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ANALYTICAL CHEMISTRY, VOL. 50, NO. 14, DECEMBER 1978
Trace Arsenic Determination by Volatilization and X-Ray Photoelectron Spectroscopy Manuel B. Carvalho and David M. Hercules” Department of Chemistry, University of Pittsburgh, Pittsburgh, Pennsylvania 75260
By coupling ESCA with volatilization techniques, a sensitive method for trace elements was obtained. It is possible to detect arsenic in solution in the parts-per-trillion range. Arsine, produced by NaBH, reduction, is trapped as an arsenide of mercury on mercuric chloride impregnated paper. A calibration curve was obtained, linear up to 650 ppb. The relative standard deviation of the method is 10% when measured at 100- and 500-ppb levels. Results agreed well with values for a NBS standard reference material. Simultaneous detection of As, Se, Sn, and Sb, each at 100 ppb in a l-mL sample, was performed. The black precipitate sometimes observed with the NaBH, method was determined to be elemental arsenic.
Electron Spectroscopy for Chemical Analysis (ESCA) is generally not considered a trace analysis technique. For a homogeneous sample, under the best conditions, it is possible t o detect a bulk component concentration of 0.1%. Typically 1 to 5% is a more reasonable estimate. However, ESCA has a very high intrinsic surface sensitivity. It is possible to detect fractions of a monolayer on a surface. This means t h a t analysis of subnanogram quantities of material is feasible (1). Thus, ESCA should be suitable for trace analysis provided t h e analyte is present in the form of a thin surface layer. m e E A for trace analysis has been accomplished in a variety of ways. Brinen and McClure ( 2 , 3 )have described an electrochemical deposition process for heavy metals and were able to detect nanogram quantities of Pb. Hercules and co-workers ( 4 ) have used chemically modified surfaces for scavenging metal ions (Pb, Ca, T1, Hg) from solution with detection limits of ca. 10 ppb. Using plasma-grafted polypropylene as an ion-exchange surface, Czuha and Riggs (5) were able t o detect a number of metal ions a t 1 ppm levels. Bancroft (6) was able to detect a small fraction of a monolayer (-lo-’ g/cm2) of Ba2+and Pb2+by evaporation on a surface of a freshly cleaved calcite crystal. T h e technique was extended by Briggs (7) to include anionic species. By evaporation of the analyte solution on aluminum substrates, it was possible to analyze many different cationic and anionic species in the nanogram level for the best case (Pb). We were interested in extending the capabilities of ESCA for trace analysis by coupling its surface sensitivity with volatilization of a sample onto a substrate. Volatilization is a classical technique in analytical chemistry (8) and offers several advantages. First, it removes the analyte from the potential interference of its matrix. Second, it is selective since volatilization removes only certain elements. In this respect ESCA is also quite specific in its measurement capabilities. It is usually possible t o find peaks for chemically similar elements that d o not overlap. Consequently, multielement analysis should be possible. Lastly, volatilization is relatively easy to perform. Specifically, we were concerned with trace arsenic determination. Interest in arsenic determination stems from many diversified areas: agricultural and food chemistry (9, I O ) , water quality control ( I I ) , forensic chemistry (12),and steel analysis (13). An extensive bibliography on arsenic in the environment is available (14). Such widespread concern, along with en0003-2700/78/0350-2030$01 .OO/O
vironmental demands, has required methods of higher sensitivity and simplicity than have been previously available (15). Use of sodium borohydride has made determination of a number of elements as their volatile hydrides very attractive ( 1 6 ) . Atomic absorption has been widely used for arsenic determination (10, 11, 17-19), primarily because of the high sensitivity obtainable via the hydride generation technique and also because of the general availability of such facilities. This paper describes a procedure for trace arsenic determination using ESCA. Arsine, produced by the action of sodium borohydride, is trapped as an arsenide of mercury on mercuric chloride impregnated filter paper. Data are presented which demonstrate that ESCA can be used effectively for determination of trace elements, when coupled with volatilization. I t has been possible to detect trace arsenic solutions in the parts per trillion range, making ESCA competitive with other trace analysis techniques for arsenic. We show our method to be both accurate and precise. Further, it is also demonstrated t h a t ESCA-volatilization allows simultaneous multielement detection a t trace levels.
