National Bureau of Standards

Library, N.W. Bldg MAY 12 1965 NBS MISC. PUBL 260-5

Standard Reference Materials:

Accuracy of Solution X-Ray Spectrometry

Analysis of Copper-Base Alloys

U.S. Department of Commerce

National Bureau of Standards
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Standard Reference Materials:

Accuracy of Solution X-Ray Spectrometric

Analysis of Copper-Base Alloys

Robert Alvarez and Richard Flitsch

Institute for Materials Research

National Bureau of Standards Washington, D.C.

National Bureau of Standards Miscellaneous Publication 260-5

Issued March 15, 1965

For sale by the Superintendent of Documents, U.S. Government Printing Office Washington, D.C, 20402 - Price 25 cents
Library of Congress Catalog Card Number: 65-60009
PREFACE

Within the framework of the NBS Institute for Materials

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iii
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iv
CONTENTS

PAGE

1. INTRODUCTION 1

2. INSTRUMENTATION 5

3. EFFECT OF FILM POSITION 7

4. EFFECT OF ACID CONCENTRATION 9

5. EFFECT OF TEMPERATURE l4

6. APPLICATION TO ANALYSIS 14

7. DISCUSSION 15

H REFERENCES . 19

LIST OF TABLES PAGE

TABLE No. PAGE

1. X-Ray Spectrometer Settings 7

2. Comparison of Chemical and X-ray Results ... 16

LIST OF FIGURES

FIGURE No. PAGE

1. Diagram of solution cell 4

2. Sample-holder assembly of spectrometer .... 6

3. Apparatus to measure the position of the film. . 8

4. Effect of changes in film position on

zinc Ka intensity 10

5. Effect of changes in film position on copper Ka intensity 11

6. Effect of changes in nitric acid concentration on zinc Ka intensity 12

7. Effect of changes in nitric acid concentration on copper Ka intensity 13

v

ACCURACY OF SOLUTION X-RAY SPECTROMETRY ANALYSIS

OF COPPER- BASE ALLOYS

Robert Alvarez and Richard Flitsch

Institute for Materials Research

National Bureau of Standards

X-ray fluorescence spectrometry has "been investigated as an independent method for the analysis of

certain major constituents in NBS standard reference

materials. The determination of copper and zinc in

cartridge and aluminum brass samples which were to he certified as standard materials, was selected for

study. After a preliminary analysis , a solution of

the unknown and a closely-matched synthetic standard

were compared without the use of an internal standard or added control element. To achieve reliable

results j close control of the type and concentration

of acid was found necessary, and, for cells employing

films as windows, the position of the film was critical. The temperature rise of the cell during irradiation was 0.1°C, which did not contribute significantly to error. Typical coefficients of variation

obtained for individual results in a group of 8 runs were 0.3% for copper and 0.k%> for zinc. The agreement between chemical and x-ray results was within

0.3%> of the amount present for both elements.

1 . INTRODUCTION

For certification of NBS standard reference materials of

composition, it is desirable to employ at least two independent

analytical methods to reduce the possibility of bias in the

results. X-ray fluorescence spectrometry, with its inherent

high precision, may serve as one of the methods providing that

the factors affecting accuracy are evaluated.

Present address: International Business Machines Company,

Poughkeepsie, New York

1
It is generally recognized that the ultimate in reliability is attained by the direct comparison of unknown with

standard when they are uniform, identical in composition, and

identically prepared [1], In addition, both should be exposed

to the excitation source in the same way. To approach these

ideal conditions, it is necessary to determine and control the

factors that are most critical in affecting accuracy.

Solutions were selected for this study because of the

advantages that they offer. They are homogeneous, and after a preliminary analysis of the unknown, a synthetic standard

closely approximating the composition of the unknown can be

prepared. Moreover, differences in absorption and enhancement

effects between unknown and standard can be reduced by dilution.

In the comparison of solutions of an unknown with a synthetic

standard, several factors may be expected to contribute to error

1. When the solution cell includes a thin film, the films

of the two ceils may distend by different amounts thus changing

the surface being irradiated.

2. The acid concentration may differ in the two cells,

producing different absorption effects.

3. Temperature changes affect the density of the solutions.

By adding the same amount of a suitable control element to the solutions of synthetic standards and unknown and taking a ratio of the intensity of the analyte to the intensity of the

control element, a partial compensation for the effect of

2
differences in film position and acid concentration can be made.

However, the selection of a suitable control element may be

difficult because of possible precipitation or interference

effects on other radiations to be measured. Therefore, to make

this method more generally applicable a control element was not

added. Our investigation was limited to the determination of

copper and zinc by x-ray fluorescence spectrometry in the cartridge and aluminum brasses which were being analyzed for certification as NBS standard reference materials [2]. Data are reported on precision and accuracy and the principal factors

affecting accuracy are discussed.

