Coherent Scattered Radiation Internal Standardization in X-Ray


Coherent Scattered Radiation Internal Standardization in X-Ray...

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Coherent Scatte red Radia tio n Inte r na I Sta nda red iz a ti o

I

I

in X-Ray Spectrometric Analysis of Solutions THOMAS J. CULLEN United States Metals Refinipg Co., Carteref,

The use of coherently scattered x-ray tube characteristic radiation is proposed to compensate for unknown and variable matrix effects due to metal ion and acid content of solutions. intensity ratios of the fluorescent radiations to that of the scattered x-ray tube characteristic radiations are plotted vs. the content. Coherently scattered radiation intensities vary with the atomic numbers of the elements in the matrix, bulk density of the sample, surface variations, and power fluctuations. The advantage of high scatter intensities from solutions is made in the analysis of solutions found in the refining of copper. Cornparisons between chemical and x-ray results are in good agreement.

X

-RAY SPECTROMETRIC ANALYSIS Of

solutions offers several advantages: homogeneity, ability to dilute matrix effects, ease of preparation of standards, ease of addition of internal standards, ability to dilute or concentrate to a convenient concentration, and the ability to carry out chemical separations. Complicating factors include: density effects, effects from temperature changes, high background levels, and incoherent (Compton) scattering of fluorescent line intensities, which may interfere with intensity measurements. Previous papers describing aqueous solution methods fall into two categories. The first involves dissolution of a solid sample in an acid medium (3, 4). An example of this is the analysis of stainless steels. Hauk and Silverman showed the effects of various acids on the intensity of background scattered radiations. The second type is a liquid sample containing variable amounts of constituents. Examples of this include tungstate and uranium solutions (6). Sodium tungstate solutions were studied in regard to absorption coefficient corrections, and sodium tungstate and uranium solutions studied with the addition of internal standards. The scope of this paper is with the latter type. Andermann and Kemp (1) reported on the use of scattered background radiation internal standardization. This 812

ANALYTICAL CHEMISTRY

N. 1.

technique has found wide use for the compensation of several instrumental and sample effects; namely, x-ray tube power fluctuations, sample to x-ray tube target distance variations, sample density, and particle size variations. These variations are in large measure compensated by determining the ratio of the fluorescent and background radiation intensities. Absorption and enhancement effects were to some degree compensated by this technique, and a qualitative mathematical relationship was derived from theoretical considerations which showed that the ratio varied with the atomic numbers, raised to the -1st t o -2nd power, of the elements in the sample. A straight fluorescent intensity measurement varies with the atomic numbers raised to the -4th power Compton and Allison ( 2 ) discuss the scattering phenomena from theoretical and experimental considerations. Coherently scattered radiation intensities vary with the atomic number raised to the 1st or 2nd power, depending on the magnitude of the atomic number. Low atomic numbered atoms scatter the x-rays in a manner as if the electrons were isolated and acted independently. In the case of high atomic numbered atoms, the distances between electrons approach that of the wavelength of the x-rays, thus x-rays scattered from different electrons are nearly in phase, resulting in an increased total intensity. Following the same derivation as Andermann and Kemp for the case of low atomic numbered elements and using only coherently scattered x-ray tube characteristic radiations, one finds that the ratio varies with the atomic numbers raised to the -1st power. This is an improvement over either a straight intensity measurement or a background ratio measurement. THEORY

The Andermann-Kemp equation for the scattered intensity is composed of two parts, that due to coherent and that due to incoherent (Compton) scattering, Io=?+-s s, 2u0

uo

+ uc

(1)

where lo is the obserw-: smttered radiation intensity, uo is the absorption coefficient a t the observed wavelength, 71, is the absorption coefficient a t the Compton wavelength, So 1s the coherent scattered radiation intecsity and S, is the Compton scattered radia tion intensity. Since the characteristic x-ray tube target radiations are measured a t the coherently scattered radiation peak the second part of the equation he. comes insignificant. Thus, Equation 1 can be simplified to:

