scale; improved burner tip and burner housing design and sample inlet passages having less adsorbing power, for shorter response time, shorter warmup time, and increased emission intensity; and use of a n optical filter having a maximum transmission a t about 380 mp instead of a t 402 mM.
tions and for all wet analyses of SOz samples, to Thomas Hauser for running the spectral transmission curve of the optical filter, to Jerome Flesch for the use of the monochromator and power supply, and to Jack Goldstein of Phoenix Precision Instrument Co. for the use of a strip chart recorder.
ACKNOWLEDGMENT
(1) Gilbert, P. T., Jr., “Analysis Instrumentation,” Plenum Press, New York,
Center, Cincinnati, Ohio (1964). (3) IIT Research Institute TerhnirAl - ... . .... Documentary Rept. X o . RTD-TDR63-1072, USAF Contract No. A F 04(611)-7543 (1963). WALTERL. CRIDER Laboratory of Medical and Biological Sciences Division of Air Pollution Public Health Service U. S. Department of Health, Education, and Welfare Robert A. Taft Engineering Center Cincinnati, Ohio 45226 Mention of products or company names does not constitute endorsement by the U. S. Public Heal1 h Service.
LITERATURE CITED
The author is indebted to Gilbert Contner and S a o m i Barkley for furnishing Mylar bags of known SOz concentra-
1964.
(2) Hochheiser, S., PHS Publication Xo. 999-AP-6,Taft Sanitary Engineering
X-Ray Fluorescence Spectrometric Determination of Chromium, Iron, and Nickel in Ternary Alloys SIR: A rapid analytical method was required for quantitative determination of 1 to 95% of chromium, iron, and nickel in ternary alloys of these elements. If necessary, some sacrifice in precision was acceptable to obtain rapid analyses. Relative standard deviations of the order of 1% seemed possible using a rapid x-ray fluorescence method. However, thcl simultaneous determination of chromium, iron, and nickel is a difficult problem in x-ray fluorescence analysis because of strong fluorescence (chromium by iron and nickel, iron by nickel) and absorption (iron by chromium, nickel by chromium and iron) effects among these three elements. Sumerous techniques, including use of a series of standards (Z), dilution ( 7 ) , and theoretical correction factors (6, 8,9), have been proposed for reducing interelement effects. Generally a large number of samples having known composition or knowledge of mass absorption coefficients and fluorescent effects is required to apply these techniques. The dilution technique which may be appropriate in this case for individual analyses, may not be sufficiently rapid for routine analyses. Another cause for error is self-absorption by the sample, but careful control of the sample size mas considered as a way of minimizing the effect of this parameter. I n the method described here, an empirical correction proposed for use in the electron microprobe analysis of binary and ternary metallic systems (IO), was applied to the x-ray fluorescence analysis of the ternary alloys. Intensity measurements of the desired x-ray lines are obtained from the pure metal, A , from binary mixtures containing 50% by weight of A and B , and from the samples. Using the relative intensity, k ~ of, element A from a 50ye binary mixture (A, B ) to the pure metal (A), a correction parameter, a ( A . B ) , is obtained which describes the effect of
B on the intensity of A . This effect is expressed by the equation:
h similar expression is obtained to describe the effect, u ( A , ~ )of, element C on A . The values for a ( A , n ) and a(A,o as measured from binary mixtures are used in the following equation in calculating the concentration of A , (CAI
:
Because the three concentrations are unknown initially, it is necessary to substitute kn and kc for cn and cc in Equation 2 and arrive a t the concentrations by successive approximations. This empirical correction approach was developed to apply to the x-ray fluorescence determination of chromium, iron, and nickel in solutions. EXPERIMENTAL
Instrumentation. A Philips Electronic Corp. x-ray spectrograph t h a t has a three-position head, a lithium fluoride analyzing crystal, and a Machlett FA 60 tungsten-target x-ray tube was used. -4 multiplier photot u b e a n d a sodium iodide-thallium iodide scintillation crystal served as t h e detector. T h e x-ray tube was operated a t 50 kv. and 30 ma. and the phototube at 840 volts. The inside of the stainless steel sample cell, provided with the instrument, was coated x i t h paraffin to prevent corrosion by the nitric acid solutions. Mylar windows for the cell were coated with a n anti-wetting agent (Desicote, Beckman Instruments, Inc.) to reduce bubble formation during irradiation.
