for the range of literature values. The

non-conformity of this system to Beer's Law undoubtedly

accounts for part of this disparity.

2 . Composite Mixtures

The preparation and spectral properties of two empirical

solutions (NBS composite and Thomson [36] solution) was

discussed previously [1]. Each of these solutions exhibits

a high degree of optical neutrality throughout the visible

and near ultraviolet regions. Because of their neutrality,

22
they are potentially useful for calibrating a wide variety

of automated, flow-type spectrophotometers. During the

past year, ampoules of these solutions were prepared and

distributed to five clinical laboratories for evaluation

(figure 5). Absorbance measurements were requested over a

Figure 5. Ampouled filters now undergoing

evaluation in cooperating clinical

laboratories

period of four months. The instructions and data sheets

provided with the filters are reproduced in table 6 . Although the final results have not been received, preliminary

data indicate absorbance variations as large as 1 percent.

The spectral instability of chromium(III) ion which is

present in these two composite solutions is primarily responsible for these variations. Studies are continuing on

ways for improving the stability of chromium( III ) solutions.

The principal variables being examined are the effects of

refluxing, acidity and choice of complexing anions (see

previous report [1] for some typical results).

3 . Organic Dyes

On the basis of the previous study [1], three watersoluble organic dyes - Cibalan Black, Neolan Elack and

Alizarin Light Grey - were selected for purification and fur23
ther evaluation. Members of the Organic Chemistry Section have

provided, expertise in attempting to purify these materials.

Relatively pure samples of each have been prepared on a milligram scale, but attempts to prepare gram quantities have not

been successful. A procedure recently developed for the purification of Alizarin Light Grey, however, shows promise of

yielding larger amounts of the dye. The crude material is

dissolved in dimethylformamide , filtered twice through Whatman

No. 1 paper and precipitated with diethyl ether. A red impurity, estimated to be 15-20 percent of the original material,

has been isolated by a Soxhlet extraction of the crude material with methanol. Work on this procedure is continuing

so that a well-characterized material may be prepared.

Table 6. Format of the interlaboratory evaluation of

two liquid filters, designated T and NBS , as potential absorbance standards.

Instructions for Use:

All absorbances are to be measured against an air

reference at 25 + 1 °C in a covered, rectangular, 1.000-

cm quartz cuvette. When a cuvette has been selected,

mark it so that a given orientation can be maintained,

and use this cuvette for all subsequent measurements.

Include any additional information regarding exact

Path length of the cuvette under "comments" on the data

sheets

.

Wavelength Calibration : Use the holmium oxide filter enclosed to check the

accuracy of your wavelength scale. If your maxima are

different from the accepted values, change your dial

settings accordingly (interpolate where necessary) so

that the wavelengths requested on the data sheets are

correct to + 1 nm.

24
Slit Width : For each wavelength, use the minimum slit width

commensurable with the sensitivity and stability of your

spectrophotometer. Also, please enclose with your first

set of results a copy of the manufacturer's dispersion

data.

Absorbance Measurements

A minimum of two independent atsorbance measurements are requested at each wavelength on successive

aliquots of both the distilled water and the samples

.

Record these duplicate values on the data sheets.

Check zero (air vs air) before each measurement.

DATA SHEET A

Date Received:

Date Measured:

Absorbances Obtained on Receipt (Within One Week)

Spectrophotometer Make and Model, including

attachments

Wavelength Calibration:

X (accepted), nm 536 453 4l8 361 334 279

max

.

X ( found) , nm

max. ' Thomson Solution (Labeled T)

Absorbance

X , nm Slit Width, nm Water vs . Air T-l vs . Air

590

560

488

410

531

318

aAmpoules are prescored immediately beneath gold

bands to facilitate opening. Disposable pipettes

are recommended for rinsing and transferring

samples into cuvettes

.

25
DATA SHEET A

Date Measured:

Absorbance Values

NBS Solution : Set 1 1- Green Tip Ampoules

Absorbance

.. Slit

X } Width Dist. Water Samples vs. Air

nm nm vs. Air NBS-1 ' NBS- 2 ¥BS-3 NBS-4

540

475

415

365

320

263

NBS Solution : Set 1 - Green Tip Ampoules

26
DATA SHEET A

Date Measured

Potassium Dichromate Solution, i» 40 and 80 mg/liter

pH ^ in P erchloric Acid:

Slit

mg

b Width Dist . Water mg

vs. Air vs . Air PH vs

.

Air pH

350

s 3 > 313

s 5 > 257

i i »

3^-42 and 78-82 mg K

2

Cr

2 0„ weighed to nearest

0.01 mg. Dissolve in distilled water, add

1.00 ml concentrated perchloric acid and dilute

to mark at 25 °C in Class A volumetric flask.

Prepare fresh before use.

Comments (Difficulties, Recommendations, Usefulness , etc . ) :

F - The Use of Chromium(VI) Solutions as Spectrophoto- metry Absorbance Standards - R. W. Burke. E. R Deardorff

and D. S. Bright

1 . Introduction

Both alkaline and acidic chromium(VI) solutions have

been used for many years as spectrophotometric absorbance

standards. Each has its advantages and limitations.

Although Haupt [37] has shown that solutions prepared

by dissolving potassium chromate in 0.05N KOH are stable

for several years when properly stored, many analysts,

especially those involved in various collaborative tests

[38], [39], [40], have preferred, instead, to use weakly

acidic solutions of potassium dichromate.

The spectral properties of the alkaline and acidic

chromium(VI) systems are quite different, as shown in

figure 6, with the latter giving the more desirable

27
LU2

<

CD

oro

(/) CD<

1.0

0.8

0.6

0.4

0.2

I

I. K2Cr2 7 /0.05N KOH

n.K2 Cr2 7 /IO"3 N HCL04

220 250 300 350

WAVELENGTH, nm 400

Figure 6. Comparison of absorption spectra of 1.0 x 10" M

K

2

Cr 2°7 ln °- 001N HC10 2| and 0.05N KOH.

arrangement of maxima and minima. Furthermore, since

the total chromium concentrations of the two solutions are

equal, it is also apparent that the alkaline form is

two to three times more sensitive than the acidic one.

Thus, small errors in weighing the milligram quantities of

material are more critical in the former than in the latter.

A further disadvantage of the alkaline form is that the

0.01-0.05N KOH solutions normally recommended are corrosive

and difficult to handle. Thus, from the standpoints of

spectral properties, sensitivity, storage and handling

characteristics, acidic chromium (VI) solutions appear superior to alkaline solutions. They possess one serious disadvantage. Chromium(VI) solutions in acidic media do not

obey Beer's law. The remainder of this section will

be concerned with the causes of these deviations. Specific

points to be covered include: (1) the redetermination

of the equilibrium constant for the reaction 2HCr0j. =

28
Cr

2 0„ + H" 2 0, (2) the use' of this constant to calculate

the absorption spectra of the HCrO^" and Cr

p

O

~ ions,

(3) combining of the equilibrium and absorption data

to predict the absorbance of any weakly acidic chromium(VI)

solution, and (4) recommendations for using acidic

potassium dichromate solutions to check spectrophotometry linearity in the ultraviolet.

2 . Chromium( VI) Equilibria

When a chromium(VI) salt is dissolved in water the

following equilibria must be considered.

K

l + H

2

CrO ij = H + HCrO^ (1)

- K

2 + HCrO^ = H + CrO^ (2)

2HCr0 1| =J Cr

2 ? + H2 (3)

Reactions (1) and (2) are clearly pH dependent, whereas

reaction (3) depends only on the total chromium concentration. The equilibrium constants K-, , K„ and K~ have

been measured by various investigators and the following

values have been reported at 25 °C:

K = 10 mole liter" 1 [4l]; K

2 = 35-5 mole liter" 1 [42]

and 33.0 mole liter-1

[43]; K^ = 3-2 x 10" 7 liter

mole" 1 [44] and 3.0 x 10" 7 liter mole" 1

[45].

Based on these values of the equilibrium constants,

several important conclusions can be drawn: (1) at a pH = 10,

at least 99-9 percent of the total chromium is in the CrOj.

form, while (2) in dilute acid (pH ^ 3), HCrCK" is the predominant chromium(VI) species since the contribution by

H

2 Cr0h does not exceed 0.1 percent; moreover, the fraction

of HCrOj," that undergoes dimerization to give Cr

2 7

~

29
(reaction 3) will depend on the total chromium concentration

(at 1.36 x 10" and 6. 80 x 10" M chromium, for example,

approximately 0.9 and 4.2 percent, respectively, of the

total chromium is calculated to be in the Cr

p

0~ form)

.

3 . Redetermination of K

^

, the Dimerization Constant for

the Reaction: 2HCrO ,

| = Cr2 + H

2

Tong and King's value [42] for K~ was determined by

extrapolation of spectrophotometric equilibrium measurements to zero ionic strength, whereas Davies and Prue [43]

employed a high precision, differential technique in which

appropriate concentrations of alkaline KpCrOj. were used as

standards to correct for the inaccuracy of their spectrophotometer. Both groups made absorbance measurements only in

the wavelength region of 366-404 nm and neither reported the

imprecision of their results. Because of the direct relationships between K~ and our need to establish accurately the

individual absorption spectra of the HCrOj." and Cr^O ~ ions,

we considered it essential to redetermine this constant.

If HCrOh" and CrpO ~ are the only absorbing Cr(vT) species

present with molar absorptivities £-, and £p, respectively,

the apparent molar absorptivity, e , will be given by

eml= (l-a)e, + l/2.ae (4)

2

where a is the fraction of total chromium in the dichromate

form. Rewriting equation (3),

2HCr0 2| =3 Cr

2 + HpO (3)

it is seen that the thermodynamic equilibrium constant is

given by

[Cr 9 7 ] y 9

K, = ^T-p • -% (5)

5 [HCr0

4 r Yi

30
or in terms of a

K^ = %r— . X, (6)

5 2(l-a)^Cr

T Yl

where Y-. and Yp are the activity coefficients of the HCrCK"

and CrpO ~ ions, respectively, and Cr^ is the total

chromium (VI) concentration. Because the ionic strength

never exceeded 0.01, the activity coefficients are given by

the Debye-Huckel expression of the type [43]

Az

2

I

1/2

...

- lo e *i ^72~ (7)

where z is the ionic charge, I is the ionic strength and A

has a value of 0.509 1/mole at 25 °C. Equations (4) and

(6) can now be rewritten to give, respectively,

e m

= a(l/2.e

2 - e ±

) + e ± (8)

?

. Tl/2

and log K- = log 2-_ A±

n /9 (9)

J 2(l-a)^ Crm l+I 17 ^

Assuming a value of K_, one can calculate from equation (9)

the a values at various total chromium concentrations. If

the choice of K_ is correct, the measured values of e should

lie on a straight line of slope (l/2.e ? - e, ) and intercept

C-. , as seen from equation (8). The best value of K~ is then

determined by the method of least squares

a. Experimental . Absorbance measurements were

performed on a commercial double-beam spectrophotometer

containing a quartz double monochromator . The absorbances

of solutions in regions of maxima or minima could be

reproduced on this instrument with a precision of +0.1

percent at the 95 percent confidence level. Linearity was

31
checked at numerous wavelengths in the range of 375-600 nm

by using various combinations of Chance ON-32A or Corning

5900 glass filters. These filters were tilted vertically

approximately 10° to the incoming beam to minimize reflection effects.

All absorbance measurements were made in a 1.023 +

0.003 mm cuvette, with air as a reference. This cuvette

was rigidly supported in the standard 10 mm square holder

supplied with the instrument by an anodized black aluminum

adapter fabricated by the NBS Instrument Shop. In practice,

only this adapter was touched when removing the cuvette from

the Instrument for emptying or refilling. Solvent and sample

were introduced through the 1 mm circular neck by means of

disposable, Pasteur-type pipettes. The casting surrounding

the sample and reference areas and the 10 mm cuvette holder

were thermostatted at 25.0 +0.1 °C by a circulating water

bath.

Chromium(VI) solutions were prepared from SRM-136c,«

Potassium Dichromate , which was dried at 110 °C for two

hours. The distilled water was free of reducing impurities

as indicated by titrating with a dilute potassium permanganate solution. Perchloric acid was used to maintain

the pH at 3.0 +0.1.

The optimum wavelength regions for determining K~ were

selected as follows: Two solutions were prepared, one containing 391 mg K

2

Cr

2 0„/l and the other, 40.0 mg KgCrpO^/l.

The more concentrated solution was placed in the 1.023 mm

cuvette while the second was added to a 10.00 mm cuvette. A

differential scan was then made on a recording spectrophotometer equipped with a 0-0.1 absorbance (full scale)

slidewire, using the 391 mg K

?

Cr

p 7 /l solution as the sample.

The resulting spectrum is shown in figure 7. Relatively

large differences in absorbance are seen in the wavelength regions near 385 and 275 nm even though the total

32
220 250 300

WAVELENGTH, nm

350 400

Figure 7- Difference spectrum of 391 nig KpCr^O^/liter

in 1.023 mm cuvette and 40.0 mg K^Cr^O^/liter in

10.00 mm cuvette; pH = 3.

chromium in each light path is the same. Thus, it is in

these two regions that maximum deviations from Beer's Law

should be observed. Three wavelengths were subsequently

selected in each region for the determination of K~ - 390,

385 and 380 nm and 280, 275 and 270 nm, respectively. Since

we did not choose to measure absorbances below 0.05 or

above 1.5, this choice of wavelengths, combined with our

decision to use only the 1 mm cuvette, restricted the

concentration range of the potassium dichromate solutions

from about 100 mg KpCrp0 ? /l to approximately 1000 mg

K

2

Cr

2

/l.

The absorbances of the four chromium(VI) concentrations

selected for the determination of K~ are given in table 7.

33
0.0410 0.0521 0.0602 0.1099 0.1196 0.1309

.1989 .2447 .2779 .4897 .5332 .5765

.3924 .4740 .5325 .9199 .9995 1.0747

.6093 .7274 .8115 1.3831 1.5037 1.6095

Table 7 . Absorbance of KpCr

2 7

solutions as measured in

1.023 mm cuvette at 25 °C; pH = 2.92 + 0.02 (HCIO^)

KpCr 2 ?

Cone

Absorbance

mg/1 390 nm 385 nm 38 nm 28 nm 275 nm 270 nm

100.06

400.48

699.86

1000.27

The pH of these solutions was 2.92 + 0.02. The corresponding

apparent molar absorptivities, e , are given in table 8.

Table 8. Apparent molar absorptivities of Ko Cr o solutions

as a function of chromium(VI) concentration

at 25 °C; pH = 2.92 + 0.02 (HCIO^).

K Cr Cone. APParent Molar Absorptivity, liter mole"1 cm" 1

mg/1 390 nm 385 nm 380 nm 280 nm 275 nm 270 nm

100.06 589.2 748.7 865.I 1579.3 1718.6 1881.0

400.48 714.1 878.6 997.8 1758.2 1914.4 2069.8

699.86 806.2 973.8 1094.0 I890.O 2053.5 2208.0

1000.27 875.9 1045.6 1166.5 1988.2 2161.5 2313.6

Using the method of least squares, these e values were

plotted against the a fractions calculated from equation (9)

for various assumed values of K_. The molar absorptivities

were weighted according to the reproducibility of the

absorbance data in table 7. The latter varied from +0.0002

at 0.05 absorbance to +0.0005 near 1.5 absorbance. A computer

was used to perform all computations.

A typical graph of the sum of the squares of the

deviations of the experimental e values from the best

straight line fit is shown in figure 8 for K~ values

of 28 through 36. The minimum in this curve is the best

34
2.0

Figure 8. Least squares determination of K~

estimate of K~ and, for this wavelength,, is 32.6. The

K_ values obtained at the six wavelengths are summarized

in table 9.

Table 9, L at 25 °C as determined at various wavelengths

Wavelength, nm

390

D 32.4

385 33.3

380 33.8

280

275

270

33.5

32.6

31.6

Avg. 32.9

S.D. 0.82

Rel. S.D. 2.5$

35
The consistency of the K„ values in the two widely

separated wavelength regions strongly supports the

original assumption that, in perchloric acid media near

pH 3, HCrCK and CrpO„ are the only absorbing chromium(VI)

species measurable.

4

.

Absorption Spectra of HCrO ^, " and Cr p07

~

The absorbances of three chromium(VI) solutions containing nominally 20, 40 and 60 mg K Cr^O /l (pH = 3.0 +0.1

in HCIOk) were measured in a 10.00 mm cuvette at 5 nm increments over the wavelength range of 220-^00 nm. A close

approximation of e(HCrOh~) was then obtained by extrapolating

the apparent molar absorptivity values calculated for these

three concentrations to zero chromium concentration. This

extrapolation is simplified since the variation of a over

this concentration range (20-60 mg KpCr„0 /l) is nearly

linear. Similar absorbance measurements were performed on

three solutions containing 1000 mg KpCrpO„/l a using the

1.023 mm cuvette, and the corresponding a values were

calculated from equation (9), with K~ = 32.9. The first

approximation of the e(CrpO~) values were obtained by

substituting the extrapolated e(HCrOn~) and the above a values into equation (8). These etHCrO^

-

) and e(CrpO ~)

values were then refined by successive approximations.

Only one recycling was necessary. The resulting individual

absorption spectra are shown in figure 9. The molar

absorptivity values for CTpO" have been divided by 2

to make the spectral data directly applicable to predicting

the direction and magnitude of the deviations from Beer's

Law.

5

.

Agreement Between Experimental and Calculated

Values of Apparent Molar Absorptivity

As previously shown in figure 6, the absorption spectrum

of a weakly acidic solution of KpCTpCu exhibits two broad

maxima at 350 and 257 nm and two broad minima at 313 and

235 nm. These maxima and minima do not shift significantly

36
250 300 350

WAVELENGTH, nm 400

Figure 9. Absorption spectra of HCrO,. and Cr^O^ from

220-400. ^ l i

over the concentration range of 20-100 mg LCr

y

/l. This

is considered a practical range since it yields absorbance

values of about 0.1-1.5 for a 10 mm cuvette.

The molar absorptivities of HCrO^" and Cr

p " at 350,

313, 257 and 235 nm were determined by the procedure

described above. These values are given in table 10. Although

Table 10. Provisional molar absorptivities of HCrO,,

and Cr

2

= at 25 °C.

gjl mole cm" Wavelength and Spectral Bandpass,

HCrO

4

1/2 Cr

2 ?

:

nm

350 (1.1) 313 (0.7) 257 (1.0) 235 (0.7)

1576

1788

711

823

2100

3156

1804

2688

37
no direct comparison has been made between our instrument and

the NBS high accuracy spectrophotometer, the agreement

between measurements on glass filters in the visible region

suggests that the £(HCrOjj~) values are accurate to + 0.5

percent. However, because of the uncertainty in K~ and

therefore a, the inaccuracy of the e(CrpO~) results is

approximately + 5 percent. As will be seen subsequently,

this will not seriously limit the usefulness of dilute

acidic potassium dichromate solutions as potential absorbance

standards

Table 11 shows the agreement between the calculated and

Table 11. Experimental and calculated values (E, C) of

Lvity,J ' e , at 25 °C

m

5

Wavelength and Spectral

apparent molar absorptivity, e , at 25 °C.

K

2

Cr 2°7 Conc - mg/1

20.22

40.09

60.12

80.17

99.92

e Band]? ass, nm

. Ill

, , -1 -1

1 mole cm 350(1.1) 313(0.7) 257(1.0) 235(0.7)

E

C

Diff.

1577.6

1577-9

-0.3

713.5

7H.9

+1.6

2113.6

2108.7

+ 4.9

1814.7

1812.1

+2.6

E

C

Diff.

1579.9

1579.8

+0.1

713.3

713.0

+0.3

2120.8

2118.8

+ 2.0

1820.2

1819.7

+0.5

E

C

Diff.

1581.4

1581.5

-0.1

713.8

713.8

0.0

2127.2

2126.7

+0.5

1827.1

1827.2

-0.1

E

C

Diff.

1584.0

1583.3

+0.7

715.0

714.8

+0.2

2137.6

2135.3

+ 2.3

1835.8

1834.4

+ 1.4

E

C

Diff.

1585.5

1584.0

+1.5

715.6

715.0

+0 .6

2144.0

2143.7

+ 0.3

1841.5

1841.4

+ 0.1

38
experimental apparent molar absorptivity values for five

concentrations of potassium dichromate. The experimental

values are the averages obtained on five individual samples

run at each concentration. The precision of any given set of

measurements was approximately 0.1 percent at the 95 percent

confidence level for each wavelength. Approximately IN

perchloric acid was used to maintain pH at 3-0 + 0.1. The

increasing values of the apparent molar absorptivity with

increasing chromium( VI) concentration at these four wavelengths

is readily apparent. Moreover, the difference between the

calculated and measured values does not exceed +0.3 percent.

Referring to figure 9 again, there are two wavelengths where

weakly acidic dichromate solutions should obey Beer's Law - ^ 319 and 3^5 run - whereas, in the region between these

wavelengths, negative deviations should be observed.

Similar conclusions can be drawn also from the difference

spectrum shown in figure 7.

6 . Using Acidic Dichromate Solutions to Check Linearity

in the Ultraviolet

While the accuracy of the molar absorptivity data in

table 10 must be more carefully assessed before acidic K

? Cr„07

solutions can be used to their fullest capabilities as

absorbance standards, it is felt that equilibrium and molar

absorptivity data are now sufficiently established that these

solutions can be employed to check linearity to + 0.2 percent

at the wavelengths of 350, 313, 257 and 235 run. This claim is

supported by the fact that, even in the most concentrated

KpCrpC* solution studied (100 mg/1), only approximately

4.2 percent of the chromium(VI) is in the Cr p ~ form. At

lower concentrations this percentage becomes even smaller.

Moreover, even though the errors in a and e(Cr o

) are

approximately equal and may be as large as 5 percent, it is

their product, a.s(Cr„0 7 ), that is required. Since these

errors have opposite signs, this product quite likely is

known to better than 5 percent

39
One approach to employing weakly acidic K„CrpO„ solutions to assess spectrophotometry linearity and, eventually,

accuracy, is by the concept of apparent weights. These

would be determined as follows

:

Apparent Wt . = Actual Wt . X Apparent Wt . Factor

[1-a] .eCHCrO^

-

) + a . l/2.e(Cr

2

=

) Apparent Wt . Factor = (10)

eCHCrO^ )

A plot of absorbance against apparent weight should yield a

straight line (within +0.2 percent) whose slope would be

used to calculate e(HCr0 1 .~). The agreement between this

e(HCrO|.