EXPERIMENTAL Reagents. Stock arsenic solutions of 1.00 mg per mL were prepared by disolving 1.320 g of arsenic trioxide (MC&B, Primary Standard Reagent) in 10 mL of water containing 4 g of NaOH (Mallinckrodt, Analytical Reagent Grade); the solution was diluted to 1 L. Standards of arsenic were prepared immediately before use by serial dilution of the stock solution. Sodium borohydride (J. T. Baker “Baker” grade) was used as a 290 (w/w) solution. Hydrochloric acid (Fisher, Reagent grade) was diluted 1:l (v/v) with water. Mercuric chloride (Fisher, Certified ACS grade) was dissolved in absolute ethanol (U.S. Industrial Chemicals Co., Reagent quality). Filter paper (No. 50, hardened) was purchased from Whatman. For the multielement experiment, 1000 ppm atomic absorption standards (Fisher, Certified) were diluted with 0.1 F HCl to the appropriate concentration prior to use. Deionized, distilled water was used throughout. All solutions were prepared and stored in Nalgene plastic labware. Apparatus. The apparatus used for the production and trapping of arsine is shown in Figure 1. A Gallard-Schlesinger Chemical Mfg. Corp. (Carle Place, N.Y.) G3103 mini-reactor, commonly used for GLC derivatizations, was modified by the addition of a 10/18 male ground glass joint. The reactor is sealed by pressure from the vial screw cap on a Teflon lined silicone septum (Pierce, Rockford, Ill.). Reagents are introduced through a small hole drilled into the vial cap. The collector is connected to the reactor with rubber bands. Mercuric chloride impregnated paper is sandwiched between two pieces of buckeye rubber compound (MIL-R-6855-6, 2A (fiO), 1Q73) and inserted in the collector. Both collector halves are clamped tightly by a spring loaded pinch clamp with a screw locking device. The whole system is leak tight up to about 5 psig. A rectangular slit approximately 0.5 by 2 cm is cut in both rubber pieces so that the sample fits properly on the spectrometer probe. Spectra were run on an AEI ES200A ESCA spectrometer fitted with an A1 K a X-ray source, and interfaced with a PDP 8/M computer. The analyzer slits were set at the largest size. An X-ray power of 400 watts was used. Samples were cooled by continuously running water through the sample probe. Procedure. The arsenic solution (1.00 mL) followed by 1 mL of the hydrochloric acid is added t o the reactor and the reactor sealed. HgC12impregnated paper, sandwiched in the two rubber 1978 American Chemical Society
ANALYTICAL CHEMISTRY, VOL. 50, NO. 14, DECEMBER 1978 0 2031
'
I
,vi-
1
'
Figure 1. Apparatus for the production and trapping of arsine
pieces, is tightly secured between both halves of the collector with a pinch clamp. The solution is continuously stirred by a Teflon encased magnetic stirring bar. Three mL of the NaBH, solution is injected, dropwise through the septum. Decomposition of the N&H4 in the acidic media evolves copious quantities of hydrogen which aids in scrubbing arsine from solution. As the gases flow through the collector, arsine is trapped as an arsenide of mercury on the impregnated paper. When borohydride addition is complete, nitrogen is flowed through the system for about 1 min to purge residual arsine from the system. The paper is removed and attached to the spectrometer probe with double sticky tape. The whole process takes about 15 min. Arsenic 3d and carbon 1s peaks are run. To minimize nonrandom effects such as variations in X-ray power or detector voltage, that may occur from sample to sample, area ratios are used. Sample Preparation. For accuracy studies, NBS standard reference material 1571 (orchard leaves) was used. The leaves were dried for 8 h in a 85 "C oven. T o 1.00 g of the sample, in a 100-mL round-bottom flask equipped with a thermometer and a condenser, 30 mL of a (4:l:l) mixture of concentrated, reagent grade HN03, H,S04, and HC10, were added. The sample-acid mixture was initially heated very gently with a Bunsen burner. After sample foaming subsided, the temperature was raised to produce steady boiling (-120 "C). Excess HC10, was boiled off and residual H2S04heated strongly to fumes of sulfur trioxide. A solution with a light yellow color was obtained. When cooled to room temperature, the digest was quantitatively transfered to a 100-mL volumetric flask. Concentrated HCl, 30 mL, ww added and the solution was diluted to 100 mL with deionized distilled water. A larger volume reactor was used to analyze 4.00-mL aliquots of the digest.
