TABLE IV. Results of si l icon determinat ion in HF acid."

Wavelength H2SiF~ Present

(nm) (%)

H2SiFs Found (%)

Calibration Standard addition

Si 251.6 0.01 0.0101 =i: 0.0001 0.0101 .¢_ 0.0001

" Mean of 10 determinations.

was found to have a shorter decay time for B, implying a lower memory effect.

The inertness of this sample introduction system to- ward HF was determined by measuring C emission at 247.9 nm after aspirating commercial HF acid. No change in signal intensity was observed. Spectra were also re- corded from 190 to 800 nm for acid blank (10% HC1 + 10% HNO3) and HF acid solutions. The spectra were similar and no CF band was observed.

For determination of silicon in commercial 48% HF (Mallinckrodt 2640), an analytical curve was established by measuring the relative intensities of five reference solutions. The analytical function was linear over the concentration range of 1 to 40/zg/ml.

ICP-AES determination of silicon in HF acid using the inert sample introduction system gave satisfactory re- sults (Table IV). The accuracy of the determination was checked using the standard addition method. Concentra- tions obtained by both methods were in close agreement. Analytical and standard addition curves were parallel and linear.

III. CONCLUSION

The inert sample introduction system compares favor- ably with the conventional system and has the important added advantage of being able to introduce HF acid

solutions. The arrangement is completely compatible with the commercial system presently in use. Also, con- venience of construction, ease of adjustment, simplicity, and low cost are some of the important features of this new introduction system.

ACKNOWLEDGMENTS

This research was supported in part by the Department of Energy Contract DE-AC02-77EV-0432.

The authors thank C. Allemand for his assistance in designing the cross flow nebulizer.

1. J. Gontter and B. R. Culver, ICP Inf. Newslett. 6, 495 (1981). 2. L. Marcieilo, Jarrell-Ash Plasma Newslett. 4 (3) 1, 4 {1981). 3. J. D. Nohe and J. M. Katzenberger, Paper 333, presented at the 8th Annual

Meeting of the Federation of Analytical Chemistry and Spectroscopic Soci- eties, Philadelphia, 1981.

4. J. Goulter, Paper 56, presented at the 7th Annual Meeting of the Federation of Analytical Chemistry and Spectroscopic Societies, Philadelphia, 1980.

5. W. Zamecheck and W. G. Pankhurst, in Developments in Atomic Plasma Spectrochemical Analysis, R. M. Barnes, Ed. (Heyden, Philadelphia, 1981}, p. 121.

6. G. F. Wallace, V. V. Pire, and R. D. Ediger, Paper 157, presented at the Pittsburgh Conference on Analytical Chemistry and Applied Spectroscopy, Atlantic City, 1982, and Can J. Spectrosc. 35, 408.

7. R. G. Schleicher, C. Shapiro, and S. B. Smith, Paper 525, presented at the Pittsburgh Conference on Analytical Chemistry and Applied Spectroscopy, Atlantic City, 1982.

8. P. W. J. M. Boumans and M. Ch. Lux-Steiner, Spectrochim. Acta 37B, 97 (1982).

9. E. Grallath, P. TschSpel, G. KSlbin, U. Stix, and G. TSlg, Fresenius Z. Anal. Chem. 40, 302 (1980).

10. L. Ehdon, M. R. Cave, and D. J. Mowthorpe, Anal. Chim. Acta 115, 179 (1980).

11. M. R. Cave, R. M. Barnes, and P. Denzer, Paper 24, presented at the 1982 Winter Conference on Plasma Spectrochemistry, 68, Orlando, 1982.

12. R. H. Scott, V. A. Fassel, R. N. Kniseley, and D. E. Nixon, Anal. Chem. 46, 75 (1974).

13. R. N. Kniseley, H. Amenson, C. C. Butler, and V. A. Fassel, Appl. Spectrosc. 28, 285 (1974).

14. R. M. Barnes and H. S. Mahanti, Paper 18, presented at the 1982 Winter Conference on Plasma Spectrochemistry, Orlando (1982).

"Quantitative" Electron Probe Analysis of Low-Atomic- Number Samples with Irregular Surfaces

KLARA KISS Stauffer Chemical Company, Livingstone Avenue, Dobbs Ferry, New York 10522

T h e quanti tat ive electron probe analys i s on irregular surfaces v ia the two-vo l tage technique w a s assessed for l ight e lements . This analys i s can b e p e r f o r m e d without the k n o w l e d g e of the local tilt angle, i.e., the take-of f angle. Data a r e p r e s e n t e d of composi t ions determined on crystal facets w i t h w i d e l y v a r i e d

orientat ions w i t h respect to the d e t e c t o r . A range of model compounds and commercial products w a s invest igated w i t h

special emphas i s on those consist ing of low-atomic-number ele- ments . The usefu lness of the technique, its d isadvantages , and l imitat ions are critically evaluated. The precis ion and accuracy obtainable in routine laboratory w o r k a r e quantif ied and t h e

effect of the select ion of the accelerat ing vol tage pairs and magnif icat ion is discussed.

