tional Bureau of Standards

Library, N.W. Bldg

I AUG 1 6 1966

Reference book not to I taken from the library.

NBS TECHNICAL NOTE

Analytical Mass

Spectrometry Section

Instrumentation and Procedures

for Isotopic Analysis

277

*Z

^

kO»t or

V

.*&

^CAU Of *

\

CO Q

U.S. DEPARTMENT OF COMMERCE

National Bureau of Standards
THE NATIONAL BUREAU OF STANDARDS

The National Bureau of Standards is a principal focal point in the Federal Government for assur- ing maximum application of the physical and engineering sciences to the advancement of technology

in industry and commerce. Its responsibilities include development and maintenance of the national

standards of measurement, and the provisions of means for making measurements consistent with

those standards; determination of physical constants and properties of materials; development of methods for testing materials, mechanisms, and structures, and making such tests as may be necessary, particularly for government agencies; cooperation in the establishment of standard practices

for incorporation in codes and specifications; advisory service to government agencies on scientific and technical problems; invention and development of devices to serve special needs of the Govern- ment; assistance to industry, business, and consumers in the development and acceptance of commercial standards and simplified trade practice recommendations; administration of programs in cooperation with United States business groups and standards organizations for the development

of international standards of practice; and maintenance of a clearinghouse for the collection and

dissemination of scientific, technical, and engineering information. The scope of the Bureau's

activities is suggested in the following listing of its three Institutes and their organizational units. Institute for Basic Standards. Applied Mathematics. Electricity. Metrology. Mechanics. Heat. Atomic Physics. Physical Chemistry. Laboratory Astrophysics.* Radiation Physics. Radio Standards

Laboratory : * Radio Standards Physics ; Radio Standards Engineering. Office of Standard Reference

Data. Institute for Materials Research. Analytical Chemistry. Polymers. Metallurgy. Inorganic Materials. Reactor Radiations. Cryogenics.* Materials Evaluation Laboratory. Office of Standard Refer- ence Materials. Institute for Applied Technology. Building Research. Information Technology. Performance

Test Development. Electronic Instrumentation. Textile and Apparel Technology Center. Technical

Analysis. Office of Weights and Measures. Office of Engineering Standards. Office of Invention and

Innovation. Office of Technical Resources. Clearinghouse for Federal Scientific and Technical

Information.**

*Located at Boulder, Colorado, 80301.

* "Located at 5285 Port Royal Road, Springfield, Virginia, 22151.
UNITED STATES DEPARTMENT OF COMMERCE • John T. Connor, Secretary

NATIONAL BUREAU OF STANDARDS • A. V. Astin, Director

NBS TECHNICAL NOTE 277

ISSUED July 25, 1966

Analytical Mass

Spectrometry Section:

Instrumentation and Procedures

for Isotopic Analysis

Edited by William R. Shields

Analytical Mass Spectrometry Section

Analytical Chemistry Division

Institute for Materials Research

NBS Technical Notes are designed to supplement the Bureau's regular publications program. They provide 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 - Price 60 cents
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

highly important. 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. At the

National Bureau of Standards such publications fit

logically into the category of a Technical Note. In

1966 we plan to issue these summaries for all of our

sections

.

W. Wayne Meinke, Chief

Analytical Chemistry Division

11
PREFACE

The basic function of the Analytical Mass Spectrometry

Section is to establish high accuracy absolute isotopic ratio

standards. To this end the section is concerned with developing instrumentation and devising analytical techniques which

will improve the precision of isotope ratio measurements; and

with performing the actual measurements. In addition, the

section is concerned with monitoring Doth natural and man-made

variations in the isotopic composition of elements. Delimiting

natural variations is vital to establishing the constancy of

atomic weights and is also of general interest to earth

scientists

This report summarizes the major works performed by the

personnel of this section over the past eight years. It is

primarily designed as a "cookbook" of analytical techniques for

isotopic ratio measurements of various elements,, and as a guide

to the pitfalls encountered in attempts at very precise or

accurate isotopic ratio measurements. The report fits the

formal description of a Technical Note in the sense that the

material is "transient". The instrumentation is continuously

being improved and modifications in analytical procedures,

which might improve precision, are always being sought.

E. J. Catanzaro and E. L. Garner wrote the descriptions of

the analytical procedures; W. A. Bowman, III drafted the schematics; and H. H. Ku, of the Statistical Engineering Section,

wrote the appendix. Mrs. Joy Shoemaker typed the report.

In order to describe the instrumentation adequately, it

has sometimes been necessary to identify certain commercial

equipment. Such identification does in no way imply recommendation or endorsement by the NBS nor does it imply that said

equipment is necessarily the best available for the purpose.

William R. Shields, Chief

Analytical Mass Spectrometry Section

iii
TABLE OF CONTENTS

PAGE

A. INTRODUCTION 1

B. ANALYTICAL PROCEDURES 16

Introduction 16

Parameter Evaluation 17

Sample Composition and Size 17

Filament Material 18

Mounting Technique 19

Filament Temperatures and Heating Techniques . . 20

Analyses of Particular Elements 21

Introduction 21

Individual Procedures ..... 25

Bromine 23

Cesium 27

Chlorine . 29

Chromium 33

Copper 37

Magnesium „ 41

Plutonium . 45

Silver 49

Uranium 53

Interim Procedures ©7

Boron 67

Lithium 69

Rubidium 71

Strontium „ 75

C. APPENDIX 77

(STATISTICAL EVALUATION OF UNCERTAINTIES

ASSOCIATED WITH THE REPORTED VALUES)

iv
LIST OF FIGURES

FIGURE NO . PAGE

1. 12" Solid Source Mass Spectrometer

2. Source Control 3

3. Magnet Supply and Control Circuit 4

4. Filament Control Circuit 5

5. Thermocouple Vacuum Gauge 6

6. Vacuum Control Circuit. . 7

7. Ionization Gauge Degas Control 8

6. Source Assembly 9

9. Collector Assembly 10

10. Expanded Scale Circuit 12

11. Experimental 2 ^5U/2 ^ U vs NBS Best Estimate 1958. . 15

12. Experimental 2 ^5 U/25 U vs Theoretical 2 ^5U/2 ^ U . . 64

(APPENDIX)

1. Normal Probability Plot of Observed Isotopic

Ratios 24/26 of Magnesium Reference Samples ... 80

2. Comparison of observed calibration factors for

isotopic ratios 52/53, 50/52, and 52/54 of

Chromium with values computed from the observed

calibration factor for the 50/54, assuming a linear relationship between the calibration

factor and the mass difference of the isotopes. . 82

v
LIST OF TABLES

TABLE NO . PAGE

la. Low-Temperature Data Acidity/Sample Size

Experiment 6l

lb. High-Temperature Data Acidity/Sample Size

Experiment 61

(APPENDIX)

MEMORANDUM AND SUMMARY TABLE 84

1. Analysis of Separated Samples 86

2a. Analysis of Concentration of Separated Sample

Solutions 87

2b. Analysis of Concentrations of Normal NaBr 3 Meq/g 88

2c. Estimated Precision of Chemical Analysis 89

2d. Analysis Of Normal NaBr 90

3. Composition of Synthetic Mixes 92

4a. Analysis of Synthetic Mixes 94

4b. Analysis of Synthetic Mixes 95

5. Y = Ratios 79/81 for Reference Sample 97

6. Comparison of Determinations I and II 98

VI
ANALYTICAL MASS SPECTROMETRY SECTION:

INSTRUMENTATION AND PROCEDURES FOR ISOTOPIC ANALYSIS

Edited by William R. Shields

ABSTRACT

This report describes the general instrumentation of

the Analytical Mass Spectrometry Section and the specific

analytical techniques which have been debised for the

measurement of isotopic ratios of Ag, Br, G±3 Cr, Cs, Cu, Mg,

Pu, and U. Interim procedures for B, Li, Rb, and Sr are

also given.

In the appendix some general statistical principles used

in the design and analysis are briefly discussed; an example

is given in detail illustrating the various steps involved

leading from original data to the reported uncertainties for

the Isotopic ratio of bromine.

Key words: Mass spectrometry,, instrumentation,

procedures, isotopic analysis.

vii

A. INTRODUCTION

The present analytical instrumentation of this section

consists of five mass spectrometers; three with 12-inch radius

of curvature and two with 6-inch radius of curvature. All

components are interchangeable within each size group and

electronic components and vacuum systems are interchangeable

among all of the instruments.

The instruments are nominally solid-source but are readily

convertible to gas-source by the attachment of a gas inlet

system and an electron bombardment source.

The basic 'instrument (figure l) is a composite of

commercial and NBS-built components^ the latter usually being

either slight modifications of commercially designed components

or copies of components designed in other laboratories. The

instruments are unique only in the combination of the various

components. Figures 2 through 7 are the NBS-built electronic

circuits

.

The more important characteristics of these instruments

concern the source, collector , and recording systems. The ion

source (figure 8) is a linear thin lens source with each

succeeding lens being approximately twice as far from the

ionizing filament as the previous lens. The source is made

as wide open as possible (no baffles or skirts) to maintain

as high a vacuum as possible in the region of the ionizing

filament, and a liquid nitrogen cold finger has been installed

in the source region to increase the pumping speed for

condensables . At the same time, the source housing is made

as large as practical to minimize the penetration of the

ground potential of the walls into the ion region.

The collector used in the NBS instruments (figure 9) was

designed to improve the collection efficiency of the conventional 50$ transmission grid collector. The major change

from the conventional collector is the six suppression grids

made of fine wire that are located between the transmission
l3* • #

CD -po

Jh -po

CD co

«j s

(Doo

co

T5H

rHO

CO

CM

PhhO

•H
e- s9aI8:

1!

ij

««ls i Ii

I* j ii;

Hou

-poo

(DoOm

OJ

CD

•H

w eg <0 ^

- X I- „-

I- ~ (0 00 <0 * s I CO •— «0 u.xS3asszir>-i-i-i5
h

e

i le

I

<\ en o q 10 < ml * < » ° 9

s- -

- --S.---N

* tt O o 2 o ?i ; = £

i .» I : i si illl |

I

I Mil!

¥ f

X z o

3

o

01 >

(1) H

ft3

O

o

o

a CD ^ W

CD 3

si CO Eh t5

CD

•H
isif

^5h

Ol ©- ^>

v---mmsuC05 — 22

-0 e <rf\j ©-

»i otr to o<

OQ-

-©"

^3- ^a c •£? "

1 1 r

CDW

•H

7
;t .I'-

ll Ull

CD CM 2%

< DO O O

A * A A ^, -<D0©

-o d d—

_»._ > ^

S » M -d>-

I?

8


grid and the defining slit of the collector. (A plate with

grid wires on both top and bottom acts as two grids.)

The suppression grids give the operator the ability to

control the secondary charged particles, both positive and

negative, and when the appropriate potentials are applied to

the grids it is possible to obtain the same measured isotopic

ratio, for a given sample, with both the expanded scale X10

and the ratio modes of measuring. The ability to control the

grid response made it possible to prove conclusively that

variations in grid response are reflected in the measurement

of ratios measured solely with the Faraday cage collector.

The magnitude of this random bias component is only a few

tenths of a percent but it varies with isotopic concentration.

One of the properties of the suppression grids is the ability

to maintain a fairly constant grid response independent of

the ion species and age or cleanliness of the collector.

A small bar magnet strapped to the outside of the

collector housing, axially aligned with the ion beam and at

45 to the defining slit, helps control secondary electrons

that are released before the entrance to the collector

assembly.

The measuring circuit consists of two vibrating reed

electrometers (Applied Physics Corps. MS -31)* a master and a

slave, and an expanded-scale recorder. The slave unit monitors

the total signal intensity while the master unit measures the

ions striking the Faraday collector. The input resistors for

the master and slave reeds are 10 and 10 ohms, respectively.

The expanded-scale circuit (figure 10) utilizes the 10 mV and

100 mV signals of the master VEE. A reference source coupled

with the push-pull decade provides the other half of the input

to the chopper stabilized difference amplifier contained in

the recorder. The recorder is a Leeds and Northrup X - Xp

model G Speedomax with 1 second response time. The slidewire

is shunted to a resistance of 110 ohms for 110 division chart

over 9 7/8 travel. The function of the expanded scale circuit

11
-EHBBEQ—r — -BB3

H.I"

iUlnJi'll

si!) Hit

»

: ' ill ' g!i 'l

1

It I

12
is to increase the normal 100 division full-scale output of

the null-balance bridge to 1000 divisions in precise proportional steps of 10% of that output. This improves the

operators ability to intercompare the potentials generated by

the various isotopes and reduces the bias errors inherent in

the servo balance system (dead zone and non-linearity) by an

order of magnitude. It should be noted that this circuit

maintains its precise proportionality independent of the

reference potential.

The function switch in the expanded scale circuit

(figure 10) allows the operator to use the recorder and the

two amplifiers in three different ways:

XI) The function switch selects the 10 mV output of the

VRE and full-scale output of the amplifier equals

100 chart divisions on the recorder.

X10) The function switch selects the 100 mV output of

the VRE and full-scale output of the amplifier

equals 10 full chart widths. By selecting the

proper position for the slidewire, with the slidewire

position switch, the operator can make the recorder

balance and the first digit of the potential being

measured is read on the slidewire position switch.

The end result is that any systematic error contributed by the recorder has been reduced by one

order of magnitude.

Ratio) With the function switch in this position, the 100

mV output of the grid VRE is used as the reference

potential of the recorder and the 100 mV output of

the Faraday VRE is fed to the recorder. This

function will measure a ratio of ratios independent

of intensity, if the collector efficiencies are

independent of intensity.

Solid source isotopic measurements generally show a bias

when referenced to the absolute ratio of the element in

question. A number of the individual bias components whose

13
sum constitutes the total bias have been identified. The order

in which the identified bias components are discussed is

essentially the actual order of their identification. This

order was chosen because in many cases it was not possible to

detect and/or identify certain terms until modifications were

made on the instrumentation.

The simplest way to demonstrate the methods used to

identify many of the error components is through the use of

well documented experimental data:

Ref. 1: Report Number 8, A.E.C. Advisory Committee for

Standard Reference Materials and Methods of

Measurement , May i960.

Ref. 2: Proceedings ASTM E-l4 Conference, May I965,

p. 157.

Ref. 3: Proceedings IAEA Symposium on Nuclear Materials

Management, September 1965.