RESULTS AND D I S C U S S I O N I m p r e g n a t i o n S t u d i e s . T h e best conditions for impregnating the filter paper with mercuric chloride were investigated. T o optimize arsine pickup, the percent mercuric chloride (w/w) used in the ethanol impregnating solution was varied. T h e filter paper was placed in solution the same amount of time (30 min) for each run. Arsenic concentration was kept constant a t 1 ppm. Data are plotted in Figure 2. T h e arsenic-to-carbon peak area ratios were normalized relative to the largest peak area ratio and called arsine pickup. Since normalized values are used, small differences in arsine pickup appear large. The actual differences in peak area ratios were rather small. Pickup increases up to 15% HgClz and then starts to drop. T h e drop is most likely caused by clumping of the HgC12. As the limit of solubility is approached (- 23%), crystals of HgC12 can be seen on the surface of the paper. Clumping leads to a decrease of HgC12available on the paper surface, and thus to a lower arsine pickup. Impregnation time was investigated by keeping both HgCl, (15%) and arsine concentration (1 ppm) constant. T h e time t h e filter paper remained in the impregnating solution was varied. T h e results are shown in Figure 3. After an hour in solution, there is no substantial difference in arsine pickup.
:
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22
Figure 2. HgCI, concentration study. Arsine pickup by impregnated paper as the percent HgCI, is varied in the impregnating solution. Arsenic concentration was kept constant at 1 ppm, impregnation time at 30 min I
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I00 80 Y,
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i I I 40
80
120
I m p r e g n a t i o n Time ( m r n 1
Figure 3. Impregnation time study. Arsine pickup by impregnated paper as the time of impregnation is varied. Mercuric chloride concentration was kept constant at 15%, arsenic at 1 ppm
All paper was impregnated in 15% HgC12ethanol solution for a t least an hour. After an hour, the paper was removed from solution, excess reagent drained, and air dried. When dry the paper was stored in a desiccator and kept in the dark. Paper was cut as needed, being careful not to use the edges since excess reagent might be present. Impregnated paper was prepared fresh as needed. C a l i b r a t i o n Curve. Typical arsenic 3d spectra used to obtain the calibration curve are shown in Figure 4. Each spectrum is an accumulation of 15 scans and took about 40 min to record. Since 1 mL of sample is used, the concentration can also be read as nanograms of arsenic. The lower part of the figure is a spectrum for 25 ng of arsenic. This corresponds to 3.3 x mol or about 2 x atoms. Consider that a monolayer consists of about 10'j sites per cm2. Since the sample area is 1 cm2, we are detecting about 0.2 of a monolayer. By varying arsenic concentration and plotting the As-to-C ratio vs. concentration of arsenic, a calibration curve seen in Figure 5 is obtained. Note that since the sample volume is always 1 mL, the x-axis also can be read directly in nanograms of As. The curve has a large linear dynamic range. For the linear portion, the correlation coefficient is 0.995 and has a relative standard deviation of 4.8%. The curve starts to flatten out above 650 ppb because the depth of the arsenic layer exceeds the electron escape depth. Essentially the paper is saturated with arsenic as far as ESCA is concerned. Initial attempts a t obtaining a calibration curve using arsenic-to-mercury peak area ratios produced a plot somewhat akin to a scatter diagram. The difficulty was explained in two ways. T o minimize paper charring, due to heating caused by the X-rays, samples were cooled by passing liquid nitrogen through the probe. This had the effect of condensing con-
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ANALYTICAL CHEMISTRY, VOL. 50, NO. 14, DECEMBER 1978
Table I. Precision Studies at Two Concentration Levels arsenic concentration, ppb trial no.