Index Headings: Electron microprobe.

Received 12 April 1982.

INTRODUCTION

A vast technical literature demonstrates that X-ray microanalysis has become an invaluable method of anal- ysis. Quantitative as well as qualitative X-ray microanal- ysis with the electron microprobe is well established for fiat, polished specimens. Quantitative energy-dispersive analysis of X-rays (EDAX) on rough irregular surfaces in the scanning electron microscope (SEM) is carried out rarely, however, because it is very time-consuming and inaccurate. Since SEM can examine "as is" samples, a rapid, precise method for quantitative EDAX analysis of such specimens is desirable.

In X-ray microanalysis, the exact knowledge of the local surface orientation of the investigated sample is of

Volume 37, Number 1, 1983 0003-7028/83/3701-001952.00/0 © 1983 Society for Aoolied Soectroscooy

APPLIED SPECTROSCOPY 19

great importance. The local tilt angle has to be deter- mined so that quantitative ZAF corrections can be per- formed. This local tilt angle is identical to the tilt angle of the specimen stage for flat, polished specimens. On the other hand, a stereoscopic method must be used for irregular surfaces. 1 This method is imprecise and cum- bersome and is limited to the case in which the local surface tilt angle is in the same direction as the tilt angle of the SEM specimen stage.

Two alternative quantitative X-ray microanalysis methods have been reported in the literature for iron and zinc sulfide, respectively, in which the determination of the tilt angle, i.e., the take-off angle is not required. The composition of the sample is determined from absolute, background-corrected intensities measured at two differ- ent accelerating voltages 2 or at two initially unknown surface tilt angles differing by a known amount. 3

This paper assesses the applicability of the two-voltage technique in routine, industrial EDAX analysis. A variety of homogeneous, analytical grade compounds as well as commercial products was investigated. Particular em- phasis was placed on the analysis of materials which consist of light elements. Various crystal facet orienta- tions were selected for spot analysis. This selection pro- vides the "worst case" scatter in the experimentally determined compositions at the various points.

I. CONCLUSIONS

The present study shows that even for the light ele- ments, the two-voltage technique provides a precise, accurate analysis for as is specimens with rough surfaces. Provided that optimal conditions are applied, sulfur, phosphorus, potassium, chromium, and copper can be determined with +_1 to 7% relative error. However, the error is _+11 to 17% with sodium and aluminum. The voltage effect is not critical when elements with an X- ray excitation potential larger than about 1.5 kV are analyzed, but careful selection of the voltage pair seems to be necessary for optimal light element analysis.

The major limitations of the method are: (1) it cannot analyze elements below fluorine; (2) it presumes a knowl- edge of the oxygen content; and (3) it is not applicable in the presence of a relatively thick surface layer.

II. P R O C E D U R E S A N D M E T H O D S

Specimens were analyzed in an AMR 1000A scanning electron microscope equipped with a Philips-EDAX X- ray fluorescence system. Energy-dispersive X-ray spectra were recorded with the beam in the spot mode. A wide range of magnifications and accelerating voltage pairs was used. The spot mode corresponds to about a 1 to 5 ~m 3 X-ray excitation volume depending on the applied accelerating voltage and the composition of the matrix. Accelerating voltages were measured by the continuum cut-off method. 4 The analysis time was 100 s. Beam current was adjusted to give a reasonable count at the lowest accelerating voltage and was not readjusted after changing the accelerating voltage.

The elemental composition was calculated using the "no standards" ZAF program from the Scientific Micro- programs Co. The no standards program fits a calculated background to the spectrum, followed by a least squares

20 Volume 37, Number 1, 1983

fit of modified Gaussian-shaped peaks for each line of the analyzed elements to determine background-corrected integrated intensities. The pure element standard inten- sities calculated from fundamental parameters are used to obtain relative K ratios which are normalized to 100%, including the stoichiometrically determined oxygen. 2'5 Oxygen concentrations are fed into the computer pro- gram in the form of oxygen ratios.