-

) value and a certified value would, in turn, be a measure of spectrophotometric accuracy. Table 12 gives the

apparent weight factors calculated for KpCr„07 concentrations

of 20-100 mg/1. These have been tabulated only for 20 mg/1

increments. However, since a is seen to vary almost linearly

for this range of concentrations, the appropriate factors

for any KpCr^O^ concentration not given, can be obtained

with sufficient accuracy by linear interpolation.

Table 12. a, 1-a, and apparent weight factors at 25 °C

for selected concentrations of K

2

Cr

2 ?

.

K

2

Cr

2 7

Cone.

mg/1 a 1-a

0.9909

350 nm

1.0012

313 nm

1.0014

257 nm

1.0046

235 nm

20.0 0.0091 1.0045

40.0 .0178 .9822 1.0024 1.0028 1.0090 1.0087

60.0 .0262 .9738 1.0035 1.0041 1.0132 1.0128

80.0 .0344 .9656 1.0046 1.0054 1.0173 1.0169

100.0 .0423 .9577 1.0057 1.0067 1.0213 1.0207

40
7 . Optimum pH and Choice of Acid for Preparing

Acidic Chromium(VI) Solutions

A 0.01 N HpSCv has frequently been employed in the preparation of acidic chromium(VI) solutions. This medium

possesses two disadvantages when compared to a perchloric

acid solution of approximately pH 3. These are: (1) larger

ionic strength and pH effects and (2) possible formation of

mixed chromate-sulfate complexes.

The higher ionic strength of the pH 2 solutions yields

a larger value for K~ and thus increases the deviations from

Beer's Law. In addition, chromium( VI) solutions at pH 2

contain not only HCrO^" and Cr

2 " ions, but also a small

amount of a third species, HpCrCv. As several Investigators

have shown [41], [46], this is the least absorbing ion of the

three and its presence decreases apparent molar absorptivities

Thus, opposing the increase in K„ and thereby higher absorbance, is the enhanced formation of H

p

CrCv which leads to

lower absorbance values. The net effect of these two factors

depends on the spectral properties of HpCrCv at the wavelengths of interest. At 350 nm, for example, the apparent

molar absorptivities decreased 0.2 percent by decreasing the

pH from 3 to 2 with perchloric acid (50 mg KpCrpCu/l) . At

the three other wavelengths - 313, 257 and 235 nm - these

combined effects did not exceed 0.1 percent.

Regarding the choice of acid, both Tong and King [42]

and Davies and Prue [43] observed small differences in the

absorbances of dilute perchloric and sulfuric acid solutions

of K

2

Cr

2 0„, after correcting for differences in ionic strength

They attributed these differences to the formation of mixed

chromate-sulfate complexes. More recently, Haight et al [46]

have shown that the conversion of HCrCv to CrSCi is

practically complete in 1M HSCv solutions and that the

41
absorption spectrum of this complex is significantly different

from the HCrOj," spectrum. At 350, 313, 257 and 235 nm our

absorbance measurements at pH 2 in either HCICU or E^SOu

agreed within 0.1 percent, however.

In conclusion, a pH near 3 is preferred over pH 2

,

since it yields a simpler mixture of chromium(VI) species. A

pH much higher than 3 is not recommended, however, since

it is difficult to reproduce and could lead to the formation

of small amounts of CrO, The choice of either HpSOj. or

HC10|, to obtain a pH of 3 appears less critical at the

0.1 percent level. Nevertheless, we prefer perchloric acid,

4

since there is no evidence that CIO,," forms mixed complexes

with HCrO^ . 8 . Effect of Temperature on the Absorbance of

K

2

Cr 2°7 at pH 3

The absorbances of a solution containing approximately

60 mg KpCTpOy/1 at pH 3 (HOICK) were measured as a function

of temperature over the range, .17-37 °C . Figure 10 shows

UJO

z

4

X

o

w

<Jz

<

CD

dco

</) co4

0.4

1 ~T 1

0.2 —

0.2

^ — "--^^313001 _

N.

->. 0.4 —

>. V ^>v350a257nm

0.6

1

1 235 nm

O.tt 1 1 20 25 30

TEMPERATURE, °C 35 40

Figure 10. Temperature dependence of a solution of K

?

Cr

? 7

at pH 3.

42
the results obtained for wavelengths of 350, 313, 257 and

235 nm. All absorbance changes are uncorrected for thermal

expansion of the solvent. The absorption minimum at 313 nm

exhibits the smallest temperature coefficient (approximately

-0.02$/ °C) while the maxima at 350 and 257 nm and the

minimum at 235 nm have temperature coefficients of about

-0 .05%/ °C over the range of 20-30 °C.

9 . K

2

Cr

2 ?

in 0.05M Na

g

HPO,, and 0.05M KOH

To overcome some of the difficulties in handling

highly alkaline potassium chromate solutions, Johnson [47]

has recommended dissolving KpCrpCu in 0.05M NapHPOj,

(pH 'v 9-1). This system has been examined briefly and

tentative molar absorptivity values are reported for it

and the corresponding 0.05M KOH system in table 13. Over the

relatively narrow concentration ranges studied, both systems

Table 13. Tentative molar absorptivity values of K

?

Cr

?

O

y

in 0.05M Na

2

HP0 1| and 0,05M KOH at 25 °C. '

0.05M Na

2

HP0 ;

| K

?

Cr

?

Cone. Molar Absorptivity, liter mole" cm"

mg/1 pH 373 nm (1.3)

a 274 nm (1.3) a

9.869 9.2 4835 3706

22.837 9-1 4833 3707

30.557 9.1 4838 3706

0.0 5M KOH

11.098 - 4838 3707

19.466 - 4835 3706

29.875 - 4842 3707

Spectral Bandpass, nm.

43
obey Beer's Law to + 0.1 percent. Furthermore, the molar

absorptivity values in the two systems agree within the

limits predicted from the equilibrium data. For example,

the ionic strength of the 0.05M NapHPOj, solution is

0.15 and, from Neuss and Rieman's work [44], one would

estimate the corresponding concentration equilibrium

constant for the reaction HCrO^ = H + CrCK to be approximately 9 x 10" 7 mole liter" 1

at 25 °C. Thus at pH 9.1,

the contribution of HCrCK

- is approximately 1 part in 900.

The molar absorptivity values in 0.05M NapHPOj, should

therefore agree with the 0.05M KOH results to within 0.1

percent

10 . Conclusion

The deviation from Beer's law of weakly acidic solutions

of KpCrp0

7 have been re-examined from the standpoints of

ionic equilibria and the differing absorption spectra of

the pertinent chromium(VI) species. As a result of this study,

the present uncertainties in the requisite equilibrium

and molar absorptivity data in acidic chromium(VI) systems

limit the potential usefulness of KpCr ? 7 as an absorbance

standard by approximately +0.2 percent.

At any given chromium concentration, the reproducibility of the apparent molar absorptivity values was about

+0.1 percent at the 95 percent confidence level. Equally

significant, the precision did not vary with wavelength. The

results obtained at 235 nm, for example, were comparable to

those obtained at 350 nm. Two precautions were taken for

these measurements. The cuvette was never removed from the

spectrophotometer between measurements of the "blank" and

sample (each measured against air) and all solutions were

transferred to and from this cuvette with Pasteur-type

pipettes equipped with rubber bulbs.

44
The various molar absorptivity values given in this

report are tentative since the accuracy of the spectrophotometer in the UV region has not been unequivocally

established. This information will be available in the

near future when comparative measurements will be made on

this instrument and on the high accuracy NBS spectrophotometer (see section lc)

.

G. The Absorptivity of Bilirubin, SRM 916 - R. W. Burke

and E. R. Deardorff

1

.

Introduction

The Office of Standard Reference Materials has announced

the availability of a high-purity bilirubin for use as a standard for clinical determinations and for verifying the

quality of bilirubin working standards . This Standard

Reference Material, identified as SRM 916 3 is provisionally

certified at 99-0 percent purity. The principal impurity is

occluded chloroform.

Solutions of bilirubin are highly colored and absorptivity measurements are frequently employed to establish

relative purity. The procedure used for the analysis of

chloroform solutions of the NBS material is described below.

2

.

Experimental

Because of the volatility of the chloroform solvent,

special attention was given to transferring the bilirubin

solutions from the volumetric flasks to the measuring

cuvette. A closed, flow-type system was employed in which

nitrogen was used to flush the samples from the flasks

through the cuvette. All transfers were made through 1 mm

O.D. Teflon tubing. The cuvette cover was made by milling

a groove (1/8 inch x 1/8 inch) into a Teflon sheet of such

dimensions that it coincided with the rectangular cross

section of the 10 mm sample cuvette. A neoprene gasket

45
provided an airtight seal. Two holes were drilled into the

cover and these were fitted with the 1 mm tubing to form

the inlet and outlet to and from the cuvette. A standard

taper, No. 22, Teflon stopper was similarly modified for

making connections to the flasks.

a. Procedure . The bilirubin samples were weighed

on a microbalance and transferred to weighed 1-liter

volumetric flasks. One hundred ml of analytical reagent

grade chloroform j stabilized with approximately 0.75% v/v

ethyl alcohol, was added to each flask and the samples were

then warmed for 15 minutes in a 60 °C water bath to insure

complete dissolution. These solutions were diluted to approximately 1 liter with. the chloroform solvent and placed in a

temperature-controlled balance room (25-0 +0.2 °C). They

were protected from light at all times either by working in a

darkened room or by covering the flasks with aluminum foil.

After thermal equilibrium, the solutions were weighed and the

corresponding volumes were calculated. The measured density

of the chloroform-ethanol solvent was 1.473g per cm at

25.0 °C. Absorbances were measured at 25.0 +0.2 °C in

a glass-covered, 10.00 + 0.01 mm cuvette at 453 nm with a high precision (1 part in 1000) spectrophotometer.

b. Results . Using the nitrogen transfer procedure,

the following results were obtained for the absorptivity of

SRM 916, Bilirubin, in chloroform (table 14).

3 . Conclusions

A closed-system transfer employing an inert gas such as

nitrogen is recommended when absorptivity measurements are

made on chloroform solutions of bilirubin. Taking into

account possible sources of error, a value of 104.5 +0.5

liter g~ cm

- is recommended for the absorptivity of NBS

bilirubin in chloroform at 453 nm and at 25 °C. It may be

of interest to point out that these solutions were relatively

stable when stored in the dark at room temperature. Absorb46
ance measurements on several solutions covered with aluminum

foil decreased less than 1 percent over a period of six weeks

Table 14. Apparent absorptivity of SRM 916,

Bilirubin, in chloroform at 25 °C

a —1-1 Sample No. Wt .. , mg Absorptivity (453) ran) liter cm g

88 5.354

1568 7.893

758 3.700

341 7.756

1053 6.765

1933 6.707

104.

6

104.

8

104.

3

104.4

104.

8

104.

4

Avg. 105.

2

S.D. 0.13

Rel S.D. 0.12?

Sample contains 0.8 percent of chloroform.

H. Spectrophotometric Study of the Dye Ratio in Sample of

Ballpoint Ink - R. W. Burke

1

Introduction

A brief study was undertaken to determine the applicability of dual-wavelength spectrophotometry to the determination of the dye ratio in a sample of ballpoint ink.

The ultimate purpose of this investigation was to establish

the accuracy of this procedure for identifying a possible

manufacturer of ink used in signing a document.

2

Experimental

The dye content of the sample was believed to be a 60/40

weight mixture of Methyl Violet and Victoria Blue. Small

samples of these two dyes identified as Methyl Violet

DY Base and Victoria Pure Blue BO Base were obtained

from a major ink manufacturer. A sample of bulk ink,

reportedly having the 60/40 dye composition, and several

scribble charts prepared from this ink were also obtained

from the same manufacturer.

47
a. Calibration . Absorption spectra were measured

on ethanolic solutions of the two dyes. A peak at 580 nm was

obtained for Methyl Violet (MV) . Victoria Blue (VB) produced

a maximum at 590 nm and a pronounced shoulder at 620 nm.

Additional measurements on the MV system showed a continual

increase in absorbance, while the VB solution appeared

relatively stable. The addition of 1-2 drops of concentrated

hydrochloric, however, immediately increased (^20 percent) and

stabilized the absorbance of the MV solution. Under these

conditions, the absorption peak shifted from 580 to 583 nm.

Acidification of the VB solution did not change the absorbance

at the 590 peak, but did increase the absorbance in the

shoulder region near 620 nm. The resulting absorption

spectrum now exhibited a double peak instead of the previous

peak-shoulder relationship. Apparently, some acid is

necessary to hasten the conversion of the colorless carbinol

base into the colored dye. Following these studies,

approximately 0.3 ml of 12N HC1 per liter was added to all

ethanolic solutions of the dyes

.

The absorption spectra of the acidified ethanolic

solutions of MV and VB are shown in figure 11. The two

wavelengths selected for the simultaneous determination

of MV and VB were 583 and 640 nm. Absorbance measurements were made on individual solutions of these dyes

over a range of concentrations (the dyes were used

as received to prepare these "standard" solutions).

The following two equations were obtained from this set

of measurements

.

0.100 (VB) + 0.242 (MV) = A (583 nm) (11)

0.080 (VB) + 0.011 (MV) = A (640 nm) (12)

s

The coefficients correspond to the absorbances measured in a

1.00 cm cuvette for dye concentrations of 1.00 mg/1 . Beer's

48
Law was obeyed by both dye solutions over the absorbance

range of 0-1.2.

400 500 600

WAVELENGTH, nm

700

Figure 11. Absorption spectra of acidified ethanolic

solutions of Methyl Violet DY Base (MV) and

Victoria Pure Blue BO Base (VB)

b . Analysis of Composite Dye Solutions . Two

composite dye solutions were prepared by dissolving weighed

amounts of each of the dyes in absolute ethanol, acidifying

with 0.3 ml of 12N HC1 and diluting to 1 liter with ethanol.

Absorbances were measured at 583 and 640 nm in a 1.00 cm

cuvette. Equations (1) and (2) were then solved simultaneously

to obtain the measured composition of the two solutions. The

results are summarized in table 15.

49
Table 15 • Analysis of known mixtures of Methyl Violet DY

Base (MV) and Victoria Fure Blue BO Base (VB)

Sample Dye Cone . , mg/1 Rel. Wt . %, Theor. Rel . Wt . % , Found

MV VB MV VB MV VB

1 3-52 2.37 59.8 40.2 59-6 40.4

2 3.52 3.54 49.9 50.1 49.1 50.9

c. Analysis of the Bulk Ink . Samples of the

bulk ink were analyzed in the same manner as the known

mixtures, but with one modification. To remove the graphite

suspension, the solutions were filtered through 0.25 micron

solvent-resistant filters. Hydrochloric acid was added,

although pH measurements on aqueous suspensions of the bulk

ink showed that it was already slightly acidic. Therefore,

the bulk ink was expected to be free of carbinol base. This

was verified by measurements on both acidified and nonacidified samples in which identical results were obtained.

Some typical values are given in table 16.

Table 16. Analysis of bulk ink.

Sample Relative Dye Composition, Wt . %

MV VB

1 60.2 39.8

2 61.5 38.5

3 61.2 38.8

4 61.9 38.1

d. Analysis of Bulk Ink After Application to

Bond Paper . A small portion of the bulk ink was placed in an

empty ballpoint cartridge and several writing samples were

then made on bond paper. Sections were cut from this paper

and reserved for analysis. Several were analyzed immediately;

others were stored and analyzed over a period of 4-5 days.

All samples were eluted with slightly acidic ethanol.

Although the major portion of the graphite remained on the

filter paper, the filtration was nevertheless performed as

a precautionary measure. No trend in results was observed.

. 50
Table 17 gives the results of these analyses.

Table 17. Analysis of bulk ink following application

and elution from bond paper.

Sample Relative Dye Composition, Wt . %

MV VB

1 60.1 39.9

2 60.1 39-9

3 60.6 39-4

eDye Composition of Manufacturer's Scribble

Chart Ink . Small portions of the manufacturer's scribble

chart were analyzed in the same manner as the bond paper

writing samples. However, the results differed considerably,

as shown in table 18. Having been assured that the scribble

Table l8. Analysis of ink on manufacturer's scribble chart.

Sample Relative Dye Composition, Wt . %

MV VB

1 53.4 46.6

2

3

4

chart was prepared from a sample of the bulk ink, only two

explanations were possible to account for the differing

results; (1) Preferential elution of the VB component

following prolonged drying and/or (2) Preferential

fading of the MV component. Experiments on the effects

of drying were carried out by heating writing samples

of the bulk ink at 75 °C for 2 to 14 hours. No differences

were observed with heating times, and the average values

obtained were 60.5 percent MV and 39.5 percent VB

f. Light Fastness . Since the effects of drying

did not explain the disparity in the bulk ink and scribble

chart results, the second alternative of preferential

fading of the MV component was then examined. Nine

51

52.8 47.2

53.7 46.3

52.7 47.3
writing samples of the bulk ink on bond paper were

prepared and exposed in an accelerometer provided by the

Building Research Division. The acceleration factor of this

instrument for fluorescent and ordinary sunlight is about

100X. The results of this study are given in table 19

Table 19 . Variation of relative dye concentration after

exposure to light

Simulated Exposure, Relat ive Dye

Sample days Concentr

MV

at ion, Wt

VB

%

1 1 59.5 40.5

2 2 57.8 42.2

3 3 57.0 43.0

4 4 54.0 46.0

5 5 53.4 46.6

6 6 52.0 48.0

7 7 52.0 48.0

8 8 50.1 49.9

9 100 29.8 70.2

Visual inspection indicated that samples 4-9 had faded

perceptibly. In fact, only the graphite residue was

apparent for the sample subjected to the 100 day exposure

(sample 9)- Thus, while both dyes fade on exposure to

light, the results in table 19 clearly indicate that MV

fades more rapidly than VB. Relative light fastness seems to

explain the differences observed between the bulk ink and

scribble chart ink analyses.

3 . Conclusion

The original objective of this investigation was to

determine if a dye ratio could be used to identify, or narrow

substantially, the commercial sources of a given ballpoint

ink that may have been used to sign a document. The light

fastness studies clearly show that for those inks containing

DY Base and Victoria Pure Blue BO Base, additional information

the light exposure is needed. Otherwise only qualitative

identification is possible.

52
2 . FLUORESCENCE

A. Introduction

Spectrofluorometry is a sensitive and selective analytical technique of widespread use in many areas such as

clinical, environmental, analytical and biological chemistry.

Important research using this technique Is being done on

biological and metabolic processes, enzyme interactions and

characterization of binding sites, determination of pollutants,

trace metal analysis and fundamental studies on energy

transfer and manifolds

.

It has long been an accepted fact that fluorescence

standards are necessary to obtain meaningful and comparative

data which are of use in these research areas [48]. A

general program was devised [1] and research was started

to develop a series of fluorescence standards to cover

the wavelength range 350 to 700 nm.

Research this past year at NBS was concerned mainly with

investigations into experimental determinations, conditions,

stabilities and reproducibilities of solution and solid

samples using commercial instruments. Relative reproducibilities of sample fluorescence intensities were found to

have a standard deviation of 4 percent for glass samples

and 6 percent for solutions on a day-to-day basis and

0.5 to 1.5 percent on measurements made during an eight

hour period. It is evident from these investigations

that for more reproducible results required for SRM

certification, a high accuracy spectrofluorometer

with closely controlled instrumental parameters is needed.

Work at Hebrew University with Dr. R. Reisfeld under a NBS PL 480 grant was concerned mainly with the evaluation

of inorganic ions in glass matrices for SRM ' s . Two ions,

thallium(l) and cerium(lll) showed good potential qualities

as SRM's. Lead(ll), manganese(ll) and copper(II) - Copper(l)

53
were also investigated briefly, and exhibited suitable

properties for a SRM series covering the desired wavelength

regions

.

Research with rare earth ions in glass matrices also

suggests their use as SRM's for special cases based on their

sharp spectral characteristics. Preliminary results also

show that energy transfer between pairs of these ions provide

a good method for exciting acceptor ions at the donor's

excitation wavelength.

More work is needed on the evaluation of stabilities

and physical and fluorescent properties of these glasses

under various experimental conditions.

B. Instrumentation Evaluation - R. A. Velapoldi

Preliminary investigations largely concerned instrumental

reproducibilities and stabilities using the well studied

quinine sulfate in . IN HpSCK [49] and a uranium glass [50]

as comparative standards.

Two representative spectrofluorometers were used;

instrument (A) which provided corrected emission and

excitation spectra and instrument B which produced uncorrected spectra. Both have a xenon arc source, excitation

and emission monochromators and photomultiplier detectors.

One was previously described [51] and may be operated in

corrected or uncorrected fluorescence modes. In the

corrected mode, excitation spectra are corrected to

constant excitation energy. Emission spectra are presented

in quanta/unit bandwidth.

Before determining instrumental parameters, it was

necessary to investigate the characteristics of the quinine

sulfate solutions and the reproducibility of repetitive

solution preparation. Excitation and emission maxima for

quinine sulfate in . IN HpSOj. were 350 nm and 455 nm,

respectively

.

54
Figure 12 shows the corrected and uncorrected

excitation and emission spectra for quinine sulfate, purified

according to Fletcher [52], in . IN H

?

SCK. Note the

dependence of the excitation spectrum on the xenon lamp

intensity in the uncorrected spectra. On the other hand,

since emission occurs over a wavelength range of relative

linearity of detector response and by one excitation wavelength, no significant dependence can be observed.

"corrected

"uncorrected

350 400 450 Wavelength, nm

Figure 12. Corrected and uncorrected emission and excitation

spectra of quinine sulfate in . IN H„SCK . Fluorescence emission spectra are usually measured as

total fluorescence area under the emission peak. Measurements

show, figure 13, that peak height is directly proportional

to peak area and thus peak height may be used as a measure of

emission area. This relationship is generally true with

species in which excited state energy levels do not change

for reasons such as agglomeration or dimerization with

increasing concentration.

55
0.6 0. Peak Area

Figure 13. Emission peak height is directly proportional to

emission peak area.

Three samples of quinine sulfate were weighed and

prepared as stock solutions (1000 ppm in . IN H-SCO and

dilutions were made in duplicate or triplicate to provide

working solutions (0.01 to 20.0 ppm).

Figure 14 is a plot of relative fluorescence versus

quinine sulfate concentration. Linearity of response was

observed to a concentration of 6 ppm. Concentration

quenching (inner filter effects) was noted at concentrations

of solute greater than 6 ppm. An absorbance versus concentration plot is linear to higher concentrations (^100 ppm)

Table 20 shows the reproducibility of the three quinine

sulfate solutions, each value being the average of 10 determinations . There were no apparent trends for the various

solutions; thus the differences for the three samples

apparently show instrumental but not methodological errors.