Compton and Allison (2) give equations for coherent scattered radiation intensity:

S, = I,ZF2 (low 2 elements)

(3)

I,, the electron scatter intensity is a constant, Z is the atomic number, and F 2 is the atomic structure factor and is dependent upon the electron-electron distances within the atom. This equation is derived from considerations of an isolated, un-ionized, and spherical atom. In case of a conglomeration of ions, atom-atom distances also play a role in the intensity of the scattered coherent radiation. In the case of solutions, the atom-atom distances are uniformly irregular and this effect should be constant. The absorption coefficient u is also dependent on the atomic numbers of the elements in the sample. The relationship is given in the following equation: iv u =

c24x3 A

(4)

where C is a constant, N is Avogadro’s number, A is the atomic weight, 2 is the atomic number, and X the wavelength observed. The usual fluorescent intensity equation of an atom is: I, =

t ui + uo

(5)

where I f is the intensity of the fluorescent radiations, t is the emission coefficient and constant for a given element, ui is the absorption coefficient of incident radiations, and ug the

Table 1. Variation of Absolute Intensity and Ratio Measurement with Instrument Changes

I CuKcv

Ratio

CuKaIWLpi

Tube voltage 7.5Yc/kv. 1.401,/kv. Tube current 4.6T07,/ma. l.47,lma. Sample position 2.0$/0.1 mm. 0.5%/0.1 mm. Table II.

Effects of Various Acids (1 0%

Acid

HKOa HZ804

HC1 HClO,

by Volume)

Intensity, C.P.S. CuKa WLpl 1932 1656 1736

6823 5840 6130 6325

1707

Ratio

C ~ K ~ , I WLB~ 3 531 3 525 3 530 3 519

Table 111. Effect of Concentration of Various Acids on Intensity Ratio CuKa/ W l ~ l bv Volume 1 5 15 20 HNOi 3.538 3.536 3.536 3.532 HpSO; 3 525 3 528 3 528 3 529 HC1 3 531 3 533 3 530 3 529 HC10, 3 533 3 531 3 536 3.530

absorption coefficient of the observed radiations. Relating the intensities to the atomic numbers, one finds:

Io

N

2-3

0.010 Soller slit, and a scintillation counter. A sample drawer was modified to hold Spex Industries liquid cell holders covered with 0.25-mil Mylar film. A preset count time of 100 seconds was used for all measurements. All solutions were prepared from pure metals or metal oxides, reagent grade acids, and diluted with distilled water. A copper concentration of 0 to 5.0 mg. per ml. was determined to be the best concentration range of that element with regard to fluorescent and scattered radiations. The best available tungsten line was the WLsl. This line is the most free from interferences and of high intensity. A copper solution of 2.5 mg. per ml. was used to determine the variation of absolute intensity and the ratio measurements with instrumental changes. Table I gives the results of this esperiment. The sample position refers to sample surface to x-ray target distances and was determined by lowering the solution cell from the sample drawer mask by means of spacers. Another series of copper solutions (2.5 mg. per ml.) was prepared, containing various acids (10% by volume). Table I1 shows that, while the net intensities

Table IV.

Metal Added t o Copper Soln. Cu only Cr Fe Ni

Pb

and

Cd

If

N

ured show predictable fluctuations according to adsorption edge locations which produce absorption and enhance ment effects. Per cent deviations are calculated from measurements of the pure copper solution. The low deviations found in the case of nickel are due to the enhancement of the CuKa radiations. The effects of absorption and enhancement on both the straight intensity and the ratio measurements decrease with dilution. Solutions submitted for analysis in a copper refinery include electrolytes, leaching solutions, and mother liquors. Examples of these are electrolytes containing copper and nickel, and copper alone, leaching solutions containing cnpper, and nickel sulfate mother liquids. All these solutions are in dilute sulfuric acid. Silver electrolyte, which contains silver and copper in dilute nitric acid, is also submitted for analysis. The metal and acid concentrations of each solution will vary from day to day. It was desired to determine the metal contents rapidly and accurately for the efficient operation of the various processes. A calibration of solutions prepared from pure copper and nickel dissolved

Effects of Various Metals on CuKa/WLsl Ratio (Metal and copper = 5.0 mg./ml.)