The experimental conditions used for each sample are summarized in Table I. K2Cr207, CuSO4, and KA1 (SO4)~. 12H20 were analytical grade products of Fisher Scientific and MCB Manufacturing Co. Sodium aluminum phos- phate NaA1Hl.~(PO4)s, trisodium phosphate 4(Na3PO4. 12H20) .NaOH, and TSP (trisodium phosphate) were commercial products of Monsanto, Stauffer Chemical, and ERCO, respectively. All samples were powdered onto double-coated Scotch Tape and carbon-coated to prevent sample charging.

III. RESULTS AND D I S C U S S I O N

The technical literature documents that absolute in- tensities as well as intensity ratios obtained from various points of irregular surfaces of chemically homogeneous specimens vary greatly and in an unpredictable manner. 6 This phenomenon is due to variations in matrix effects, especially in the extent of the absorption of X-rays. Consequently, absolute intensities or intensity ratios can- not be used to determine elemental composition or to establish concentration gradients in such samples. The quantitative X-ray microprobe analysis of polished spec- imens is straightforward and well established, but polish- ing is very time consuming and unsuitable for many samples because embedding, cutting, and polishing may cause damage and introduce artifacts. While embedding and polishing are excellent sample preparation tech- niques for most specimens in metallurgy and geology, they are likely to be very harmful for polymers, for heat-, water-, air-, and solvent-sensitive particulates, ca- talysts, foams, etc.

Thus, quantitative X-ray microanalysis often has to be carried out on irregular, rough surfaces of as is specimens. The present work shows that "quantitative" X-ray mi- croanalysis, based on the iterative calculations of the two-voltage technique, provides an analysis with good precision and accuracy. The term quantitative is placed in quotation marks because the results are inferior to wet chemical methods or even to X-ray microprobe analysis on polished specimens. This is particularly true, as shown

TABLE I. Summary of experimental conditions.

Compound Grade Magnifica- tion Voltage

K2Cr207 Analytical × 380 19, 29 CuSO4 Analytical × 2000 17.5, 26.2 KAl(SO4)~. 12H20 Analytical × 130 11, 16, 20.4, 26 KAl(S04)2.12H20 Analytical × 380 10.8, 13.6, 18, 26 KAl(SO4)2.12H20 Analytical × 1000 10.5, 14, 18, 26 KAl(SO4)2.12H20 Analytical x 2000 10.5, 14, 18, 25 Na:~A12H~.5(PO4)s Commercial x 2000 8.8, 19.5 4(Na:~PO4.12H20). Commercial x 170 10, 20

NaOH TSP Commercial × 74 4.6, 8, 18, 25.6

,, 100 p m

FIG. 1. Quantitative analysis of K2Cr207 via angle iteration.

Experimental point Aver- Theory 1 2 3 4 age

K Wt. % 25.8 25.9 25.2 22.3 24.8 26.6 Cr Wt. % 36.2 36.1 36.9 40.1 37.3 35.4 O Wt. % 38.0 38.0 37.9 37.6 37.9 38.1

19 kV 29 kV Point Inten- Inten- K/Cr Inten- Inten- K/Cr

sity K sity Cr sity K sity Cr 1 25 374 18 344 1.38 78 139 66 050 1.18 2 21438 15 942 1.35 53 550 47 731 1.12 3 8,658 7,755 1.12 26 882 30 747 0.87 4 2,503 2,459 1.02 6,410 7,716 0.83

later, for aluminum and sodium, the lightest elements investigated in this study.

The application of the two-voltage technique to the analysis of a crystal of analytical grade potassium bi- chromate is shown in Fig. 1. The loci of the analysis are marked by numbers on the SEM photograph. T h e y are on crystal facets with greatly different orientat ions with respect to the detector; thus, the take-off angles are widely different, yet the experimental ly determined com- position compares favorably with the theoretical values of K2Cr2OT. For comparative purposes, the absolute in- tensities and intensity ratios are also shown. It is appar- ent tha t the absolute intensities and the elemental ratios vary widely which would lead to the incorrect conclusion that the various facets of the crystals have greatly differ- ent composition.

In order to s tudy the effect of the accelerating voltage and magnification on the precision and accuracy of the analysis, potassium aluminum sulfate dodecahydrate was studied. The areas of the crystals examined are shown on Fig. 2, and the results are summarized in Tables II, III, and IV for the × 380 data. A different crystal was analyzed at each magnification and each crystal was analyzed on a different day. Data were collected at four accelerating voltages giving a total of six voltage pairs.

The tables show the experimental concentrat ions of aluminum, sulfur, and potassium, respectively, at spots 1 to 5 for each voltage pair combination. The tables also

compare experimental and stoichiometric concentrat ions and list the averages, s tandard deviations, and relative errors for rows and columns. The standard deviations and relative errors through rows reflect the variation of the composition at one particular spot due to the selec- tion of the voltage pair; those through the columns measure the variation of the composition due to the location of the spots at one part icular voltage pair.