Figure 11 is a pictogram that clearly displays several

sources of experimental bias. In fact it supplies an example

of both types of bias (systematic and random) as evaluated

through the use of NBS isotopic standards. The data were

supplied by laboratories in both Europe and the United States

and shows a first partitioning of the variance components that

constitute the uncertainty of a large number of isotopic

analyses done by surface or thermal ionization. In general,

the error components identified are not restricted to a single

element, but rather are found to exist for all elements at

various levels. All of the data shown had previously been

corrected for all known sources of bias by the workers at the

various laboratories (ion multiplier discrimination, etc.) and

had a calculated analytical precision of better than a 0.25$.

A very cursory examination of figure 11 reveals three obvious

bias factors: one due to the difference in the mode of

ionization, 'evidenced by the average 1.1$ bias of the single

filament - dc amplifier results; and another due to the mode

of collecting and measuring the ions, evidenced by the

14

average 0.55$ bias superimposed on the results of both modes

of ionization when a multiplier is used. Finally., superimposed on both of these systematic errors, is a random error

or average limit of error of bias of approximately 0.5$. The

random error is made up of several components, the most

important of which is due to the strip chart recorder.

Manufacturers of strip chart recorders have for years supplied

the information that new recorders have possible errors of

0.1$ (full scale) in dead zone and 0.25$ (full scale) in

linearity. Still, the recognition that these parameters are

the limiting factor (0.5$ LE of bias) on the precision of

most isotopic analyses is, unfortunately, not widespread.

The expanded scale recorder was developed to circumvent this

limitation. The modified recorder, rather than a variety of

available digital output systems, was chosen because it is

felt that visual interpretation and integration is necessary

to do the most precise work.

B. ANALYTICAL PROCEDURES

1. Introduction

After the instrumental bias components have either been

removed or have been controlled and corrected for, most solid

source mass spectrometr ic isotopic analysis still show a bias.

For a given instrumental setup and element this bias is more

or less constant. The magnitude and range of variation of

this bias component can be controlled. The fact that elements

isotopically fractionate during thermal ionization is well

known. The fact that this fractionation factor is variable

and is a function of the sample size and sample composition

interacting with the filament material and filament temperature is not so well known. This variable fractionation

factor (bias) has been observed in all elements analyzed at

this laboratory. For many of the elements analyzed in this

laboratory slight changes in the chemistry of the sample, or

the manner in which the sample is mounted on the filaments,

will have dramatic effects on the observed ratio. The manner

16
in which the filaments are adjusted to temperature (fast,

slow) and the final temperature (high, low) also affect the

final observed ratio.

Direct comparison of analyses on the same mass spectrometer are strictly valid only if the sample size, sample

composition, mounting procedure, and mass spectrometric

filament-heating and data-taking time are identical in all

analyses, or are as identical as has previously been determined

necessary to preclude them as the limiting factor in the

statistical error. Any changes in any of these parameters may

introduce significant changes in the total bias which would

negate the use of correction factors.

2. Parameter Evaluation

a. Sample Composition and Size

The total composition of the sample generally has an

important effect on the ionization efficiency, and the best

sample composition must be evaluated by experimentation. This

experimentation should be concerned not only with the major

extraneous anion or cation, but also with pH, concentration

of the element under study, and, in general, total composition

of the sample solution. All chemical factors may affect the

temperature and efficiency of volatilization and ionization,

as well as the work function of the filament material. Even

those elements or radicals which are removed during the drying

of the sample on the filament are generally important because

of their possible effect on the distribution of the dried

sample and the sample-filament contact.

Extraneous elements are generally detrimental to- the

analytical precision of isotopic analyses, and all samples

should initially be processed in such a manner so as to give

a uniform purity. The practice in this laboratory has been

to establish an analytical procedure using the reference

sample, usually a very pure salt of the element to be analyzed.

The next step is the mineral survey, and the initial chemistry

done on the mineral samples is held to a minimum but is uniform

17
for all samples. As the mineral survey progresses the results

from some of the samples usually fall outside the statistical

limits of the reference sample. Any sample whose analytical

results show a difference is returned to the chemist and

carefully repurified, with some effort being made to identify

the trace impurities that are removed. By doing the survey

in this manner it is possible to identify particularly

deleterious impurities and/or to establish the necessary

purity for good analytical results. Treating the samples in

this manner insures that any difference is real and not the

result of some impurity, and in the majority of the elements

investigated in this laboratory apparent isotopic differences

between samples have disappeared after reprocessing.

b. Filament Material

The usual filament materials are used in this laboratory,

and it is the practice of the laboratory to try all of the

different filament materials when starting an investigation

on a new element. The best filament material and method of

ionization (single or multiple filament) for a particular

element must be determined experimentally.

The Saha-Langmuir equation may provide a clue as to

whether the element in question can be ionized, but in general

it has no direct application because the true work function

of the contaminated filament surfaces is not known. The

elements whose ionization potential is between 7.9 and 11.7

electron volts in general cannot be ionized thermally. There

are of course a few exceptions. The filament materials

themselves will emit ions of the base element and it is,

therefore, possible to observe ions of iridium (I. P. 9« 2 V)

.

Boron (I. P. 8.2 V) as a complex ion (meta borate) has a very

high ionization efficiency from a platinum or tantalum filament, but, in general a much poorer ionization efficiency

from the other filament materials.

whenever possible, triple-filament sources are to be

preferred, since in general they have a higher ionization

18
efficiency. The increased efficiency will usually allow the

analyst to devise an analytical procedure that will minimize

the effect of fractionation which will result in a lower rate

of change of the ratios with time.

c . Mounting Technique

Since almost all samples are placed on the filaments as

solutions or slurries, they must be dried before being inserted

into the mass spectrometer. This is usually accomplished by

the use of heat lamps and/or electrical currents. The manner

in which the samples are dried is very important. Too rapid

heating can cause the formation of bubbles or curling of

solid material. This will lead to flake-off, or, at least,

poor sample -filament contact. For volatile salts, such as

bromides and chlorides, the drying temperature must be kept

low enough to insure against loss of the element under study,

which would change the sample size. For elements which form

multiple oxides, it is important to find the best oxide for

analyses and to achieve this form in the drying procedure.

To this end, it is very helpful to have fiduciary points

such as color changes in the sample (see uranium procedure).

In general, since the temperature and manner of drying

can affect the physical chemistry of the sample, as well as

the physical shape or form of the sample and the samplefilament contact, it is important to mount all samples in an

identical manner.

An example of the importance of finding the proper

mounting technique can be seen by comparing silver and copper.

Both of these elements are mounted as the metal but silver is

deposited on the filament as a salt (AgNCu) and then converted

to silver metal by heating in a hydrogen atmosphere, while

copper metal is directly electrodeposited on the filament.

Although, in both cases, the end result is the metal, the two

methods are not interchangeable. Copper which Is deposited

by converting the salt in a hydrogen atmosphere gives a very

19
unstable ion beam, and silver which is electrodeposited

directly onto the filament does not ionize efficiently.

d. Filament Temperatures and Heating Techniques

Since the integral of time-signal strength is frequently

the determining factor in the total bias of the results, it

is extremely important to devise and follow a standard

technique for all analyses that are to be intercompared.

Filament temperatures which will yield strong, stable

ion beams are determined experimentally. Generally, at a sufficiently high temperature, ion beams will first increase

in strength, then level off and finally decay. The objective

of the experimentation is to determine a filament temperature

and heating pattern which gives an extended leveling-off

period at an ion current sufficiently large for accurate

measurement of all isotopes.

In triple-filament sources, the temperature of the

ionizing filament (Re) is monitored by the Re signal. The

currents in the sample filaments are separately adjusted so

that each contributes 1/2 of the total ion beam which is to

be obtained at a specific time in the heating pattern. Specific

heating patterns for various elements are discussed in the

section on procedures, with some examples that demonstrate

that the heating pattern will alter the rate of fractionation

thus changing the average bias.

If all samples of an element are prepared and analysed in

truly identical manner, they should theoretically have the

same ion beam characteristics. However, since slight differences in chemical purity and sample size are always present

there will always be variations in the maximum stable ion

currents of the analyses. For this reason, the standard

signal strength to be used in all analyses should be made low

enough (within the bounds of accurate measurement) to be

consistently attainable in most analyses. Any individual

samples which cannot consistently attain this level of

ionization should be chemical.ly re-processed. Also, any

20
individual analyses which deviate strongly from the general

signal pattern of the majority of the analyses should be

discarded.

5. Analyses of Particular Elements

a. Introduction

A number of elements have been extensively studied,

isotopically, at this laboratory, including Br, Cs, CI, Cr,

Cu, Mg, Pu, Ag, and U. On the following pages are described

the chemical and mass spectrometr ic techniques used for each

element. Many details such as voltages, sample size, datataking period, and standard beam strength are given for each

element studied. The general pattern of controlling all

parameters is never varied, that is to say that time, temperature, sample size, and preparation are controlled, but the

fine details of the procedures are adjusted for each of the

elements. The beam strength, in particular, is a direct

function of the instrumental efficiency and the element and

the signals achieved at this laboratory may be impossible to

duplicate with less-efficient instruments, and far too large

for instruments equipped with more sensitive detectors. Other

details, particularly sample composition and mounting procedures

are the results of extensive experimentation and, in general,

attempts to mount elements in a common form other than the

one described will frequently lead to unusable or inferior ion

signals, and/or inferior statistics.

A major purpose in the isotopic analysis of multiple

samples is to determine the extent, if any, of natural variations in the isotopic abundances of an element. The correct

evaluation of real differences, as opposed to analyticallybiased differences, is based on two procedural safeguards.

The first is the continued attempt to maintain absolute

uniformity in sample handling and analytical procedure. For

some elements this is difficult and as many as 30 to 50% of

all analyses attempted are rejected. These analyses are

rejected for not maintaining the standard growth, intensity,

21
or stability pattern and are never calculated. Once a chart

has been accepted and calculated the data must be retained.

The second safeguard is to reprocess any sample that gives

results statistically different from the reference sample.

If after the prescribed number of analyses the average

isotopic values for a sample are significantly different from

the reference values, the sample is chemically re-purified to

insure against the possibility that the different analytical

results are due to chemical differences. The purified sample

is then re-analyzed the prescribed number of times and if it

is still significantly different from the reference the

difference is considered real.

When real differences are found it is generally useful

to re-sample the original "odd" material to determine the

characteristics and extent of the isotopic differences.

Frequently the differences are confined to surface layers,

where they may result from oxidation-reduction phenomena.

22
1. Analytical Procedure for Bromine

Chemistry

Water-soluble samples , brines, bitterns , and sea water

are treated as follows: A measured quantity is dissolved in

500 ml of HpO + 1 ml HNCu. The solution is transferred to a

distilling apparatus and 50 ml of the solution is distilled

into dilute ammonium hydroxide solution, to remove iodine.

Seventy-five milliliters of nitric acid are then added and

the bromine produced is distilled into dilute ammonium

solution. Since chlorine is usually the dominant halide in

natural samples, it is necessary to repeat the oxidation and

distillation to free the bromine from chlorine. After the

second distillation, the solution is made acidic with nitric

acid and AgBr is precipitated. The solution is filtered and

the precipitate dried in an electric oven (110 C). A weighed

portion of the AgBr is transferred to a small vial and

sufficient concentrated ammonium hydroxide is added so that

the resulting solution contains 3 mg Br/ml.

Samples of the mineral bromyrite (AgBr) are dissolved in

ammonium hydroxide and the silver is electrodeposited onto a

platinum gauze electrode. The solution is then made acidic

and treated as in the water-soluble samples.

Mass Spectrometry

A triple-filament rhenium-ribbon source is used. One

drop of the ammoniac al AgBr solution (60 u.g Br) is deposited

on each sample filament and dried under a heat lamp for 10

minutes. The spacing between ionizing and sample filaments

is increased by 1/32 of an inch. This reduces vaporization

of the sample by radiant heat from the ionizing filament but

does not appreciably decrease the ionization efficiency.

The Br" signals are measured at an accelerating voltage

of -3780 volts. In all negative thermal ionization work it

is necessary to keep the accelerating voltage and ionizing

filament temperature low, so as to reduce the source

efficiency for electrons.

23
The draw-out and discriminator plates are set at the

average position giving optimal signal strength, determined

during the preliminary experiments,, and never changed.

Results have shown that changing these plates can affect the

apparent isotopic ratio. Ionization is probably some

combination of thermal ionization and electron impact

ionization, with the e" impact being very minor during a

good analysis. The source and collector slit openings are 7

and 40 mils, respectively. Analyses are begun when the

—7

pressure in the tube is < 4 x 10 torr.

The following analytical procedure is used:

Time from start

(minutes

)

Procedure

The ionizing filament current is set to give a

temperature of ~ l600°C (~ 3.0 A). The 79Br

signal should be < 5 niV.

5 The sample filament currents are set at 0.5 A.

79 The vBr signal should be ~ 5 mV.

10 The sample filament currents are adjusted to

79 give a 10 mV vBr signal.

15 The sample filament currents are adjusted to

79 give a 15 mV ^Br signal. At this point the

signal should begin to grow slowly. If it has

not grown to 25 mV by 30 minutes, the signal is

increased to this level by increasing the

ionizing filament current. This current

should be 3.0 - 3.2 A and must never be raised

above 3.4 A because excessive electron emission

will occur. The baseline reading is adjusted

to zero.

40 Data-taking begins. The ^Br signal should be

30-40 mV (3-4 x 10 ^ A). Ten peak sets are

taken over the next 10-12 minutes. Each peak

is monitored for 1/2 minute and magnet changes

are made in discrete steps.

24
Bromine signals are easy to control and very stable.

All analyses should be made on slowly growing signals. Signals

which do not increase by more than 25$ during the analyses

will not have an observable change in ratio (less than 0.1$)

over a 12 minute period.

Notes

The use of NaBr as the sample material leads to erratic

Br" signals. The voluminous electrons given off by the

sodium overload the field and cause signal instability by

changing potentials on plates and defocusing the ion beam.

Attempts to obtain significantly higher beam intensities,,

as a regular procedure, while maintaining stable signals were

not successful. This element, and several others that have

a fairly sharply defined maximum signal intensity, can be

used to estimate the relative transmission efficiency of

different mass spectrometers. Attempts to double or triple

the ion intensity will result in a completely different

signal pattern, generally a very rapid decay.

In general, background signals are completely negligible

but for analyses of highly separated isotopes the ionizing

filament should be pre-baked. Also, the analysis of highlyseparated isotope sample may have a residual effect on

succeeding analyses, so that the source should be thoroughly

cleaned after such analyses.

25
26
2. Analytical Procedure for Cesium

Chemistry

Cesium nitrate is the salt form used and the final

solution is adjusted to 0.1 N HNO^ and 3 M-g Cs/ml.