I
I
1 2 3 4 5
I
\
100
500
108 93
514 423
106 100
517 551
mean
81 97.6
U
10.1
RSD, %
10.4
-
501 54.8 10.9
-~ Table 11. Analysis of Arsenic in NBS Orchard h a v e s (SRM 1 5 7 1 )
As found, l g / g of
As/C area ratio
sample 11.0
0.336 0.305 0.27 6 0.302 0.291
9.9 8.9 9.8 9.5 9.8 0.8 9.8
mean 0
value at 95% CL NBS value I
I'
Y
i
lo?2
0.9
I
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I
I
46
44
42
40
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Figure 4. Arsenic 3d spectra for various concentrations of arsenic used for the calibration curve 6C
lp-_ 8OC Ppb A s
45
43
41
E,(eV,
Flgure 5. Analytical curve sed for quantitative trace arsenic analysis. Since I-mL samples were used, the x-axis can also be read as nanograms of arsenic. Correlation coefficient, 0.995; relative standard deviation, 4.8% for the linear portion of the curve
Figure 6. Enhancement of detection limit Arsenic 3d spectrum obtained from a solution 300 parts per trillion in arsenic using large sample size (165 mL)
taminants on the sample surface. Uneven contact of the sample with the probe could lead to nonuniform cooling. This would then cause disparities in the contaminant coating, both within and between samples, leading to erratic results. Another factor was the volatility of the mercuric chloride. Depending on t h e amount of mercury arsenide present and also on the uneven cooling effect, varying amounts of mercury would be volatilized. Consequently, mercury was not constant as assumed. Proof of this was that a plot for the same samples using arsenic-to-carbon ratios bore greater similarity to a straight line. Consistent results were obtained by using arsenic-to-carbon ratios rather than arsenic-to-mercury ratios. Also cooling with liquid nitrogen was discontinued, and water cooling was found to cause only minimal charring. P r e c i s i o n and Accuracy. T h e precision of the method was evaluated by running five replicate samples. The relative standard deviation (Table I) was 10.4% and 10.9% for 100 and 500 ppb of arsenic, respectively.
Accuracy of the analytical method was tested by analyzing a NBS biological standard reference material. T h e arsenic concentration in the orchard leaves as certified by NBS is 10 f 2 pg As per g of sample. For five replicate analyses (Table 111, our value, 9.8 f 0.9 a t the 95% confidence limit, agrees well with the NBS value. Detection Limit. A lower limit of detection, taken as the concentration of arsenic necessary to give a signal twice the standard deviation of the noise, was calculated to be ca. 3 ppb. However, the volatilization method has an attractive feature which is not easily applicable to other methods for trace arsenic analysis. I t is possible to do lower concentrations by using a larger sample volume. Figure 6 shows a n arsenic spectrum obtained from a solution containing 300 parts per trillion of arsenic. This was accomplished by interfacing a three-neck, one-liter flask, equipped with a 500-mL dropping funnel, thermometer, and magnetic stirring, with the collector in Figure 1. A sample volume of 165 mL with the same relative
-
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,
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Se 36 i
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ANALYTICAL CHEMISTRY, VOL. 50, NO. 14, DECEMBER 1978
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Figure 7. Simultaneous detection of As, Se, Sb, and Sr each at 100 ppb in a 1-mL sample
ratio of other reagents was used. It should be possible to analyze lower concentrations using larger sample volumes. Multielement Analysis. Another advantage of the ESCA-volatilization method is the possibility for simultaneous volatilization of a number of species which are chemically similar. Coupled with the specificity of ESCA, simultaneous multielement analysis is feasible. Figure 7 demonstrates the simultaneous detection of As, Se, Sb, and Sn, all at 100 ppb using a 1-mL sample. All spectra are normalized to the same height and smoothed. Quantitative simultaneous multielement analysis should be possible by constructing a set of independent calibration curves. Black Precipitate. As experience was gained with the borohydride method, it became obvious that hydride yields are not as quantitative as originally reported (20,211. At high concentrations of arsenic, it was observed that either a transiently colored solution or a finely divided black precipitate was formed. This was surprising since in the initial literature search no mention concerning the formation of a black precipitate was found. A closer examination of how sodium borohydride acts as a reducing agent revealed that, depending upon reaction conditions, products other than volatile hydrides may be obtained. Alternatively, the reactants may be reduced to a lower oxidation state (16),form insoluble borides, which may act as catalysts for the hydration of sodium borohydride (22),or the free elements may be produced (23). Of the few papers (24-27) reporting formation of a black precipitate, it was the majority opinion that it was the finely divided metal. T o determine the nature of the precipitate, a large amount (10 mg) of arsenic was used. Addition of the borohydride caused copious gas evolution and immediate formation of the black precipitate. The solution was filtered, rinsed three times with 10-mL portions of water, and air dried. Since the precipitate adhered to the filter paper and could not be removed, analysis was done on the filter paper. The 3d spectrum of the black precipitate, and reference spectra of As”, As3+, and Aso are shown in Figure 8. As can be seen, the black precipitate is elemental arsenic. Formation of metallic arsenic has not been observed with trace arsenic solutions after many repetitive reactions done in the same flask. T h e analyte concentration is so low t h a t conditions favor formation of the hydride over the element. I t is interesting to note, however, that studies (24-27) which mention formation of a black precipitate were concerned with interferences of various elements a t high concentrations.