Previous work using x-ray fluorescence of solutions for the

determination of copper includes the determination of that element in bronze heat-treating materials by Blank and Heller [3]

who reported "a limit of error at the 95$ confidence level" of

approximately 1% expressed as a percentage of the mean value.

Zinc was used as an internal standard. It was reported that no significant error was introduced in the chemical preparation.

Bertin and Longobucco [4] determined copper, nickel, silver and

gold in plating baths. Designed primarily as a rapid method,

the agreement between chemical and x-ray methods was within

1 to 3 percent for all elements.

2 . INSTRUMENTATION

A commercial, multichannel spectrometer of the curved

crystal type was used for this investigation. Although the

main features of the equipment were described previously [5],

3
additional information pertinent to this work is described

below.

A horizontally-mounted x-ray tube having a platinum target

and a 1.0 mm beryllium end window was operated at 50 kV and

55 mA to irradiate the solutions. The solutions were contained

in cells of methyl methacrylate plastic having the dimensions

indicated in Figure 1. After cementing a 0.0006-mm polyester

TAPERED HOLE

0. D. * 7

D. = 6

ALL DIMENSIONS IN mm

Figure 1. Diagram of solution cell.

(1/4 mil nominal Mylar) film with adhesive (Pliobond) [4] across the 25-mm opening, approximately 10 ml of solution was introduced into the cell and a tetraf luoroethylene polymer

stopcock used as a closure. To minimize the possibility of

4
damage to the x-ray tube due to leakage , the spectrometer had

been mounted on its side, thus placing the cell in front of,

rather than above, the tube. This arrangement is possible

since none of the components are dependent on gravity for

operation.

The solution cell was irradiated by inserting it into the

circular cavity of the sample holder, moving the holder horizontally approximately one-half meter into the spectrometer,

alining the gear segments of the holder and x-ray tube shutter,

and then twisting the holder to open the shutter. The sample

holder serving also as a radiation shield during excitation,

weighed almost 2 kg and was therefore awkward to move into

position. To facilitate sample handling for irradiation, the

holder was modified as shown in Figure 2, by attaching to the

end of the holder a 25-mm diameter rod, 0.5 m long with a longitudinal keyway and by attaching to the side of the holder

a roller bearing. Supported by a bracket attached to the side

of the spectrometer, the sample holder assembly could be rolled into position with the keyway facilitating alinement of the

gear segments. A hinged plate actuated by the sample holder

assembly served as an additional safety shield.

While irradiating the sample through the Mylar film, element line intensities were integrated by capacitors connected

to detectors. Details of spectrometer characteristics are given in Table 1. At the same time, a brass sample was also

irradiated by the primary x-ray beam and the integrated,

5
Figure 2. Sample -holder assembly of spectrometer.

non-dispersed radiation used to terminate the integration

period of approximately two minutes. A digital voltmeter provided a measurement proportional to the voltage developed on each capacitor during the integration period. These numbers

6
Table 1. X-Ray Spectrometer Settings

Crystal

Element Wavelength Radius Material

A .. cm

Atmospherej Detector

Air Proportional8- Air Proportional8-

a,Counter measures average current for integration.

were displayed visually and were automatically typed on an electric typewriter. Linear analytical curves were developed

for each element relating intensities expressed as digital

voltmeter readings to the concentrations of the synthetic solutions for the range covered. The digital voltmeter response

could be adjusted to read concentration directly. After a preliminary analysis of the unknown, another standard was then made

up having this analysis and additional determinations were made.

In the direct comparison method, cells containing solutions

of the unknown and closely-matched standard are interchanged and

ideally all conditions should remain constant except for the

substitution of one solution for another. However, the hydrostatic pressure of the liquid distends the polymer film approximately 0.4 mm, an amount which may differ for each cell. To

evaluate the magnitude of this effect on the results, the

measuring system shown in Figure 3 was devised.

Three cells were used--two having tetrafluoroethylene

polymer stopcocks, the other a plug of the same plastic.