Mass Spectrometry

A single-filament platinum-ribbon source is used. One

drop of solution ( ~ 0.06 [ig Cs) is placed on the filament

and dried under a heat lamp for 10 minutes.

+

The Cs signals are measured at an accelerating voltage

of 10 kV. The source and collector slit openings are 7 and

40 mils s respectively. Analyses are begun when the tube

—1

pressure is < 4 x 10 torr.

The following analytical procedure is used:

Time from start

(minutes) Procedure

0-1 The filament current is set at 1 A.

3 The filament current is adjusted to give a

3 mV Cs grid signal. The signal should grow

steadily.

15 The Cs grid signal should have grown to about

30 mV. The filament current is adjusted to

give a 150 mV grid signal. The signal should

continue to grow.

40 Data-taking begins. The grid signal should

have grown to about 300 mV (3 x 10" A). Ten

peak sets are taken over the next 10-12

minutes. Each peak is monitored for 1/2

minute and magnet changes are made in discrete

steps

.

All data should be taken on constant or slowly growing

signals. In such cases,, the observed ratio changes by <0.1$

over a 30 minute period.

27
Notes

The onset of cesium emission is very sharp and occurs at

a filament current of 1.0 to 1.2 A. The signal grows slowly

and steadily but may be erratic; there are '.momentary gross

changes in signal intensity during the first 1/2 hour of

heating. The exact cause of this erratic behavior is not

known and is not observed during each analysis,, but the signal

instabilities normally cease before the start of data-taking

(40 minutes)

.

Efforts to get a 150 mV (grid) growing signal may require

several temperature adjustments. If the signal decays strongly,,

wait 2 minutes and readjust the signal to 150 mV. Up to 10

minutes (5 adjustments) are allowed to obtain a growing signal.

The Cs emission is very sensitive to small changes in temperature and a cautious approach is used to obtain a growing

signal in order to avoid overheating the sample and accelerating the rate of sample depletion. Filament currents of 1.2

to 1.4 A are normally sufficient to give the required signal.

The filaments are prebaked at 2.2 A for 1/2 hour in a

vacuum and under a potential field,, to eliminate the

possibility of barium background signals from the filament

material.

Single-filament platinum- ribbon sources give far more

efficient ionization of cesium than do single-filament

rhenium, tungsten , and tantalum,, or triple-filament rheniumribbon sources. With a platinum filament ^ the sample size

can be small enough so that very radioactive samples may be

analyzed without undue hazard to the analyst or excessive

contamination of the mass spectrometer.

No extensive study of Cs minerals has been made at this

laboratory; only occasional service analyses.

28
3. Analytical Procedure for Chlorine

Chemistry

The natural samples,, halite and sylvite^ are dissolved

in HpO. A few drops of nitric acid and an excess of silver

nitrate are added. The precipitated silver chloride is

allowed to stand in the dark overnight. The solution is

filtered through a fine-porosity porcelain filter and the

precipitate is washed with distilled water. The silver

chloride is weighed (in the filter ) } dissolved in 1 ml of

concentrated ammonium hydroxide and drawn through the filter

into a small vial. The filter is washed twice with 0.5 ml

of ammonium hydroxide. The final solution is adjusted to

15 mg Cl/ml by adding the appropriate amount of concentrated

ammonium hydroxide.

Mass Spectrometry

A triple-filament rhenium-ribbon source is used. One

drop of ammoniac al silver chloride solution ( ~ 300 u.g CI)

is placed on each sample filament and dried in the following

manner: 5 minutes with heat lamps only; 2 1/2 minutes at

1 A; 2 1/2 minutes at 1.5 A. The current is then increased

until the deposit flows and then redrys (1.8 - 2.0 A). The

spacing between ionizing and sample filaments is increased

by 1/32 of an inch. This reduces vaporization of the sample

by radiant heat from the ionizing filament but does not

appreciably decrease the ionization efficiency.

The Cl" signals are measured at an accelerating voltage

of -3780 volts. In all negative thermal ionization work it

is necessary to keep the accelerating voltage and ionizing,

filament temperature low so as to reduce the source efficiency

for electrons.

The draw-out and discriminator plates are set at the

average position giving optimal signal strength^ determined

during the preliminary experiments , and never changed.

Results have shown that changing these plates can affect the

apparent isotopic ratio. Ionization is probably some

29
combination of thermal plus electron impact i with the e~

impact being very minor during a good analysis. The source

and collector slit openings are 7 and J4-0 mils,, respectively.

Analyses are begun when the pressure in the tube is <4 x

-7

10 torr.

The following analytical procedure is used:

Time from start

(minutes) Procedure

The ionizing filament current is set to give a

CI grid signal of 1/2-1 mV ( ~ 3.2 A).

3 The sample filament currents are set at 0.5 A.

35 The ^Cl signal should be ~ 10 mV.

5 The sample filament currents are increased to

35 give a CI signal of 150 mV. The signal will

generally decay to about 100 mV in the next

few minutes.

10 The sample filament currents are adjusted to

35 give a -CI signal of 150 mV. At this point

the signal will begin to grow and should

continue to grow steadily for the remainder

of the analysis.

35

25 The -^Cl signal should have grown to 250-300

mV. If not. it is increased to 250 mV

-12 . (2.5 x 10 A) by raising the sample filament

currents. Baseline readings are taken.

40 Data-taking begins. The CI signal should

be ~ 350 mV. Ten peak sets are taken over the

next 10—12 minutes. Each peak is monitored

for 1/2 minute and magnet changes are made in

discrete steps.

Notes

The use of NaCl as the sample material leads to erratic

CI*" signals. The voluminous electrons given off by the sodium

overload the field and cause signal instability by changing

potentials on plates and def ocusing the ion beam . 30
Chlorine ions are virtually the only background signal

in the negative ion spectrum and, though small, the background interferes with the analyses of the separated isotopes

The method used to circumvent this problem was to use the

same set of filaments over and over until the background

spectrum of these filaments approached that of the separated

isotopes.

Also the analysis of a highly separated isotope sample

may have a residual affect on succeeding analyses , so that

the source should be thoroughly cleaned after such an

analysis.

31
32
J

4. Analytical Procedure for Chromium

Chemistry

Chromite and Cr-Silicate samples are crushed and fused

with sodium carbonate. The melt is dissolved in an excess

of dilute perchloric acid and the solution is evaporated at

140-150 C until fuming begins. This precipitates the silica

and oxidizes the Cr (III) to Cr (VI). After filtration, the

solution is adjusted to pH 8 with ammonium hydroxide and

hydrogen peroxide is added to insure complete oxidation of the

chromium. The solution is then filtered to remove insoluble

hydroxides, and sulfurous acid is added to reduce the Cr (VI)

to Cr (III) and thus cause precipitation of Cr(OH)^. The

Cr(0HK is filtered, ignited to Cr

? 0^ at 800 C, and weighed.

The Cr

p 0^ is taken into solution by digestion with perchloric

O acid at 140-150 C. Sulfurous acid is then added to reduce

the Cr (VI) to Cr (III) and Cr(0HU is precipitated by

adjusting to pH 8 with dilute ammonium hydroxide. The Cr(0H) Vi is caught on a filter and dissolved in hot HNO^ (1+9).

This solution is evaporated to dryness and the residue is

dissolved in an amount of HNO^ (1+99) so that the Cr

concentrated in 3 mg/ml.

Two minerals are taken into solution by other methods.

Crocoite (PbCrO^) is dissolved in perchloric acid, the lead

is removed by electrodeposition, and the chromium is precipitated as above. Merumite (Cr

2 0, • H

2 0) is dissolved in

hot perchloric acid at l40-100°C and treated as above.

Mass Spectrometry

- A single-filament platinum-ribbon source is used. The

sample is mounted in the following manner: A drop of solution

( - 60 [ig Cr) is placed on the filament and dried with a heat

lamp and 1.3 A of current for 5 minutes; the current and lamp

are turned off and a drop of 10$ HN0, or distilled water is

placed on the filament; the sample is then re-dried with the

heat lamp and a current of 1.2 A for 5 minutes ; 1.6 A for 1

minute, and 2.2 A for 1 minute. The drop of acid or water

33
serves to spread the sample evenly on the filament; the two

reagents appear to work equally well.

The Cr signals are measured at an accelerating voltage

of 10 kV on the 12-inch mass spectrometer and 7.75 kV on the

6-inch mass spectrometer. The source and collector slit

openings are 7 and 40 mils, respectively, for the 12-inch

instrument and 6 and 30 mils, respectively, for the 6-inch

instrument. Analyses are begun when the tube pressure is

< 4 x 10 ' torr.

The following analytical procedure is used:

Time from start

(minutes) Procedur e 0-1 The filament current is set at 2.2 A. The

52 Cr signal is found and focused.

2 The filament current is increased to 2.3 A.

4~l4 The filament current is increased 0.1 A every

2 minutes until the Cr signal begins to grow

rapidly.

52 10-15 The Cr signal should have grown very rapidly.

"When it reaches 20 V, the filament current is

52 reduced so that J Cr = 2 V (generally 2.0 - 2.3 A).

15-25 The signal should grow slowly. It is kept

between 2 and 3 V by appropriate adjustment

of the filament current.

25 The final filament current adjustment is made

52 to give a Cr signal of 2 V. Baseline readi:

are taken.

52

37 Data-taking begins. The Cr signal should

be 2—3 V and slowly growing. Each of the

three less abundant isotopes is measured

52 relative to the ^ Cr isotope. Groups of four

results are obtained for each ratio in the

following sequence: 50/52, 53/52, 54/52,

53/52, 50/52 giving 8 results for the 50/52

34
and 53/52 ratios and 4 for the 5^/52 ratios.

Since the sequence is symmetric any changes

due to fractionation will balance out and no

secondary time dependent bias is introduced

between the minor isotopes. The Gr signal

should increase slowly but steadily throughout

the data-taking period (20-25 min. ) . Each

peak is monitored for 1/2 minute and magnet

changes are made in discrete steps.

With a stable or slowly growing signal, the 50/52 ratio

will generally decrease by ~0.1$ during the data-taking

period (20-25 min.). However the observed ratio may change

much more rapidly if a decaying signal is used.

Notes

If large amounts of the alkali elements are present in

a sample, they will inhibit the ionization of the chromium

and the chromium signal will not grow until the alkali ion

signals have degraded to a reasonable level.

A number of different mounting procedures were tried,

including drying in air to a green oxide, drying in air to a black oxide, drying in an 2 atmosphere, and drying in a H

2

atmosphere, all without the 10% HNO^ (or H

2 0) drop. All of

these procedures lead to poor sample-filament contact and

generally resulted in erratic analyses.

S2

Very strong ( > 20 V) Cr signals may be obtained quite

easily but such signals are generally erratic, decay rapidly,

and are difficult to standardize.

Triple-filament Re sources give strong, stable Cr signals

but are unusable because of a large Cr background signal from

the high temperature ionizing filament. In addition, when

the side filaments or triple filament sources are heated hot

enough to vaporize the chromium they also give a strong ion

beam.

All filaments are prebaked at 2 A for 1 hour in a vacuum

and under a potential field.

35
36
5. Analytical Procedure for Copper

Chemistry

A weighed quantity (1 to 3 gm) of mineral is treated

with dilute (1:2) HNCu and digested overnight on a steam bath.

The solution is filtered and evaporated to dryness, and the

residue is dissolved in 75 ml of an aqueous solution containing 1 ml of HNCU and 1 ml of HpSOh. The copper is electrodeposited onto a platinum gauze cathode at a current of about

0.2 A. The cathode is then removed and washed with water

while the smaller platinum gauze anode is cleaned with dilute

HNO^ and Hp0„ to remove any oxides of lead and manganese which

may have been deposited. The original electrolyte is discarded

and replaced with a fresh batch. The polarity of the

electrodes is reversed so that the original cathode becomes

the anode, and the copper is electrolytically stripped from

the anode and deposited on the cathode. The cathode is then

removed, washed with water, dipped in ethanol, and dried for

1 minute at 105 C. The weight of the copper is determined

from the weight of the electrode before and after deposition.

The copper is dissolved from the electrode with dilute HNO^

and the solution is evaporated to dryness. The residue is

redissolved in dilute HNO^ and the solution is brought up to

a volume so that the copper concentration is 10 mg/ml.

Samples whose mass spectrometric results indicate the

need for further purification are treated as follows: The

sample solution is mixed with dilute (1:2) HN0-, and heated

overnight on a steam bath. The solution is filtered and the

filtrate is evaporated to dryness. The residue is dissolved

in dilute HC1 and re-evaporated to dryness to remove HNO^,.

The residue is then dissolved in 100 ml of IN HC1 and the

copper is precipitated with H

?S. After filtration, the

precipitate is dissolved in dilute HN0-,, evaporated to dryness,

dissolved in HC1 and re-dried again to remove the HNO-^. The

residue is dissolved in dilute HC1 and the solution is made

ammoniacal, filtered, and treated with Hp

S . The solution is

37
then filtered and the precipitate of CuS is ignited to CuO

at 500°C. The CuO is dissolved in dilute HNCU and the solution

is evaporated to dryness. The residue is dissolved in dilute

HNCu-HpS(X solution and the copper is electrodeposited once,

with the final solution being prepared as described above.

For samples in which the copper concentration is low,

the copper is first concentrated by precipitation with HpS

and then purified by the electrodeposition method described

above

.

Mass Spectrometry

A triple-filament rhenium-ribbon source is used. One

drop of solution is placed on each sample filament and the

copper is electrodeposited onto the filament. Platinum wire

is used as the anode. The potential difference is 3 1/2 volts

and a 1 ma current is passed through the solution for 5

minutes. The estimated sample size is 200 |±g Cu per sample

filament. The spacing between ionizing and sample filaments

is increased by 1/32 of an inch to reduce sample evaporation

by radiant heat from the 2100 C ionizing filament.

The Cu signals are measured at an accelerating voltage

of 10 kV. The source and collector slit openings are 7 and

40 mils, respectively. Analyses are begun when the tube

pressure is < 4 x

10**' torr.

The following analytical procedure is used:

Time from start

(minutes

)

Procedure

0-1 The ionizing filament current is set to give

a 50 mV Re grid signal (5-6 A), The sample

filament currents are set at 0.5 A, while the

Re signal is being monitored.

4-6 If necessary, the ionizing filament current is

readjusted to give a 50 mV Re grid signal. The

sample filament currents are slowly increased

63 until the Cu signal is detectable. The Cu

peak is located and focused. The sample

38
filament currents are adjusted to give a 6 mV

5Cu signal (0.8 - 1.0 A).