’
Reagents. Solutions containing chromium, iron, or nickel were prepared from analytical reagent grade chromium nitrate, iron metal, or nickel shot, respectively. Each reagent contained less t h a n 500 p.p.m. of each of t h e other two metals. T h e chromium solution was analyzed by oxidation to dichromate with perchloric acid, addition of excess standard ferrous ammonium sulfate solution, and titration of excess ferrous ion with ceric sulfate (4). The iron was determined gravimetrically as the oxide ( 5 ) . Sickel was determined by constant potential electrodeposition from ammoniacal solution ( 3 ) . Reagent grade nitric and hydrofluoric acids and triple-distilled water were used throughout the analyses. Recommended Procedure. dpproximately 200 mg. of t h e ternary alloy were weighed accurately, transferred t o a beaker made of Teflon, and dissolved in 1 6 s nitric acid or a mixture of 16N nitric and 29iV hydrofluoric acids. T h e solution was evaporated t o incipient dryness, and t h e residue was dissolved with distilled water and diluted to 10.00 ml. with 2.0 ml. of 1 6 s nitric acid and distilled water. -45-ml. aliquot waq transferred to the wax-coated sample cell, and duplicate measurements of the intensities of the K a x-ray lines for chromium, iron, and nickel were made. The number of counts accumulated at each wavelength was selected so that 30 to 100 seconds were required for measurement of each x-ray intensity. Count rates were then measured from solutions containing 200 mg. of either chromium, iron, or nickel. The background correction for chromium was obtained as the average chromium x-ray intensity from the solution containing 200 mg. of iron and the solution containing 200 mg. of nickel. Similarly, background corrections were obtained for iron and nickel from measurements of the nickel and chromium solutions and the iron and chromium solutions. Each measured count time was corrected for detector dead time by subtracting
V O L . 37, NO. 13, DECEMBER 1965
1773
Table 1. Evaluation of Correction Parameters for X-Ray Spectrometric Determination of Cr, Fe, and Ni
System Cr-Fe
Element Cr Fe
Cr-Ni
Cr
Xi
Fe-Ni
Fe Ni
~(A,B)
0.952 1.270 0.956 1.264 0.931 1.357
Std. dev. 0.018 0.013 0.016 0.019 0.017 0.017
Table I I . Precision of X-Ray Fluorescence Spectrometric Determination of Cr, Fe, and Ni (200-mg. Samples)
Concn.,
93
Cr 0.7 1.4 0.9 1.9 2.7
75 50
25 5
1
Rel. std. dev. Fe Ni 0.9
1.3 1.6 2.2 2.9
0.8 1.0 1.1
1.4 1.7
4.0 seconds per lo6 counts accumulated, and for x-ray background by subtracting the appropriate background correction. Corrected count rates for samples were divided by the appropriate corrected count rate for 200 mg. of pure element to obtain the values kc,, kFe, and kN1. Values for the correction parameters, U ( A , B ) and u ( A , c ) , were determined as described below, and the concentration, CC,, of chromium was calculated using Equation 3. 1 - kcr -kc,
-
A similar equation was used to determine the concentration of nickel, cNi. Then, using the calculated values c C r and cN, rather than kcr and kN,, the concentration of iron, cFe, was determined. Evaluation of Correction Parameters. Correction parameters, U ( A . B ) and U ( A . C ) for t h e ternary system chromium-iron-nickel were calculated from d a t a obtained during a six-month period. Several solutions containing 200 mg. of chromium, iron, or nickel individually, or 1: 1 binary combinations of the elements in 3.3 f 0.3Y nitric acid were prepared. Intensities of KC^ x-rays for chromium, iron, and nickel were measured, and values for u ( A . ~ were ) calculated using Equation 1. The data obtained are shown in Table I in which each value is the average of 24 determinations. DISCUSSION