In the method using dilution by weight, the instrumental

56
-o- Instrument B

-a- Instrument A

Concentration Quenching

10u

Quinine Sulfate Concentration, ppm

101

Figure 14. Relative fluorescence intensity versus quinine

sulfate concentration showing "concentration

quenching." ©- instrument B, -o- instrument A

Table 20 . Reproducibility of preparation and measurement of

quinine sulfate solutions in 0.1N HpSO^ peakheight measurements

.

Peak Heights

Concentration .5 ppm 1 ppm 1 ppm

a

5 ppm 5 a ppm 10 ppm 10 ppm

Sample

1

2

3

.058

.057

.061

.100

.107

.103

.103

.104

.105

.496

.501

.492

.514

.512

.515

0.900

• 907

.917

0.907

• 915

.911

Dilution by weight

57
variation is the limiting factor (1 percent), while dilution

by volumetric techniques shows a 1.5 to 2 percent variation.

Fluorescence measurements of the emission maxima and

spectra were made on a day-to-day basis at the same instrumental settings. Substantiation for instrumental variations

of measurements on a day-to-day basis are shown in figure 15

Statistical analyses for these data (38 days B instrument,

25 days A instrument) are given in table 21. With the

10

Instrument A (quinine sulfate)

(quinine sulfate) Instrument B

Figure 15. Instrumental stability and reproducibility

as a function of time using liquid and

solid samples.

Table 21. Statistical analysis of instrumental

stabilities using quinine sulfate and

uranium glass as standards.

A

Quinine Quinine Uranium

Sulfate Sulfate Glass

Average Pe>ak 0.739 0.398 0.383 Height

a .011 .027 .018

% 1.49 6.89 4.80

58
corrected Instrument, instrumental variations are less than

those for the uncorrected spectrofluorometer . Evidence for

xenon lamp deterioration is seen for both instruments after

25 days (Instrument A) and 38 days (Instrument B) by the

definite change in emission intensities. These observations

could also be interpreted as quinine sulfate deterioration;

however, the uranium glass measured in the uncorrected

instrument also follows the same trend, and it is unlikely

that two 'standards' would show similar degradation

characteristics

.

For a single day, good instrumental stability can be

obtained, as shown by data evaluation in table 22. The

large percent standard deviation for the 1 ppm solution is

undoubtedly due to errors in measuring small areas.

Table 22. Statistical analysis for eleven fluorescence

determinations of quinine sulfate during an

eight hour span using a corrected spectrofluoro meter

.

5 10

0.557

.0033

0.59

1.116

.0050

0.49

ppm 1

Avg. Peak Area 0.1047

a .0045

% a 4.34

Operating Instrument A in an uncorrected mode

necessitates longer warm-up times for lamp and detector

response reproducibility, figure 16 . An aluminum plate,

placed at a 45° angle in the same compartment served as the

sample. Approximately 2-1/2 hours instrumental warm-up time

are needed. Most of this time is required for source

stabilization. In the corrected mode, fifteen minutes warm-up

is sufficient to give reproducible results, since fluctuations

in source intensity are eliminated by emission to source

intensity ratioing.

59
0.6-

>

0.4-

°= 0,2

-OD- -«&- O EP -©tsr

O 5/25/70 D 5/26/70

loo ' 35o~ ~W

Time, minutes IbT

Figure 16 . Typical warm-up times for a commercial instrument

in uncorrected mode using aluminum foil as

reflecting sample.

Standards are very useful in determining instrumental

characteristics and are diagnostic tools for detecting

instrumental problems. Standards evaluation is limited

to reproducibilities of 2-5 percent, depending on

instrumentation used. No discussion is presented on

the accuracy of these determinations, because all measurements were made in a relative and not absolute manner.

Greater reproducibility and accuracy can be obtained

with a high accuracy spectrofluorometer . The design and

construction of such an instrument is one of the objectives

of the fluorescence program.

One prerequisite for providing a standard is the determination of quantum efficiencies. The quantum efficiency of

quinine sulfate has been reported between 0.4 and 0.6 [49, 53].

Although some questions have been raised as to the suitability

of quinine sulfate as a standard, it has been extensively

studied and very widely used as a comparative standard.

Statistical analyses of quantum efficiency data from fourteen

60
laboratories (using several different methods and acid

concentrations) indicate a value of 0.5^4 + 0.033.

Using these data, a comparative quantum efficiency for

purified fluorescein [5^] was determined in quadruplicate and

a value of 0.87 + 0.03 was obtained. Literature values

reported in sixteen studies give an average quantum efficiency

of 0.884 + 0.04l for fluorescein. However, there does appear

to be a significant increase in the quantum efficiency for

quinine sulfate in different acid concentrations, e. g.,

0.5^6 for IN H

2

S0^ [531. Preliminary results in this

laboratory show a 6 percent relative decrease between IN H

p

S0j,

and 0.1N H

2

S0^.

C

.

Fluorescence - Inorganic Ions in Glass Matrices - R. A. Velapoldi and R. Reisfeld [55]

Fluorescence standards may be solids or solutions. The

research at Hebrew University was to investigate and

determine the fluorescence characteristics of various metal

ions in glass matrices. The use of these solids as standards

is attractive for several reasons: they are rapidly and

easily used; have low temperature coefficients of expansion;

have only two reflections; scattering is controlled; are

stable; and have no cell dimension problems; all of which

decrease methodological errors. In addition, solids are

convenient for cooperative testing.

The research at Hebrew University covered investigations

of (a) fluorescence of inorganic ions in glasses which have

broad excitation and emission spectra, (b) rare earth

fluorescence and (c) energy transfer between pairs of ions.

D. Inorganic Ions with Broad Spectra

1 . General

Three metals, thallium(I), cerium(III) and lead(II),

were found suitable for standards considering positions of

emission wavelengths and half-bandwidths . Thallium and

61
cerium are two good non-organic UV emitters, while lead

emits in the blue region. All have relatively broad

emission and excitation half-bandwidths (see table 23).

Table 23. Emission and excitation wavelength dependence on

matrix for lead, cerium and thallium.

Emission

Ion Matrix Waveler.gth, nm Half Bandwidths

Excitat;ion Emission nm

Tl KC1 247. 5 300 40

Tl Phosphate 219. 302 70

Tl Borate 233. 325 90

Ce Phosphate 297. 334 55

Ce Borate 310 364 80

Ce BoraxD 311 .0 365 65

Ce c Silicate 317 .0 380 85

Pb Phosphate 241 • 5 309 70

Pb Borate 9" 261 .0 424 140 - Pb Borax"3 241 .0 425 165

Pb Germanate 295 .0 440 125

aMixture of boric acid and boric oxide.

Sodium borate only.

c Sample prepared at NBS

.

Cerium(III) is included because of its similar spectral

characteristics to the other ions.

The glasses used in these studies are disks prepared as

previously described [56]. In some cases, these disks are

susceptible to air and water vapor degradation (fogging)

which affects the optical properties, thus making the glasses

unsuitable as standards. This problem can be circumvented

by use of stable phosphate, borate and silicate base glasses

developed at NBS by the Inorganic Glass Section, Inorganic

Materials Division [57]

.

2 . Quantitative and Qualitative Analysis

Each ion has specific excitation and emission spectra

and may be qualitatively identified by fluorescence. For

62
low concentrations, a plot of the relative fluorescence

intensity versus concentration is linear, figure 17, and

provides a means for the quantitative determination of

these ions.

O Ce in Borate D Pb in Borate (matrix 1) • Pb in Borate (matrix 2) Tl in Phosphate (wt, 2x10)

0.08 0J0 0.'12 Metal Concentration, Wt, %

o'.m 0,'l6 0.18 0.20

Figure 17. Relative fluorescence intensity versus concentration in weight percent for cerium(III),

lead(II) and thallium(I).

3. Matrix Effects

The positions of the emission and excitation band maxima

are dependent on the type of glass matrix used. The variation

of band position for cerium, thallium and lead are shown in

figure 18 and tabulated in table 23. These effects are

attributable to fluorescent center-matrix interactions.

63
1 - Tl in Phosphate

2 - Tl in Borate

3 - Ce in Phosphate

4 - Ce in Borate

5 - Pb in Borate

6 - Pb in Germanate

6

Wavelength, nm

Figure 18. Wavelength dependence of emission peaks on matrix

for cerium, lead and thallium.

The coulombic field of the active ion is approximated

by (1)

F = (1)

where F = coulombic field

r = ionic radius

Z. = ionic charge.

Calculations of the coulombic fields for various ions are

presented in table 24 and show that inorganic ions, including

rare earths, enter the glass matrix as network modifying ions

64
Table 24. Ionic field strengths of cations in glasses.

o Ion Ionic Radius, A Field Strength Effect

[58] Z/r 2

75.0

43.2

23.8

2. 16

, 45

2. 04

1. 11

3. 32

3. 23

3. 52

3. 64

3. 41

B(III) 0.20

P(V) .34 43.2 Network Forming Ions

Si(IV) .41

Ce(III) 1.18

T1(I) 1.49

Ca(II) 0.99

Na(I) .95

Eu(III) .95 3.32 Network Modifying Ions

Sm(III) .964

Tb(III) .923

Dy(III) .908

Gd(III) .938

The observed matrix effects (actual ligand field

perturbations) may be approximated also by (2) [56]:

Z.Z. b

E = Z -i-i + — (2) n

r r where b, r, Z, and E represent the accepted terms. It is

seen from table 24 that stronger matrix perturbations are in

the order borate > phosphate > silicate resulting in shifts

of emission spectra to longer wavelengths. Cerium in silicate

does not follow this pattern; however, NBS matrices had

different starting compounds, were higher melting and were

prepared under different melt conditions. In addition,

cerium prepared at Hebrew University is quantitatively

cerium(III) and not cerium( III)-cerium( IV) mixtures as

prepared at NBS. Results are as predicted with rare earths

whenever glasses were produced under similar conditions.

Emission shifts may be interpreted by Ligand Field Theory

which states: an increase in the separation of the crystal

field-split degenerate levels indicates an increase in the

ligand field. In cases of transitions between two nondegenerate energy levels, an increase in the ligand field
causes the upper levels displacement toward lower energy

levels, thus causing longer wavelength fluorescence,

summarized in figure 19.

NON-DEGENERATE STATES

WEAK FIELD - STRONG FIELD- A~

T t

DEGENERATE STATES

GROUND STATE

c

\

Ep EB

WEAK FIELD (PHOSPHATE) STRONG FIELD (BORATE)

Figure 19. Matrix dependent ligand field effects on

degenerate and ion-degenerate states.

In addition, as discussed later for the rare earths,

weaker matrix interactions (use of phosphate rather than

borate) results in less fluorescent center interactions

by energy transfer. The lower probabilities for energy

transfer produces less concentration quenching, thus making

phosphate a somewhat better matrix for standards.

Infrared studies of the matrices show that borate has

the highest stretching frequency, table 25, which is more

compatible with the energy gaps of transitions for rare earths

and thus has a higher probability of phonon assisted energy

transfer from the metal to the borate matrix resulting in

increased radiationless transitions. Shorter bond lengths

similarly show higher 'effective' ion charge-to-size ratios

again resulting in increased matrix-metal ion interactions

.

66
Table 25. Infrared stretching frequencies and bond lengths

for active Ion-oxygen interactions in the matrix

Stretching

Matrix Frequency [ 59] Bond Length

(cm

-1

) (A)

Silicate (Si-O) 1010-1115 1.62

Metaphosphate (P-0) 1140-1300 1.57

Borate (B-0) 1310-1380 1.39

4 . Quantum Efficiencies

Quantum efficiencies are measures of the efficiency through

which ions or molecules convert absorbed light into emitted

light, equation 3, and are characteristic of the individual

Q

_ energy emitted _ photons emitted /^>

energy absorbed photons absorbed

species. Quantum efficiencies are of practical and theoretical

importance since they: (1) permit evaluation of sensitivity

of proposed methodology, (2) permit evaluation of material

purity, (3) permit evaluation of quenching and specie

interferences, (4) are needed for calculating thresholds

for laser action, (5) determine suitabilities of species

as energy donors, (6) permit assignments of electronic

transitions, (7) determine extent of radiationless

transitions, etc. They may be calculated by several methods:

(a) Absolute [60]

(b) Comparative [52, 6l]

(c) Lifetime measurements [62]

(d) Calorimetrically [63].

We are currently performing quantum efficiency measurements by the comparative (b) and lifetime (c) methods because

of rapidity and ease of measurements; however absolute methods

67
will be developed. Method (b) will be discussed here and

method (c) will be discussed later.

Comparative quantum yield and oscillator strength have

been determined for the 'A T band of thallium(I) chloride in

pressed polycrystalline disks of KC1 [64] using solutions

of quinine sulfate (QS) in .'IN HpSOj, as the standard.

Maximization of the emission intensity was necessary

since disks (1 cm diameter, 1-2 mm thick) or disk-shaped

cuvettes were used. Emission intensity is dependent on

the angle of excitation, figure 20, with the result

that angles of 15-20° give maximum relative intensity.

0.5

t

°' 3

\- ~ 0,2" >

i—<£ 0.1

—

D

TlCl in KCl 'O Quinine Sulfate

(b)

Sample Compartment

Emitted Light

10 ~~2Q

30 W~ 50 Angle of Disc With Exciting Light,*.

-**

Exciting Light

(a)

Sample

Meas

Figure 20. Fluorescence intensity dependence on sample

angle for thallium(I) in KCl and quinine sulfate

in 0.1N H

2

SO i|

.

The quantum efficiency was calculated from (4) [51, 6l].

2

QTl 'QS

PAT1 A

QS Xex

QS n " Tl

2 FAQS AT1 XexTl n QS

(4)

68
where

Q = quantum efficiency

FA = integrated area under emission peak

A = absorbance

A = exciting wavelength

D

ex

= refractive

&o

index

Both thallium and quinine sulfate were excited at a wavelength of 247.5 nm. Although the absorption maximum is at

250 nm, it has been reported that the quantum efficiency of

quinine sulfate does not vary by more than 5 percent over the

excitation wavelength range of 240-390 nm (except for

position of minimal absorbance at 272 nm) (Eq. 5) • The

refractive indices of the KC1 were taken from LandbltBornstein Tables. An average of the respective indices of

the silicate windows and solution was calculated. Absorbance

was measured with a Cary Model 14 double-beam spectrophotometer with expanded scale and 1-cm cells or the pressed disks

The calculated quantum efficiency of 0.51 + 0.04 for

thallium is in excellent agreement with another experimental

value of 0.52 [65].

The oscillator strength, calculated from Smakula's [66]

equation (5), was 0.073 + 0.003 which agreed well with other

results, table 26.

F P

f = ( p* n

p K^-2-) /cx(E)dE (5)

2tt he N eff

where

f = oscillator strength

N = number of luminescent centers per cm

m* = mass of electron

X] = refractive index of matrix

E = electric field in matrix

Eeff

= electric field in vicinity of ion

69
Tab

a = absorbance/cm as a function of energy

c, e, h are physical constants.

le 26- Oscillator strengths for T1C1 in KC1.

Band Oscillator

Half Width Strength Reference

f • Temper- Absorption

ature Peak

°K nm

293

77

77

293

248.2

247.0

247-0

247.5

ev

0.199

.11

12

0.0764 [671

.075 C68]

.075 [69]

.200 .073 this work

The oscillator strength is related to radiative lifetime

as expressed by equation (6) [70]..

f = 1.499 x 10" 8

X

2

(6)

nat

where

t , = calculated radiative lifetime

ila C \ = wavelength in microns

Quantum efficiency may be determined by method (c) (discussed

later) from equation (7)

Q = ^— (7)

nat

where

x = measured lifetime

Thus a check on the experimentally determined quantum

efficiency may be calculated from t and the absorbance

spectrum. Unfortunately, available instrumentation only allows

measurements of t in the milli- to microsecond range and not

the nanosecond range. An excitation source with a very fast

rise and decay time (^1 nanosecond) such as a tunable dye laser

is needed.

70
5 . Future Research

It is necessary to determine the stabilities of these

species in various matrices. It has been reported [71] that

cerium(III) is photo-oxidized to cerium(IV); however, preliminary results here show that only a limited amount of

cerium(III) is photo-oxidized and thus a stable condition may

be reached by pre-irradiation.

The determination of comparative quantum efficiencies for

thallium, cerium and lead in various glass matrices is also

planned. These three species have maxima which cover the

wavelength range 300-480 nm. It would also be of interest to

study the manganese(II) and copper(II)-copper(I) ions in

glass matrices to complete a series of glass standards. The

emission maxima for manganese(II) are 5^0 and 620 nm [72]

while copper emission maximum is at 530 nm.

E. Lanthanides

1. Analytical

The lanthanides, as with cerium, thallium and lead, may

be determined analytically by the linearity of a plot of

relative fluorescence intensity versus concentration,

1.0

0.8"

:0.6-

;0.4

0,2

D Eu(IIl) in Phosphate H Eu(III) in Borate ° Er(III) in Borate w Tm(III) in Phosphate (%x10) • Tm(III) in Borate ttxlO) ATb(III) in Borate AGd(III) in Borate (MOO)

ODy(III) in Borate

Rare Earth Concentration, Wt. Z To"

Figure 21. Relative fluorescence Intensity versus rare earth concentration in weight percent

.

71
figure 21. These relative fluorescence intensities were

determined using excitation wavelengths and emission wavelengths in table 27 which gave maximum emission intensities.

Table 27. Excitation and emission wavelengths for lanthanides

in glass matrices yielding maximum emission

intensities

.

Metal Ion Matrix Excitation X Emission X nm nm

Er(III) Borate 365 546

Dy(III) Borate 365 578

Tb(III) Borate 378 543

Eu(III) Borate 39^ 617

Eu(III) Phosphate 39^ 612

Eu(III) Silicate 394 610

Tm(III) Borate 265 457

Tm(III) Phosphate 265 453

Gd(III) Borate 274 312

The lanthanides have several excitation and emission

peaks, any of which may be used for analytical determinations,

if convenient or when necessary, because of interferring

fluorescent species.

The usual narrow band (half widths are 0.5-5 nm) emission

spectra of the rare earths, exemplified by europium(III) in

silicate, figure 22, are due to f shell electronic transitions which are 'LaPorte forbidden' for centrosymmetric

fields. In solids or liquids where the center of symmetry

is lowered or destroyed, the electronic transitions become

"allowed" and due to mixing of f orbital transitions and

Interactions with the effective crystal field of the glass

matrices, broadening and splitting of the rare earth energy

levels occur.

2. Matrix Effects

Analogous to the matrix effects discussed earlier in this

report, similar effects are noted with the rare earths,

table 28.

72
r^r

A

• 1

1

1

1

1

300

j!

,1! in

i.i i

1 1

1

• ii

'!

\ i!

|i i J

|1 I '

'»

!

1

1

i ;«

«»—

r -%*^

Wavelength, m

Europiiw(III) in SiLICATE GL«S. 0.3

Emission

excitaticw

Absorption

too 700

Figure 22. Corrected excitation, emission and absorbance

spectra for europium(III) in a silicate matrix

Table 28. Shifts of major europium, samarium and thulium

emission peaks as a function of matrix.

Emission Maximum, nm

Matrix

Silicate

Phosphate

Borate

Eu(III)

610

612

617

Sm(III)

596

601

605

Tm(III)

553

557

Lower matrix interactions are also supported by less

quenching in the matrices which exert weaker perturbations

on the fluorescent center: e.g. 0.5 wt percent samariumC III)

in a borate matrix showed concentration quenching, while

3 wt percent samarium(III) in silicate showed no concentration

quenching [73].

Further evidence for rare earth-matrix interactions were

obtained by the measurement of fluorescence lifetimes, presented in table 29. On the average, fluorescence lifetimes

were shorter in the order borate > phosphate > silicate

73
showing better facilitation of energy dissipation through

stronger rare earth-matrix interactions and thus shorter

lifetimes

.

Higher intensities of fluorescence were obtained by use

of borate versus silicate matrices, table 30, although preliminary results for thallium in borate and phosphate appear

to give equal emission intensities when corrected exciting

lamp intensities are used.

Table 29. Average fluorescence lifetimes of rare earths in

various matrices

.

Lifetimes, Milliseconds

.E.(III) a Silicate Phosphate Borate

Eu 2.55 2.32 2.05

Sm 2.60 1.87 1.60

Tb 3-90 3.30 2.80

Dy 0.95 0.87 1.00

Gd 4.32 5.25 4.30

L Rare earth concentration = 1 wt . percent

Table 30. Effects of glass matrix on fluorescence intensities

of the rare earth ions

.

Fluorescence Ratio

R.E.(III) Silicate/Borate

Eu 0.30

Sm .65

Tb .76

Dy .74

Gd .59

3 . Quantum Efficiency Determinations of Eu(III) in

Silicate Matrix

Quantum efficiencies have been determined for

europium( III) in solution, in crystals or doped in other rare

earth matrices [74], but essentially no research has been

done on the determination of quantum efficiencies in glass

matrices

.

74
The quantum efficiency of europium (III) has been

determined in a silicate matrix by two methods, comparative

and lifetime measurements for the D

n to F transitions [75].

The comparative quantum efficiency measurement was made by

using Eu(NOo) ? in water as the standard [7^a]. The solution

7 5 and solid were excited at the F, -* D

n wavelength (592.5 nm

for solution and 587.5 nm for solid). Major emission peaks

obtained for these excitation wavelengths are given in

table 31. It is necessary, however, to sum the areas under

all emission peaks when determining comparative quantum

efficiencies (PA in equation 3). Figure 23 presents emission

Table 31. Major emission peaks for europium(III) in water

and silicate matrices.

Assignment Emission Wavelength, nm

H

2

Silicate

5 Do->

7F 580.0 578.5

5 D -> 7 F! 592.5 592.2

5 D + 7 F 2 617. a

610.

a

5 D + 7 F 3 652 a

655.

a

5 D -> 7 F, 695

a

693.

a

5 D -> 7 Fi* 706.a

aEmission peaks obtained by exciting at 7 Fi+ 5 D transition.

spectra for europium( III) in water and silicate when excited

at 396 and 393.8 nm, respectively. The ratios of the

emission areas are constant regardless of excitation wavelength, thus ratios may be made for the emission areas when

excited at 396 and 590 nm for the solution, and the total

emission area at 590 excitation determined. Similar calculations were made for the europium(III) in the glass matrix.

Use of equation 3 necessitates careful absorbance

measurements. Dawson and Kropp [7^b] have shown that

corrections must be made if the absorption band is smaller

than a triangle constructed on the absorption spectra from

75
Figure 23.

Eu(III) in Silicate

Eu( III) in Water (as NITRATE SALT)

650 Wavelength, nm Major emission peaks of europium(III) in

silicate matrix and water solution.

an experimental determination of the half-bandwidth for the

excitation wavelength. In our case, no correction was necessary since the half-bandwidth of the excitation wavelength

(2.5 nm) was smaller by factors of 2-4 than the half-bandwidth

of the absorption peaks as determined from absorption spectra.