12 The sample filament currents are adjusted to

63 give a 50 mV Cu signal. The signal may

decay for a brief period but should then begin

to grow slowly ( ~ 1 mV/min:).

63 18 If the ^Cu signal has not grown to at least

55 hiVj the sample filament currents are

increased to obtain this intensity. The signal

should continue to grow slowly and no further

adjustments should be made.

63 25 Data-taking begins. The Cu signal should be

40-50 mV. Ten peak sets are taken over the

next 10-12 minutes. Each peak is monitored

for 1/2 minute and magnet changes are made in

discrete steps.

All data should be taken only on constant or growing

signals. Under these conditions the observed ratio does not

change with time. Data taken on rapidly decaying signals may

show a rapid change in observed ratio with time.

Notes

During the first 12 minutes of heating the primary

objective is to allow the ion source to warm up and approach

thermal equilibrium. The temperature of the ionizing filament is approximately 2100 C and is hot enough for efficient

ionization but cool enough so that radiant heat effects are

negligible.

The accumulation of copper on the ion source during the

normal lifetime of 30 to 40 sample-loadings is not large

enough to cause memory or give a detectable source background

signal. The rhenium filaments contain a minute quantity of

+

' -16

copper which gives a Cu current of less than 1 x 10 A at

2100 C } and is detectable only with an ion multiplier. Only

after extensive periods of baking is it possible to reduce

the filament background signal to zero intensity.

39
Of all the isotopic ratios of elements studied at this

laboratory,, the copper ratio is probably the most sensitive

to variation in analytical technique or heating pattern. For

careful work the 95$ confidence limit of a single determination

is-'0.15$ of the ratio. The average long-term observed

^Cu/ 5 Cu ratio is 2.2750, but ratios 1 to 2% lower than this

value can be consistently obtained by simply overheating the

filaments or changing the time-signal strength relationship.

Failure to adhere to a strict time-signal strength relationship can lead to a significant expansion of the analytical

limit of error , thereby eliminating the possibility of

detecting small but real differences in the copper isotope

ratios of various minerals. It was not possible to routinely

obtain stable signals greater than 50 mV (5 x 10 A)

.

This element, and several others that have a fairly

sharply defined maximum signal intensity, can be used to

estimate the relative transmission efficiency of different

mass spectrometers. Attempts to double or triple the ion

intensity will result in a completely different signal

pattern, generally a very rapid decay.

40
6. Analytical Procedure for Magnesium

Chemistry

Acid soluble materials are taken into solution with dilute

hydrochloric acid. Silicate materials are fused with sodium

carbonate before being dissolved in dilute hydrochloric acid.

The magnesium is purified and recovered by a variation of the

method of Bricker and Parker^ '. An ammoniacal solution of

(ethylenedinitrilo) tetraacetic acid is used to precipitate

the magnesium as magnesium dihydrogen (ethylenedinitrilo)

tetraacetate hexahydrate (MgC-^H^Og]^ • 6^0). A double

precipitation is used to insure the removal of sodium which

would present a major problem in the mass spectrometry.

Although the recovery of magnesium_, by this procedure , is high (>95$)j> it is not quantitative; about 0.5 mg of

magnesium is left in solution when ~ 10 mg of magnesium is

precipitated. However mass spectrometric examination has

shown that the isotopic composition of the magnesium left in

solution is identical with that of the precipitated magnesium,,

so there is no significant fractionation during the sample

preparation.

The magnesium complex is ignited to MgO at 1000 C and

dissolved in dilute HC1. Since pure MgO does not adhere well

to rhenium filaments some uranyl nitrate is added to the

solution to act as a binding agent. The final solution

contains 5 mg Mg/ml + 5 mg U/ml and lOfo HNO-^.

Mass Spectrometry

A triple-filament rhenium-ribbon source is used. One

drop of solution ( ~ 100 |ig Mg) is placed on each sample

filament and dried with a heat lamp and an electrical current

which is increased until the uranium forms an orange oxide.

The electrical current is adjusted in the following manner:

mv

' Bricker, C. E. and Parker, G. H., Anal. Chem. _29, 1470-

1474, (1957).

41
1.5 A for 5 minutes; 1.8 A for 1/4 minute; slowly increased

(period of ~ 1/2 min, ) until orange uranium oxide forms

(2.0 - 2.5 A).

One internal modification is made on the 12-inch mass

spectrometer,, for Mg analyses. The opening of the first

baffle on the collector is narrowed to 5/8". Since the

masses are about 1/2" apart in this mass range, a l" opening

25 24'

puts the Na peak on the edge of the baffle when the ' Mg is

being collected, causing troublesome edge-effects.

The Mg signals are measured at an accelerating voltage

of 8000 volts*. Higher voltages will increase the size of

the Mg signal but will also greatly increase the number of

secondary electrons from the Na beam with consequent

deleterious effects on the baselines under the Mg peaks.

The source and collector slit openings are 7 and 40 mils,

respectively. Analyses are begun when the tube pressure is

< 2 x

10"* 7 torr.

The following analytical procedure is used:*

Time from start

(minutes

)

Procedure

0-1 The ionizing filament current is adjusted to

give a 100 mV Re grid signal ( ~ 6 A, T«2100°C)

1 The sample filament currents are set at 2.4 A' while the Re signal is being monitored.

4 The ionizing filament current is readjusted to

give a 100 mV Re grid signal.

24

5 The Mg signal is found and focused on. If

this signal is > 200 mV the sample filament

currents are lowered and then readjusted so

24 that each one gives 75 mV of Mg signal.

10 The sample filament currents are adjusted to

24 give a 200 mV ' Mg signal. The signal will

generally grow slowly.

For the 6-inch instrument.

42
15 The sample filament currents are adjusted to

i 24 give a 400 mV Mg signal. The signal will

increase slowly for the remainder of the

analysis.

20 Baseline readings are taken.

27 Data-taking begins. Ten peak sets are taken

over the next 20-25 minutes. A complete peak

set includes five mass peaks: 24-25-26-25-24,

with each interior 24 peak being the last and

first peak of successive sets. Each peak is

monitored for 1/2 minute and magnet changes

are made in discrete steps.

There is significant fractionation during the mass

spectrometric analysis. Over a 25 minute period, the

Mg/ Mg ratio will usually decrease by approximately 0.5$.

In view of this it is very important to follow a strict time

schedule of filament heating and signal strength and to

restrict the data-taking to a particular time interval. It

is generally possible to obtain growing signals in all Mg

analyses, and such signals have relatively slow rates of

change in observed ratios with time. However, rapidly decaying signals usually have rapidly changing observed ratios and

such analyses should always be discarded; they invariably

24 give results which are low in Mg.

¥hen a 12-inch instrument is used for magnesium analysis

the Re grid signal is set at 75 mV and Mg signals obtained

at the various stages are only 1/2 those obtained with the

6-inch instrument. For magnesium the 6-inch instrument is

approximately twice as efficient as the 12-inch instrument

and although 400 mV signals can be obtained with a 12-inch

instrument they are generally decaying and a high percentage

of the analyses are unusable.

43
Notes

Attempts to mount the magnesium from chloride and nitrate

solutions with no uranium lead to erratic (and abortive)

results because of frequent partial or complete flake-off.

Analyses using 1-5$ HNCU solutions (with uranium) did not

have the signal stability attained when 10$ HNCU was used.

The center filaments are always prebaked (1/2 hr . at

5 A) in a vacuum and under a potential field (500 V) to reduce

the background Na signal from the Re ribbon. Aside from an

alcohol wash, the sample filaments are not pretreated in any

manner; prebaked sample filaments increase the likelihood of

flake-off. The background signal of the entire filament

set-up is negligible, being virtually non-existent ( < 1 mV

A small amount ( ~ 1$) of source memory can sometimes be

detected if samples of widely different isotope composition

are analyzed back to back on the same source. The memory

appears to occur only after an analysis in which particulate

sample matter has flaked-off the filaments and adhered to the

source plates. The succeeding analyses then may show memory

of the flaked-off sample.

It is of primary importance to keep the sodium signal at

a minimum. Although there is no problem at all in resolving

25 24 the Na and Mg peaks, the scattering of secondary electrons

produced by the Na beam can cause sharp and significant

25 26 depressions in the baselines under the ^Mg and Mg peaks.

The Na signal is minimized by careful chemistry and by

prebaking the ionizing filament. The effects of the small

but invariably present Na beam are made negligible by

restricting the size of the baffle and by the use of the

usual, but in this case particularly important, suppressor

grids in the collector.

44
7 . Analytical Procedure for Plutonium

Chemistry

Plutonium sulfate tetrahydrate is prepared from highpurity metal. Any americium present in the sample is

removed by the use of a small column of Dowex 1x4 ionexchange resin. An 8 N nitric acid solution of the sample is

put through the column and the plutonium is absorbed while the

americium is not. The plutonium is then stripped from the

column with 0,3 N nitric acid and the final solution is

adjusted to 0.1 mg Pu/ml, in 2N HNO, . Mass Spectrometry

A triple-filament rhenium-ribbon source is used. One

drop of solution ( ~ 2 u.g Pu) is placed on each sample

filament and dried by the use of a heat lamp and an electrical

current. The current is adjusted in the following manner:

1 A for 5 minutes; 1.2 A for 3 minutes; slowly increased to

approximately 2 A,, over a 1 minute period, to convert the

plutonium sulfate to plutonium oxide.

The Pu signals are measured at an accelerating voltage

of 10 kV. The source and collector slit openings are 7 and

25 mils,, respectively.

After the sample is loaded into the mass spectrometer

and a pressure < k x 10 torr is achieved, the filaments are

heated in order to dehydrate the sample and eliminate this

source of oxygen, which would otherwise lead to the formation

of a strong PuO signal. The dehydrating procedure is as

follows: The ionizing filament current is adjusted to give a rhenium grid signal of 70 mV. The sample filament currents

are set to give a Pu grid signal of K0 mV. All filaments are

left on for 10 minutes, and then turned off and allowed to

cool for at least 20 minutes. Analyses are begun when the

tube pressure is < 1 x 10 torr.

45
The following analytical procedure is used:

Time from start

(minutes

)

Procedure

0-1 The ionizing filament current is adjusted to

give a 100 mV Re grid signal. (The corresponding Re signal is 65O mV) . The sample

filament currents are set at 1.4 A. The Pu

signal is usually not detectable.

5 Check the focus of the rhenium signal. The Re

signal will usually decay during the first few

minutes of heating, so the ionizing filament

current is readjusted to give a 650 mV Re

signal.

6 The sample filament currents are increased

until the Pu signal is located and then

adjusted to 40 mV grid.

11 Check the Re signal intensity and, if

necessary, adjust to 650 mV.

12 The sample filament currents are increased to

give an 80 mV Pu grid signal.

18 The sample filament currents are increased to

give a 120 mV Pu grid signal.

24 The signal should be between 120 and 15 mV

and growing slowly. No further adjustments

should be made.

30 Data-taking begins. The Pu grid signal should

be approximately 150 mV and nearly constant.

Ratio determinations are made in the following

order

1. 23 W240

Pu

2.

240Pu/24lPu/242

Pu

3.

2 ?9pu/240

Pu

Each peak is monitored for 1/2 minute and

magnet changes are made in discrete steps.

46
239 240 The Pu/ Pu ratio will usually decrease by about

O.lfo over a j)0 minute period.

Notes

The behavior and characteristics of plutonium analysis

are similar to that of uranium analysis. The temperature of

the sample filament for plutonium analyses (2.0 - 2.3 A) is

slightly lower than that for uranium analysis s and can be

controlled for both low and high temperature analyses.

47
48
8. Analytical Procedure for Silver

Chemistry

The sample is dissolved in nitric acid. The solution is

filtered and the silver is precipitated as silver chloride.

The dried silver chloride is placed on a layer of sodium

carbonate in a porcelain dish, and is reduced to metallic

silver by directing a gas-oxygen flame on it from above. The

metal is washed free of sodium carbonate and dissolved in

nitric acid. The solution is adjusted to a concentration of

10 mg of silver per ml of 1:25 nitric acid.

Mass Spectrometry

A triple-filament rhenium-ribbon source is used. One

drop of solution ( ~ 200 u.g Ag) is placed on each sample

filament and dried with a heat lamp and an electrical current

of 1 A for 5 minutes, in air. The sample is then converted

to metallic silver by placing the filament in a hydrogen

atmosphere and increasing the electrical current until the

filament begins to glow red (2.0 - 2.8 A). The spacing

between ionizing and sample filaments is increased by 1/32

of an inch to inhibit sample evaporation by radiant heat from

the white-hot ionizing filament.

The Ag signals are measured at an accelerating voltage

of 10 kV. The source and collector slit openings are 7 and

40 mils, respectively. Analyses are begun when the tube

pressure is <,4 x 10 torr.

The following analytical procedure is used:

Time from start

(minutes

)

Procedure

0-1 The ionizing filament current is set to give

a Re signal of 650 MV. The sample filament currents are set at 0.25 A.

3-4 The ionizing filament current is readjusted

187 107

to give a Re signal of 650 mV. Ag is

located and the sample filament currents are

;ive

49

107 increased to give a Ag signal of 70 mV
(7 x 10 A). During the next few minutes

the signal may decay at first but should then

begin to grow.

15 The sample filament currents are increased to

give a 107Ag signal of 120 mV (1.2 x 10~ 12 A).

The signal should continue to grow.

107 20 Data-taking begins. The Ag signal should

be, stable at ~ 150 mV. Ten peak sets are

taken over the next 10-12 minutes. Each peak

is monitored for 1/2 minute and magnet _ changes

are made in discrete steps.

Under the above conditions the Ag/ Ag ratio will

generally decrease by about 0.25;$ over a 20-minute period.

Notes

If large signals are not necessary it is possible to use

200 u.g (total) samples and reduce the sample filament

temperatures to give 50 to 50 mV signals which will grow

slowly throughout the analyses. Under these conditions the

ratio will not change with time ( < 0.1%).

With very small samples ( < 5 M-g) > the above procedure

must be modified. The desired signal strength is achieved

sooner and data-taking is begun after only 15 minutes.

Generally, when very small samples are used the signal will

grow, level off., and decay very rapidly, so the time schedule

must be condensed.

Care must be taken not to overheat the sample filaments.

If they are overheated the signal will grow rapidly and then

decay rapidly, giving rise to rapidly changing observed

ratios. Once the decay process has begun, cooling the

filaments will not suspend it.