Net Intensity, C.P.S. CuKcr WLBl 1411.6

9532.1 8202.6 7905 6 8635 1 8377.1 8703 6

1273.5 1253 7 1266 8 1378 2 1367 7

Ratio 6.75 6.44 6 31 6 81 6 08 6 36

% Deviation Intensity Ratio ... ... -13.9 -17 1 -9 3 -12 1 -8 7

-4.5 -6 5 +o 9 -9 9 -5 8

2-4

thus:

If -2-1 70

Thus, theoretically, the ratio measurement should be less sensitive to matrix change than a straight intensity or a background ratio measurement. Location of absorption edges with regard to the fluorescent and scattered radiation wavelengths, however, would change the ratio. The intensity measurements must be free from interference by other radiations emitted by the sample. Interference of this nature is the most severe limitation of the use of this technique; however, x-ray tube targets such as tungsten omit several lines of the L spectra so that a certain latitude of choice is available. EXPERIMENTAL DATA

A General Electric XRD-5 x-ray spectrograph was operated a t 35 kv. and 20 rna., using a tungsten target x-ray tube, litthium fluoride analyzing crystal,

of both the CuKa and JVLp1 change, the ratio C U R ~ / W L Bremains ~ nearly constant. Table I11 shows that the effect of 1 to 2001, of the acid is minor with regard to the ratio measurement. Higher acid concentrations. will attack the Mylar film. A series of 5.0 mg. per ml. copper solutions was prepared containing 5.0 mg. per ml. of various metals, specscally chosen as to the location of absorption edges. Table I V shows that the ratio is less sensitive to absorption edge location than the straight intensity measurements. The intensities meas-

Table VI.

Type of Soln. Tank house electrolyte Cu powder electrolyte Nickel parting soln. Tank house stripper Slime leaching soln. Silver electrolyte Silver electrolyte

cu 38.41 8.3 0.7 28.20 38.25 98.30 79.51

Table

v,

Calibration of Nickel Solution

hfg./Ml. Cu Ni0.0 1.25 2.50 1.28 2.50 0.0 2.5

0.0 125 2.50 2.50 1.25 2.5 0.0

copper-

Ratio CuKa/

Ratio NiKa/ WLB~ W L ~ 0.32 1.95 3.52 1.94 3.53 .30 3.53

0.09 1.74 3.42 3.41 1.74 3.41 0.09

Comparison of Results

Chemical Ni

Ag

Cu

X-ray Ni

-4g

25.10

.. .

38.33

24.80

, . .

4i:i2 18.74

... ... ...

...

8.2

27:i8 18.16

0.5 28.51 38.70 97.50 80.20

... .. .

, . .

42:33 18.58

... ...

...

VOL 34, NO. 7, JUNE 1962

...

...

26:98 19.00

813

in nitric acid was prepared. Table V shows the results of ratio measurements of the various standard solutions which contain from 0 to 2.5 mg. per ml. of the metals. When the ratios of either the CuKa or NiKa to the WLpl intensities are plotted us. the metal content, straight line calibrations are obtained. Table VI shows the results of determining the copper and nickel content of various solutions. The sample solutions were diluted to a suitable concentration range. Silver electrolyte was also analyzed using synthetic standards. The x-ray beam reduces the silver in the solution, thus inaccuracies result.

standards is shown theoretically and experimentally to compensate for instrumental variations and absorption and enhancement effects. Solutions are most advantageously analyzed by this technique since high scatter intensities give statistically accurate counts in a minimum time. Short count times are of importance in solution techniques because of the action of the x-ray beam on the sample. Comparisons of chemical and x-ray determinations of typical copper refinery solutions show deviations of about *l%. ACKNOWLEDGMENT