Solvent. T h e acid used in sample dissolution is important in x-ray fluorescence solution analysis because t h e solvent comprises a large portion of t h e matrix. Usual solvents for steel samples are hydrochloric, sulfuric, and perchloric acids. Initial studies showed that variations in hy1774
ANALYTICAL CHEMISTRY
drochloric acid concentration of 0.04 ml. per 10 ml. of solution caused differences in chromium, iron, and nickel x-ray intensities as large as 1%. This stringent control of acidity is a difficult task. =1 similar problem is expected when other acids such a? perchloric or sulfuric that have high mass absorption coefficients are used. Therefore, a solvent system having a mass absorption coefficient more nearly equal to that of water was considered. Nitric acid, or a mixture of nitric and hydrofluoric acids, has a low mass absorption coefficient and iq a suitable solvent for stainless steel arid chromium-iron-nickel alloys. Therefore, these solvents were used in subsequent work. Variations in nitric acid content of approximately 0.4 ml. per 10 ml. of total solution produce deviations in intensity measurements that are not greater than 1%. Effect of Sample Size. A sample size was selected t h a t would dissolve readily in 10 ml. of t h e solvent and provide 10,000 counts a t the 1% concentration level in a reasonable counting time to obtain a relative standard deviation of about ly',. The lowest count rate was expected from chromium (approximately 10 counts,' second,'mg.jml. above a background of 80 counts/second). To obtain 10,000 counts within 2 or 3 minutes, approximately 2 mg. of chromium or 200 mg. of sample per 10 ml. are required. T o determine the effect of sample size on the method, values for the correction parameters, U ( A . B ) and u ( ~ , were ~ ) , measured using as samples a series of 1 : l binary mixtures that varied in mass from 175 to 225 mg. These mixtures were diluted to 10.00 ml. with 3.3N nitric acid and the values for the correction parameters were obtained as described. The correction parameter for each binary combination varies linearly with sample mass. The following equations were derived to define these relationships:
+ 0.152 y 0.956 + 0.116 y
u ( c ~ , F=~ 0.952 )
(4)
u(cr.xI) =
(5)
u(re,cr)= 1.270 f 0.82 y
(6)
0.931 f 0.108 y
(7)
C L ( N ~ , C ~= )
1.264 4- 0.78 y
(8)
u ( N ) . F ~ )=
1.357 f 0.91 y
(9)
u(F,,N,)
=
sample, mg. -1 Y= 200
A second series of binary mixtures, each weighing 175 or 225 mg., was prepared in which the concentration of each constituent was varied over the range of 10 to 90%. These samples were analyzed using values for the correction parameters calculated from the above equations. The results showed that the
correction parameters are generally adequate for Concentrations approximabely in the range of 25 to 75%. However, deviations in excess of 3yo relative n-ere observed in some samples, and generally rvere most pronounced a t 10 and 90% concentration levels. It is possible that these deviations result from a self absorption fact,or t,hat has not been included in the correction equations. To avoid the deviations observed for samples that weigh l i 5 or 225 mg., the recommended sample size is restricted to 195 to 205 mg. If significant concentrations of elements such as silicon and carbon, which are lost during sample dissolubion, are present, the sample iveight used in Equations 4 to 9 should be decreased by the amounts of these elements that are lost. Application of Successive Approximations. The measured intensity ratios, k~ and kc, are substituted init'ially in Equation 2 as first allproximations of the unknown concentrations of elements B and C when determining the value for c A . Then by