Attempts to analyze silver as the nitrate, sulphide, and

chloride, without converting it to metallic silver, were not

successful.

50
Significant amounts of zirconium, either in the filament

or the sample , can lead to the formation of a zirconium

oxide ( y Zr 0) background signal at the 107 mass position,

as well as at the 106, 108, 110, and 111 mass positions.

Baking out the filament eliminates the zirconium from the

source, but if zirconium is present in the sample in

significant amounts it is sometimes impossible to reduce the

mass 107 zirconium oxide signals to a negligible value before

the data-taking period begins.

51
52
9. Analytical Procedure for Uranium

Chemistry

Uranium standards, as U^,0o, are dissolved in dilute

nitric acid (1:1) and the final solutions are adjusted to

contain 5 mg of uranium per milliliter, in 5$ nitric acid.

Mass Spectrometry

A triple-filament rhenium-ribbon source is used. One

drop of solution (100 pig U) is placed on each sample filament

and dried with a heat lamp and an electrical current. The

electrical current is adjusted as follows: 1 A for 5

minutes] 1.2 A for 3 minutes; slowly increased (period of 1

minute) until the yellow-orange oxide forms (1.8 - 2.5 A).

Heating too quickly may cause the sample to flake-off. Overheating the filament will convert the sample to a black

uranium oxide, which is to be avoided since the yellow-orange

oxide generally gives superior performance.

The U signals are measured at an accelerating voltage

of 10 kV. The source and collector slit openings are 7 and

25 mils, respectively. The sample is degassed for 10 minutes

and then allowed to cool and pump down for at least 30

minutes before the actual analysis is begun. The primary

purpose of the degassing step is to dehydrate the sample.

This eliminates a source of oxygen which would otherwise lead

to the formation of a large U0 signal. A secondary function

of degassing is to drive-off uranyl nitrate that is not

converted to U0^ during sample preparation. The magnitude of

this signal at low sample filament temperatures (1.5 to 2.0 A)

is occasionally greater than 5 x 10"* A, but is decaying too

fast for a precise ratio determination. Degassing is begun

when the tube pressure is 4 x 10 torr. An accelerating

voltage of 8 kV is used, to decrease the likelihood of high

voltage breakdown during the first few minutes of heating,

when large amounts of volatile material are driven off.

53
The signal strength necessary for a precise analysis is

determined by which isotopic ratios are to be measured. The

U and U abundances generally amount to at least 98% of

235 238 the total sample and if only a U/ U ratio is to be

measured, a total uranium signal of ; x 10"" A is sufficient.

Such an analysis is called a "low-temperature" analysis and

requires sample filament currents of 2.2 to 2 5 A, with a corresponding uranium grid signal of 0.3 volts.

Precise determination of the isotopic abundances of all

uranium isotopes in a sample requires a large ion current

(1-4 x 10~ A), since the individual U and U abundances

are usually less than 1%, Such an analysis is cabled a "high-temperature" analysis and requires sample filament

currents of 2.6 to 3.0 A, with a corresponding uranium grid

signal of 1.2 volts (4 volts on instrument equipped with Z

lens )

.

The following analytical procedure is used:

Degassing of Samples

Time from start

(minutes) Procedure

0-1 The ionizing filament current is adjusted to

give a detectable Re grid signal. The Re

peak is located and focussed. The ionizing

filament current is adjusted to give a 600 mV

Re signal. The sample filament currents

are set at 1.5 A.

3-4 Check the focus .of the rhenium signal. If

necessary, the ionizing filament current is

readjusted to give a 600 mV 'Re signal.

The uranium is found and the sample filament

currents are increased to give a 100 mV U grid

signal. This signal will usually decay during

the degassing step.

10 All filaments are turned off.

54
Low-Temperature Analysis

0-1 The ionizing filament current is adjusted to

give a detectable Re grid signal. The sample

187 filament currents are set at 1.5 A. The Re

peak is located and focused for maximum

intensity. The ionizing filament current is

then adjusted to give a 650 mV Re signal.

The corresponding Re grid signal is ~ 100 mV

and the filament temperature is ~ 2150 C. The

uranium signal will usually be less than 3 x

10" 15 A.

5 Check the focus of the rhenium signal. The

Re signal will usually decay during the first

few minutes of heating so the ionizing filament

current is readjusted to give a 650 mV Re

signal. The U signal is located.

6 The sample filament currents are increased to

give a 100 mV U grid signal.

11 Check the Re signal intensity and,, if

necessary,, adjust to 650 mV.

12 The sample filament currents are increased to

give a 200 mV U grid signal.

18 The sample filament currents are increased to

give a 300 mV U grid signal.

2k If the signal intensity has changed significantly (+ 50 mV) , the sample filament currents

are readjusted to give a 300 mV U grid signal.

30 Data-taking begins. The U grid signal should

be approximately 300 mV. Ten scan sets are

taken. Each peak is monitored for 1/2 minute

and magnet changes are made in discrete steps.

55
High-Temperature Analysis

0-1 The ionizing filament current is adjusted to

give a detectable Re grid signal. The sample

i P\i filament currents are set at 1.5 A. The Re

peak is located and focused for maximum

intensity. The ionizing filament current is

then adjusted to give a 650 mV Re signal.

The corresponding Re grid signal is ~ 100 mV

and the filament temperature is ~ 2150 G.

The uranium signal will usually be less than

5 x 10~1:5A.

5 Check the focus of the Re signal. The Re

signal will generally decay during the first

few minutes of heating, so the ionizing

filament current is readjusted to give a 650 mV 'Re signal. The U signal is located.

6 The sample filament currents are increased to

give a 600 mV U grid signal. (Sample filament

currents approximately 2.5 - 2.7 A).

I87 11 Check the Re signal intensity and, if

necessary, adjust to 650 mV.

12 The sample filament currents are increased to

give a 1.2 volt U grid signal. The signal will

generally decay during the next few minutes.

However, if it should grow, or decay and then

grow, it should not be allowed to exceed 1.2

volts.

18 If the signal intensity has changed significantly (+ 200 mV) , the sample filament currents

are readjusted to give a 1.2 volt U grid

signal.

24 If the U grid signal is ~1.1 volts and is

constant, slightly growing or decaying, no

further adjustments are necessary.

56
30 Data-taking begins. Seven peak sets of

U/ U are taken first; each peak monitored

for 1/2 minute and magnet changes are made in

discrete steps. Then 2 ^U/255 U and 2 ^ U/2;55U

ratios are determined by continuous mass

scanning 5 which is necessary for the accurate

measurement of baselines under the small

peaks. Ten peak seta are taken. Finally

23R 258 another 7 peak sets of U/ U are taken, as

before.

Less than 5 minutes are normally required to obtain a constant Re signal of 6.5 x

10"* A. Provision is made in

the analytical procedures for filaments that require more than

5 minutes to yield a constant Re ion current. The intensity

and constancy of the Re signal for such filaments must always

be checked before the sample filaments are adjusted to yield

more intense signals. If the Re signal intensity is not

—12 —12

constant or slowly changing between 6 x 10 and 7 x 10" ' A

after 18 minutes of heating, the analysis is discarded.

When the Re signal is significantly outside this range,

the ionizing filament temperature is either too hot or too

cool. This means that the sample filaments temperature will

have to be too cool or too hot, in order to obtain the

standard signal intensity. The net effect of this temperature

compensation will be to decrease the precision of the

measurements.

The uranium signal has been observed to go through a

growth cycle shortly after a significant signal is obtained.

This growth cycle is, in most cases, less than 10 minutes in

duration. After the growth cycle, there may be a period of

short duration when the signal is constant. This cycle is

followed by decay. Continued adjustment of the sample filaments will improve the slope of the decaying signal but does

not cause an immediate reversal to the growth cycle. During

57
periods of extensive high-temperature work, the U signal

decayed most of the analytical time (approximately 1 1/2

hours) before approaching a constant intensity. Occasionally,

the signal will decay, stabilize, and grow during data-taking

for high-temperature analysis. The behavior (growing,

constant, or decaying) of the uranium signal is not attributed

to sample size since only a small portion of the total sample

(200 |ig) is used for analysis.

The U signal will normally decay for most hightemperature and low-temperature analyses. The isotopic

ratios are, within the limits of analytical precision,

independent of signal behavior with the possible exception of

rapid and long-term growth. Strongly growing signals during

data-taking are rare and can be considered a good indication

of overheating for the particular technique used and the

isotopic ratios are likely to show the effect of increased

fractionation. If the signal is decaying during data-taking,

the limiting factor is the ability of the analyst to accurately

plot and read peak-heights with the expanded scale recorder.

Therefore, if there is excessive growth or decay, the

analysis is discarded.

Notes

Uranium has been analyzed more extensively than any

other element at this laboratory. We now have the ability

to control the bias for long periods of time (6 months or

more) with consistent average biases of 0, . 15$* .25$^ .3$ > and .5$. This assortment of biases is due to different

combinations of mass spectrometric, analytical and chemical

parameters. The full efffect of all parameters associated

with uranium analyses by thermal emission mass spectrometry

is not completely understood, but it has been possible to

identify some of the sources of bias and show that they can

be controlled, at the 0.1$ level for extended periods.

Since the installation of an expanded-scale recorder in

the measuring circuit, and the improved collector, all

58
identified bias components are associated with the filaments,

except when the spectrometer is equipped with an ion

multiplier. This filament bias is dependent upon the sample

size, sample purity, temperature of the filament system,

physical chemistry at the filament surfaces, acidity of the

solution, and the time-signal strength relationship. Gross

changes in transmission efficiency have the effect of

requiring a temperature compensation to yield a standard

signal intensity; therefore, malfunctions or defocused

conditions which reduce transmission are likely to affect

the rate of fractionation of the uranium isotopes, and a significant difference is reflected in the experimental ratios

measured under these conditions, when compared with experimental ratios measured under focused conditions.

Variation of uranium isotopic ratios with time is

related to the rate of depletion of the sample which is, to

a large extent, controlled by the temperature of the filament

system. The temperature of the ionizing filament for all

-i Ory

analyses is approximately 2150 C. The Re ion current from

a 2150 C filament is approximately 6.5 x 10" A and is used

as a fiduciary for making a direct and accurate temperature

setting of the ionizing filament. This temperature was

selected because it is high enough for efficient triplefilament ionization and can be maintained at 2150 C for long

periods of time without danger of burning-out the filament;

a lower ionizing filament temperature will not drive out the

inter

b e am

.

interstitial

p

which is one of the sources of a U0 ion

Since the ionizing filament is maintained at a nearlyconstant temperature which does not cause serious sample

depletion, the temperature of the sample filaments becomes

an all important parameter. Sample size, purity and acidity

of the solution all have some effect on the temperature

necessary to obtain a standard signal. The magnitude of

these effects as reflected by the sample filament currents,

59
is 0.3 A or less. Impure samples generally require higher

temperatures than pure samples to produce a standard signal

intensity. Significant increases in the sample size require

lower than normal temperatures to yield a standard signal

intensity.

Data are given in Tables la and lb showing the results

of a preliminary study of uranium solutions of variable

sample size and acidity. The NBS U500 standard was put into

solution in concentrations of 1 } 5, and 10 milligrams of

uranium per milliliter. For each concentration there was a neutral solution and acid solutions containing lfo } ^>% } and

10$ HNO^. Each of the 12 solutions was analyzed under lowtemperature and high-temperature conditions with separate

sample loadings for each technique. Statistical analysis

of the data shows that any differences between the averages

of two sample sizes that exceed 0.000^-05 can be considered

as significant at the 95$ confidence level. Similarly, any

differences between the averages of two acidity levels that

exceed 0.000530 can be considered as significant at the

95$ confidence level.

For low-temperature data it can be seen that:

1. The results for the 400 u.g sample size are higher

than those of the smaller sizes.

2. The results for neutral solutions are lower than

those for acid solutions.

For high-temperature data it can be seen that:

1. The results for the 40 u.g sample size are lower

than those of the larger sample sizes.

2. There are no differences in results for the lower

acidity levels.

60
Table la. Low-Temperature Data

Acidity/Sample Size Experiment

23 r) 2^8 Experimental ^U/ ^ ' Ratios for U500 Standard

40 u.g 200 u.g 400 p.

g

Sample size Sample size Sample size

Average 1.0040

5

1.0040

7

1.0046

Average

Neutral 1.00^4 1.0035 1.0041 1.00367

ifo Acid 1.0044 1.0039 1.0046 1. 00430

5$ Acid 1.0042 1.0045 1.0048 1.00450

10$ Acid 1.0042 1.0044 1.0049 1.00450

Table lb. High-Temperature Data

Acidity/Sample Size Experiment

2~5 CS 2^>8 Experimental yyU/ ^ U Ratios for U500 Standard

40 iig 200 (ig 400 iig

Sample size Sample size Sample size Average

Neutral 1.0008 1.0024 1.0027 1.00197

1% Acid 1.0006 1.0030 1.0028 1.00213

5% Acid 1.0008 1.0026 1.0026 1.00200

10$ Acid 1.0012 1.0021 1.0035 1.00227

Average 1.0008

5

1.0025, 1.0029

61
The problems caused by gross variation in sample size,

purity, or acidity are best controlled by good chemical

preparation. Solutions must be essentially free of alkali

elements, and in particular, potassium which is the source

of complex ions that make significant background contributions at the 2^4 and 236 mass positions. Even with good

chemistry it becomes essential to develop and observe rigid

procedures for mounting the samples for analysis. The

volume of solution per filament, the conversion of the nitrate

to oxide, and positioning of the sample filaments with respect

to the ionizing filament must be uniform and is, as far as

possible, identical for each sample loading.

Efforts to control the temperature of the filaments

are further complicated by variations in the dimensions of

the rhenium ribbon. This dimensional variation is large

enough to cause a difference of 1 .A. in producing a 2150 C

temperature. Sample filaments are operated at temperatures

less than 1650 C and the current range due to dimensional

variation is normally 0.3 A. Therefore, two filaments

drawing identical currents are not necessarily at the same

temperature, and it is dangerous to rely solely on current

readings as an indication of temperature. If it is assumed

that sample filaments at the same temperature will yield

approximately equal signal intensities, a signal-current

relationship can be established and is an effective means of

controlling the temperature of the sample filaments. The

signal-current relationship for low-temperature analysis is

- 11

3 x 10 A for a sample filament current range of 2.2 to

2.5 A and is based on the long term performance of many

sample loadings. Significant deviation from this relationship is cause for rejection of the analysis.