Figure 8. Arsenic 3d spectra for As,SO,, As,O, the black precipitate, and elemental arsenic. Spectra show that the black precipitate formed with NaBH, at high arsenic concentrations is elemental arsenic
Table 111. Absolute Arsenic Detection Limit of Various Techniques detection limit, ng technique atomic absorption ( 2 8 ) flame flameless atomic fluorescence ( 2 9 ) atomic emission ( 3 0 ) neutron activation ( 3 1 ) anodic stripping voltammetry ( 3 2 ) GC/MS (33) this work
4 0.08 2
1 15 0.1 10 3
Auger Studies. We also tested the feasibility of performing trace analysis by Auger spectroscopy using the volatilization technique. This led t o limited results. Charging effects were so severe with the filter paper that sparks were observed coming from the sample. To overcome this, thermal decomposition of arsine to arsenic: over a heated metal substrate was attempted. Excellent spectra for milligram samples were obtained. However, at trace levels, arsenic was not detected. Several variations of the decomposition apparatus gave no better results. Thcrmal decomposition at trace levels may be limited by the kinetics of the process. The rate of decomposition is probably proportional to arsenic concentration. As the amount of arsine decreases, the rate of decomposition is so slow that the arsine escapes before being decomposed. CONCLUSIONS Our results suggest, in contrast to general opinion, that photoelectron spectroscopy can be used effectively for trace analysis. By coupling the surface sensitivity of ESCA with volatilization onto a substrate we have obtained a sensitive
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ANALYTICAL CHEMISTRY, VOL. 50, NO. 14, DECEMBER 1978
technique for quantitative analysis for trace amount>s of arsenic' Of detection limits 'I1) demonstrates that the ESCA-volatilization technique is quite competitive with other trace analytical methods. Likewise, our analytical method shows both good accuracy and precision. Further, volatilization can be performed in the field making remote sampling facile. T h e possibility of doing quantitative simultaneous multielement analysis a t trace levels also has been demonstrated.
ACKNOWLEDGMENT T h e authors express their gratitude to F. LV. Plankey and E. M. Heithmar for loan of the atomic absorption standards and for some helpful discussions; also t o S.Erickson for her assistance.
LITERATURE CITED (1) T. A. Carison, "Photoelectron and Auger Spectroscopy", Plenum Press. New York, 1975 p 268. (2) J. S. Brinen and J. E. McClure, Anal. Lett., 5, 737 (1972). (3) J. S. Brinen and J. E. McClure, J . Electron Spectrosc. Relat. Phenom.. 4, 243 (1974). (4) D. M. Hercules, L. E. Cox, S . Onisick, G. D. Nichols, and J. C. Carver, Anal. Chem., 45, 1973 (1973). (5) M. Czuha and W. M. Riggs, Anal. Chem., 47, 1836 (1975). (6) G. M. Bancroft, J. R. Brown, and W. S . Fyfe, Anal. Chem., 49, 1044 119771. \ ~ , (7) D. Briggs, V. A. Gibson, and J. K. Becconsall, J . Electron. Spectrosc. Relat. Phenom., 11, 343 (1977). (8) H. F. Walton, "Principles and Methods of Chemical Analysis", 2nd ed., Prentice-Hall, Englewood Cliffs, N.J., 1964, Chap. 9. (9) H. s. Satterlee and G. Blodgett, h d . h g . Chem., Anal. Ed., 16,400 (1944). (10) J. A. Fiorino, J. W. Jones, and S. G. Capar, Anal. Chem., 48, 120 (1976). (1 1) U.S. Public Health Service, "Drinking Water Standards', U.S. Dept. of Health, Education, and Welfare, Washington D.C., 1962.