DISCUSSION

The use of coherently scattered x-ray tube characteristic radiations for internal

The author thanks United States Metals Refining Co., a subsidiary of American Metal Climax, Inc., for

permission to publish the results of this investigation. LITERATURE CITED

(1) Andermann, G., Kemp, J. W., ANAL. CHEM.30, 1306 (1958). (2) Compton, A. H., Allison, S. K.,

“X-Rays in Theory and Experiment,”

2nd ed., pp. 116-40, Van Nostrand,

New York, 1935. (3) Hauk, W. W., Silverman, L., ANAL. CHEM.31, 1069 (1959). (4)Jones, R. W., Ashley, R. W., Ibid., 31, 1629 (1959). (5) Liebhafsky, H. A., Pfeiffer, H. G., Window, E. H., Zemany, P. D., “X-Ray Absorption and Emission in Analytical Chemistry,” pp. 168-70, TViley, New York, 1960. RECEIVED for review January 16, 1962. Accepted April 13, 1962. Pittsburgh Conference on Analytical Chemistry and Applied Spectroscopy, Pittsburgh, Pa., March 1962.

Measurement of Skeletal Densities of High Surface Area Inorganic Oxides with a Gas Pycnometer JOHANNES TUUL and R. M. DeBAUN American Cyanamid

Co., Sfamford,

Conn.

,A commercial gas pycnometer is used to determine skeletal densities with helium. The results of these determinations are compared with those obtained with liquids. Reasons for the substantial differences between helium and water are suggested. Experiments with different catalyst materials are described, and an explanation of the results is attempted. Possible deviations of the helium skeletal densities from the true values, due to the finite size of the helium atom and adsorption of helium, are also considered.

T

air comparison pycnometer, manufactured by Houston Instrument Corp., Houston, Tex., is an instrument for measurement of volumes and skeletal densities of irregularly shaped, porous, or pgwdered materials. The details of the method are given in “Prelimipary Operating Instructions, Model 200 Air Comparison Pycnometer,” supplied by the manufacturer. The instrument consists essentially of two cylinders with a piston in each. A differential pressure gage indicates when the pressure in the two cylinders is equal. The displacement of air by the sample is obtained from the positions of the pistons. A scale in the form of a counter enables the observer to read the volume directly. The reading accuracy is 0.01 cubic em. HE

814

ANALYTICAL CHEMISTRY

EXPERIMENTAL

With low surface area samples, the instrument performed satisfactorily. However, when i t was used in our laboratory for examining catalyst materials with large surface areas, unrealistic and even negative volume readings were obtained. This indicated that a substantial amount of air was adsorbed on such specimens during measurement. The instrument was then returned to the manufacturer to be modified so that other gases besides air could be used in it. Essentially this amounted to adding another valve and recalibrating the instrument. The main portion of this investigation was carried out on alumina base catalysts. These were obtained in the

Table 1.

form of pellets or extrudates. Some measurements were also carried out with samples of silica and silica-alumina which were obtained in the form of fine powders. The characteristics of the materials with which the bulk of this work was done, are given in Table I. A skeletal density measurement was made as follows. The cup of the pycnometer was cleaned and weighed on an analytical balance. Then the cup was filled with the sample, its edge was thoroughly cleaned, and the cup was weighed again and clamped to the pycnometer. The instrument was evacuated with a Cenco Hyvac mechanical pump. The evacuating was done slowly by gradually opening the valve which connected the pycnometer with the vacuum pump. This pre-

Materials

Original Calcina-

Surface

Notation A AM AMC

s

Composition Physical State Gamma-alumina 1/16-inchextrudates 90% A1203 10% Moos x 3/18 inch pellets 82% Al,O, 15% MoOa 1/16-lnCh extrudates

+ +

Si02

microspheres

Area Compacted tion (S), Bulk TemperaMeter2/ Densitg, ture, O C. Gram Gram/ c

+ 3% COO

240

0.56

593

200

1.oo

593

270

0.59

482

700

0.43

200