Under the conditions described for low-temperature and

2 "3 5 238 high-temperature analyses the experimental U/ U ratios

are always greater than the theoretical ratios. This bias

is in the range of 0.50$ for low-temperature and 0.25$ for

62
high-temperature. The procedure for both of these techniques

is identical during the first six minutes of heating when all

effort is concentrated on obtaining a stable Re signal of

— 12

6.5 x 10 A. During the remaining 24 minutes, until data

are taken, the procedures differ in sample filament temperature

and consequently, in signal intensities. This temperature

differential is the prime factor in causing different

fractionation rates which result in a characteristic average

bias for each technique. Even after 2 hours of heating the

biases of high-temperature and low-temperature analyses are

still significantly different but are nearer zero-bias. This

is especially true of high-temperature analyses where the

bias is expected to decay to less than 0.15$. The bias of a low-temperature analysis after 2 hours of heating is normally

greater than 0.3$. Once the ratio has decreased significantly

for either technique the direction of change cannot be

reversed by lowering the filament temperature. However,

depending upon the combination of parameters selected, and

with ; proper care the bias can be controlled at any given

level over about a 1$ range with better than 0.1$ precision.

23S /238 The ratio of the corrected experimental U/ U ratio

2^c5 238

to the "best estimate" of the theoretical U/ U is a dimensionless number and provides an effective means of

comparing data taken at different times in the history of

this laboratory. Three groups of data are presented in

Figure 12 for comparison. The 1959 data were taken on a standard-recorder and each point on the graph is the average

of 2 analyses corrected for zero bias. This group of data

followed closely the first effort of this laboratory to

measure uranium isotopic abundances and the graph is typical

of standard-recorder data before i960. The 1963 data were

taken on an expanded-scale recorder and each point is a •single ^U/ U analysis corrected for 0.15$ bias. Both

the 1959 and 1965 data were measured under high-temperature

63

conditions but neither is characterized by control of acidity

or the rigid analytical procedures of later measurements.

The interval between the first expanded-scale measurements of 1963 and the 1966 expanded-scale data was a period

of identifying bias components and developing procedures to

control them. All aspects of an analysis - chemistry,

mounting procedures, and the mass spectrometer procedure - were rigidly controlled for the 1966 measurements. A single

low-temperature analysis was made for each standard and the

experimental U/ U ratios were corrected for 0.46$ bias.

The 95$ limit of error per determination for the 1959j> 19^3^

and 1966 data are 0.96$, 0.24$, and 0.07$, respectively.

65
66
1. Interim Procedure for Boron

Chemistry

Weighted quantities of boric acid and Na

?

CCu are mixed

to yield a Na/B ratio of 1:1 and the solution is evaporated

to dryness. The final solution is adjusted to a concentration

of 1 mg of boron per ml.

Mass Spectrometry

A single-filament tantalum source is used. One drop of

solution ( ~ 20 u.g of boron) is placed on an ionizing

filament and dried with an infrared heat lamp for 10 minutes.

Boron is measured as the (NapBOp) ion at an accelerating

voltage of 10 kV. The source and collector slit openings are

7 and kO mils_, respectively. Analyses are begun when the

tube pressure is < 4 x 10 torr.

The following analytical procedure is used:

Time from start

(minutes) Procedure

0-2 The ionizing filament current is slowly

increased until the boron signal is detectable

on the grid. This will require a current of

approximately 1.1 A. Locate mass 88

(Na? B Op) and focus for maximum signal

intensity. Increase the filament current

until the (Na

2

10B 0p)

+ signal is 25 mV (2.5

x 10

-1^ A).

6 Increase the filament current to give a 50 mV

(Na

2

10

B

lo

2

) signal.

15 Data-taking begins. The (Nap B Op) signal

should be 20-30 mV and decaying. Ten peak

sets are taken over the next 10-12 minutes.

Each peak is monitored for 1/2 minute and

magnet changes are made in discrete steps.

Notes

The boron signal is normally decaying during most of the

analytical time. On the basis of a limited number of sample

67
loadings it appears that, if the (Na9 B Q ) signal is

-13 adjusted to 5 x 10 A, the signal will decay to an intensity

-13

of approximately 3 x 10 A within 10 minutes. During the

next 15 minutes the- rate of decay is significantly less and a

precise ratio determination can be made. (Na B o ) signal

-12 intensities in the 10 A range are obtainable but the rate

of decay is excessive and the signal will often reach 1 x, -13

10 A before the analysis is completed.

10 11 Under carefully controlled conditions the B/ B ratio

does not change significantly (0.2$) during the data-taking

period. The filament is operated between 1.1 and 1.3 A, and

at this current range a very faint red glow can be detected

in the filament. When the filament is operated significantly

above 1.3 A or at red-heat, very large (10*" A) ion currents

are obtained, but are decaying rapidly. Under these conditions the ratio shows significant fractionation.

Boron samples of known Na/B ratio (1:1, 1:2, and 2:1)

were used to evaluate the effect of sodium content on the

B/ B ratio determination. All samples were analyzed using

the same analytical procedure. The observed B/ B ratio

for the solution with the 2:1 Na/B ratio was lower (1$) than

the observed ratios for the other samples.

All filaments are pre -baked at 3 A for 1 hour in a vacuum and under a potential field. Baked filaments are

free of background signals at the mass 88 and 89 positions

when operated at 2 A. Tantalum was selected as the filament

material because significant (NapB0?

) ion currents were not

formed off rhenium or tungsten. Preliminary investigations

with platinum ribbon have since indicated that it is a more

efficient ionizing surface for the (NapB0

p

) ion. Ion

-11 currents in the 10 A range were obtained without changing

the sample size.

68
2. Interim Procedure for Lithium

Chemistry

Lithium is dissolved in nitric acid and the final

solution is adjusted so that the concentration is 100 u.g

Li/ml

.

Mass Spectrometry

A triple-filament rhenium-ribbon source is used. One

drop of solution ( ~ 2 |ig Li) is placed on each sample

filament and dried with a heat lamp for 10 minutes. The

spacing between ionizing and sample filaments is increased by

1/32 of an inch to eliminate sample evaporation by radiant

heat

.

The Li signals are measured at an accelerating voltage

of 5 kV. The source and collector slit openings are 5 and

40 mils j respectively. Analyses are begun when the tube

pressure is < 4 x 10 torr.

The following analytical procedure is used:

Time from start

(minutes) Procedure

0-1 The ionizing filament current is set to give

a temperature of 1600 C (3.2 A), which is

measured with an optical pyrometer. The

sample filament currents are set at 0.1 A.

The Li signal is very small and frequently

not detectable.

5 The sample filament currents are increased

until the Li signal is 8 mV. The signal

should begin to grow rapidly.

15 If the Li signal is less than 500 mV^ (5 x

—12

10 A), the sample filament currents are

increased until this intensity is achieved.

The signal should double during the next 15

minutes and no further adjustments should be

made

.

69
7 30 Data-taking begins. The Li signal should be

approximately 1 volt. Ten peak sets are

taken over the next 10-12 minutes. Each peak

is monitored for 1/2 minute and magnet changes

are made in discrete steps.

6 ,7 The observed Li/ Li ratio changes very rapidly with

time and strict adherence to a rigid analytical procedure is

essential. If for any reason the data cannot be taken

between 30 and 40 minutes after the filaments are turned on,

the analysis is rejected. The most common cause of rejection

is excessive signal strength and/or excessive rate of signal

growth.

Notes

Lithium is very sensitive to small changes in sample

filament currents and there is constant danger of generating

far more signal than is required for an analysis. Caution

must be used in adjusting sample filament currents since

7 0.3 A is hot enough to yield growing Li signals as large as

-11

5 x 10 A. The Li signals grow strongly throughout the

analysis and it is difficult to control or slow down the

growth rate once the filaments approach thermal equilibrium.

The smaller than usual (5 mils) source slit opening reduces

the signal intensity which helps balance the growth rate but

does not completely solve the problem. No extensive study

of Li has been made at this laboratory, only occasional

service analysis.

70
3. Interim Procedure for Rubidium

Chemistry

Mass spectrometric experimentation showed that the

presence of other alkali elements in the rubidium samples

does not generally interfere with the rubidium isotope

analysis and no attempt is made to obtain pure rubidium in

the usual chemical procedure.

Rubidium-bearing silicate minerals are decomposed in

platinum dishes, with a mixture of hydrofluoric and sulfuric

acids. After- evaporation, the excess sulfuric acid is fumed

off and the residue is ignited for 1 to 2 minutes over a Meeker burner. The ignited residue is digested with water

and filtered. This procedure eliminates most cations; only

magnesium accompanies the alkali metals in significant

amounts in the filtrate. However magnesium has a deleterious

effect on the rubidium analyses and, if it is present in the

sample, a supplementary chemical procedure must be employed

for its removal. Final solutions are adjusted to contain

10 |ig of Rb per milliliter in the form of RbpSOk in neutral

solution.

Mass Spectrometry

A triple-filament rhenium-ribbon source is used. One

drop of solution ( ~ 0.2 u.g Rb) is placed on each sample

filament and dried with a heat lamp and a 0.25 A current for

5 minutes. The RbpSOh deposit is colorless and not

detectable with the naked eye, but a slight stain, due to

impurities, is noticeable in most mineral samples. The

ionizing filament is set 1/32 of an inch lower than normal

in the source block. This reduces vaporization of the sample

by radiant heat from the ionizing filament but does not

appreciably decrease the ionization efficiency.

The Rb signals are measured at an accelerating voltage

of 3800 volts. This relatively low voltage is used to reduce

the number of secondary electrons released by the reflected

71
ions. The source and collector slit openings are 7 and 40

mils, respectively. Analyses are begun when the tube

pressure is < 4 x 10 torr.

The following analytical procedure is used:

Time from start

(minutes) Procedure

0-1 The ionizing filament current is set to give

a temperature of 1600 C ( ~ 5.2 A), measured

with an optical pyrometer. The sample

filament currents are set at 0.25 A. The Rb

signal is not normally detectable.

5 The sample filament currents are increased in

increments of 0.1 A until the Rb ion current

is detectable on the grid. The signal is

focused for maximum intensity and the sample

filament currents are adjusted to give a 0.5

85 volt Rb signal. The signal should grow

steadily.

15 The sample filament currents are adjusted to

85 give a Rb signal of 1 volt.

85 25 Data-taking begins. The Rb signal should

have grown to approximately 1.4 volts (1.4 x

10~ A) . Ten peak sets are taken over the

next 10-12 minutes. Each peak is monitored

for 1/2 minute and magnet changes are made in

discrete steps.

Rubidium signals are easy to control and stabilize, but

sensitive to small changes in filament current. 1/10 A too

much current is sufficient to cause overheating and excessive

signal. All analyses should be made on constant or slowly

growing signals. Under these conditions the observed ratio

does not change over the 12 minute period required for taking

data.

72
Motes

The presence of magnesium has a very deleterious effect

on the rubidium isotope analyses. The magnesium apparently

suppresses the rubidium ionization and sample filament

temperatures must be made abnormally high in order to obtain

the standard signal intensity. Under these conditions the

observed ratio always changes rapidly with time. All such

analyses should be discarded and the sample should be

reprocessed for the specific removal of the magnesium.

The standard Rb ion signal intensity is obtained with

sample filament currents of 0.4 to 0.8 A. If these currents

are increased to > 1 A, ionization of rubidium will occur on

the surfaces of the sample filaments and the mixing of these

ions with the normal thermally-formed ions will produce a bias effect. For this reason, any analysis which requires

sample filament currents > 0.8 A is rejected.

The ionizing filaments are pre-baked at 5 A for 1/2 hour

in a. vacuum and under a potential field. Unbaked filaments

produce a small background signal, but this becomes barely

detectable after baking. Clean source assemblies give no

background signals but after a source has been used for 10 to

12 analyses a background Rb signal becomes detectable and

increases in magnitude with each additional analysis. To

eliminate this background, without having to remove and clean

the source, a copper, silver, or uranium (a number of elements

will do) sample is analysed after each 10 to 12 rubidium

analyses. These analyses reduce the rubidium background signal

by baking out or coating over the rubidium deposits on the

source

.

If a sample solution contains an excess of HpSO^, it will

not dry properly on the filament. Attempts to force drying by

increasing the temperature will result in partial evaporation

of the rubidium and cause discrepancies in the mass spectrometric measurements. It is therefore essential that all

sample solutions be neutral.

73
74
4. Interim Procedure for Strontium

Mass Spectrometry

A triple-filament rhenium-ribbon source is used. One

drop of solution (5 |ig Sr as SrCTp) is placed on each sample

filament and dried with a heat lamp and an electrical current

which is increased until the filament has a faint red glow.

The electrical current is adjusted in the following manner:

1.2 A for 5 minutes; 1.5 A for 1 minute; faint red glow

(2.0 - 2.5 A) for 2 minutes.

The following analytical procedure is used:

Time from start

(minutes

)

Procedure

The ionizing filament current is set at 4.8 A.

The sample filament currents are raised to

give a 10 mV Sr grid signal ( ~ 0.5 A).

2-3 The ionizing filament current is reduced to

3.8 A, (the signal decays). The sample

filament currents are increased to give a 100 mV grid signal. The signal should be

growing and no further changes are necessary.

15 Baseline readings are taken.

20 Data-taking begins. The grid signal should be ~ 200 mV and should continue to grow slowly 3 or level off, for the remainder of the analysis

With a slowly growing or stable signal, the observed

ratios will not change over the data-taking period (20-25

minutes). If the signal should start to decay the run should

be rejected; decaying signals generally show a significant

change in observed ratio with time.

No extensive study of Sr has been made at this laboratory. The procedure given above was developed by Carl Hedge,

of the U.S. Geological Survey.

75
76
C . APPENDIX

STATISTICAL EVALUATION OF UNCERTAINTIES ASSOCIATED WITH THE

REPORTED VALUES

The determination of isotopic abundance ratios and atomic

weights of selected elements by mass spectrometric methods as

described above affords a good opportunity to illustrate how

statistical methodology can be employed to advantage in the

treatment of precision data. In the following, the general

principles will be discussed first, followed by a detailed

analysis for /the determination of the atomic weight of bromine.

Some of the additional problems that arise when there are more

than two isotopes involved, e.g., chromium, are also briefly

discussed.

The basic procedure used here, the same as in any other

well-conceived calibration process, is to compare measurements

on an unknown quantity with measurements on a known standard

of similar magnitude by the same measurement process, and to

derive the magnitude of the unknown from that of the standard.

In performing an experiment of this type, the experimenter

may wish to follow certain statistical principles and practices

so as to strengthen the validity of his results. The essential

ones that are used in this laboratory are discussed and

illustrated below.