(12) R. S. Braman, L. L. Justen, and C. C. Foreback, Anal. Chem., 44,2195 (1972). (13) N. G. Elenkova, R. A. Tconeva, and Tc. K. Nedeitcheva, Talanta, 23, 726 (1976). (14) "Arsenic in the Environment - An Annoted Bibliography", Oak Ridge National Laboratory, Oak Ridge, Tenn., ORN-E1S-73-16 (1973). (15) "Official Methods of Analysis of the Association of Official Analytical Chemists", 11th ed., 1970, p 399. (16) T. F. Jula, "Inorganic Reductions with Sodium Borohydride: Principles and Practice", Ventron Corp., 1974. (17) K. T. Kan, Anal. Lett., 6,603 (1973). (18) F. J. Fernandez, At. Absorp. News/., 12, 93 (1973). (19) H. H. Walker, J. H. Runnels, and R. Merryfield, Anal. Chem., 48,2056 (1976). (20) F. J. Schmidt and J. L. Roger, Anal. Lett., 6, 17 (1973). (21) K. M. Mackay, "Hydrogen Compounds of the Metallic Elements", Spon, London, 1966, p 133. (22) H. I. Schlesinger, H. C. Brown, A. E. Finhoit, J. R. Gilbreath, H. R. Hockstra, and E. K. Hyde, J . Am. Chem. SOC.,75, 215 (1953). (23) H. C. Brown and A. C. Boyd, Anal. Chem., 27, 156 (1955). (24) A. E. Smith, Analyst (London) 100,300 (1975). (25) R . Belcher, S. L. Bogdanski, E. Henden, and A. Townshend. Analyst (London), 100, 522 (1975). (26) F. D. Pierce and H. R. Brown, Anal. Chem., 48, 693 (1976). (27) J. E. Drinkwater, Analyst (London), 101,672 (1976). (28) K . G. Brodie, Am. Lab., 3, 73 (1977). (29) E. M. Heithmar, Ph.D. Thesis, University of Pittsburgh, 1976. (30) R. S . Braman et al., Anal. Chem., 49. 621 (1977). (31) L. E. Wangen and E. S. Gladney, Anal. Chim. Acta, 96,271 (1978). (32) G. Forsberg, J. W. O'Laughlin. and R. G. Megargle, Anal. Chem.. 47, 1586 (1975). (33) R. D. Kadeg and G. D. Christian, Anal. Chim. Acta, 88, 117 (1977).
RECEIVED for review May 17, 1978. Accepted September 1,
1978, presented at the 29th Pittsburgh conference on Analytical Chemistry and Applied SPeCtroSCopY, Cleveland, Ohio, Feb. 27-March 3, 1978, paper no. 229. This work was by the Science Foundation under Grant CHE76-19452.
Quantitative Ion Probe Analysis of Glasses by Empirical Calibration Methods J. D. Ganjei and G. H. Morrison* Department of Chemistry, Cornell University, Ithaca, New York 74853
The range of applicability of the empirical sensitivity factor approach in ion probe analysis of glasses was investigated. Accurate results were achieved only when the standards and sample contained the same major element (excluding oxygen) in the glass matrix. With the above criterion satisfied, modification of the basic external standard approach was not necessary since other interelement matrix effects were minimal. Analyses in the range of error factors of 1.15 were obtained using average relative sensitivity factors.
Secondary ion mass spectrometry (SIMS) is one of the most sensitive techniques for surface elemental analysis. However, the potential of SIMS, particularly in the ion probe mode, is currently limited by the difficulty in quantification of the secondary ion signals. This problem has been previously discussed ( 1 ) but, briefly, t h e uncertainty in secondary ion interpretation is mainly due to the variability of the ratio of measured ion intensities to sputtered atoms. T h e relative secondary ion ratios can be affected by residual gas surface adsorption, primary bombarding ion species, sampling con0003-2700/78/0350-2034$01 O O / O
ditions, and the sample composition (commonly called the matrix effect). Of these factors, the matrix effect is certainly the most perplexing since the analyst can regulate instrumental parameters, whereas sample composition is beyond his control. In attacking the problem of quantitative analysis, the authors have selected an empirical approach utilizing external standards to calibrate the elemental secondary ion signals. The accuracy of external standards derived sensitivity factors is critically dependent on the matrix effect. If the sample matrix is sufficiently different from the standard, the change in secondary ion ratios between the matrices will result in large analytical errors. In fact, the popularity of semitheoretical quantitation methods which are accurate only to a factor of 2 or 3 is due to the uncertainty of the empirical standards' approach. Many SIMS users have rejected the empirical approach for analyses because of the difficulty of obtaining standards whose elemental sensitivity factors will be applicable to the sample. As Christie and Smith (2) have recently pointed out, criteria for sample-standard matrix match currently do not exist. C 1978 American Chemical Society