The quantum efficiency as determined by the comparative

method, was 0.93 + 0.05, a value expected for species with

no or few radiationless transitions.

Lifetime measurements were also made. The measured

lifetime, x, was determined by an apparatus previously

described [76] and found to be 2.73 + 0.03 msec. As

known, x was independent of excitation wavelength and

emission wavelength as long as the emission was measured

5 7 for a D -> F transition.

The natural radiative lifetime,s x , , was calculate

nat

7 5 using the absorption spectrum for the F+ D„ transition

76

d
It was necessary to perform Gaussian analyses on the

7 5 absorption spectrum because of overlap with the F + D~

transition, figure 24. The lifetime is calculated from the

Lewis-Kasha [77] equation (8)

0.036'

0.030

0.024

LUz

g 0.018- o

CO

PQ "^ 0,012-1

0.006

0.000-

O O O EXPERIMENTAL POINTS COMPUTER GAUSSIANS

GAUSSIAN SUM

16500

Wavenumber, cm

Figure 24. Gaussian Analysis for absorbance spectra of

europium in silicate 7 Fo,i->5 D transitions.

^— = ZA

x = 2.88x10 9

( gl /gu

)n

2 v

2 /e(v)dv (8)

nat

where

A ^1 = transition probabilities from upper to lower

state

g = degeneracies

r\ = refractive index

v = wavenumber of emission maximum

e = molar absorptivity as a function of wavenumber

1, u = lower and upper states

Although there are some "Stokes' shifts" (^5 ran), it was not

necessary to use the lifetime equation (9) given by Forster

77
[78], since calculations using both equations yielded the

same results:

1 _Q 9 ( 2V "V ) 3

^— = 2.88x10 y

( gl /gu

)n / 2 e (v)dv (9)

nat

where v is the wavenumber center for the mirror image

o ° relation.

For europium, = EA _ expands into equation (10)

nat u

1

nat 5 D ^7 F

5 D

+7-p 5 D

^.7 p

5 D

^.7 p

5 D ^.7 p

1 2 3 4

+ A + A (10) V F

5 V F

6

It was determined that A,- „ and A,- „

are small (less than 1 percent) as compared to the other

transition probabilities and may be neglected for all

practical purposes. Equation (10) may now be written in

shorthand notation as (11)

= A n ^n + A^-, + An _ + An _ + An ,,. (11)

t , 0+0 *0+l 0+2 0+3 0+4*

nat

7 5 By using the absorption spectrum and only the F +^Dn transition, we calculate only l/x n

~ since A„ ,- = *0

Aj- 7 [79]. The areas of the fluorescence spectrum,

•^DU + F

figure 23, for the europium(III) in silicate are directly

proportional to the transition probabilities from 5 n

"to

7 1 each F state. Thus, was calculated by equation (12)

T nat

78
nat

A A A A , 0+1 • 0+2 0+3 0-5-4 x = A ( 1 + —— — + —— — + —— — + —- —

A0+0 u +

.A„ . n kn . n A_ A, (12)

0+0 0+0 '0+0 0+0

and found to be 3-10 msec. The quantum efficiency was

calculated from equation (7) and was equal to 0.90, which is

in excellent agreement with the comparative quantum efficiency of 0.93 + 0.05.

P . Energy Transfer

Energy transfer has been observed between donor and

acceptor sites for pairs of different rare earth ions.

The objectives of this research include the enhancement of

emission intensities and excitation of fluorescent species

at higher wavelengths and thus lower energies so that

degradative problems such as photo-oxidation is minimized.

Two rare earth pairs have been studied - samarium( III)-

europium( III) and gadolinium(III) -terbium(III)

The emission Intensity of europium(III) is increased,

figure 25, when excited at the samarium(III) excitation

wavelength of 402 nm.

O Eu(III) E Eu(III) + Sm(III)

Rare Earth Concentration, Wt, 7.

Figure 25 Fluorescence intensity enhancement of

europium(III) by energy transfer from

samarium(III) to europium( III) , A = 402.0 nm, GXC

79
Terbium and gadolinium also gave interesting results.

Excitation at 275 nm (Gd excitation wavelength) resulted

in terbium emission enhancement by at least 50 percent

figure 26. The excitation spectrum of a mixture of terbium + gadolinium monitored at the terbium emission wavelength of

480 nm, showed excitation peaks at 275 nm (Gd excitation)

and 312 (usual Gd emission), figure 27, proving energy transfer

from gadolinium to terbium. Calculations are in progress which

will lead to the estimation of some of the system transition

probabilities, integral overlaps [79, 80], quantum efficiencies by lifetime measurements and whether the transfers are

Tb(III) -" Tb(III) + Gd(III)

Wavelength, nm

Figure 26. Fluorescence intensity enhancement of terbium(III)

by energy transfer from gadolinium(III) to

terbium(III) ,

5 A = 275.0 nm.

exc

80
Tb(III)

Tb(III) + Gd(III)

Wavelength, nm Figure 27. Excitation spectra obtained showing

gadolinium(III) spectra (275.0, 312.0 nm) when

monitoring terbium(III) emission at 490.0 nm,

Dit->7 P 6 transition.

due to dipole-dipole or dipole-quadrupole interactions.

G . . Data and Spectra Storage

One of the objectives of this research was the compilation, storage and presentation (see computer section)

of corrected excitation and emission spectra of the rare

earths and other species in various matrices. Typical

corrected spectra are presented in figure 22. The transition

assignments and relative peak intensities (areas for emission

peaks) are given in table 32. The intensities are given

relative to the most intense peak [393.9 nm for europium(III)

]

excluding the very intense charge transfer band found in

the 200-250 nm region. 0nol
Table 32. Transition assignments and relative intensities

of major emission and excitation bands for

europium(III) in silicate and phosphate matrices

Emission - relative peak areas; Excitation - relative peak heights.

82

Emission

Assignment

[8?] Relative IntensitySilic

v cm"

ate

X , nm

Phos

-1

v cm

phate

X , nm

Silicate Phosphate

17.25 579-7 17.29 578.5 5 D +7 Fo .0374 .0325

16.88 542.2 16.90 591.7 5 D -> 7 Fi .2058 .2853

16.38 610.2 16.34 612.1 5 D + 7 F 2 1.000 1.000

15.25 655.7 15.30 653.7 5 D + 7 F 3 .0606 .0558

14.42 693.2^/l4.43 693.1 ^o^Fi* .0308 .0817

14.16 706.2 14.26 701.2 5 D -> 7 F lt .1002 .2387

13.24 755.2 5 D + 7F 5 .0041

12.71 788.2 5 D +

7F 6 .0038

Excitation

32.84 304.5 32.87 304.2 7 Fi-> 5 F 3 .378

31.13 321.2 31.31 319.4 7 Fo+ 5 H .320 • 352

27.58 362.6 27-57 362.7 7 F -^ 5 D lf .229 .361

26.53 377.0 26.56 376.5 Fo -*" G

3

.202 .163

26.18 382.0 26.16 382.2 7 Fi+ 5 G 3 .350 .231

25.38 393.9 25.38 393.9 7 Fo+ 5 L 6 1.000 1.000

24.81 403.3 - - 7 Fi+ 5 l*6 .208 - 24.06 415.7 24.01 416.5 7 Fo+ 5 D g .077 .017

21.54 464.2 21.49 465.3 7 F + 5 D 2 .528 .173

18.99 526.7 19-00 526.4 7 Fo+ 5 D! .122 .019

18.75 533.2 18.70 534.7 7 F 1 -> 5 D 1 .162 .034

17.29 578.7 17.22 580.6 7 Fo+ 5 D .028 .008

17.02 587.5 16.89 592.2 7 Fi+ 5 D .036 .011

shoulder.
This compilation will eventually be produced for all

fluorescent spectra recorded here at NBS and circulars

will be issued containing these spectra with all operating

conditions

H. Computer Programs - R. A. Velapoldi

In any compilation of spectra and data, it is necessary

to develop computer programs for data handling, feedback, etc

Programs were developed for simple data conversion to wavenumbers or energy from wavelength. A BMDX85 (Health Sciences

Computing Facilities, University of California at Los

Angeles) program was used for Gaussian analyses (see for

example figure 14). Prom this it is possible to separate

overlapping absorption or emission bands and determine areas

and percent areas of each band and each Gaussian. A

Gaussian plot program shows graphically the results of the

Gaussian analyses program.

A simple cal-com plot program was developed which will

plot with different symbols up to 10 data sets with various

options on the same plot. This is very useful for comparison

and presentation of data.

Other programs will be developed to store and recall

data sets for spectra on tape.
3. ANALYTICAL FLAME SPECTROSCOPY

A

.

Introduction

Flame emission and atomic absorption spectroscopy

continue to play an important role in helping to solve some

of the analytical problems associated with our current

concerns in health and environmental pollution. As in the

past year, emphasis in this Section was placed on instrumentation and methodology to improve the sensitivity

and maintain the desired precision and accuracy. Again

as in previous years these competences contributed a great

deal to the characterization of Standard Reference Materials

(SRM's). The new developments encompassed both instrumental designs and testing of procedures for handling

special materials. The research in Instrumentation included

the construction and evaluation of components while the

methodology studies dealt with simultaneous analyses of

multicomponent systems and the analysis of a volatile element

in an organic matrix.

The present report describes some of the developments

of flame sources, multipath light gathering systems, ultrasonic nebulization of sample, simultaneous analysis of six

elements or species, a repetitive optical scan for background correction, and a non-flame method for mercury.

These efforts provided manifold improvements in sensitivity

and yielded data for establishing the accuracy of simultaneous

determinations of sodium, potassium, magnesium and calcium.

Finally, these studies Involved newer methods or modifications

for the determination of aluminum In steel and eight elements

including mercury in orchard leaves for the SRM program.

B. Evaluation of the Multichannel Direct Reading Spectrometer - R. Mavrodineanu

The spectrometer described previously [1] was designed

and built to take advantage of the fundamental properties

of flames which emit and absorb simultaneously at all

84
transitions originating from the chemical species. As a consequence of this property, the measuring system was

designed to detect and measure simultaneously a number of

individual radiations emitted or absorbed by the excitation

o source in the wavelength interval of about 2000 A to

o 10,000 A. This instrument was tested for stability and

accuracy and the results obtained are given in this section.

A multichannel direct reading spectrometer for

simultaneous flame emission and atomic absorption measurements using electronic integration of the optical signal

and digital readout was described in NBS Technical Note

5^4 [1] . This spectrometer was tested to determine its

characteristics using the emission and atomic absorption

modes in conjunction with a premixed acetylene-air flame

and pneumatic sprayer.

The stability of the 8 channel electronic circuitry,

including the photomultipliers , was measured previously

using a constant light source provided by a fluorescent

screen activated with 2 Ci of tritium [82]. This source,

having an output of about 56 u lm, was placed at the focal

plane preceding the photomultiplier tubes. An integration

time of 10 sec was used for each channel and a series of

measurements was obtained for each photomultiplier tube.

The standard deviation of a single measurement of the

constant illumination was 0.1 to 0.3 percent.

Calcium, sodium and potassium were determined by

emission, and magnesium was determined in the atomic

absorption mode. Lithium was used as an internal standard

for the emission mode. These elements were selected because

of their importance in biochemistry and their compatibility

with the excitation capabilities of combustion flames.

The pneumatic sprayer is supplied through a twoway Teflon stopcock in succession with the analytical

solution or with the blank solution of distilled water [83].

This dual arrangement simulates to some extent a double85
beam operation. Indeed, if one assumes that the emission

source (hollow cathode) and the flame, with its supply syste

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 sec which is required

to change from sample to blank is determined by the memory

of the sprayer and burner units. Since we use 20 sec

integrations for the sample and the blank, the total time

required to measure the analytical sample and the blank is

35 s.ec., respectively. Hence, one sample- blank cycle

requires 70 sec. This technique was used to obtain the

analytical data reported in table 33. For these measureTable 33. Accuracy of analytical results for magnesium, potassium, calcium and sodium determined simultaneously

Element Determined, ug/ml

No Mga = 4.00 K

b

= 60.00 Cab

= 15.00 Na = 8.00

of Q."5 "5 g."5 g.

"o 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 -2.5 57.68 -3.87 14.84 -1.07 7.93 -0.88

By flame atomic absorption.

By flame emission.

ments, the instrument was calibrated with 8 standard solutions containing all four elements and lithium in aqueous

media was used as an internal standard. Each standard solution was run In 6 replications, each measurement consisting

of a blank and a sample reading. To determine the accuracy

of the measurements, while avoiding the interferences due tc

matrix effects, the "unknown" sample was selected from the

standard solution and run from 3 to 25 times. Each determi86
nation consisted of a blank and sample solution. The results,

table 33, show that the differences between the true values

and the values found vary from 0.0 to 3.87 percent and that,

in general, a systematic negative bias is evident. The

reasons for this bias are not known at this time.

C . Long Path Flames

Further work was carried out to improve the sensitivity

of the measurement without losing accuracy. Gouy and

Lundegardh demonstrated that the emission intensity of

radiations from a flame can be increased by lengthening

the optical path; similarly, Walsh has shown that the

absorption of a flame can be augmented by increasing

its length. This can be achieved by producing a long

flame or by increasing the optical path through the

flame using multiple reflection arrangements. Both means

are discussed here.

The prototype long path burner made of copper and

described previously produced a 30-cm long flame [1, pg. 69],

and was reproduced from stainless steel SAE type 316. This

was then adapted to a Teflon spraying vessel and burner

body. The pneumatic sprayer was also made of 316 stainless

and was provided with an impact wall. The air was supplied

to the sprayer at a pressure of 39 psi, and at a rate of

2100 liters per hr

Acetylene was supplied to the burner body through a

tantalum nozzle to produce a stoichiometric premixed flame.

The consumption by the sprayer of the analytical solution

was 25 ml/min and the sample entering the flame was 1.3g/min.

The optical system used with the 30-cm flame consists of two

quartz lenses which produce an optical beam having an

average cross section of about 10 mm to 15 mm. Under these

conditions, the sensitivity was increased by a factor of 3,

when magnesium was determined in the atomic absorption

mode using the electronics discussed previously [1, pg. 59,

fig. 60].

87
The optical system just described which uses two

lenses will be replaced with a two Cassegrain mirror

arrangement [84] to eliminate the chromatic aberrations

common to all lens systems

.

D . A Triple Pass On-Axis Optical System for Analytical

Flame Spectroscopy

Z , (a) Herget

R. Mavrodineanu and W. Muller-

; The on-axis optical system is designed for atomic

absorption and emission measurements and provides for three

passes through a given flame. A chopper wheel can be

included. It is based on the use of Mangin-type mirrors

In conjunction with an asymmetric biconvex quartz lens.

The specified dimensions and the schematic arrangement of the proposed system are shown in figure 28. The

TRANSFER

LENS INTERMEDIATE

IMAGE TRIPLE PASS CELL

LENS

Figure 28. Proposed system and basic dimensions for

the triple pass, on-axis optical arrangement

(dimensions in mm).

spectral range is 240 nm to 440 nm and can be extended to

cover the visible regions of the spectrum. A minimum

clearance of 6.0 mm is provided for the chopper assembly at

the intermediate image position (Pz).

(a) Philips Laboratories, Briarcliff Manor, New York, N. Y

10510.

88
The flame cavity is designed as a concentric triplepass system with parallel light in the second pass. Because

of heat and corrosion, it is advantageous to use rearsurface 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 from 240 nm to 436 ran with the design center at 330 nm. The object (at Pz) is

imaged through the cavity with unit magnification. The

conjugate points are both outside, providing 6.48 mm

clearance beyond the mirror's back surface. The minimum

optical diameters required are 6.80 mm for the bore and

35-0 mm outside diameter of the mirror.

The hollow cathode is imaged by an asymmetric biconvex

quartz lens at the intermediate image position. It is

designed for the same spectral range with the design center

at 330 nm. The spherochromatics are minimized, but are not

suppressed In a simple lens. The aberrations of the Mangin

cell are only 2 percent of the transfer lens-blur illustrated

in figure 28.

The hollow cathode image Is demagnified (1:0.635) and

is inverted at position Pz and upright at position P

(figure 28)

.

If five passes through the flame are required, both

Mangin mirrors may be exchanged for two other mirrors

of corresponding design, while retaining all previous

dimensions. The focal length f, of the mirrors for the

five-pass cell will be, in approximation:

f = 2.618 x d = 398.98 mm

89
for required clearance of d = 152.4 mm between the mirrors.

The required diameter will be approximately 30 percent

larger.

The two Mangin-mirror arrangement is illustrated in

figure 29 which gives the dimensions for reproducing the

system. The triple-pass unit is placed on the optical

bench along the optical axis of the spectrometer. The

analytical flame is placed symmetrically between the two

mirrors and at a height selected for best sensitivity. This

system was tested with the equipment described in [[1],

pg. 58 to 65]. When measurements were made in the atomic

3 ADJUSTING

SCREWS (PUSH)

3 MIRROR

HOLDING SCREWS

ALL DIMENSIONS SHOWN ARE IN INCHES

Figure 29. Schematic drawing of the on-axis triple pass

system.

90
absorption mode, using magnesium, an increased sensitivity of

2.7 was observed when compared with the sensitivity obtained

without the triple pass unit. In the emission mode, sodium

was used for measurements with the following results: 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 refracting, optics to replace the

two lenses mentioned above.

E. Premixed Acetylene-Oxygen Flame Supplied Through an

Ultrasonic Spraying System - L. Owen^ 9-) ancj

R. Mavrodineanu

The purpose of this work was to combine the high

temperatures of premixed acetylene-oxygen flames with the

highly efficient ultrasonic nebulizers.

The burner used to provide the premixed CpH„-0p flames

was developed following the principle and design of a unit

described earlier [85] . It consists of a ring-shaped

enclosure of stainless steel SAE type 316 and is provided

with two separate chambers . The premixed CpHp-Op obtained

from a miniature welding torch unit is supplied to the lower

chamber. The gas mixture passes through a number of small

(a) Guest worker associate at NBS

91
tubes which connect the lower chamber with the upper surface

of the burner. These tubes are water cooled in the upper

chamber. The entire unit is assembled by welding in a

hydrogen furnace and is flash-proof. The sample is supplied

to the premixed C

2

H 2~°2 flames, which burn at the extremity

of the tubings, and indirectly through the central opening

of the unit, as illustrated in figures 30, 31 and 32.

12 HOLES *69 DRILL ON %" DIA CIRCLE

12 HOLES **69 DRILL ON 7/8

" DIA CIRCLE

ULTRASONIC—

GENERATOR

Figure 30. Premixed C

?

H -0

?

burner and ultrasonic nebulizer,

The sample solution aerosol is produced in a newly designed

ultrasonic generator [86]. The solution is pumped directly

across the vibrating face of a piezoelectric transducer

(1.4 MHz) at a flow rate from 0.25 to 3 ml/min. The

efficiency of nebulization is about 50 percent.

92
Figure 31. The unit from figure 30 in operation.

This ultrasonic nebulizer-CpHp-Op burner system was

tested by atomic absorption measurements using magnesium

as the analyte. An absorbance of 0.09 was obtained with

0.1 ug per ml of solution which is an indication of the good

efficiency of the system. Further work is planned to assess

the characteristics of the unit for other elements.

F . Flame Emission Spectrometry With Optical Scanning -

T. C. Rains

Aluminum is an element of interest in a variety of

materials in the NBS Standard Reference Materials Program.

Hence, a detailed study of aluminum by flame emission was

93
Figure 32. The unit from figures 30 and 31 placed on the optical bench in front of a lm grating spectrometer; shown here

for measurements in the atomic mode. At right, the hollow

cathode followed by a lens and a chopper. At lower right, the

generator which drives the ultrasonic generator and the electrically operated sample pump which supplies the ultrasonic

sprayer with the analytical sample solution.

9

4
undertaken. In an oxygen-hydrogen flame, the aluminum

o lines at 39^ and 3962 A are not observed. However, under

reducing conditions using a nitrous oxide-acetylene flame,

these lines are quite intense. The major interference,

however, is the strong CH band system which lies directly

under these lines, as shown in figure 33. The band structure

70 r

3962

3925 3970

WAVELENGTH, A

Figure 33. Flame emission spectra of aluminum over wavelength region of 3925 to 3970 A from a nitrous

oxide-acetylene flame (amplifier gain 20 mV)

o under the 39^ A aluminum line could not be resolved with

the present 0.75m monochromator ; hence, further study of this

line was discontinued. On the other hand, the aluminum line

o at 3962 A has two undefined bands which lie under its peak.

By repetitive optical scanning [87 3 88, 1] and by selecting

the length of scan, it was possible to correct for any contri95
bution to the aluminum signal by this band structure.

A typical calibration curve is shown in figure 3^ • The

80

70

60

50

40

30

5.0

20

1.0

4.0

3.0

2.0

H20-C2

H2

Flame

Time Constant, 1 sec

H2

Dark Current /

Al, ug/ml

Figure 3^. Calibration curve for aluminum using repetitive

optical scanning.

contribution to the emission signal from water and from flame

gases was about the same as that obtained with the dark

current (excluding all light from photodetector )

.

The height of the nitrous oxide-acetylene flame was

scanned under varying flame conditions. The optimum for the

best signal-to-noise ratio was to focus on the center of

the pink plume within the flame when the pink plume was

1 to 1.5 inches in height.

9 6
By comparing the data in figure 35, obtained with normal

100i3962 ^^

Lv-Twvn

"VvWAVELEN6TH, A

Figure 35. Comparison of flame emission spectrum of 0.1 ug/

ml of aluminum and H o using a nitrous oxideacetylene flame." Conditions: Time constant, 1 sec;

voltage applied to photomultiplier tube, 1000 V;

amplifier gain, 1 mV; flame, fuel rich (1-1/2 inch

pink plume)

wavelength scan, and the data in figure 36, the relative

advantages of the latter method are apparent. The scan in

figure 35 was made with and without 0.1 yg/ml of aluminum.

In a series of measurements of the intensity of the

o aluminum line at 3962 A, 0.1 u g/ml could not be repeatedly

97
n

LyVvJ Lvi" w

HI. P!/"l Figure 36. Relative intensity of aluminum using repetitive

optical scanning.

(A) Optical conditions the same as in figure 35

(B) Voltage applied to photomultiplier tube

increased to 1200 V.

detected in the presence of the band structure. Under the

same instrumental conditions (voltage to photodetector and

gain to amplifier), 0.1 ug/ml of aluminum was positively

detected using repetitive optical scanning, as shown in

figure 36

.