1. THE SEQUENCES AND NUMBERS OF MEASUREMENTS ARE CHOSEN SO

AS TO INSURE THAT THE UNCERTAINTY DUE TO RANDOM ERRORS

CAN BE REALISTICALLY ESTIMATED . It is a practice in this laboratory to make the analyses

of the unknown and standards in an alternate pattern, by

different operators and instruments, and over periods of weeks

or months. In between times, "runs" on other elements are

routinely performed and the instruments taken apart for cleaning

and adjustments. The results should show agreement within the

indicated uncertainties unless the precision was over-estimated

or there exist systematic errors which have not been accounted

for.

77
2. BOUNDS TO SYSTEMATIC ERRORS ARE INVESTIGATED EXPERIMENTALLY

AND REPORTED . All measured isotopic ratios for a particular determination

are corrected by the same "calibration factor" which is

determined by measuring isotopic ratios of synthetic mixes of

separated isotopes having "known" ratios. The uncertainties

in the fractions of the isotopes in the separated isotopes

,

and in the chemical procedure devised to make the synthetic

mixes, persist to the end as systematic errors in the calibration factor for the particular determination. The bounds to

these errors are ascertained, generally from data obtained

through multiple analysis of standards, and reported separately

from the precision of the mass spectrometric ratio determination,

Other sources of possible systematic errors are also

investigated. Known sources related to systematic errors of

comparatively large magnitudes are traced and eliminated, if

possible, in the preliminary investigation.

3. THE ASSUMPTIONS UNDERLYING THE STATISTICAL ANALYSIS ARE

EXPERIMENTALLY VERIFIED IF POSSIBLE . (a) In the statistical analysis the measured isotopic

ratios and functions of these ratios are frequently used.

107

For instance, the fraction f

1

of 'Ag in a separated sample

"Ag 107" is calculated from the observed ratio r^ of

107Ag/109Ag by the formula

r l

f-, =

1 1 + r 1

J

and the standard deviation of f., is computed by the approximate

formula „ , ~ s . d. of r,

s.d. of f-. « 7T- 1

(1 + r^

A confidence interval for f, is then computed by the use of

a Student's "t" statistic.

The justification of the use of this type of formula for

the standard deviation of a ratio and for the use of confidence

intervals based on Student's "t" statistic rests on a theorem

78
given by E c C. Fieller (see, for example , Handbook of Physics,

Condon and Odishaw, p. 1-150). This theorem shows that such a ratio will be approximately normal with approximate standard

deviation given by the classical propagation of error formula,

if the variables are normally distributed, and if the ratio of

average to standard deviation of the variable in the denominator

is greater than 5. For all measured isotopic ratios the ratio

of the average to standard deviation is greater than 100. The

near linearity of 53 measurements of magnesium 24/25 ratios

plotted on normal probability paper in figure 1, gives evidence

in support of the assumption of approximate normality. In the

bromine example given below, the individual values are really

averages of 9 determinations which in itself is usually

sufficient to achieve approximate normality.

(b) From the observed ratios of the separated isotopes,

the isotopic ratios of the synthetic mixes can be calculated.

The comparison of observed to the computed ratios of the synthetic mixes yields the first calibration factor for the instrument. The observed ratios of the separated isotopes are then

corrected by this calibration factor and the process repeated

again to give a second calibration factor, etc. Although we

have not proved mathematically that the results of the iteration

process necessarily converge to a constant, in practice two to

three iterations usually give the required accuracy. The five

steps of the iteration for the calibration factor for silver

were as follows: Calibration Factor

first round (assumed)

A

1

second round

A

2

third round

A

3

fourth round

A

4

fifth round

1.0

(.00532 96695)

.99467 03305

(.00022 37934)

.99444 65371

(.00000 94272)

.99443 71099

(.00000 03976)

.99443 67123

79
r igt ire 1 4ar ma Pi ob< ibil ity Pic>t f Obse rvf:d 1 sot 3pi( : Rati< >S 24, '?\

of Magru sii m *ef ere nee S( imp>le

71

J

l f 1 < *7f1 " "V

- • » • —4

»

:

•

71" '"in ir 1 ' 3U

• •. •

••

71/ ;^n

• •

r i <:)3V

•

p f

71*5lO ••

• # ••

• f t>>1U

*

> • »•

i c • <

1 ca

) • •

i >

c > i ««

71 ii?n

_tf

I if lj.IV

i

1 •

i

j

i

i *

;

i 7 1 1 P A

i

1 i • • i r 1 |0U

i

i

1

j »

*

!

i

1

1 7i<K^n #

>

1

i r nyovj

1 1 ! 1 1 1 1 1 1 1 1 1

i

i 2 5 5 | 10

|

i 20 50 40 )0 60 i '0 80 9 9 5 97,5

[

i

i

|

i

1 ., ..j 1

j ..1 8o
(c) In the case of chromium, four isotopes 50, 52, 53*

and 5^ are involved. The calibration factors, k, for 50/52,

kp for 53/52, and k^ for 5V52, were all computed from the

calibration factor k for 50/5^ using Mix I and Mix II. This

was done on the theoretical ground that if measurement errors

were the same, the calibration factor for 50/5^- would have the

maximum relative precision. The values of k, , k„, and k-, thus

obtained were checked against values obtained directly from an

independent auxiliary experiment using Mix III and Mix IV.

The result, shown in figure 2, lends support to the assumption

that the calibration factors for ratios are linearly related

to the mass differences of the isotopes.

(d) In computing the final uncertainties, all the known

systematic errors and the random errors are propagated through

the sequence of calculations involving ratios and functions of

ratios. If the numerator and denominator are functionally or

statistically related, the propagation of error formulas usually

shown for independent variables no longer apply. For functionally related variables, it is easiest to express all the

variables in terms of a single variable. Take chromium again

as an example. Let f,-p be the atom percent of isotope 52 in

the reference sample, a, b, c be the observed ratios of 50/52,

55/52, and 5V52, with calibration factors k. , kp, and k^

respectively, then

1 1

f

1 + k

x

a + k

2

b + k^c ±+ + _^_ + 1_

1

1 + k_ k

±

k

l

2

+ k

l N —

_ _ ——

,

—— = _

k-^a + k (a +l) + k

1 (l + 2b + c)+c D

81
ohn FJgure 2

! .J..! ..,„..

i zfvxf ln$ tru ai£j its ' M 3-1I M 3-' 1 - - j M

M

ix 1 ix 2

lor31

(or^

o

oQ 6!

'

• 7firO^ K

^

^

<K

i \\

Q *3fi

h v~ 5>'V» \\V<^

,©

>

v ^

t- \

i*

Q OR

i lN

• S7 O 3 s>

s \N niti al F [our id

\

'":! the 5 3 a< \N Mix ss 1 82

Hotfin

\

,^f ou

\ \

At ter 3 1 loui ids \

>f I fere Ho » \

r\ 7K \i ^ v < »<*

kXf f D

\

VDA

>

i \>

n 71

M( ss Difl ere nee -l! iOtO >ic Ral ios

i k

kit. I %

< > ? i i

} 52 <53< . ..__j \Q/{ >2 r 52/ 54 - 50

1 '54

Comparison of observed calibration factors for isotopic ratios

52/53, 50/52, and 52/54 of Chromium with values computed from

the observed calibration factor for the 50/5^, assuming a

linear relationship between the calibration factor and the

mass difference of the isotopes.

82
Thus, the systematic error in fVp due to the error in determination of k-, is

Af « -^-^(2^ + 1) - N[3ak + 2(a +1)^ + (1 + 2b + c) ]1.

For variables that are statistically related the problem

is much more complicated, since a reliable estimate of the

degree of correlation is usually not obtainable from the

available data. The experimenter must therefore strive to

obtain measurements that are not correlated, through proper

scheduling of his measurements and through careful consideration

of the physical factors involved. A plot of one variable

against the other, e.g., isotopic ratios r, against r ?

, would

usually bring out any significant correlation between the

variables. Other simple statistical tests may also be used.

In our analysis these tests showed lack of significant

correlation.

In addition, the use of propagation of error formulas does

not take into account sources of error that are not included in

their formulation, e.g., the possible differences between

instruments and operators. These two sources of error, believed

to be the most important, were investigated by performing

independent experiments, using different instrument-operator

combinations. The results were compared against their assigned

uncertainties to check the validity of the assumed formulation.

Such comparisons were made for the experiments on copper,

bromine, and chromium, and found to be satisfactory.

The complete analysis for the determination of the atomic

weight of bromine (in the form of the original memo, with minor

modifications for clarification) is given below as an illustration of the various steps involved in arriving at the reported

uncertainties. The summary page gives the values and the total

and component uncertainties reported in Tables 5 and 8 in

"Absolute Isotopic Abundance Ratio and the Atomic Weight of

Bromine" by Catanzaro et al., J. Research NBS 66A , No. 6, 1964.

83
UNITED STATES GOVERNMENT

Memorandum

U.S. DEPARTMENT OF COMMERCE

NATIONAL BUREAU OF STANDARDS

TO : Mr. W. R. Shields, 15.06 DATE: January 28, 1964

FROM : H. H. Ku, 11.05

subject: Determination of Atomic Weight of Bromine

Results on the analysis of data from you and Mr. T. Murphy

for the isotopic ratio and atomic weight of bromine are

summarized in the table below. The detailed calculations are

given in the succeeding pages.

SUMMARY TABLE

Isotopic Ratio

Det. I Det. II

Machine 4

Machine 1

1.028090

1.027807

Average isotopic ratio

Uncertainty components:

% confidence limits on ratio det.

1.027803

1.027669

1.02784 + .00105

(Possible error in composition of separated

samples [isotopic]

(Possible error in synthetic mixes [chemical]

Composition of Reference Sample ^Br : 50.686$

Br: 49.314$

Uncertainty components:

(95$ confidence limits on ratio det.

(Composition of separated samples [isotopic

]

(Synthetic mixes [chemical]

+ .00051)

+ .00033)

+ .00021)

+ .025

+ .025

+ .0123)

+ .0080)

+ .0051)

Atomic Weight of Bromine: 79.90363 + .00051

Uncertainty components:

(95$ confidence limits on ratio det.

(Composition of separated samples [isotopic]

(Composition of synthetic mixes [chemical]

+ .00025)

+ .00016)

+ .00010)

84
A. Isotopic Compositions of the Two Separated Isotope Samples

"Br 79" and "Br 8l".

Seven runs were made on separated sample "Br 79" (on

instrument No. 4) and six runs on "Br 8l" (on instrument

No. 1), yielding ratios r as listed in Table 1. The correction

factors used, .998364 (instrument No. 4) and .999513

(instrument No. 1), were based on results of measurements on

synthetic samples in determination I and II after two

iterations. No further iteration was found to be necessary.

The averages and computed standard deviations for r and

f are as follows:

f

/ T-, , . Half nan width wuoi

Separated s.d. (^racLion s>d> Qf

^

c>1

Sample n r of r of Y ^Br) of f on f

"Br 79" 7 378.480 1.895 .9973648 .0000132 .0000323

"Br 8l" 6 258.108 1.822 .0038594 .0000271 . OOOO697

The maximum uncertainty (Af) in the fraction determinations

was taken to be .00008. Systematic error was assumed to be

negligible

.

B. Synthetic Samples and Analytic Error in Concentration

Determinations

.

The concentrations of the "Br 79" and "Br 8l" solutions

for the two determinations were as follows:

Pet. I Pet. II

"Br 79" solution .340394 .361610

"Br 81" solution .327176 .352875

The standard deviation of the analytic method for an analysis

was computed to be .00004736 meg/g, based on 17 analyses of

the separated isotope solutions and 17 analyses of three

solutions of Normal NaBr . The detailed data are given in

Tables Ila, b, and c. The standard error of the concentrations

(average of four analyses used) of separated bromine solutions

was computed to be . 00002368 with 27 degrees of freedom.

85
Table 1

Analysis of Separated Samples

"Br 79" "Br 81"

Run No. r (79/81) r (corr .

)

Run No. r (81/79) r (corr .

)

4-2601 578.49 377.87 1-1902 260.30 260.43

-2602 586.73 386.10 -1903 253.62 253.74

-2605 372.41 371.80 -1904 253.87 253.99

-2604 379.84 379.22 -1905 262.33 262.46

-2605 383.57 382.94 -1906 254.56 254.68

-2606 377.34 376.72 -1907 263.22 263.35

-2607 375.32 374,71

r = 378.480 r - 258.108

f = .9973648 f = .0038594

S

2

(r) 25.1349 19.9198

s

2

(?) 3.5907 3.3200

S

r 1.895 1.822

1.895

(379.48)'

.0000132

1.822

(259.108)

2 = .0000271

t

(6 0<5)

S

r = 2 '^7 x .0000132 2.571 x .0000271 = .0000697

.0000323

86
Table 2a

Analysis of Concentration of Separated Sample Solutions

DETERMINATION I

"Br 79" 'Br 81"

Wt. of C Wt. of C

Sol. sol. meq. meq./g.

.340 504

Sol

A

• sol. meq. meq./g.

A 7.53019 2.56406 9 .09645 2.97587 .327 146

B 8.21278 2.79583 .340 424 B 8 .65398 2.83156 .327 197

C 8.54475 2.90845 .340 379 C 8 .07874 2.64328 .327 190

D 8.26555 2.81273 .340 296 D 7 .81712 2.55753 .327 170

E 7.77453 2.64618 .340 365

G = .340 394 G = .327 176

S

2

(C) = .0000 0000 5917 3 S^ = .0000 0000 0524 25

S(C) = .0000 7692 S = .0000 2290

DETERMINATION II

"Br 79" "Br 81"

Wt. of c Wt. of C

Sol. sol. meq. meq./g. Sol

A

sol. meq. meq./g.