3. EFFECT OF FILM POSITION

7
Figure 3. Apparatus to measure the position of the film.

Approximately 10 ml of a 1-Molar nitric acid solution of brass

having a concentration of 10 mg of metal per ml was introduced

into each cell. The position of the film could he changed by

operating a small syringe attached temporarily to the stopcock,

after which the stopcock was closed. The micrometer head, which

could be read to 0.01 mm, had a pointed sleeve of black plastic

to facilitate observing the position of contact. Placing the

cell on edge, in the position that it would normally be irradiated, a reading was taken at the surface of the cell to which

the film had been cemented and a second reading taken at the

center of the film--the difference being a measure of the amount
of bowing. Intensity readings were taken for each position

of the film surface , after which the film position was again

checked. No change in film position was observed. Intensity

readings taken of the two cells with stopcocks were alternated

with those of the cell with a plug. Since the film position

of the latter remained constant, it served as a reference cell

to correct for instrumental drift that might occur. Figures 4 and 5 show the effect of changing the position

of the film surface on the intensity of the Ka radiations of

zinc and copper respectively. The film is distended by hydrostatic pressure approximately O.k mm and bowing the film an additional 0.10 mm from this position causes the zinc Ka intensity to increase by 0.19$. For a solution having a zinc

concentration of 30.30$ by weight based on metal content, a value of 30.36$ would be obtained. For copper, a 0.10 mm

change in film position produces a 0.069$ change in intensity

and a copper value of 79- 35$ > by weight based on metal content, instead of 79.30$.

4. EFFECT OF ACID CONCENTRATION

To determine the effect on intensity of a difference in

acid concentration between solutions of unknown and standard,

intensities were measured for solutions having different acid

concentrations. Three nitric acid solutions of NBS C1120

were prepared, each containing the same concentration of

brass, 10 mg per ml. The excess nitric acid was removed by

evaporating the solution to dryness on a steam bath, adding

9
POSITION OF MYLAR FILM SURFACE, mm

Figure 4. Effect of changes in film position on

zinc Ka intensity.

water, re-evaporating the solution to dryness and repeating

this procedure. Nitric acid was pipetted to each solution

to obtain concentrations of 0.64, 1.3, and 3.2 Molar. The

10
O CELL A

A CELL B

-2.0 1.0 0 1.0 2.0 3.0 POSITION OF MYLAR FILM SURFACE, mm

4.0 5.0

Figure 5. Effect of changes in film position on copper Ka intensity.

intensities of the fluorescent radiations were measured and

graphs made relating intensities to concentrations of acid.

Figures 6 and 7 show the effect of a change in the nitric

11
1.85

1.80

.75

1.70 I . . l 1 1 J 0 12 3 CONCENTRATION OF hno 3 , moles per liter

Figure 6. Effect of changes in nitric acid concen- tration on zinc Ka intensity.

acid concentration on the Ka radiations of zinc and copper

respectively. Adding 1.6 millimoles of nitric acid to 10 ml

of the brass solution, which was 1 Molar with respect to nitric

acid, resulted in a 0.38$ decrease in zinc intensity. This was

12
7.4 I I , I I I 0 12 3 concentration OF HN0

3

, moles per liter Figure 7. Effect of changes in nitric acid concen- tration on Copper Ka intensity.

equivalent to adding 0.1 ml of 70$ nitric acid to the cell

volume which would have resulted in a zinc value of 18.05$ by

weight instead of the actual concentration of 18.12$. For

copper, the same increase in nitric acid concentration resulted

in a 0.48$ decrease in intensity and therefore a copper value

13
of 79.89^ "by weight would have been obtained instead of the

actual concentration of 80.29$.

5. EFFECT OF TEMPERATURE

The temperature rise occurring in a cell during the two minute irradiation period was measured with a calibrated thermistor placed inside the cell so as to avoid the primary x-ray

beam. A change of resistance equivalent to a temperature rise

of 0.1°C was obtained. The resulting small change in density

of the solution would not produce a significant change in the

intensity measurements.

6. APPLICATION TO ANALYSIS

Since close control of acid and film position was found to be necessary for accurate results, the following procedure was

used. A 500-mg sample of brass was dissolved in 10 ml of water

and 4 ml of concentrated nitric acid. To remove excess nitric

acid; the solution was evaporated to dryness on a steam bath,

several ml of water added to the residue s evaporated to dryness

again and the procedure repeated. A portion of 20 ml of a nitric acid solution composed of one volume concentrated nitric

acid plus four volumes of water was pipetted to dissolve the

residue. The solution was transferred to a 50-ml volumetric

flask and made to volume with water at 20.0°C. The same procedure was used to prepare synthetic standard solutions

from metals having 99-99^ purity.

14
Solution cells were filled with solutions of synthetic

standards and unknown and, after a preliminary analysis of the

unknown from a graph relating intensity to concentration,

another synthetic standard was prepared having this analyzed

composition. After introducing the new standard into a cell,

the position of the film was measured by means of the system

shown in Figure 3 and then adjusted to the same position as that

of the cell containing the unknown. These two solutions were

then intercompared to obtain the final concentration of the unknown. After the solutions were prepared it was relatively easy

to make multiple runs on the x-ray spectrometer.