A detection limit of 0.01 yg/ml of aluminum, which is

equivalent to two sigma above the flame background, was

obtained for repetitive optical scanning with a RC time

constant of 1 second on the lock-in amplifier. See

figure 37-

The selection of time constants from 0.1 to 10 seconds

was also evaluated. It was found that detection limits

could be improved with a longer time constant. At 3 and 10

98
Al, 0.2 fig/ ml

39.5 38.2 39.4

Al# 0.1 fig/ml

Figure 37- Relative intensity of 0.1 and 0.2 yg of aluminum

with repetitive optical scanning using a nitrous

oxide-acetylene flame. Conditions: Same as in

figure 35, except voltage applied to photomultiplier tube, 1300 V.

seconds and a signal of 50 percent transmittance , maximum

peak heights were obtained in 15 and 60 seconds, respectively

The slight improvement in detection limits did not justify

the longer time constant which consumed a larger amount of

analyte

.

Table 34 lists the values for aluminum in 0.4 percent

carbon steel (20g). This Standard Reference Material was

analyzed by three cooperating laboratories using different

methods. The certified value is 0.040 percent aluminum.

G. High-Preoision Atomic Absorption Spectrometry for

Major Components - T. C. Rains

In an effort to improve our precision and accuracy in

the determination of major components by atomic absorption

spectrometry, zinc and chromium were investigated respectively in a lead-base alloy and a steel.

99
Table 34. Determination of aluminum in 0.4$ carbon steel

by flame emission spectrometry.

ample Al, ug/g

lc

d

396

396

5c

d

400

396

8c

d

399

398

lie

d

396

402

13c

d

392

396

17c

d

399

397

S .D.

X 397

6

Rel. S .D. % 1.6

The determination of zinc by atomic absorption is

extremely sensitive; however, its sensitivity is dependent

upon the path length of the flame. Sensitivity is defined

by the slope of the calibration curve. As shown in

figure 38, 1 and 10-cm flame cell path-lengths were evaluated

Zn, pg i Figure 38. Effect of flame cell path-length on the

absorbance of zinc at

100

138 A.
While the 1-cm flame cell path-length extended the operating

range to 100 u g/ml of zinc, the precision was not improved,

due to the lower slope of the curve over a wider' range of

concentrations. With a 10-cm flame cell path-length, a slope of 6 was obtained over the concentration range from

to 10 ug/ml. Also, the slope dropped to 1.2 over the

concentration range from 10 to 20 u g/ml . Maximum precision

was attained with a double-beam spectrometer at concentrations

from 12 to 14 u g/ml and operating with scale expansion to

give an absorbance reading of 2.100 to 2.500. A summary of

the results and a comparison with classical procedures

are given in table 35. The values reported by the cooperators

Table 35. Determination of zinc in SRM 124d ( ounce metal)

.

AAS Wet Chemistry

Sampl

No. of

3 Replicates

6

4

3

4

Zn, %

5.087

5.082

5.073

5.030

S.D.

Rel.

S.D. , 0.15

.45

• 23

.45

%

(ZnS-ZnC)

Zn, %

A

B

C

D

0.075

.023

.012

.023

5.07

5.11

5.03

5.07

5.04

5.06

S.

Rel. S.

D.

D.,

X

%

5.073

0.016

• 31

5.06

0.028

.55

were determined by the classical method (the ZnS-ZnO method)

The technique for the determination of zinc was applied

to the determination of chromium in Cr 18-Ni 10 stainless

steel samples. However, several interferences were encountered in the determination of chromium that were not

present when zinc was determined. Using an air-acetylene

flame, nickel severely interfered in the determination of

chromium. A slight change in acidity (HpSCv or HpS0|.-

H_PCv) greatly affected the absorbance of chromium. In a nitrous oxide-acetylene flame the interference of nickel

and variations in acid concentration were removed.

101
The higher temperature flame ionized the chromium and

reduced its sensitivity. To overcome these difficulties,

500 yg/ml of potassium was added to the standards and unknown solutions. The results are summarized in table 36.

Table 36. Determination of chromium in SRM 121d.

AAS Potentiometric Titration

Sampl.e Cr, %

A 17.67

17.50

17.54

17.54

B 17.62

17.49

17.56

17.58

Rel.

S

S

.D.

X

%

17.56

0.06

• 34

Cr, %

17.58

17.59

17.56

17.58

17.62

17.59

.02

.12

a _ Values from NBS and cooperative laboratories. The values

of 17.48 and 17.43 % from two laboratories were excluded.

H . Effect of Spectral Bandwidth in Atomic Absorption

Spectrometry - T. C. Rains

Any radiation in the monochromator bandwidth which

is not due to the analytical line has a detrimental

effect. By increasing the proportion of the undesired

radiation, the slope of the working curve and the sensitivity of the analyte are decreased. There is also an increase

in nonlinearity . A typical example of this type of interference is illustrated with iron. The spectrum over the

o wavelength region of 2450 to 2505 A, as shown previously

[1], is rich in iron lines. The most sensitive ground

state resonant line of iron commonly used in atomic absorption

o is located at 2483 A. However, it is very close to a nono resonant line at 2484 A having a lower energy level of

888 kaysers . Only by means of a high resolving mono102
chromator can these two lines be completely resolved. If

the extraneous radiation cannot be eliminated by selecting

a narrow slit width, the working curve will become more

nonlinear, as shown in figure 39. The wider slit width

Figure 39- Effect of spectral bandwidth on absorbance of

iron at 2483 A.

does contribute to a lower signal-to-noise ratio. In our

o work we have selected a 0.6 A bandpass.

I . Analytical Application to Standard Reference Materials - T. A. Rush and T. C. Rains

1 . Determination of Calcium, Magnesium, Silver and

Zinc in Special Steel

The determination of these trace elements in special

steel containing a low concentration of impurities is a challenge to the flame photometrist . The first major problem

is the dissolution of the sample without contamination. To

overcome this difficulty, a mixture of HN0„, HC1(X and HP

was used to dissolve the samples in Teflon beakers with

covers. Excess fluoride was removed by heating to fumes of

perchlorate

.

103
Calcium was determined by flame emission with optical

scanning. Since iron suppressed the radiant intensity of

calcium, the standard solutions were prepared to contain the

same concentration of iron as the unknown. The iron used in

the preparation of the standards was high-purity Mossbauer

iron. To increase the sensitivity of the determination of

calcium in a high iron matrix, the final working solutions

were diluted with acetone to provide a 40 percent solution.

A nitrous oxide and air ( 80 : 20) -acetylene flame was used

o to excite the calcium resonant line at 4227 A. The addition

of acetone to the aqueous solution produced extreme fuel

rich conditions which required an adjustment in the normal

acetylene flow rate. To prevent flame-flashback due to the

lower fuel flow, a 40 percent acetone solution was nebulized

between standards and unknowns.

Magnesium, silver and zinc were determined by atomic

absorption spectrometry using a double-beam system with an

air-acetylene flame. The standard solutions were prepared

to contain the same concentration of iron as the unknowns.

However, due to the high solid content in the standard and

unknown solutions, the deuterium arc was used to correct for

light scatter. Similar results were obtained by using an

o o appropriate non-absorbing line, such as Ga-2874 A, Ne-3369 A,

o Re-22l4 A and subtracting the absorbance due to light scatter.

The results are given in table 37-

2 . Determination of Calcium, Potassium, Magnesium , Sodium, Rubidium, Cadmium, Zinc and Manganese in

Orchard Leaves - T. C. Rains and T. A. Rush

It is of interest to recall that the application of

flame methods to agronomic problems played an important role

in the development of flame spectroscopy. In the 1920 T s and

o early 1930 ' s the Swedish agronomist, H. Lundegardh, developed

the technique of flame spectroscopy for the analysis of plant

material. In this early work, an air-acetylene flame and

photographic detectors were used. Today the flame photo104
Table 37

NBSSRM

Summary of SRM analyses by flame emission and

atomic absorption spectrometry.

Material

121D 18 Cr-10 Ni Steel

1171

123c

1172

1571 Orchard Leaves

1261 Special Steel

361 Special Steel

1262 Special Steel

No.

Ele- of Rel.

ment Detms

.

Cone . mg/g

S.D. S.D.

Cr 2 176.3 1.8 1.0

Cua

2 1.209 ' 0.001 0.1

Cra

4 174.9 1.6 .9

n a Cu 4 1.206 0.008 .6

n a Cr 6 175.5 .7 .4

/-1 a Cu 4 1.040 .003 .3

Cr 4 174.9 1.6 • 9

Cu 4 1.0 39 0.008 .8

Ca 24 20.95 .20 .95

n a Ca 6 20.77 .15 .74

K

b

19 14.73 .17 1.1

K

a

7 14.65 .09 0.66

Mg 20 6.21

yg/g

.07 1.2

Na 20 82 6 7

Rb 20 11.9 0.3 3.0

Cd 12 0.11 .01 9

Zn 11 25.2 1.4 5

Mn 11 91.9 2.0 2.2

Ca 2 <1

Mg 2 1.2 +0.5

Ag 3 4.0 + .5

Zn 3 1.0 + -5

Ca 2 <1

Mg 2 2.0 +0.5

Ag 2 4.0 + .5

Zn 2 1.0 t -5

Ca 2 2.0 +0.5

Mg 2 6.0 .5

Ag 2 10.0 .5

Zn 2 5.0 .5

105
Ca 2 2.0 +0.5

Mg 2 6.0 .5

Ag 2 10.0 .5

Table 37 Cont'd

362 Special Steel

Atomic absorption.

Flame emission.

metrist has at his command a variety of oxidant-fuels and

photodetectors for flame emission or atomic absorption.

In 1968, the Association of Official Analytical

Chemists (A.O.A.C.) was the first organization to adopt

atomic absorption spectrometry as an "official" method for

the determination of selected major and minor elements in

fertilizer and feeds. Since that date, other organizations

such as ASTM have recognized AAS methods as official. The

lack of certified standards is a major problem in the

analysis of agronomic materials. The preparation and

certification of Standard Reference Materials such as

orchard leaves and related materials is the first attempt to

fill this void.

Atomic absorption and flame emission methods have been

developed for the determination of major and trace elements

in orchard leaves. Both techniques were applied to the

determination of the major components such as calcium,

potassium and magnesium, while the technique with the highest

sensitivity was used in the determination of the trace

elements such as sodium, rubidium, cadmium, zinc and

manganese

.

a. Preparation of Samples . The samples are dried

in a vacuum oven at 90 °C for 24 hours and then cooled in a desiccator over anhydrous calcium sulfate. Depending upon

the concentration of the analyte, a 0.5 to 2g test portion

is transferred to a Teflon beaker, and dissolved in a mixture

of HNO^, HF and HClCv. After evaporation to strong fumes

106
of perchlorate, the solution is transferred to a volumetric

flask and diluted to calibrated volume.

b . Determination of Constituents . Calcium and

potassium are two of the major inorganic constituents in

orchard leaves. Calcium was determined by flame emission

with optical scanning using an oxygen-hydrogen flame and an

air-acetylene flame was used to obtain the absorption

measurements. Since high concentrations of phosphate

suppress the radiant intensity and absorption of calcium,

glycerol and perchloric acids [89] were used as releasing

agents. As shown in table 37, the values, 20.95 and 20.77

mg/g of calcium for the two techniques are in good agreement and indicate comparable precision.

As an alternate method, the nitrous oxide-acetylene

flame was investigated and the interference of phosphate was

eliminated; however, additional potassium was added to

standards and unknowns to suppress ionization. The average

value for calcium using the nitrous oxide-acetylene flame

compares with oxygen-hydrogen or air-acetylene, but the

precision was 1.5 to 2.0 percent.

In the determination of potassium, both the emission and

absorption techniques were used and show good agreement

between values (table 37). An improvement in precision

was obtained by atomic absorption, and no interferences were

observed by either method. Additional information will be

necessary before concluding that the determination of

potassium by atomic absorption is more precise than by flame

emission.

Magnesium, a minor constituent in orchard leaves, was

determined by atomic absorption using an air-acetylene flame

Under our experimental conditions, phosphate did not interfere, and no releasing agents were added to either standards

or unknowns.

107
Sodium, rubidium, manganese, cadmium, zinc and mercury

are present as trace constituents in orchard leaves. Sodium

and rubidium were determined by flame emission while

manganese, cadmium and zinc were determined by atomic

absorption. In the determination of rubidium, manganese

and zinc, standards were adjusted to contain an equal concentration of the major cations that were present in the

orchard leaves. This procedure compensates for some of

the minor chemical effects which suppress or enhance the

signal of the analyte

.

Since cadmium was present at less than 1 ug/g, it was

preconcentrated by extraction with ammonium pyrrolidinodithiocarbamate in a mixture of methyl isobutyl ketonecyclohexone (3:1). The cadmium was stripped from the

organic phase with IN HC1 prior to the nebulization and the

o absorption measurement was made at 2288 A.

The results are summarized in table 37.

3 . Determination of Mercury in Orchard Leaves -

T. C. Rains

An increasing awareness of environmental pollution and

the need for Standard Reference Materials have led to an

investigation of the flameless atomic absorption technique

for determining mercury. This technique is possible because mercury has a favorable vapor pressure.

As early as 1930 Muller [90] took advantage of the

high vapor pressure of mercury to determine it in air.

In 1939 Woodson [91] described an instrument used in the

determination of mercury in air. Since the publication of

Muller, many workers have used the same general approach,

but the final analysis was completed by colorimetric or

volumetric methods.

In atomic absorption studies using an air-acetylene

flame, Poluektov and Vitkun [92] observed an increase in

sensitivity when stannous chloride was added. They also

observed that the absorption of mercury could be measured

108
in the vapor phase without the use of a flame. Hatch and

Ott [93] applied the same technique to a closed system in

which the mercury vapor was continuously recirculated until

a maximum signal was attained. The system was then opened

to exhaust the mercury vapor. The technique developed by

Hatch and Ott was first investigated but the precision and

accuracy required for Standard Reference Materials could not

be attained. This led to the investigation of various

parameters which included the dissolution of sample, reduction of mercury to the atomic state, trapping of volatile

vapors and the absorption measurement. A schematic diagram

of our system is shown in figure 40

M0N0CHR0MAT0RS

0.25 m

DIGITAL READOUT RECORDER

LOGARITHMIC

DIGITAL VOLTMETER

ARGON

REDUCTION CELL

Figure 40. Schematic diagram of flameless atomic absorption

apparatus for mercury determination.

109
a. Dissolution of Sample . Since mercury is easily

reduced and subsequently lost in the vapor phase during the

destruction of organic material, several methods of

dissolution were tested. Combustion by the Schoenger

method was examined; however, this technique is limited to

the amount of solids or organic material which can be

ignited. The sealed tube method [94] employing nitric and

sulfuric acids was used to dissolve organic material

(orchard leaves); however, two difficulties arose. The

sample size was limited to 0.2g and the oxides of nitrogen

which were liberated during the dissolution caused excessive

frothing in the subsequent reduction step. The oxides of

nitrogen were removed after transferring the solution from

the sealed tube by gentle heating, but the possible loss of

mercury existed.

The most successful dissolution procedure is a modification of the one described by Munns and Holland [95] . The

sample (1 to 5g of orchard leaves) is transferred to a

250-ml flat bottom digestion flask equipped with a 24/40

standard taper joint. After adding 25 ml of 1:1 sulfuric

acid and 20 ml of 1:1 nitric acid, an Allihn condenser with

circular water was attached to the flask. (The condenser

was filled to a height of 50 mm with Raschig rings and

glass beads.) The sample was gently heated at 155 °C for

15 minutes using a heating mantle. The temperature was

then raised to 190 °C and maintained for 45 minutes. After

the solution was cooled for 10 min, a mixture of nitric

and perchloric acids was added to the condenser, followed

by a rinse with water. The water to the condenser was

removed and the sample was heated at a temperature of

200-220 °C, with a heating mantle, until dense fumes

of perchlorate appeared, and then heated for an additional

10 minutes. The sample was cooled and transferred to a clean

100 -ml volumetric flask. The heating mantle provides more

uniform heating and reduces frothing during dissolution. This

110
203 dissolution procedure was checked with Hg and the recovery

of the tracer was complete within the counting statistics of

the counter, i.e. +1 percent:. Without the addition of

Raschig rings and beads to the Allihn condenser, the loss of

203Hg was 5-10 percent. If the dissolution was made in an

Erlenmeyer flask equipped with a tightly-fitted cover, the

203 loss of Hg was 10-15 percent. Using a beaker, the loss

203 of Hg was 60-70 percent. In one test, a solution dissolved under reflux conditions, was transferred to a beaker

203 and evaporated to a 2-ml volume. The loss of the Hg

tracer was 25 percent.

b. Reduction of Mercury . Two methods for reducing

mercury to atomic vapor were investigated. In one, the

mercury was reduced to the free atomic state with a solution

of hydroxylamine hydrochloride and stannous chloride. The

atomic vapor was flushed from the reduction cell with argon

into a heated 30-cm absorption cell.

Hypophosphorous acid was also used to reduce mercury

and it was about as effective as stannous chloride for

solutions of mercury in dilute nitric acid. In a solution

of orchard leaves, excess foaming occurred and was swept

into the vapor trap and into the absorption cell. This

excessive foaming resulted in the discontinuance of the work

with H

3

P0 . In some tests foaming also occurred with stannous

chloride. To overcome this difficulty, the system was

purged with argon prior to the addition of the reducing

solution. Under these conditions, 25 to 75 percent of the

mercury was lost. To overcome the problem of foaming, it

was determined that all organic material must be removed

from the sample solution. To aid in the removal of organic

material after the sample was fumed with HCIOn, the reflux

condenser was rinsed with water and the sample was reheated

to strong fumes of perchlorate

.

Ill
c. Trapping of Volatile Vapors . Several methods

are used to trap volatile vapors which contribute to

light scattering or molecular absorption. A cold trap

in a mixture of salt and ice (-10 °C) or drying agents,

such as magnesium perchlorate or anhydrous calcium sulfate

are often used. Each method was tested and found satisfactory for short operating periods only. The procedure

then is to remove the drying agent, clean and dry the trap

and recharge the drying agent. The vapors were passed intc

a clean, dry glass chamber where the large droplets would

collect on the walls of the container. The fine particles

and mercury vapors are then swept into a heated absorption

cell where the absorption measurement is made.

d. Absorption Cells . A number of absorption cells

having the following sizes were investigated: 1 x 10-,

0.8 x 20- and 0.8 x 30-cm. As expected, the longer cellpath gave an improvement in absorbance of 1:2:2.5, respectively. With our optical arrangement, the 30-cm cell was

selected. To prevent slight memory effects, molecular

absorption and fogging of end windows, the cell was heated

to 170 °C with heating tape.

e. Contamination Problems . The first major

problem was contamination of glassware and reagents

with mercury. All glassware was rinsed with 10 percent

nitric acid. Polypropylene flasks were best suited for

storage, over a period of several days, of sample

solutions. When samples were stored in glass volumetric

flasks, mercury values increased by 50-100 percent over a

one-week period. To illustrate the degree of contamination

of reagents by mercury, three bottles of hydroxylamine

hydrochloride, all from the same lot, were analyzed and

contained 0.0008, 0.027 and 0.031 Pg Hg/g. In another lot,

the mercury was present at <0.001 yg/g. Ultra-high purity

112
acids were also tested, since some lots of nitric acid

contained 0.03 yg of Hg/ml

.

Samples of orchard leaves, dried in a vacuum oven at

90 °C for 24 hours and then cooled in a desiccator over anhydrous calcium sulfate, showed mercury values which were

25 to 50 percent higher than the undried samples. The

source of this contamination is under investigation.

f . Measurement . Absorption measurements were made

with a single-beam and a double-beam (ratio method [1])

system. A schematic diagram of ratio system is shown in

figure 40. With this ratio method, a non-absorbing aluminum

o line at 2652 A is compared with the absorption of mercury at

o 2537 A and a ratio is taken. The values for mercury in

orchard leaves, obtained with the single beam and ratio

method, are given in table 38.

Table 38. Determination of mercury in orchard leaves by

flameless atomic absorption.

Hg, yg/g

Sample Single-Beam Syst em Ratio System

Number 1 0.155

.153

.155

.155

.167

.164

0.159

.153

.160

.157

.165

.170 .172

X 0.160

S -D. .006

Rel. S.D. , % 4.0

0.160

.006

4.0

4. Service Analysis - T. A. Rush and T. C. Rain

Atomic absorption and flame emission spectrometry were

applied to a wide variety of samples submitted by Divisions

of the Bureau as well as other Government agencies. The

elements determined and the types of materials analyzed

include: lead in paint; chromium and zinc in solutions

used in paper products; mercury in lithium bromide coolant;

113
sodium, potassium, magnesium and strontium in reagent grade

calcium carbonate; nickel and silver in reagent grade

sodium hydroxide; sodium in organic dyes; Cd, Ni, Ca, Cu, Mg.

Zn, Pb , Mn and Hg in effluent from river sediments; Al, B,

Ba, Cr, Cu, Fe , Mn, Mo, Pb , Sr and Zn in effluent from

sewage treatment facilities.

Lead was determined by atomic absorption spectrometry

in a variety of paints submitted by the National Commission

on Product Safety. The range of lead concentrations was

from 0.12 to 9. 5 percent. Samples of the dried paint were

wet oxidized with HNO Q and HClOi, and lead was determined

3 o *» using the lead line at 2833 A. Over the concentration range

investigated, no interferences were encountered and the

precision of the method was one percent.

Mercury was determined in lithium bromide coolant for

the National Institutes of Health. The flameless technique

described earlier was used; however, bromide interfered.

This interference was removed by repeated evaporations

with nitric acid. As a check on mercury loses, 50 and 100 ng

of mercury were added to test portions of the sample and

then determined. The recovery was complete in each test.

A study was made of the extract of trace elements

with 20 ug/ml of NTA from river sediments. In natural water

effluents, few interferences are encountered by atomic

absorption spectrometry. Phosphate and sulfate are known

to interfere in the determination of calcium and magnesium

using an air-acetylene flame; however, under the test

conditions, these anions did not extract or they were so

low in concentration that no interference was observed.

The analytical conditions and range of concentrations of

cations under various extraction conditions are summarized

in table 39-

114
Table 39. Analytical conditions and concentration ranges

for trace elements in water effluents with and

without NTA.