A 7.77112 2.810027 .361 599 7.78525 2.74700 .352 847

B 7.88535 2.851350 .361 601 C 7.58655 2.67682 .352 838

C 7.32444 2.648832 .361 643 D 7.09924 2.50502 .352 857

D 7.73096 2.795487 .361 596 E 7.47154 2.63715 c352 959

C = .361 610 c = .352 875

s

2 = .0000 0000 0495 58 s

2 = .0000 0000 3177 58

s = .0000 2226 s = .0000 5637

87
Table 2b

Analysis of Concentrations of Normal NaBr_, Meq/g

3A

.336 280

.336 275

.336 289

.336 321

.336 296

.336 295

.336292667

.288 819

.288 865

.288 789

.288 811

.288 884

.2888336

6B

.303 309

.303 1^9

.303 254

.303 181

.303 197

.303 191

.3032135

s (c) .0000 0000 0261 1 .0000 0000 1559 8

s(c) .0000 1616 .0000 3949

.0000 0000 3351 1

.0000 5789

88
Table 2c

Estimated Precision of Chemical Analysis

Analysis of Normal NaBr

Sample

Degrees of

n freedom

6 5

5 4

6 5

of Separated Samples

S

2

3A

5

6B

Analysis

.0^261066

.0

8

15598

.0

8 335H

Det. I

"Br 79"

"Br 8-1"

5 4

4 3

.0

8

59173

,0%2425

(n-l)S0000 oooo 13053

0000 0000 62392

0000 0001 67555

0000 0002 36692

0000 0000 15728

Det. II

"Br 79"

"Br 81"

Total

4

4

3

3

27

.0949558

.0

8 31776

.0000 0000 14868

.0000 0000 95328

.0000 0006 05616

Combining the above

P

For averages of 4 det. : SFor averages of 5 det.: S-

= .0000 0000 22430

= .0000 4736

s p

= .00002368

y[T

S

P

: .00002118

V^

89
Table 2d

Analysis of Normal NaBr

Set No,

Weight

Solution

Bromide

Calculated

meq

Bromide

Found

meq

Assay

3A 1

2

3

4

5

6

8.27566

7.51737

8.05733

7.82072

8.21018

8.27691

2.78461

2.52946

2.71115

2.63153

2.76258

2.78504

2.78294

2.52790

2.70959

2.63027

2.76105

2.78348

Average

99.940

99.938

99.9^3

99.952

99.9^5

99.944

99.944

1

3

4

5

6

7.32550

8.28530

8.89337

9.55065

10.71678

2.11707

2.59446

2.57019

2.76014

3.09716

2.11574

2.39333

2.56831

2.75833

3.09391

Average

99.937

99.953

99.927

99.934

99.960

99.942

6B

Average

1 8.16336 2.47695 2.47602 99.962

2 7.56806 2.29632 2.29425 99.910

3 10.18410 3.09008 3.08837 99.945

4 8.65015 2.62465 2.62256 99.920

5 9.62452 2.92029 2.91813 99.926

6 9.13134 2.77065 2.76854 99.924

.931

90
The ratio of the bromide meq found by this method of

analysis to that of bromide meq calculated for the three normal

NaBr solutions averaged to better than 99.93$ as shown in

Table 2d.

Five synthetic mixes were used in determination I and six

in determination II. These mixes were prepared from about

equal weighted amounts of solutions "Br 79" B.n.6. "Br 8l". Let

G = (W8l/W

?9 ) (meq 8l/meq 79)

then AG « (W^/W^) A(meq|i),

o-, the weighing error in W being negligible. Taking (meq ttq) to

0-1 be the half-width of the 95$ confidence interval on meq r^ ,

a/ 8l N _ n „r- meq 8l A(meq 7g)« 2.05 2 xj^ 1

+ 1

(meq 8l) 2 (meq 79)

2

_

1/72

x .00002568

The value of AG for both determinations was taken to be

.0002 (see Table 6).

The computed isotopic ratio \i for a synthetic mix is

1 + G , ^ ~~ f

1

+ f 2 G

-1

where

f

1 = .0026352 = fraction of 8l in "Br 79'

f = .9961406 = fraction of 8l in "Br 8l'

Then A u. « C 1

, , +. fi) ,,—_ Af + — i—« AG . (f x + f

2 G)^ (f 1

+ f

2 G)^

The values of u. are listed in Table 3. The value of a U

was calculated to be .000556 with components as follows:

..000328 due to A f (.00008)

.000208 due to A G (.0002) . 91
Table 3

Composition of Synthetic Mixes

DETERMINATION I

Mix

No.

W

79

w

8l w

8l/w

?9

meq

(81/79)

.96II689

11n

11

11

G

1

3

4

5

6

2.20305

2.03553

2.07995

2.11338

2.17520

2.23035

2.08166

2.11262

2.11904

2.24607

1.0123919

1.0226624

1.0157071

1.0026782

1.0325809

.9730796

.9829513

.9762661

.9637431

.9924846

1+G f2G f1+f2G 1+G

f-j+l'gG H

1,

1,

1,

1.

1.

9730796

.9829513

,9762661

9637431

9924846

.9693241

.9791577

.9724983

.9600236

.9886542

.9719593

.9817929

.9751335

.9626588

.9912894

2.030002

2.019725

2.026662

2 . 039916

2.009993

1.03000

1.01972

1.02666

1.03992

1.00999

DETERMINATION II

Mix TT No. W

79

W

81 w

8l/w

?9

1.0366285

meq

(81/79)

.975844

G

1 1.04618 1.08450 1.0115877

2 1.09963 1.09671 .9973446 .9732527

3 I.09990 1.10046 I.OOO5091 .9763408

4 1.09030 I.09721 1.0063377 .9820286

5 1.18382 1.07291 I.9063118 . 8844189

6 1.. 10534 1.20550 1.0906147 1.0642693

1+G f

2

G f l+f2°

1+G

f^+fgG M"

2.0115877 I.0076836 1.0103188 1.991043 .99104

1.9732527 .9694965 .9721317 2.029820 I.02982

1.9763408 .9725727 .9752079 2.026584 I.02658

1.9820286 .9782386 .9808738 2.020676 1.02068

1.8844189 .8810056 .8836408 2.132562 1.13256

2.0642698 1.0601624 I.0627976

92

1.942298 .9^230
The error in |i due to bias in assay is negligible

(approximately .000002) for solutions of ' Br and ^Br of

about the same concentration.

C. Errors in the Correction Factors . Duplicate determinations of ratios were carried out for

each synthetic mix for the two instruments. The standard

deviation of a determination was estimated from the differences

of these duplicates (Table 4a and 4b).

Let C denote the correction factor and

n

x.

' 1

i

then since the value of G is near 1,

a _ «

c

1 • 1

o X

AC" ~ c

AMand •

A

Values of A u and G

p are listed in Table 6. ^" Isotopic Ratios of Reference Sample . In Table 5 } the observed isotopic ratios of the reference

sample Y are listed for the two determinations and the two

machines. Computations were carried out for

Y = C Y y A?* « !(aS), and

V

t|52 (%)2

+ ?

2

( a

r>)

2

are listed in Table 6. Since the range of the four values of

Y was well within the estimated uncertainty of an individual

value, the four values were averaged, yielding an isotopic

9>
Table 4a

Analysis of Synthetic Mixes

DETERMINATION I

INST. 4 INST. 1

Syn.

Mixes

X M. C 4

= -

X

X C

l =- x X

1 1.0306

1.0301

1.0303

1.0291

1.03035 1.03000 .999670 1.0297 1.000301

5' 1.0213

1.0211

1.0202

1.0209

1.0212 1.01972 .998561 1.02055 .999197

4 1.0286

1.0287

1.0282

1.0282

1.02865 1.02666 .998075 1.0282 .998512

5

1 . o4o8

1.0417

1.0412

1.0409

1.04125 1.03992 .998723 1.04105 .998915

6 1.0120

1.0127

1.0115

1.0120

1.01235 I.OO999 .997679 1.01175 .998270

Average 1.02526 .9985416 .9990390

y~V cliff, bet. clup.)

2 = 387 x 10

o S x 387 x 10 = .0000001935 X

Ad

X

1

2 x 10

.00044

.00044

1.025^10

94

= .000136
Table 4b

Analysis of Synthetic Mixes

DETERMINATION II

INST. 4

M< INST. 1

M< Syn. X M. G

Mixes 4

"

X v=- __ _ X

-1 X

1 .99341 .9909

.99196 .991^

.992685 .99104 .998353 .99115 ,999899

2 1.0330

1.0315

1.0293

1.0299

1.03225 1.02982 .997646 1.0296 1.000214

3

I.0295 1.0271

1.0280 1.0276

1.02875 1.02658 .997900 1.02735 .999260

4 1.0222 1.0207

1.0232 1.0218

1.0227 1.02068 .998025 1.02125 .999442

5

1.1341 1.13H

1 . 1344 1.1326

1.13425 1.13256 .998510 1.13185 1.000627

6 .9^-323 .9422

.94386 .9^15

.9^3545 ,94230 .998681 .9^185 1.000478

Average I.02383 .9981858

/](diff . bet. dup.) 2 = 1289.94 x 10"

S

1

X 2 x 12 x 1289.94 x 10-8

9999866

00000054

s x

= .000735

c .000735

1.024 >Jl2

95

= .000210
ratio of Y = 1.02784 for this experiment. Table 6 lists

the values, systematic errors, and standard errors, for results

of determinations I and II.

E. Composition of Reference Sample. The isotopic composition P of the reference sample is

given by

dP = —?* 1.02784 K n£ft£U

-^

= g; g7b4 = .50o864

—

2

AP « _2-_ (A?) = ( 2>0^?84 ) (.00035 + .00021)

= (.000080 + .000051)

where the first component is due to Af and the second due

to AG. n

a -, 2 a p ~ ( 2.02YB4 ) (.000252) = .000061.

The half width of a 95$ C.I. on P = 2.020 x .000061 = .000123.

F. Atomic Weight of Reference Sample . W = N8l - (N8l - N

?9

)P = 79.90363

based on

N

Q = 80.916344

N

7g

= 78.918348 .

AW « 2.00 AP = (.00016 + .00010)

A A

ct ¥

« 2.00 ap= .000122

Half width of 95$ C.I. on W = 2.020 x .000122

.00025 .

96
Table 5

Y = Ratios 79/81 for Reference Sample

DETERMINATION I DETERMINATION II

Instrument Instrument Instrument Instrument

No. 4 No. 1 No. 4 No. 1

1.0292 1.0289 1.0296 1.0284

294 285 295 270

298 287 302 279

302 294 305 269

294 290 288 273

296 286 294 261

293 291 299 286

295 293 290 286

300 288 296 283

297 298 2 99 283

296 274 2 99 268

294 280 300 280

Ave . 1.029592 1.028792 1.029675 1,027683

S

2 .000000086 .00000041 .00000024 .OOOOOO69

S Y .00029 .00064 .00049 .00083

SY

.00008 .00018 .00014 .00024

97
HHa

as H

rao

•H

-p

CO

•Hs

!m

CD -P

CD pO

P!o

CQ H

?HCOftoo

VD

(1) H

.a

cO EH

HHoH

EhHSwHOH

EhgH

EhP

CO

SI

IS- c—

CV1 OJ 00 CO

vo vo UA

KA KA

H

0J CVJ OA f =$ ^ o o O

0J OJ

O UA H CVO CO H CM VO UA

KA KA

-=t CO

UA

o-0A

31

LP; -=t

-=t VO

OA CKA H O tKA KA

OA

VOH

VO

OA

W

CO

t- ISCVJ OJ 00 CO H VO vo H KA H

OJ OJ OA t -=t ^t O o o

CO

coHooo

oo

i>- ooHoooH

00

COHooooH

OAHOOo

OJooo H

EhP

CO

cO

CDs

cO 03

OO

-P -P

-P -P

•H -H

oj vo

UAO

KA -^h 00

0J VO

H

O

O OJ VO o O OA O O OA

VO UA

OJH

KA [>- H OJ O O

H

H

H

H

v -«v—--

H

H

O

O

v -•

HH -3- ^T

OJ OJH 0J KA ka KA KA O o O O o

-p

-p

•H

t3

CO

in ooH

00

OA

0A

CO 00

0JO

KAOJ O OO O O VO • H

UA

VO 00

KA OA

UA OA OOO OH

vo

KAH

KAO

VO

VO

CO

OA

0A

OA

OA

H

EhWP

CQ

cO

CD

CO mo o

oj o

KAOJ O O

OA

O

KA • • O

OA

OA 0J OA UAO OH

vo

KAH

KAO

UA

IS- vo

OA

OJO o

OJC^

UA

0A

0JOHH

COo

KA 00

VO

l>- OJO

HH

0JOA

IS- co

0JoHH

OAH

KAO

O

>R. ."

N ' v UA & -=t KA OA CD r: e VO H v ^ o CDA H o KA UA

OJ <

-p IS- 00 -P 00 OA \ OA OA H H

cO pq pq

H

cO OA• 0A• H 0A 3 n ^ 3

CQ II CO IS- 0< \ \ D1 H

CD IIS

J is CD

H

H CD II II IO 3 00 co <] H HlOA

COIL^-

O <H

cd OA

HH 0A <] < -=tH '!h > !A- CO 00 t>- D1

II CD

II II CDIIO II

OO

a1 a1 & CD

M J*i ^au

CD CD CD S3

O cO 1

OJ cO =L -=t

H IO -=t . H

a £ £ < <] EH Ch <H EH <

IO IO <| l>H 1 >H

98
00 KA ua OJ

OJ KA

KA KA O O

H

EHH

KA OA Q

o VD oo VO w

IS- t- CO 0J OJ o o CD

• • S H H CO CO

OAVO

OJ O

KAOJ O O

O

o o

o IS- • o OA o

O co

00 t- UA

OJ OJ KA o o UA

• • O

H H OO

KA VO

VO KA H OJ KA KA O O

o

•H-P

CO «

o

•H

ftl o

-po

W

H

T3

CD -P

fHO

ft

CD «

H

CO

•H

<P O

<< • o o <U p -p • 73

CD CD 3 3 -=j- 73 73

OAVO 4^> OJ O •H KAOJ £ O O

O OO

• •

L(A OJ 3- KA UA

co UA OJ o- O O

OJo

*

11 t> *

II >H *ll>H < <b

^-^

-=t KA H H UA

00 KA OJ UA O

t- O o O H

OJ o O O

o o o

H

II o CO !>3 •H CD -p -P H CO • OJ a

CO ft CD H UA •H « £ * •OJ CO

CO •H o o -P

fl CO 3 o Sh O ^o CD 73 o UA • CJ >» CD •H OA C

-P P P 3 a CO CD <P, X

•H ^H ti O H

CO CO 4J CO P> ft fl A o P

is CD >S -P OJ O

CD CO CO 73 O Eh O •H • g o o £ OJ 3 p p 1 H CD CD H II CO 3 3 CO -P O P H! O

En

l>H l>HH

IO

* -3" l>H * H *

l>H ll>H<

99

U.S. GOVERNMENT PRINTING OFF ItE : 1 966 —251 -01 7/<t


U.S. DEPARTMENT OF COMMERCE

WASHINGTON, D.C. 20230

POSTAGE AND FEES PAID

U.S. DEPARTMENT OF COMMERCE

OFFICIAL BUSINESS