This procedure was applied to the determination of copper

and zinc in several brass NBS standard reference materials.

The x-ray results and measure of their precision are compared

with the provisional chemical values [2] in Table 2. In this

table, a run is a concentration value derived from an intensity

measurement with the cell having been removed and reinserted

into the sample holder between each measurement.

Aluminum was not determined because its soft radiation is

absorbed to a large extent by the film resulting in poor sensitivity and precision.

7. DISCUSSION

The coefficient of variation for an individual run in a group of eight was found to lie between 0.17 to 0.46$, the precision of the method being limited mainly by instrumental instability. By taking the mean of eight runs the effect of drift

15
Table 2. Comparison of Chemical and X-Ray Results.

NBS

Sample

Numb e r Element

Chemical

Results a X-Ray

Results"

Standard

Deviation

of the Mean0 Coefficient

of Variation

of the Meand C1100 Cu

Zn

%

' 67.4

5 2 - 2 0 67.3.

32.2;?

0

0 07

03

0.10

.09

CHOI Cu

Zn

69.5.0 69.

Q 30.2°

0

0 0704

.10

.12 C1102 Cu

Zn 72

27

72.

6 27.0^

0

0 06

05

.08

.12 C1118 Cu

Zn

75

21

75. 0

6 21.9^

0

0 0904

.12 .16

C1119 Cu

Zn

77

20 H

77.2

20

0

0 05

03

.07

.15 C1120 Cu

Zn

80

18 it

80.2

l8 y l§

0

0 05

02

.06

.12

Average reported on provisional certificate.

^Mean of 8 runs . c \ / d

2 Standard deviation of the mean s =\/n ^ n _2

> where d is the

difference of the determination from the mean, and n is the

number of runs. ^Coefficient of variation of the mean,, C.V. =

S where C is the average concentration in percent.

was partially corrected. However, the drift was irregular and

could not be entirely compensated for. Improvement in the

x-ray method should lie in the direction of better control of

drift.

Since the accuracy of the method depends on closely matching the composition of solutions of samples and synthetic standards, a preliminary examination of the sample should be made.

16
! It is necessary to note particularly the presence of elements

I having principal x-ray radiations lying several tenths of an Angstrom below the absorption edge of the analytical line of

| the analyte and also those elements which have large mass

I absorption coefficients at the wavelength of the analytical

I line. The preparation of synthetic standards should include

these elements at approximately the same concentration as in

the sample.

Significant distention of the thin film cover on the sample

cell was observed when so mounted that the film was vertical.

This indicates the necessity for determining and adjusting the

film position to the same relative position for both sample

and synthetic standards.

The same acids must be used to prepare the synthetics as were used to dissolve the samples and the final acid concentration should be the same for both standard and samples.

It is of interest to compare the x-ray method with chemical

methods for the determinations made. Copper can be determined

by electrodeposition with a coefficient of variation of 0.01 to 0.02$ and interferences are not serious. This represents one of

the simplest and most accurate chemical procedures and is preferred for the determination of major amounts of copper.

Because of difficulty in the chemical determination of

zinCj even high concentrations are often reported "by difference.

However, zinc can be determined with a coefficient of variation

of 0.1 to 0.2% by a double precipitation with hydrogen sulfide

17
followed by ignition to the oxide. Here the x-ray method is more competitive , especially for lower concentrations of zinc.

The differences between results by the chemical and x-ray

methods lay between 0.02 and 0.10$ except for zinc in C1118

where it was 0.17$.

8 . REFERENCES

[1] Liebhafsky, H. A., Pfeiffer, H. B., Winslow, E. H.

Zemany, P. D., X-Ray Absorption and Emission in

Analytical Chemistry, John Wiley and Sons, New York,

i960.

[2] Michaelis, R. E., Wyman, L. L. , Flitsch, R., Preparation

of NBS copper-base spectrochemical standards, NBS Misc.

Publ. 260-2 (1964).

[3] Blank, R. R. , Heller, H. A., X-ray spectrometr ic determination of copper, tin and uranium in bronze heattreating materials, Advances in X-Ray Analysis _4, 457^

Plenum Press, New York, i960.

[4] Bertin, E. P., Longobucco, R. J., X-ray spectrometr ic analysis. Nickel, copper, silver and gold in plating

baths, Met. Finishing _6q, 54-58 (1962).

[5] Michaelis, R. E., Alvarez, R. , Kilday, B. A., Determination of minor constituents in low-alloy steels by x-ray

spectroscopy, J. Research NBS _65C, 71 (1961).

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