Range of Scale

Ele- Wavelength Calibration Curve Expansion Concentration

ment A ug/ml Factor Range, ug/ml

Ca 4227 0.1 - 1.0 1.0 0.02 -200

Cd 2288 .1 - 1.0 2.5 .02 - 0.04

Cu 3247 .1 - 1.0 7.0 .03 - .03

Hg 2537 .001- 0.005 2.5 < .0005- .016

Mg 2852 2.0 -10.0 - .6 - 60

Mn 2795 0.1 - 1.0 5.0 < .005 ->50

Ni 2320 .1 - 1.0 10.0 < .01 - 0.2

Pb 2833 .2 - 1.0 5.0 < .05 - .3

Zn 2138 .1 - 1.0 6.0 < .001 - .5

115
4. THERMAL ANALYSIS

A . Introduction

Differential Thermal Analysis (DTA) as a physical,

analytical method for characterizing materials is gaining

wider interest. Both from the theoretical standpoint and

quality control the effects of phase changes with temperature

on the physical and chemical properties of materials provide

useful information. These investigations extend over a broad

spectrum of science and technology and include biochemical

or health systems such as the studies of lipid systems in

reference to artherosclerosis ; or space technology involving

high temperature refractories; or properties of polymers in

controlling the quality of new material and the manufactured

product. In a current symposium sponsored by this Section

at NBS a few of these areas of applications have been

described and were published [96].

Further evidence of the growth of this field is indicated

by the activities of national and international societies.

The Third International Conference on Thermal Analysis is to

be held at Davos, Switzerland in 1971, and the Third Annual

Meeting of the North American Thermal Analysis Society will

be held on February 7 and 8, 1972 in Waco, Texas. Finally,

one can note from the records of the current Analytical

Chemistry Laboratory Guide that over thirty-three commercial

instruments are available indicating a continual Increase

in number over the past years.

To the present, the Section's efforts have been limited to

providing temperature scale calibration SRM's in the

temperature region of 125 °C to 9^0 °C. This work is

summarized in the following section.

There is at present a further need to extend the

temperature range to subambient and to higher temperature

(^ -150 °C to 2500 °C) to cover the diverse areas of interest.

116
B. Certification of SRM's for Calibrating the Temperature

Scale of DTA Apparatus - D. S. Bright and 0. Menis

In conjunction with the International Confederation on

Thermal Analysis (ICTA) a series of materials was certified

for temperature scale calibrations of Differential Thermal

Analysis (DTA) apparatus. This represents the culmination

of efforts described more fully last year [1] . These

materials consisted of eight inorganic substances, exhibiting

solid IHSolid II transition and two metals, and covered

the temperature range from 125 °C to 940 °C.

As shown in table 40 and figure 4l, the mean temperature

Table 40. Mean values of onset heat temperatures of

ICTA substances.

Temperature Value °C

Substance Literature a ICTA

Mean

Std. Dev.

KNO 127.7 127.6 4.9

KCIO^ 299-5 299.4 6.4

Ag

2

S0

4 - 424.1 7.5

Si0

2 573 570.7 5.0

K

2

S0

4 583 581.6 7.1

K

2

CrO i| 665 664.4 7.3

BaCO 810 807-7 8.4

SrCO 925 928.4 7.6

In 157 154.6 5.4

Sn 232 230.5 5.0

aNBS Circular 500, "Selected Values of Chemical Thermodynamic

Properties," 1952.

values are based on data obtained by 34 cooperating laboratories in 13 countries employing 28 different commercial

or custom-made instruments. From these data, the mean

values of onset heating temperatures by DTA are only

one to two degrees higher than the literature equilibrium

values. The standard deviation, independent of the

1-1?
30 "

•

20 -

•

•

• • •

•

• 10-1

2 3 •

•

3

2 • 2 •

• • 5 5 3 ^- MEAN

4

8

5

8

8

8

2X

7

5

6

6

3

2

8

8 UJ ft 5 5 ? 4 9 4 7 ; ac 4 2 • 8 • 3 2 • LU 3 4 3 2 3 4 • ea

2

2•

• 3 •

3

2

2

•

2 -10- •

• •

2

2

•

•

2• •

•

2 •

2

-20- •

•

•

•

•

•

•

-30

KCIOj Ag

2 S04 SiOo K

2 S04 K2Cr04 MATERIALS

BaCO* SrC03 In Sn

Figure 4l

30-i

20-

10-

«2 MEAN £ oC3

-10-

-20-

-30

Data from 34 cooperating investigators for each

of the 10 substances (submitted to the

Standards Committee of ICTA)

. 2 4

2 2 4

2 • •

• •

2 2 . 2 w~m-

• 4 • • 2

2 . 3

4 2 -4-r24 •

2 2 •

6 2 2.32.

2 (2.64

-w- 3 . • . m

4

• 4 • 2 2 4 2 • 2.55 2

2 2 • • 3 4 • -3-3-» 2-, > 1 5-

2 •

• 3

3 2 •

• • •

• 2

2

• 2

• 2

2 .

2 4 5 6 8 13 15 16 IT 18 21 22 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 40 41 42 43 44

47'C INVESTIGATOR Figure 42. Deviations from the mean value as reported by

the 34 investigators (submitted to the Standards Committee of ICTA).

substance measured was 5 to 8 °C. This reflects the

bias among laboratories as well as the measurement errors.

As shown in figure 42, some of the investigators had

118
more pronounced differences, however, their values were

included. The complete discussion of these data will be

published in the Proceedings of the Third International

Conference on Thermal Analysis, Davos, Switzerland 1971,

and in a NBS Special Publication - 260 series. The SRM ' s 758, 759 and 760 were issued to cover three temperature

ranges and the certificates are reproduced in the

following pages.

Finally, as chairman of the Standard Section of the

ASTM Provisional Committee on Thermal Analysis, in cooperation

with Dr. P. Garn of the University of Akron, an ASTM

procedure, "Standard Method of Calibration of Temperature

Scale for Differential Thermal Analysis" was written and

submitted for membership approval.

(a)

C. Scanning Calorimetry - D. S. Bright, E. Prosen and

0. Menis

1. Introduction

In addition to the Standard Reference Materials already

available for use in checking the performance of DTA equipment

[1], there is a need for establishing accurate enthalpy

values for these transition temperatures. While calorimetric

data for many of these compounds are available in the literature, there is a continuing demand to verify these values

or to obtain data on new materials. The classical calorimetric methods, while capable of providing very accurate

values are time-consuming and require complex measuring

techniques. Ozawa described a relatively simple scanning

calorimeter which could be calibrated independently by

electrical means and for which he claimed an accuracy of 1 to

2 percent [9 7].

A brief exploratory experiment was undertaken to

evaluate Ozawa' s design and procedure. A rudimentary

(a)

E. Prosen, Physical Chemistry Divition, IMR.

119
U. S. Department;

1 of Commerce

MauriceiB. Stans

National BureSfl ofi Standards

L. M. Bran^rnb; Director Certificate

NBS-ICTA Standard Reference Material 758

DTA Temperature Standards (125 - 435 °C)

These standards include: high-purity potassium nitrate, indium, tin, commercial grade potassium

perchlorate, and analyzed reagent silver sulfate. They are for use in calibrating the temperature scale on differential thermal analysis and related thermoanalytical equipment under the operating conditions. They are to be used only in the heating mode.

Transition Temperature Data (°C)

DTA Mean Values

Equilibrium Extrapolated

Material Value [1] Onset Peak

KN03 127.7 128 135

In (metal) 157 154 159

Sn (metal) 231.9 230 237

KC104 299.5 299 309

Ag2 S04 -< a > 424 433

l a 'The value for the transition temperature reported for this material [1] is currently under review. A more recent value of 430 °C

lias been reported |2).

For potassium nitrate only, the first heating of the material is not a reliable measure of the

transition temperature. This sample must be cycled through the transition temperature in situ before recording data.

The extrapolated onset temperature is defined (Point A, Figure 1) as that temperature found by

extrapolating the base line, prior to the peak, and the leading side of the peak to their intersection. The peak temperature (Point B, Figure 1) is defined as the temperature corresponding to the point

of maximum deflection of the differential temperature curve.

Based on the mean value from 34 cooperating laboratories, the standard deviation was 5-8 °C,

which includes the bias among laboratories as well as measurement errors. In the heating mode, onset and peak temperatures have about equal precision. However, the mean extrapolated onset temperatures arc closer to the equilibrium transition temperatures.

The materials for these standards were furnished by the Committee on Standardization of the 1CTA.

The International Test Program leading to the data on which certification is based was coordi- nated by II. G. MeAdic, Chairman, ICTA Committee on Standardization.

Coordination of efforts leading to certification by NBS was under the chairmanship of O. Menis.

The technical and support aspects involved in the preparation, certification, and issuance of

these Standard Reference Materials were coordinated through the Office of Standard Reference

Materials!))' T. W. Mcars.

Washington, D. C. 20234 J. Paul Cali, Chief June 1 1, 1971 Office of Standard Reference Materials 120
The transition temperatures given are the means of values obtained on 28 different types of commercial or custom-made instruments, and do not necessarily agree with the equilibrium

values [1, 2]. Generally, values from DTA are somewhat higher than the equilibrium values and

vary in a complex manner for different instruments and heating rates. Details of the instruments

used, the test conditions and complete data will be given in an NBS Special Publication in the 260

series (in publication).

AT

EXTRAPOLATED ONSET

TEMPERATURE

-\

b/

PEAK \TEMPERATURE

[1 ] Nat. Bur. Stand. (U.S.), Circ. 500 (1952).

|2] Hedvall, J. A., Lindner, R.,and Hartler, N., Acta. Chcm. Scand. 4, 1099(1950).

121
S. Department' of Commerce

MauriceiJH. Stans

ffec/^ai*

National BureaQ of Standards

L. M. Bransoomb, Director Certificate

NBS-ICTA Standard Reference Material 759

DTA Temperature Standards (295 - 675 °C)

These standards include: analyzed reagent silver sulfate, potassium sulfate, potassium chromate;

commercial grade potassium perchlorate; and natural quartz. They are for use in calibrating the temperature scale on differential thermal analysis and related thermoanalytical equipment under

operating conditions. They are to be used only in the heating mode.

Transition Temperature Data (°C)

DTA Mean Values

Equilibrium Extrapolated

Material Value [1]

299.5

Onset Peak

KC104 299 309

Ag2 S04 .(a) 424 433

Si0 2 573 571 574

K2 S04 583 582 588

K2 Cr04 665 665 673

' a 'The value tor the transition temperature reported for this material [1] is currently under review. A more recent value of 430 °C

has been reported [2].

The extrapolated onset temperature is defined (Point A, Figure 1) as that temperature found by

extrapolating the base line, prior to the peak, and the leading side of the peak to their intersection. The peak temperature (Point B, Figure 1) is defined as the temperature corresponding to the point of maximum deflection of the differential temperature curve.

Based on the mean value from 34 cooperating laboratories, the standard deviation was 5-8 °C,

which includes the bias among laboratories as well as measurement errors. In the heating mode,

onset and peak temperatures have about equal precision. However, the mean extrapolated onset temperatures are closer to the equilibrium transition temperatures.

The materials for these standards were furnished by the Committee on Standardization of the ICTA.

The International Test Program leading to the data on which certification is based was coordi- nated by H. G. McAdie, Chairman, ICTA Committee on Standardization.

Coordination of efforts leading to certification by NBS was under the chairmanship of 0. Menis.

The technical and support aspects involved in the preparation, certification, and issuance of

these Standard Reference Materials were coordinated through the Office of Standard Reference

Materials by T. W. Mears.

Washington, D. C. 20234 J. Paul Cali, Chief June 11, 1971 Office of Standard Reference Materials 122
The transition temperatures given are the means of values obtained on 28 different types of commercial or custom-made instruments, and do not necessarily agree with the equilibrium

values [1, 2]. Generally, values from DTA are somewhat higher than the equilibrium values and

vary in a complex manner for different instruments and heating rate~. Details of the instruments

used, the test conditions and complete data will be given in an NBS Special Publication in the 260

series (in publication).

AT

EXTRAPOLATED ONSET

TEMPERATURE \

\

B/

PEAK \TEMPERATURE

[l] Nat. Bur. Stand. (U.S.), Circ. 500 (1952).

[2] Hedvall, J. A., Lindner, R., and Hauler, N., Acta. Chem. Scand. 4, 1099 (1950).

123
U. S. Department' of Commerce

MauriceSJS. Stans

gecreter*

National Bureau of Standards

L. M. Brauscomb, Director

(dttixixtnit

NBS-ICTA Standard Reference Material 760

DTA Temperature Standards (570 - 940 °C)

These standards include: natural quartz; analyzed reagent potassium sulfate, potassium chro- mate, barium carbonate, and strontium carbonate. They are for use in calibrating the temperature

scale on differential thermal analysis and related thermoanalytical equipment under operating conditions. They are to be used only in the heating mode.

Transition Temperature Data (°C)

DTA Mean Values

Equilibrium Extrapolated

Material Value [1]

573

Onset Peak

Si02 571 574

K2 S04 583 582 588

K2 Cr04 665 665 673

BaC03 810 808 819

SrC0 3 925 928 938

The extrapolated onset temperature is defined (Point A, Figure 1) as that temperature found by

extrapolating the base line, prior to the peak, and the leading side of the peak to their intersection. The peak temperature (Point B, Figure 1) is defined as the temperature corresponding to the point

of maximum deflection of the differential temperature curve.

Based on the mean value from 34 cooperating laboratories, the standard deviation was 5-8 °C,

which includes the bias among laboratories as well as measurement errors. In the heating mode,

onset and peak temperatures have about equal precision. However, the mean extrapolated onset temperatures are closer to the equilibrium transition temperatures.

The materials for these standards were furnished by the Committee on Standardization of the ICTA.

The International Test Program leading to the data on which certification is based was coordi- nated by H. G. McAdie, Chairman, ICTA Committee on Standardization.

Coordination of efforts leading to certification by NBS was under the chairmanship of O. Menis.

The technical and support aspects involved in the preparation, certification, and issuance of

these Standard Reference Materials were coordinated through the Office of Standard Reference

Materials by T. W. Mears.

Washington, D. C. 20234

June 11, 1971

J. Paul Cali, Chief

Office of Standard Reference Materials

1?4
The transition temperatures given are the means of values obtained on 28 different types of commercial or custom-made instruments, and do not necessarily agree with the equilibrium

values [1] . Generally, values from DTA are somewhat higher than the equilibrium values and vary

in a complex manner for different instruments and heating rates. Details of the instruments used, the test conditions and complete data will be given in an NBS Special Publication in the 260 series (in publication).

EXTRAPOLATED ONSET

TEMPERATURE

v

\

AT

b/

PEAK \TEMPERATURE

[1] Nat. Bur. Stand. (U.S.), Circ. 500 (1952).

125
Cooperating Laboratories

Dr. David E. Baum, Monsanto Australia Limited, West Footscray, Victoria, Australia

Prof. Marcel Bourgon, Universite de Montreal, Montreal, Quebec, Canada

Dr. H. G. McAdie, Ontario Research Foundation, Sheridan Park, Ontario, Canada

Dr. Klaus Heide, Friedrich-Schiller-Universitat, Jena, Germany

Dr. J. Wiegmann, Berlin, Germany

Dr. G. R. Cizeron, Universite de Paris, Paris, France

Dr. M. Harmelin, C.N.R.S., Paris, France

Mme. J. Jegoudez, Universite de Paris, Paris, France

Dr. M. Foldvari, Hungarian Geological Institute, Budapest, Hungary

Mr. J. Paulik, Technical University of Budapest, Budapest, Hungary

Prof. H. Kambe, University of Tokyo, Tokyo, Japan

Prof. C. Kato, Waseda University, Tokyo, Japan

Prof. K. Oinuma, Tokyo University, Tokyo, Japan

Prof. R. Otsuka, Waseda University, Tokyo, Japan

Mr. J. E. Kruger, National Building Research Institute, Pretoria, South Africa

Dr. R. S. Forsyth, Aktiebolaget Atomenergi, Studsvik-Nykoping, Sweden

Dr. G. Krien, Institut fur Chemisch-Techn. Untersuchungen, Leverkusen, Germany

Prof. H. R. Oswald, Universitat Zurich, Zurich, Switzerland

Dr. T. Peters, Universitat Berne, Berne, Switzerland

Dr. H. G. Wiedemann, Mettler Instrumente A. G., Greifensee, Switzerland

Prof. L. G. Berg, Gosuniversitet, Kazan, U.S.S.R.

Prof. E. K. Koehler, Academy of Sciences of the USSR, Novosibirsk, USSR

Dr. I. S. Rassonskaya, Institut Obshchei i Neorganicheskoi Khimii, Moscow, USSR

Dr. G. 0. Piloyan, Academy of Sciences of the USSR, Moscow, USSR

Mr. E. L. Charsley, Stanton Redcroft Limited, London, England

Dr. R. C. Mackenzie, The Macaulay Institute for Soil Research, Aberdeen, Scotland

126
Prof. D. A. Smith and Mr. J. Youren, Queen Mary College, University of London, London, England

Dr. P. K. Gallagher, Bell Telephone Laboratories, Murray Hill, N.J., U.S.A.

Prof. P. D. Garn, University of Akron, Akron, Ohio.U.S.A.

Dr. 0. Menis, D. S. Bright, and J. T. Sterling, National Bureau of Standards, Washington, D.C.,

U.S A.

Mr. T. D. Oulton, Englehard Minerals & Chemicals Corp., Edison, N.J..U.S.A.

Dr. R. K. Ware, Owens-Illinois, Toledo, Ohio,U.S.A.

International Confederation for Thermal Analysis Committee on Standardization:

Chairman (Canada) H. G. McAdie

Vice-Chairman (U.S.A.) P. D. Garn

Secretary (U.K.) R. C. Mackenzie

National Delegates (France) C. Mazieres

(German) K. Heide

(Hungary) F. Paulik

(Japan) H. Kambe

(Sweden) R. S. Forsyth

(Switzerland) H. G. Wiedemann

(U.S.S.R.) I. S. Rassonskaya

(U.K.) D.A.Smith

(U.S.A.) R. W. Pfeil

Representative ASTM (U.S.A.) Vacant

Delegates ISO Vacant

NBS (U.S.A.) 0. Menis

Soc.Anal.Chem. (U.K.) C.J. Keattch

SAMA (U.S.A.) Vacant

Ex-Officio ICTA (U.S.S.R.) L. G. Berg

Delegates (U.S.A.) C. B. Murphy

(U.K.) R. C. Mackenzie

(U.K.) J. P. Redfern

127
scanning calorimeter similar to Ozawa's was assembled.

Several compounds, whose enthalpy values ranged from room

temperature to 300 °C, were measured to determine the

characteristics of the instrument. Essentially, these

materials were used to measure the proportionality

coefficient, k, of the instrument. At the same time,

independent proportionality coefficients were measured by

an electric heat source. The measured values of the

coefficient, k, showed a relative spread of 15 percent which

indicates that this cell design is suitable for calorimetric

enthalpy transition measurements to within only about

10 percent

.

2 . Principle and Measurements

Calorimetric measurements are based on the determinations

of heat flow to a sample cell across a thermal barrier. If

the temperature gradient across the barrier is linear, then

the temperature differential is proportional to the heat

flowing through the barrier.

A system used in such measurements is described in

figures 43 and 44. The sample and a reference material

are placed in a sample holder which, in turn, is contained

in a ceramic thermal barrier. They are placed in a copper

block which is heated at a slow and uniform rate.

The temperature difference between A and B (A-B)

,

measured with "small" thermocouples, will be proportional to

the rate of heat flowing into the sample, both to heat the

sample and thermocouple and to supply heat for any transitions

Also, (C-D) is proportional to the rate of heat flowing into

the reference material and will be constant, if the heat

capacity of the reference material remains constant. If the

reference material has the same heat capacity as the sample

material, then (C-D)-(A-B) will be zero between phase

transitions, and will be proportional to the rate of heat

flowing into the sample due to the transition only. In an

128
VA/IRFQ !•

.

' • * •• •'•!• '

.

' •»*•• ••'•

'.

' •»

' .

' ..

'.'."..' .' ...*

.' .•••* ' .

'.••". •

'.'.••'• •••'. '.

'.•'.•.' '.'•••".'.'.' ••'

.

' .

'

• •.'••' • • '•?•• "lillil

7.5 cm v£;>.V:

ARGON

^®}®#£GLASS WOOL ^SVVV-I

IN i ^"-* '*••*»*'-*•-'**•"-'«•*•-*-•**-*' *-***' •**- CAN

12.5 cm

ARGON

OUT

(NOT DRAWN TO SCALE)

Figure 43- Cross section of calorimeter chamber

129
75 cm

/

5 cm

COPPER PLUG (Sliding fit)

NICKEL (15 mm long

Sample 6mmdiam)

HOLDER

CERAMIC

INSULATOR

(1.5mm thick)

COPPER

THERMOCOUPLE

HOLDER

(Imm thick walls)

\

AT THERMOCOUPLES

FURNACE PROGRAMER IMOCOUPLE

THERMOCOUPLE

Figure 44. Cross section of copper block.

130
ideal case, if the copper block is at a uniform temperature,

(C-D)-(A-B) = B-D, then only temperatures at B and D need

to be monitored. If the reference material differs in heat

capacity from the sample, then (C-D)-(A-B) will not be zero,

but a constant value called the baseline, and the area of a

peak above the baseline is still proportional to the heat of

transition. Since this is also true for A-B, the reference

cell is not needed, but with a reference cell, there would

be a smaller baseline signal than just (A-B), making the

baseline less susceptible to changes in the heating rate.

3 . Apparatus

a. Sample Holder . Ozawa [98] has recently used

cylindrical sample holders containing 10 thermocouple

junctions on each side of the thermal barrier. The apparatus

used in this work Is similar to Ozawa's earlier design

(figure 44) consisting of only one thermocouple on the

inside of each thermal barrier.

The copper block and heater were enclosed in a

container which was insulated by air, loose thermofax and

glass wool (figure 43). All wires leading from the container

were enclosed in a rubber hose which could be pinched off

by a clamp to provide seals suitable for maintaining an

argon atmosphere.

This apparatus differed from Ozawa's [97], [99] in the

following five ways:

1. Instead of splitting the block to provide

access to the sample chamber, small plugs were provided

in the block. The one-piece block promotes more homogeneous

block temperature.

2. For increased sensitivity, the inside AT

thermocouples were inserted into the copper cup wall rather

than touching the side.

131
3. To more closely approach the ideal case, the

thermal barrier was split at the top, rather than near

the middle, making thermal gradients in the barrier more

constant near the sample itself. In our experiments, the

nickel sample cup was partially filled and fitted smoothly

into the copper cup.

4. The temperature of the copper block was

measured as near as possible to each thermocouple in the

copper cup (figure 44) to help compensate for nonuniformities in block temperature. Ozawa's later design

also had this feature [98].