application of successive approximations, a more accurate value is calculated for C A . However, as u ( A . B ) approaches U ( A , ~ ) ,Equation 2 reduces to Equation 1 and successive approximations are not necessary. I n analyzing the chromium-iron-nickel ternary alloy, the effects of chromium and iron on the determination of nickel are approximately equivalent, and the effects of iron and nickel on the determination of chromium are almost equal (Table I). Therefore, application of successive approximations should have little effect on the determination of chromium and nickel. Experimentally, this was found to be the case. However, the difference between the effects of chromium and nickel on the determination of iron (Table I) indicates that a second approximation may appreciably affect results for iron. Use of calculated chromium and nickel concentrations, cCr and cNi, did change values for iron by as much as 2% relative compared to values obtained using intensity ratios, kcr and kKi. Therefore, in analysis of ternary alloys, chromium and nickel concentrations are calculat'ed first, using measured intensity ratios, and these calculated concentrations for chromium and nickel are used in determination of iron content. Fluorescence Effect. I n applying a theoretical correction for secondary fluorescence, it has been proposed (I) that significant,ly higher correction factors are required for low concentrations of an element, 4 , than for high concentrations of this element in the presence of high concentrations of a strongly fluorescing element, B. Thus, Equation 2 would not be valid and high values would be obtained when
Table 111. Effect of Other Elements on the Determination of Cr, Fe, and Ni
Other ele- Added, yG ment A1 10 A1 5 110
Discrepancy, av. relative 7ca Cr Fe Ni -0.5 1.3 -1.2 0.5 0.0 0.0 -2.5 0.3 -0.7 5 -2.1 -1.5 -1.0 v hln 10 -0.3 0.1 0.7 0.. 11.2 co 10 -1.0 cu 10 -0.4 0.5 2.4 -1.2 0.5 Zr 10 -1.0 -0.2 0.2 Zr 5 -0.4 -1.0 1.9 110 10 -2.3 0.0 -0.6 RIo 5 -0.4 -2.0 0.3 SI1 10 -1.5 -0.9 -1.0 Sn A -1 3 Ta 10 6.0 -0.6 1.9 w 10 1.2 1.9 1.9 10 -2.7 -1.8 0.2 Pb -0.5 -1.4 Pb 5 -1.4 -5.5 -2.8 G 10 -7.2 - 1 . 1 -1.6 u 2 -0.7 a Average of three determinations. ~~
~~
determining loiv concentrations of iron in the presence of high concentrations of nickel. or low concentrations of chromium in the presence of high concentrations of iron and nickel. To determine the reliability of the proposed technique in these cases, a series of 200mg. qainples that contained 5.00, 2.50, or l . O O ~ oof iron, 90, 50, or 15% of nickel, and the remainder of the sample being chromiuni was prepared in duplicate and analyzed. The deviations in the iron found from the quantities added are no greater than 0.2, 0.4, and 4yo, respectively, for 5.00, 2.50, and l.OOyo of iron. Therefore the correction parameters are adequate provided the sample size and dilution factors are kept within the limits described. Recause control of sample size and dilution factors eliminated significant deviations in the fluorescence effect of nickel on iron, it was assumed that the effects of nickel and iron on chromium also were insignificant under similar experimental conditions. Precision. The precision of t h e nwthod was calculated a t five concentration levels for each of t h e three elements. Solutions were prepared t o contain various amounts of chromium, iron, and nickel in 3 . 3 5 nitric acid and were analyzed using the procedure described. For each concentration level, for one element, t h e concentrations of t h e other two elements were varied over a wide concentration range. Relative standard deviations, based upon a minimum of 10 determinations at each concentration, are summarized in Table 11.
Table IV.