5. The copper thermocouple holder, corresponding

to Ozawa's cell [99] did not contain a screw-on cap. The

cap was part of the plug and merely rested on the holder.

Subsequent experiments indicate that this thermal contact

was not sufficient to maintain the required uniform temperature around the sample.

b . Copper Block and Housing for Heater and Thermocouples . For convenience, the heater consisted of heavy

chrome 1-alumel glass insulated thermocouple wire which was

attached directly to the copper block with Colco furnace

cement. The heater voltage was supplied by a conventional

Stone Model B furnace programmer.

The AT signal was transmitted by fine-wire glass

insulated thermocouples (see figure 44 for connections) and

amplified by a DC amplifier.

The temperature of the copper block was measured with

chromel-alumel thermocouples referenced to an ice bath, and

recorded with one channel of a Honeywell Electronik 19 dual

pen recorder. The recorder scale was calibrated with a

Leeds and Northrup K-3 potentiometer.

4 . Electrical Calibration

Electrical calibration was made with an Ohmite Series 99

3000 ohm resistor heater. Any heat generated by the lead

132
wires was less than 0.5 percent of the total calibration

heat

.

Voltage was monitored with a 3-digit voltmeter and was

supplied by a Fluke Model 406 power supply. Current was

monitored by measuring the voltage across a Leeds and

Northrup 4025-B precision 10 ohm resistor in series with the

heater with an NLS U34A digital voltmeter (4 digits). Time

was monitored with a Hewlett Packard 5216 Electronic Counter

referenced to the NBS 1 kHz frequency standard and switched

with auxiliary contacts of the heater switch. The timing

was estimated to be good to + 0.001 sec.

The resistor was inserted into the reference copper

thermocouple holder and connected to the power source by

No. 40 glass insulated wire which extended through grooves

cut along the copper plug to the copper wire. All wire

connections were made with silver solder.

5 . Observations

All peak areas were normalized to the same amplifier

gain. Typically, peaks were 5-8 in. broad and 5-8 in. high

where 1 in. = 1 min, time scale, and 1 in. = 1.10 uV (0.02'

to 0.2 °C for copper-constantan thermocouples). This

varies by 50 percent over the range 100-300 °C.

When calibrated, heat is equal to the normalized area

of the curve times the proportionality coefficient, k,

where k is dependent on the geometry and thermal conductivity

of the thermal barrier. In this work, the results,

in table 41 are given in terms of k, because the purpose

was to determine the calorimetric characteristics of this

scanning calorimeter cell design and not to determine heats

of transition by means of electrical calibration.

The proportionality coefficient, k, was also affected

by the heating rate, the type gas surrounding the copper

block, and the time of heat during calibration.

133

>
Table 4l. Calibration data - determination of proportionality

coefficient, k, for transition (melting) on

heating

.

Temo

.

Weight Value Normalized

n Material °C g cal/g

47.5

Area k

a

Stearic acid 71 0.0753 0.8252 4.33

Benzoic acid 125 .1183 35 1.081 3.83

^ ^ • • jl. ^ d ,e Potassium nitrate

1st heating

subsequent heating

127

127

.2824

.2824

12.85

12.85

0.9496

.4941

3.82

(7.34)

Indium 157 .0909

1.281

6.79

6.79

.1654

1.758

3.73

4.16

c Tin 235 0.5043 14.23 1.917 3.74

Potassium perchlorate 303 .3561 23.7 2.234 3.77

Ohmite resistor 20

280

- - - 3.8

4.1

f Potassium dichromate - - -

Literature value of enthalpy/sample weight

k (area x amp. gain/sample weight)

electrical heat

area x amp. gam

The AT curves, as in figures 47 and 48, were obtained at a

chart speed of 1 in./min, to make a broad peak for more

accurate measurement of the area. Areas were measured

twice with a Dietzgen Oir-planimeter , usually duplicating

areas to better than 0.5 percent. The baseline is extremely

important for accurate measurement, as demonstrated by the

lower left point of figure 45. The 2 percent correction up

to the line corresponded in raising one end of the baseline

(figure 46) about .05 inch so that it falls on the line in

figure 45. Very small errors in the baseline change the

area by a few percent

.

Some curves such as the tin melt (figure 1 47) have the

straight left edge, expected for a pure substance, plus

the expected rapid fall-off. Other curves such as the

benzoic acid melt (figure 48) had a leading tail, plus

a levelling off, which may be due to poor thermal conductivl3i|
ity within the sample. The benzoic acid was NBS Standard

350, and therefore pure, but it may have reacted with the

nickel sample cup.

For KN0„, the k of 7-34 + 3 percent was obtained for

heating Jrates of 1.2°/min to 4.5°/min. The first heating

value of k was characteristically smaller than the subsequent

heating values. The standard heat of transition is, therefore, for a non-preheated sample. For the subsequent

heating values, the sample is cooled through only one

apparent transition, and immediately heated. The heat of

transition is about 6.7 cal/g. Although a run was omitted

because both sample weight and the thermal conductivity of

the thermal barrier was uncertain, it did show that when the

sample was cooled 25 °C below the first cooling transition,

taking about one-half hour, a very slow transition takes

place. The sample was heated before the transition was

complete, and the 127 °C transition on heating was about as

large as on initial heating. On the subsequent heating

this 127 °C transition was half as large as noted above.

This is in agreement with the early observations by Kracek,

who reported three transition phases [99].

Q Indium and KN0~ were measured with and without_ argon. Two

Indium measurements were made in air and two in argon.

There was a relative 5 percent spread within the pair,

16 percent spread between pairs, which may be a result of

the design in the apparatus . It might explain the apparent

spread of 15 percent in k for the 6 samples,

fCompound determined but not reported. The transition rates

were four times as slow as for other substances and covered

four times the temperature range. The average temperature

of heating and cooling was within 2 percent of the 242 °C

of the literature value, but the relative standard deviation

of the area was 15 percent or about three times that for

other compounds studied. The shape of the peaks suggest

poor packing of the sample, or poor thermal conductivity

of the sample. This was the only crystalline sample. The

others were powders or very small beads (metals). The data

were omitted because of the high relative standard deviation

and because of the odd shapes of the peaks

.

The data collected are presented in figure 45. If the

cell was heated at 1.2°/min (the rate at which the samples

were run) in an argon atmosphere, and the calibrating time

was 25 sec or less, than k was within 0.8 percent standard

deviation of the straight line shown in figure 45- The

change in k, shown in figure 45, with temperature is

135
PROPORTIONALITY COEFFICIENT, K vs TEMPERATURE

5.0 a

Q

a IN AIR ATMOSPHERE * 4.8 o IN ARGON ATMOSPHERE

a IN COOLING MODE

2

UJ * NOT CORRECTED FOR BASE LINE o 4.6

LlLu

UJ8 4.4

>-H

_J 4.2 <

o

H 4.0 O

Q.g 3.8

Q.

o

1

i i i

ARGON

o

i i i

o A

1 1 1 1 1 1 1 l 20 40 60 80 100 120 140 160 180 200 220 240 260 280 300

TEMPERATURE, °C Figure 45. Proportionality coefficient, k, vs temperature.

presumably a combined effect. While the thermal conductivity

of the thermal barrier increases with temperature, this

effect is partially cancelled by the increase of sensitivity

of the copper-constantan thermocouples. The two values of k,

which were measured when the apparatus was cooling at the rate

of l°/min, were 5 percent low (shown in figure 45 as A). The

k value measured in an atmosphere of ambient air instead of

argon was about 30 percent high. For the data presented in

this figure, the current was maintained for less than

20 sec. When the current was applied to the resistor for

more than 40 sec, the AT curve was different from figure 46

(18 sec). It tended to round off at the top, and the value

for k was high, which indicated heat loss.

The change of k with a change of atmosphere suggests

heat leakage from the cell that has not been accounted for.

The following is postulated as a cause of this leakage: The

Ohmite resistor barely fitted the sample chamber; the

original leads were cut to <0.5 mm. A No. 40 wire was silver

136
mV

TIME

ELECTRICAL CALIBRATION CURVE

INCORRECT

BASELINE

Figure 46. Electrical calibration curve.

137
TIME

TIN HEATING CURVE

Figure 47. Tin heating curve.

ImV

soldered to the remaining leads forming protrusions at each

end of the resistor. There was no packing between the

resistor and the copper holder; instead it contained thin

Teflon tape electrically insulating the bottom protrusion

on the resistor from the bottom of the thermocouple holder.

The top protrusion of the resistor nearly touched the cover

cap of the thermocouple holder. Since the protrusions were

138
mV

TIME-*

BENZOIC ACID HEATING CURVE

Figure 48. Benzoic acid heating curve.

probably hot spots on the resistor, the top one presumably

heated the cap above the thermocouple holder. This hot cap

would serve as a path for heat leakage which could miss the

thermocouple

The possibility of unaccounted heat loss introduces

a bias on the electrical calibration value of k. This value

of k (with an argon atmosphere), however, approached that

obtained for the materials under the same conditions.

6 . Conclusion

This very brief experimental exploration of the

scanning calorimetric system indicates a potential usefulness of the method for obtaining, conveniently, accurate

enthalpy values. It served to acquaint ourselves with

this technique. Moreover, it provided the necessary information for a design which would extend the accuracy from

10 percent to at least 2 to 1 percent.

139
5 . STANDARD REFERENCE MATERIALS

A. Introduction - J. I. Shultz

The characterization of approximately 40 Standard

Reference Materials covering a wide range of materials

accounted for about 35 percent of the activities of the

Section. The variety of materials submitted for analysis

and the analytical techniques employed reflect the adaptability and flexability of the staff. The analytical

problems become more complex as new types of materials are

encountered or as additional elements are certified in

replacement Standard Reference Materials. Newly developed

high-temperature alloys, for example, containing combinations

of elements such as niobium, tantalum, titanium, zirconium,

cobalt, nickel, etc. frequently require complex separations

involving cation and anion exchangers for preconcentration

prior to the final determinations by gravimetric or spectrophotometric techniques

.

Atomic absorption and flame emission spectrometry,

spectrophotometry and gas evolution thermal conductivity

techniques continue as effective means for determining many

elements whose concentrations may be at the parts-per-million

level. While the application of gravimetry and titrimetry to

Standard Reference Materials certification have been

relatively limited during the period, they remain as important

and reliable techniques for determining many major constituents of metals and alloys, biochemical, botanical and

clinical materials.

New Standard Reference Materials were developed in the

areas of spectrophotometry and differential thermal analysis.

Recently, calibrated glass filters have been issued for

use in the calibration of the photometric scale of spectrophotometers. Transmittance values of the filters are

certified with a relative uncertainty of +0.5 percent.

The accuracy of calibration was confirmed by the high-accuracy

spectrophotometer which was designed and constructed in the

140
Section. The accuracy of this instrument was established

by light-addition measurements.

Complementing the two DTA standards (KNO„ and SiO p ),

already available } are an additional 3 sets (5 samples per

set) of temperature standards covering the range from 125 °C

to 9^0 °C. Each set includes overlaping temperature ranges.

These materials comprise a total of 8 inorganic substances

which exhibit phase transitions and 2 high-purity metals.

The certification of these new standards was carried out

in collaboration with the International Confederation for

Thermal Analysis.

Homogeneity testing of proposed Standard Reference

Materials is a continuing important role of the Section.

This testing, which precedes the general analysis, verifies

the aim composition of a given material and provides a measure of acceptable uniformity of composition. When

homogeneity is established, quantitative analyses by at

least 2 independent methods, together with data from cooperating laboratories, provide for the provisional and

eventual final certification.

The magnitude of the Standard Reference Materials

activity in the Section is graphically illustrated in

figure 49-

B. Standard Reference Materials Information and Summary

The following background information on selected

Standard Reference Materials which were analyzed during

the year may be of interest

1. Stainless Steels 121d and 1171

These low carbon stainless steels (AISI type 321)

containing 17 Cr, 11 Ni, and 0.3 Ti are being issued in the

form of chips (121d) for checking "wet" chemical methods

of analysis, and in solid form (1171) for use in calibrating

optical emission and X-ray spectrometric methods of analysis

The materials are austenitic and are stabilized against

141
Figure 49 . Types of Standard Reference Materials

characterized.

intercrystalline corrosion at elevated temperatures by the

addition of titanium. The standards will be especially

useful in control analyses for elements such as carbon,

nickel, chromium and titanium; however, they are certified

for 11 elements (table 42)

.

2. Stainless Steels 123c and 1172

Two austenitic stainless steels (AISI type 348) are

stabilized 18-8 steels for service at elevated temperatures,

and are similar in composition to Standards 121d and 1172,

except that niobium (0.6 percent) instead of titanium, has

been added as the stabilizer. They will serve as "wet"

chemical and spectrochemical standards respectively, in the

same manner as Standards 121d and 1171. Their primary use

will probably be as controls for niobium + tantalum determi142
I Table 42. Stainl

Ele- No. Detms

.

ment

C

121d - 1171

4 4

Mn 3

P 4 4

S 7 6

Si 3 2

Cu 2 4

Ni 2

Stainless Steels 121d and 1171.

Average, %

Method 121d - 1171

Thermal conductivity O.O67, 0.068

Periodate photometric 1.80

Molybdenum-blue photometric 0.018, 0.018

Combustion-titration .012, .013

HClOn gravimetric .535, .537

Atomic absorption .121, .121

Dimethylglyoxime-gravi- 11. 16

metric

Cr 4 - S

2 0g oxidation, Fe 17.46

titration

Mo 4 2 Thiocyanate photometric 0.164, 0.164

Ti 3 - Diantipyrrylmethane photo- .341

metric

Co 4 = Nitroso R photometric .104

nations in these and similar types of alloy steels, although

in addition, they are certified for 12 additional elements

(table 43)

.

Stainless Steels 123c and 1172.

Method

Thermal conductivity

Periodate photometric

Molybdenum-blue photometric 0.024

Combustion-titration

HClCv gravimetric

Atomic absorption

Dime thy lglyoxime -gravimetric , , Cr 4 4 S 2°8 oxidation, Fe

titration

Mo 3 2 Thiocyanate photometric

Nb 4 - Ion-exchange-gravimetric

Ta 4 - Ion-exchange-photometric

Co 4 1 Nitroso R photometric

3. Special Steels 361, 1261; 362, 1262; 363, 1263; 1264

In previous reports, we reported on the progress of the

certification of the 300 to 1200 series of Special Steels

which were designed to replace the very popular 1100 series

143

Table 43. Stainl*

Ele- No. Detms.

ment 123c - 1172

C 4 4

Mn 4 1

P 4 4

S 4 5

Si 3 2

Cu 4 4

Ni 2 2

Average, %

123c 1172

.056, .056

1 .76 , 1 .72

.024, .024

.015, .014

.585, .592

.104, .104

17 .41 , 17 .44

.21 , .21

.654

< .001

.130, .131
of spectrographs steel standards. During the year, work

continued on the project which resulted in the determination

of 14 elements, each, for 363 and 1263 involving 112 determinations. Atomic absorption, flame emission, volumetric,

gravimetric, spectrophotometric and combustion techniques

were used in this certification work. Samples were analyzed

by 4 cooperating laboratories and the provisional certificates

were issued in September 1970.

The analysis of another of the series, SRM 1264, was

completed at NBS for 10 of the major elements and approximately 17 of the minor and trace elements. The sample has

been analyzed by selected cooperator laboratories and is

being issued with a provisional certificate of analysis.

The 10 major elements, involving 57 determinations, were

C, Mn, P, S, Si, Cu, Ni, Cr, V, Mo. The minor and trace

elements included: B, Mg, Ca, Sb , Ag, Zn, La, Au, W, As,

Ti, Pb, Zr, Ta, Nb and Co and N.

Standards 362 and 1262, also in the series, were issued

with provisional certification after analyses by '4 cooperating laboratories. During the period, 9 elements each

for 362 and 1262 were determined in the Section involving

67 determinations.

As the program continues toward final certification of

approximately 35 elements on each standard in the 1200

series, the Section will undoubtedly be involved in the

determinations of many of the minor and trace elements that

remain before the final certification is complete.

J. R. Baldwin T. C. Rains

R. K. Bell T. A. Rush

E. R. Deardorff S. A. Wicks

R. W. Burke

144
J. R

R. K

D. S

R. W

E. R

6. PERSONNEL AND ACTIVITIES

A. Personnel Listing

Oscar Menis, Section Chief

James I. Shultz, Assistant Section Chief

Mary B. Pantazis, Section SecretaryBaldwin R. Mavrodineanu

Bell T. C. Rains

Bright (part time) T. A. Rush

Burke R. A. Velapoldi

Deardorff S. A. Wicks

B. Diamondstone

B

.

Publications

1. 0. Menis and T. C. Rains, "Sensitivity, Detection

Limit, Precision and Accuracy in Flame Emission and Atomic

Absorption Spectrometry ,"" Analytical Flame Spectroscopy - Selected Topics (R. Mavrodineanu, ed.), Springer-Verlag,

New York, 1971, pp. 47-77,

2. John A. Dean and T. C. Rains, "Standard Solution for

Flame Spectrometry" in Flame Emission and Atomic Absorption

Spectrometry ("John A. Dean and Theodore C. Rains, eds . ) , Dekker, New York, 1971, pp. 327-339.

3. John A. Dean and Theodore C. Rains, eds., Flame

Emission and Atomic" Absorption Spectrometry , Dekker, New York,

1971.

4. R. A. Velapoldi and 0. Menis, "Formation and

Stabilities of Free Bilirubin and Complexes with Transition

and Rare-Earth Elements, accepted for publication in Clinical

Chemistry

.

5. R. Reisfeld, A. Honigbaum and R. A. Velapoldi,

"Thallium Doped Potassium Chloride Discs as UV Fluorescence

Standards, accepted for publication in J. Opt. Soc. Amer

.

6. R. A. Velapoldi, R. Reisfeld and L. Boehn, "Spectral

Characteristics and Quantum Efficiencies of Eu(III) in a Silicate Matrix," Proceedings of 9th Rare Earth Research

Conference . n ,, _ 145
7. R. Reisfeld, R. A. Velapoldi and L. Boehm,

"Transition Probabilities of Eu(III) in Phosphate Glasses,"

submitted for publication to J. Phys . Chem.

8. R. Reisfeld, R. A. Velapoldi and L. Boehm, "Quantum

Efficiency and Radiationless Transitions of Eu(III) in

Phosphate Glasses," submitted for publication to J. Phys.

Chem.

9. 0. Menis, Editor, "Status of Thermal Analysis,"

NBS Special Publication 338, Oct. 1970.

10. 0. Menis and C. S. P. Iyer, "Spectrophotometry

Determination of Vanadium and Iron with 3-Isopropyltropolone,

Anal. Ch±m. Acta 55, 89 (1971).

11. R. Mavrodineanu, Editor, "Analytical Flame

Spectroscopy, Selected Topics," 772 pp., McMillan, London;

Springer, New York, 1970.

12. R. Mavrodineanu, "Bibliography on Flame Spectroscopy,

Analytical Applications," pp. 651 to 752 in Analytical Flame

Spectroscopy, Selected Topics , R. Mavrodineanu, Editor,

McMillan-Springer, 1970.

13. R. Mavrodineanu, "Discussion of Some Experimental

and Fundamental Conditions in Analytical Flame Spectroscopy,"

in Methodes Physiques d

r Analyse , Proceedings from the 3rd

International Congress of Atomic Absorption and Flame

Fluorescence, Paris, France, 1971.

14 . . R. C. Hughes, R. Sweet, R. Mavrodineanu, "Ferrite

Analysis" in Vol. 7 of Encyclopedia of Industrial Chemical

Analysis, Interscience , Wiley, 1971.

15. R. Mavrodineanu and J. R. Baldwin, SRM-930, Glass

Filter for Spectrophotometry, NBS, 1971.

16. 0. Menis and J. I. Shultz, "Analytical Coordination

Chemistry Section: Summary of Activities July 1969 to June

1970," NBS Technical Note 544, Sept. 1970.

146
C. Talks

1. T. C. Rains and 0. Menis, "Determination of Trace

Elements by Flame Emission Spectrometry with Repetitive

Optical Scanning," Rock Mountain Section of Society for

Applied Spectroscopy, Denver, Colorado, Aug. 3-4, 1970.

2. T. C . Rains , "Atomic Absorption and Flame Emission

Spectrometry Trends for the Future," Analytical Chemistry

Division, Oak Ridge National Laboratory, Oak Ridge, Tennessee

(Invited Presentation)

.

3. T. C. Rains , "Applications of Atomic Absorption

Spectrometry in the Analysis of Major Constituents in NonFerrous Material," 9th Annual National Meeting of the

Society for Applied Spectroscopy, New Orleans, Louisiana,

Oct. 4-9, 1970 ("Invited Presentation and Session Chairman.)

4. T. C. Rains , Clinical Standards for Atomic

Absorption Spectrometry," (In association with NIH Symposium.)

Standardization for Clinical Analysis, NBS , Oct. 9, 1970.

5. T. C. Rains , T. A. Rush and 0. Menis, "The Analysis

of Simulated Lunar Glass by Atomic Emission and Absorption

Spectrometry," NBS, Oct. 12, 1970.

6. T. C. Rains and" . "Menis , "Determination of Aluminum

in Ferrous Material by Flame Emission Spectrometry with

Repetitive Optical Scanning," 18th Anachem. Conference,

Detroit, Michigan, Oct. 14-16, 1970.

7- 0. Menis and T. C. Rains , "Application of Flame

Emission and Atomic Absorption Spectrometry to Particulate

Matter," 14th Conference on Analytical Chemistry in Nuclear

Technology, Gatlinburgh, Tennessee, Oct. 13-15, 1970.

8. T. C. Rains and 0. Menis, "Application of Repetitive

Optical Scanning for Flame Emission Spectrometry," 17th

Spectroscopy Symposium, Ottawa, Canada, Oct. 26-27, 1970

(Invited Presentation, Session Chairman and Discussion

Leader)

.

147
9. T. C. Rains , T. A. Rush and 0. Menis, "Critical

Evaluation of Resonant Lines for Atomic Absorption

Spectrometry," Eastern Analytical Symposium, New York, New

York, Nov. 18-20, 1970 (Invited Presentation and Session

Chairman)

.

10. T . C. Rains , "Atomic Absorption Spectrometry,"

American Chemical Society Short Courses on AAS, New York,

March 19-20, 1971.

11. T . C . Rains and 0. Menis, "Flame Emission

Spectrometry with Optical Scanning," l6lst National Meeting

of ACS, Los Angeles, California, March 28-April 2, 1971

(Invited Presentation.)

12. T. C. Rains and 0. Menis, "An Evaluation of Atomic

Absorption and Flame Emission Spectrometry for Air Pollution

Analysis," l6lst National Meeting of ACS for Los Angeles,

California, March 28-April 2, 1971 (Invited Presentation).

13. T . C . Rains , "Atomic Absorption and Flame Emission

Spectrometry in a National Laboratory," Montana State

University, Bozeman, Montana, April 5, 1971 (Invited

Ppresentation.

)

14. 0. Menis, R. W. Burke and E. R. Deardorff,

Solution Standards for Checking the Absorbance Scales of

Spectrophotometers in the Wavelength Region of 200-600 nm,"

Anachem Meeting, Detroit, Michigan, October 13-15, 1970.

15. E. R. Deardorff , R. W. Burke and 0. Menis,

"Spectrophotometry Characterization of the 500 and 50 ppm

Trace Element Glasses for Iron, Nickel, Manganese and Gold,"

Trace Glass Symposium, ASTM Committees E-2 and C-14,

National Bureau of Standards, Gaithersburg, Maryland,

April 13-14, 1971.

16. R. A. Velapoldi , "Series on Determination of

Absolute Quantum Efficiencies, Hebrew University, Jerusalem,

Israel, January 10-15, 1971.

17. R. A. Velapoldi , "Metalloporphyrins : Novel

Preparation and Reactions," Hebrew University, Jerusalem,

Israel, February 20, 1971. 148
18. R. Mavrodineanu , "Multichannel Simultaneous Flame

Emission and Atomic Absorption Spectrophotometer,"

Pittsburgh Conference on Analytical Chemistry and Applied

Spectroscopy, 1971.

19. R. Mavrodineanu , "Discussion de Quelques Conditions

Experimentales et Fondamentales en Spectrophotometrie de

Flamme," invited plenary lecture at the 3rd International

Congress of Atomic Absorption and Flame Fluorescence, Paris,

France, 1971.

20. L. Owen and R. Mavrodineanu , "Premixed

Acetylene-Oxygen Flame with Separate Sample Supplying

Arrangement," ibid.

21. L. Owen and R. Mavrodineanu, "A Premixed OxygenAcetylene Burner with Independent Aerosol Introduction,"

10th National SAS Meeting, Oct. 1971, St. Louis, Missouri.

22. 0. Menis, "Optical Scanning Methods in Flame

Spectroscopy," Chemistry Department, University of Maryland,

College Park, Maryland.

23. 0. Menis , "Current Development in Spectrophotometrie

Standards," Pittsburgh Conference on Analytical Chemistry

and Applied Spectroscopy, 1971.

24. 0. Menis, "Clinical Standards for Spectrophotometry,"

Standardization for Clinical Analysis Symposium (in

association with NIH) , NBS, Oct. 9, 1970.

D . Committee Activities

Oscar Menis

Member ASTM, Subcommittee E13.01, Apparatus

Specifications

Member ASTM, Committee E-3, Div. I., Thermal

Analytical Test Methods Subcommittee on Standards.

1

Guest Worker

149
Member ASTM, Committee E-10 Subcommittee I.

Editorial Board Member, Chemical Instrumentation,

A Journal of Experimental Techniques in Chemistry and

Biochemistry

.

Member American Chemical Society.

Member American Association for the Advancement

of Science.

Institutional Representative, Standards Committee,

International Confederation on Thermal Analysis.

Participant, National Committee Clinical Laboratory

Standards (NCCLS) - Instrumentation.

Titular member of the International Union of Pure

and Applied Chemistry Commission V3 on Analytical

Nomenclature.

Referee, "Critical Review in Analytical Report."

Recipient of the Department of Commerce Silver

Medal Award.

James I. Shultz

Member ASTM, Advisory Board, Committee E-3

.

Member ASTM, Vice Chairman, Div. P., Committee E-3.

Member ASTM, Secretary Editorial Subcommittee,

Committee E-3.

Member ASTM, Committee E-3, Div. M.

Member ASTM, Committee E-3, Div. G.

Coeditor, ASTM Handbook of Methods of Chemical

Analysis of Metals.

Member American Chemical Society

U. S. Delegate to ISO, Technical Committee 17,

Subcommittee 1.

Recipient of the 1971 Lundell-Bright Memorial

Award, presented by ASTM at Atlantic City, New Jersey,

June 1971.

150
T. C. Rains

Member ASTM, Committee E-2, Subcommittee X.

Eastern Analytical Symposium, Governing Board.

Society of Analytical Chemists, Editorial Board.

R. K. Bell

Member ASTM, Committee E-3, Div. N.

Member American Chemical Society.

E. R. Deardorff

Member ASTM, Task Force Chairman, Div. F.,

Committee E-3.

R. W. Burke

Member National Committee Clinical Laboratory

Standards (NCCLS) - Instrumentation (UV-Vis

Spectrophotometry)

B. Diamondstone

Member American Chemical Society.

S. A. Wicks

Member American Chemical Society.

Radu Mavrodineanu

Honorary member of Groupement pour l'Avancement

des Methodes Physiques d' Analyse.

K. A. Velapoldi

Member American Chemical Society.

Member Sigma Xi

Member Japanese-United States Chemists Association

151
7. REFERENCES

[1] Menis, 0. and Shuitz, J. I., Editors, NBS Technical

Note 544, 1970.

[2] Beattie, I. R., J. Soc . Glass Technol. 36. , 37 (1952).

[3] Morey, G. W., "The Properties of Glass", 2nd ed.

Reinhold, New York, 1954.

[4] Shand, E. B., "Glass Engineering Handbook", McGraw

Hill Book, London, 1958.

[5] Putner, T., Brit. J. Appl. Phys. 10, 332 (1959).

[6] Elster, J. and Geitel, H., On the Comparison of Light

Intensities by Photoelectric Methods, Ann. Physik, Chemie

48, 625-635 (1893).

[7] Elster, J. and Geitel, H., The Photoelectric Effect on

Potassium at Low Light Levels, Phys. Ztsch. 13, 468-

976 (1912).

[8] Roller, L. R. and Breeding, H. A., Characteristics of

Photoelectric Tubes, General Electric Rev. 31, 476-479

(1928).

[9] Campbell, N. R., The Experimental Proof of a Fundamental

Photoelectric Law, Trans. Opt. Soc. (London) 3_2_, 61-65

(1930-1931).

[10] Fleury, P., An Addition Method for the Precise Study of

the Variation of the Current Output of a Photoelectric

Cell as a Function of the Incident Luminous Flux, Compt

.

Rend. Acad. Sci. (France) 199., 195-197 (1934).

[11] Preston, J. S. and McDermott, L. H., The Illumination

Response Characteristics of Vacuum Photoelectric Cells of

the Elster-Geitel Type, Proc. Phys. Soc. (London) 46 , 256-272 (1934).

[12] Preston, J. S. and Cuckow, F. W., A Photoelectric

Spectrophotometer of High Accuracy, Proc. Phys. Soc.

(London) 48, 869-880 (1936).

152
[13] Buchmuller, P and K*6nig, H., Precision Measurements on

Incandescent Lamps Using Selenium Photocells, Assoc.

Suisse Electriciens, Bulletin 2_8, 89-99 (1937).

[14] Atkinson, J. R., Campbell, N. R., Palmer, E. H. and

Winch, G. T., The Accuracy of Rectifier-Photoelectric

Cells, Proc. Phys . Soc . (London) 50_, 934-946 (1938).

[15] Barbrow, L., A Photometric Procedure Using BarrierLayer Photocells, NBS Research Paper RP 1348, J. Res.

NBS 25, 703-710 (1940)

.

[16] Kaiser, H., Photographic-Photometric Calibration of Step

Filters, Spectrochimica Acta 3, 518-537 (1947-1949).

[17] Harding, H. G. W., Precautions Necessary for Accurate

Measurements of Optical Density Standards, Photoelectric

Spectrometry Group Bulletin 4, 79-86 (1951).

[18] Kortum, G. and Maier, H., Dependence of Photo Current

and Illuminating Intensity in Photocells and Photomultipliers, Z. Naturforschung 8A, 235-245 (1953).

[19] Hansen, G., Check of Photometric Function of Spectrophotometers, Mikrochimica Acta 1955 , 410-415.

[20] Hermann, W., Noise and Linearity Check on Photomultipliers, Z. Naturforschung 12A, 1006-1013 (1957).

[21] Reule, A., Checking the Photometric Scale of Absorption

Measuring Instruments., Zeiss-Mitt. 1, 283-299 (1959).

[22] Hoppmann, H., An Instrument for Checking the

Proportionality of Physical Radiation Receivers,

Technisch-Wissenschaftliche Abhandlungen der OsramGesellschaft 7, 306-312 (1958).

[23] Gibson, G. L., Hammond, H. K. Ill, Holford, W. L. and

Nimeroff, I., Calibration of Photometers, NBS Manuscript,

Nov. 1, i960, (Personal Communication).

[24] Bischoff, K., Measurement of Proportionality of Radiation

Receivers Over a Large Radiation Intensity Range, Z.

Instr. 69, 143-147 (1961).

153
[25] Jones, 0. C. and Clarke, F. J. J., A New Photometric

Technique Using a Variable Shutter Device, Nature 191 , 1290 (1961)

[26] Cordle, L. C. and Habell, K. J., Photometry of Telescopes

and Binoculars, National Physical Laboratory, Notes on

Applied Science No. 14 , 18 pp (1962).

[27] Sanders, C. L., A Photocell Linearity Tester, Applied

Optics 1, 207-271 (1962).

[28] Nonaka, M. and Kashima, T., Linearity Characteristics of

Multiplier Phototubes, Japanese Journal Applied Physics

2, 785-791 (1963).

[29] Hofert , H. J. and Loof, H., Calibration of Photometric

Scales of a Reflection Photometer, Z. Instrumentenkunde

72, 139-143 (1964).

[30] Clarke, P. J. J., Time Ratio Photometry, Lecture to the

Optical Group of the Institute of Physics and the

Physical Society, June 9th 1966.

[31] Clarke, F. J. J., High Accuracy Spectrophotometry of

Three Glass Filters, NPL Report No. 3042, 14 pp , Nov. 4,

1968.

[32] Reule, A., Testing Spectrophotometer Linearity, Appl.

Optics 7, 1023-1028 (1968).

[33] Kunz, H., Representation of the Temperature Scale Above

1337.58 K with Photoelectric Direct Current Pyrometers,

Metrologia 5, 88-102 (1969).

[34] Kostkowski, H. J., Personal Communication.

[35] Edisbury, J. R., "Practical Hints on Absorption

Spectrometry", p. 190, Plenum Press, New York, 1967.

[36] Thomson, L. C, Trans. Faraday Soc . 42, 663 (1946).

[37] Haupt, G. W., NBS J. Res. 48_, 4l4 (1952).

[38] Gridgeman, N. T., Photoelectric Spectrometry Group

Bulletin, No. 4, 67 (1951).

[39] Ketelaar, J. A. A., Fahrenfort , J., Haas, C. and

• Brinkman, G. A., Photoelectric Spectrometry Group

Bulletin, No. 8, 176 (1955).

154
[40] Vandenbelt, J. M., J. Opt. Soc . Am. 50, 24 (I960).

[4l] Bailey, N., Carrington, A., Lott , K. A. K. and Symons,

M. C. R., J. Chem. Soc, p. 290 (I960).

[42] Tong, J. and King, E. L., J. Amer. Chem. Soc. 75., 6l80

(1953).

[43] Davies, W. G. and Prue, J. E., Trans. Faraday Soc. 51 , 1045 (1955).

[44] Neuss, J. D. and Rieman, W. , J. Amer. Chem. Soc. 56 , 2238 (193 1*)

.

[45] Howard, J. R., Nair, V. S. K. and Nancollas, G. H.,

Trans. Faraday Soc. 54. , 1034 (1958).

[46] Haight, G. P., Jr., Richardson, D. C. and Cob urn, N. H.,

J. Inorg. Chem. 3, 1777 (1964).

[47] Johnson, E. A., Photoelectric Spectrometry Group

No. 17, 505 (1967).

[48] Chapman, J. H., Forster, G., Kortum, G., Parker, C. A.,

Lippert, E., Melhuish, W. H. and Nebbia, G., Applied

Spectry. 17, 431 (1963) . [49] See for example: C. A. Parker, "Photoluminescence of

Solutions," Elsevier Publishing Co., New York, N. Y.,

1968; D. M. Hercules, "Fluorescence and Phosphorescence

Analysis," Interscience Publishers, New York, N. Y., 1966;

W. H. Melhuish, J. Opt. Soc. Amer. 54, 183 (1964).

[50] C. Zeiss Inc., Silver Spring, Md

.

[51] Turner, G. K., Science l46_, 183 (1964).

[52] Fletcher, A. N., Photochem. and Photobiol. 9, ^39 (1969).

[53] Dawson, W. R. and Windsor, M. W. J. Phys . Chem. 72,

3251 (1968); G. K. Turner Associates, "Notes on the

Determination of Quantum Efficiency", Palo Alto,

California; Drobnik, J., and Yeargers , E., J. Mol.

Spectry. 19_, 454 (1966); Ruskawicz, R. and Testa, A. C.

J. Phys. Chem. 7_2, 793 (1968); Eastman, J. W., Photochem.

Photobiol. 6, 55 (1967); Chen, R. F., Anal. Biochem. 19,

374 (1967); Borreson, H. C, Acta Chem. Scand. 19_, 2089

(1965).

155
54] Koch, L., J. Assoc. Off. Agr . Chem. 39, 397 (1956).

55] Hebrew University, Jerusalem, Israel, PL 480 grant

continuation.

56] See for example: Reisfeld, R. and Greenberg, E., Anal.

Chim. Acta 47_, 155 (1969); Reisfeld, R. and Biron, E.,

Talanta 17, 105 (1970).

57] Cleek, G., Inorganic Glass Section, Inorganic Materials

Division, NBS

.

58] McMillan, P. W. , "Glass Ceramics," Academic Press, New

York, N. Y. (1964); Moeller, T., "Chemistry of the

Lanthanides ," Reinhold, New York, N. Y., 1963

.

59] Colthup, N. B., Daly, L. H. and Wiberly, S. E.,

"Introduction to Infrared and Raman Spectroscopy,"

Academic Press, New York, N. Y., 1964.

iO] a. Parker, C. A. and Rees , W. T., Analyst 85., 587 (I960);

b. Weber, G. and Teale, F. W. J., Trans. Faraday Soc.

52, 646 (1957); c. Melhuish, W. H., J. Phys . Chem. 65.,

229 (1961).

a] Himel, A. M. and Mayer, R. T., Anal. Chem. 42_, 130

(1970).

2] Strickler, S. J. and Berg, R. A., J. Chem. Phys. 37, 8l4

(1962).

3] Seybold, P. G., Gouterman, M. and Callis, J., Photochem.

Photobiol. 9, 229 (1969).

4/] Reisfeld, R., Honigbaum, A. and Velapoldi, R. A.,

submitted for publication to J. Opt. Soc. Amer., 1971.

5] Halperin, E. and Braner, A., Private communication,

Physics Department, Hebrew University, Jerusalem, Israel.

6] Dexter, D. L., Solid State Physics 6, 353 (1958); Dexter,

D. L., Phys. Rev. 101, 48 (1956).

7] Wagner, W. V., Z. Phys. l8l, 143 (1964).

8] Fukuda, A., Sci. Light 13, 64 (1964).

9] Delbecq, Ch . J., Ghosh, A. K. and Yuster, Ph. H.,

Phys. Rev. 151 (1966)

.

156
[70] Mavrodineanu, R. and Boiteux, H., 'Flame Spectroscopy,'

J. Wiley and Sons, New York, N. Y., 1965, p. 516.

[71] Stroud, J. S., J. Chem. Phys . 35, 844 (196I).

[72] Turner, W. H. and Turner, J. E., J. Amer. Ceram. Soc.

53, 329 (1970); Goldberg, P., Ed., "Luminescence of

Inorganic Solids", Academic Press, New York, N. Y.,

1966.

[73] Reisfeld, R. and Kirshenbaum, L . , unpublished results.

[74] a. Haas, Y. and Stein, G., Accepted by J. Phys. Chem.,

(1971); b. Dawson, W. R. and Kropp , J. L., J. Opt. Soc.

Amer. 55., 822 (1965); c. Rinck, B., Z. Naturforschg

.

4l2b, 406 (1948); d. Weber, M. J. and Schaufele, R. P.,

J. Chem. Phys. 43, 1702 (1965); e. Blasse, G. and Bril,

A., Philips Res. Repts . 23_, 189, 201 (1968).

[75] Velapoldi, R. A., Reisfeld, R. and Kirshenbaum, L., to

be published.

[76] Reisfeld, R., Honigbaum, A., Michaeli, G., Harel, L.

and Ish-Shalom, M., Israel J. Chem. 7, 613 (1969).

[77] Lewis, G. N. and Kasha, M., J. Amer. Chem. Soc. 67_, 994

(1945).

[78] Forster, Th., "Fluoreszenz Organischen Verbindungen,

Vandenhoeck and Ruprecht, Gottingen, 1957-

[79] Inokuti, M. and Hirayama, F ., J . Chem. Phys. 43_,

1978 (1965)

.

[80] Nakazawa, E. and Shionoya, S., J. Chem. Phys. 47_,

3211 (1967).

[81] Carnall, W. T., Fields, P. R. and Rajnak, K., J. Chem.

Phys. H9_ 3 4550 (1968); Dieke, G. H., "Spectra and Energy

Levels of Rare Earth Ions in Crystals," Interscience

Publishers, New York, N. Y., 1968, pages 242-7.

[82] Mavrodineanu, R. and Hughes, R. C, A Multichannel

Spectrometer for Simultaneous Atomic Absorption and

Flame Emission Analysis, Applied Optics 7, 1281-1285

(1968).

157
[83] Mavrodineanu, R., Sprayer and Burner Units for Flame

Emission and Atomic Absorption Measurements,

Develop. Appl. Spectry. 8, 18-32 (1970).

[84] Muller-Herget , W. , Some Considerations on Optical

Design and Selection of Spectroscopic instruments,

Chapter 3, pg. 79 to 144 in "Analytical Flame

Spectroscopy, Selected Topics", R. Mavrodineanu

Editor, Springer-McMillan, 1970.

[85] Mavrodineanu, R., Some Considerations on Burner for Flame

Spectroscopy, Develop. Appl. Spectrosc. 5, 371-401 (1965).

[86] Owen, L. E., AFML-TR-68-354 , AF Materials Laboratory,,

WPAFB, OH 45433.

[87] Snelleman, W. , Rains, T. C, Yee , K. W. , Cook, H. D.

and Menis, 0., Anal. Chem. 42_, 394 (1970).

[88] Menis, 0., Editor, NBS Technical Note 504, 1969.

[89] Rains, T. C, Zittel, H. E. and Ferguson, M., Talanta

10, 367 (1963).

[90] Muller, K., Z. Physik. 65., 739 (1930).

[91] Woodson, T. T., Rev. Sci. Instr. 10, 308 (1939).

[92] Poluektov, N. S. and Vitkun, R. A., Zh . Anal. Khim. 18,

33 (1963).

[93] Hatch, W. R. and Ott, W. L. , Anal. Chem. 40, 2085

(1968).

[94] Hillebrand, W. P., Lundell, G. E. F., Bright, H. A.

and Hoffman, J. I., "Applied Inorganic Analysis"

2nd ed., pp. 347-51, Wiley, New York, 1953-

[95] Munns , R. K., and Holland, D. C, private communication with Food and Drug Administration, Dec.

1970.

[96] Menis, 0., "Status of Thermal Analysis," NBS Special

Publication 338, Oct. 1970.

158
[97] Ozawa, T., Monota, M., Osozaki, H., Bull. Chem. Soc

Japan 40, 1583 (1967)

.

[98] Ozawa, T., Osozaki, H., Negishi, A., Thermochimica

Acta 1, 5^5 (1970)

.

[99] Ozawa, T., Bull. Chem. Soc. Japan 39, 2071 (1966).

159
FORM NBS-114A (1-71) U.S. DEPT. OF COMM. BIBLIOGRAPHIC DATA

SHEET

1. PUBLICATION OR REPORT NO. NBS-TN 584

2. Gov't Accession No.

3. Recipient's Accession No.

4. TITLE AND SUBTITLE

Analytical Coordination Chemistry Section: Summary

of Activities July 1970 to June 1971

5. Publication Date December 1971

6. Performing Organization Code

7. AUTHOR(S), . , _ _ _, -. tpa-4. Oscar Menis and James I. Shultz, Editors 8. Performing Organization

10. Project/Task/ Work Unit No. 310 0900

9. PERFORMING ORGANIZATION NAME AND ADDRESS

NATIONAL BUREAU OF STANDARDS

DEPARTMENT OF COMMERCE

WASHINGTON, D.C. 20234

11. Contract/Grant No.

12. Sponsoring Organization Name and Address

Same

13. Type of Report & Period Covered Interim

Jilly 1970 to June 1973

14. Sponsoring Agency Code

15. SUPPLEMENTARY NOTES

16. ABSTRACT (A 200-word or less factual summary of most significant information. If document includes a significant bibliography or literature survey, mention it here.) Progress in research in spectrophotometry,

spectrofluorometry , flame emission, atomic absorption, thermal analysis

and related programs is described. Efforts to improve accuracy of

spectrophotometry measurements led to th_e construction of a high accuracy

spectrophotometer, certification of glass filters, establishment of a more

accurate value for molar absorptivity of the acid dichromate system, and

the evaluation of spectrally neutral filters of interest for automation

in clinical studies. The linearity of the single beam spectrophotometer

was calibrated by the light addition technique. Glass filters were

calibrated at h wavelengths with an uncertainty of . 5 percent. Data for

the dichromate system in perchloric acid led to new values for the

dimerization equilibrium constant.

Studies were made of new fluorescence filters incorporating Ti(l),

Ce(lll), Pb(ll) and rare earths in a variety of glass matrices. Quantum

efficiencies of Tl in K'Cl and Eu in a silicate matrix were determined.

Developments in instrumentation led to increased sensitivity and

improved precision and accuracy by flame spectroscopy. M

were determined with the multichannel spectrometer. A newly designed

optical system and an ultrasonic spraying system provided increased signal

and efficiency. The repetitive optical scan of flame emission overcame

matrix and flame background problems for the determination of aluminum in

steel. Certification was made on eight new materials in cooperation

with the International Confederation on Thermal Analysis.

Ca, K and Na

1 7. KEY WORDS (Alphabetical order, separated by semicolons) Alumi num in steel" bilirubin - DTA standards; fluorescence filters Tl(l), Ce(lll), Pb(ll); glass filters IbBM 930 1; jaexcury in botanical ; quantum efficiencies.

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