Determination of Cr, Fe, and Ni in NBS Stainless Steel Samples by the Proposed X-Ray Fluorescence Method
lOlc
S B S certified concn., Cr Feb Xi 18.21 70.68 9.27
160
19.12
66.97
8.91
121b
17.68
68.36
11.14
sample number
Found, Yoa Cr Fe Ni 18.48 70.76 9.21 18.51 72.23 9.18 18.41 70.25 9.25 18.98 67.21 8.92 19.07 67.03 8.86 19.03 67.22 8.85 17.71 68.05 11.08 18.04 68.60 11.09 17.92 68.14 11.10
Difference, yo relative Cr Fe Ni 1.5 0.1 -0.6 1.6 2.2 -1.0 1 -1 -n- . -6 -n- . -3 -0.7 0.4 0.1 -0.3 0.1 - 0 . 6 -0.5 0.4 -0.7 0.2 -0.5 -0.5 2.0 0.3 -0.4 -0.4 -0.4 - 0 . 4
Result of a single determination. By difference.
Effect of Impurities. Although t h e method was specifically designed for analysis of chromium-iron-nickel alloys, t h e effects of other elements were investigated t o extend t h e applicability of the method t o analysis of various steels. Elements t h a t have high mass absorption coefficients or t h a t might cause enhancement were selected for this investigation. Samples were prepared in triplicate t o contain 12 to 50% each of chromium, iron, and nickel, and u p t o 10% of various other elements. The differences between the concentrations of chromium, iron, and nickel added and the values found, expressed as relative per cent (Table 111), show that 29Z0 concentration of any of the other elements investigated, and 10% of many of these elements, can be tolerated without introducing significant analytical errors. The most serious interference is caused by elements such as uranium and lead which have high mass absorption coefficients. The effect of the impurities generally is greatest in the measurement of chromium because of the larger absorption a t the longer wavelength of the chromium K a line. Interference by enhancement was significant only when absorption interference was absent (nickel enhanced by copper). dpplication of an additional correction factor should minimize errors if high concentrations of other elements are anticipated. Analysis of Steel Samples. Ternary chromium-iron-nickel alloys t h a t had known compositions were not available for analysis. Therefore, t h e method was applied t o analysis of three stainless steel samples certified b y t h e National Bureau of Standards (KBS). Three separately weighed portions of each sample were analyzed (Table IV). T h e differences between t h e certified concentrations and those found b y analysis are less than the relative standard deviations of
the method, showing that the method is reliable. Difficulties were not esperienced in the application of this method to analysis of these stainless steels. ACKNOWLEDGMENT
The authors are indebted to R. G. Hurley for obtaining much of the analytical data and to J. W. Dahlby for standardization of the nickel solutions, A computer program for performing the calculations was written by Joe Duran. The assistance of C. F. Lletz, under whose direction this work was performed, is gratefully acknowledged. LITERATURE CITED
(1) Birks, L. S., “Electron Probe Microanalysis,” p. 118, Interscience, New
York. 1963. (2) Briksey, R. M., A N ~ L CHEM. . 25, 190 (1953). , (3) Dale, J. &I., Banks, C. V., “Treatise on Analytical Chemistry,” 11. XI. Kolthoff, P. J. Elving, eds., Part 11, Vol. 2, p. 408, Interscience, Yew York, 1963. (4) Hartford, W. H., Ibid., Part 11, Vol. 8, p. 324. (5) Kolthoff, I. XI., Sandell, E. B., “Textbook of Quantitative Inorganic Analysis,” 3rd ed., p. 310, Macmillan, New York. 1952. (6) Mitchell; B. J., ANAL. CHEM. 33, 917 (1961). (7) Sherman, J., “Proceedings Sixth Annual Conference on Industrial Applications of X-Rays,” W. 11. hheller, ed., p. 231, Denver Research Institute, University of Denver, 1957. (8) Sherman, J., Spectrochzm. Acta 7, 283 11955). ( 9 j Sheiman, J., Ibid., 15, 466 (1959). (10) Ziebold, T. O., Ogilvie, R. E., ANAL. CHEM.36, 322 (1964). \ - - -
E. A. HAKJLILA G. R. WATERBURY The University of California Los Alamos Scientific Laboratory Los Alamos, N. 11. WORK performed under the auspices of the U. S. Atomic Energy Commission.
VOL. 37, NO. 13, DECEMBER 1965
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