From the data, it is apparent that: (1) the selection of the voltage pair does not have a major effect on the determined composition provided tha t the analyzed spot is in line of sight to the detector and at high take-off angle (i.e., unobstructed) (spots 1 and 2). When the analyzed peak is in such a position tha t the X-rays must travel a long distance through the crystal before emerging to the surface to be detected, the selection of the voltage pair has a major effect (spots 4 and 5).

(2) An optimal voltage pair seems to exist at which the sensitivity of the analysis to the location of the analyzed spots is minimal. The opt imum seems to vary for differ- ent elements. In this work, for example, the opt imum for A1 and K was the 13.6-18 kV pair, whereas the 13.6-26 and 18-26 pair were both optimal for sulfur.

(3) The errors are highest for aluminum. This is con- sistent with the fact tha t A1 X-rays have the longest wavelength, and they suffer the highest degree of absorp- tion in the matrix.

(4) The operating voltages must be carefully selected for optimal results. Standard deviations determined for the various elements should be pooled to obtain the best voltage pair.

Fig. 3 exhibits the pooled s tandard deviations on a plot of the low voltage vs high voltage components of the voltage pairs. The pooled s tandard deviations were cal- culated from the column standard deviations of Tables II, III, and IV by the following equation

. / ( S . D . A t ) 2 ..b (S.D.8) 2 -I- (S.D.K) 2 S.D. pooled

3

Efforts to mathematical ly fit the pooled data and obtain a master curve were unsuccessful due to the randomness of the pooled standard deviations, but the plot clearly shows that, for the studied analysis, the 13.6-18 kV pair is the optimum.

To determine the effect of magnification, the relative errors obtained at the various magnifications were statis- tically compared. The comparison revealed that the error was the smallest at × 380 magnification. The relative error was especially high when shielded regions were analyzed at × 130 magnification. This high error was expected because the X-rays must travel several hundred microns through the crystal before reaching the surface and, during this process, are largely absorbed. Tilting and /o r rotating the crystal to bring the analyzed spot into a favorable, unobstructed position decreases the error considerably. The reason for the higher errors at × 1000 and × 2000 magnification is not understood, and further work will be needed to verify this observation. The effect of magnification on the relative error is dis- played in Table V for the 13.6-18 kV voltage pair. The effect was similar in the case of the other voltage pairs.

The present work demonstrates tha t the selection of an optimal voltage pair is very impor tant when the

APPLIED SPECTROSCOPY 21

FIG. 2. KAI(SO4)2.12H20 crystals used in optimization study. A, x 130; B, x 380; C, × 1000; D, x 2000.

TABLE II. Precis ion and accuracy of a luminum analysis."

Accelerating voltage, kV (pah's) Point No. Average

10.8-13.6 10.8-18 10.8-26 13.6-18 13.6-26 18-26 S.D. Relative

error

Wt.%a~minum 1 7.1 7.4 6.7 6.6 6.6 6.8 6.87 0.32 4.7 2 7.7 6.9 6.7 6.1 6.1 7.6 6.85 0.70 10.2 3 10.3 7.1 6.8 6.0 6.2 6.7 7.18 1.58 22.0 4 4.8 4.4 6.0 4.2 9.2 6.4 5.83 1.87 32.1 5 5.0 7.4 9.0 6.6 2.4 2.9 5.55 2.6 46.8

Average 6.98 6.64 7.04 5.90 6.10 6.08 S.D. 2.25 1.27 1.14 0.99 2.43 1.83 Relative error 32.2 19.7 16.2 16.8 39.8 26.9

At x 380 in KAI(SO4)2.12H20; calculated: 5.7 wt. %.

sample consists of low-atomic-number elements. Similar conditions are anticipated when low energy L-lines of heavier elements are used in the analysis. In the analysis of elements with an X-ray absorption edge larger than about 5 keV, there is less choice because an overvoltage of higher than 3 (possibly 4 or 5), necessary for the efficient X-ray excitation, limits the selection to the 20 to 30 kV range. The voltage effect here is not as critical because the matrix absorption of the short wavelength, highly energetic X-rays is minimal.

The SEM photographs of crystals of the commercial products analyzed are shown in Fig. 4 and the data on all

of the samples examined in this work are summarized in Table V. With one exception, the agreement between theory and the experimental values is excellent. In the lone exception, the experimental sodium and phosphorus concentrations in the Stauffer T S P are very close to the stoichiometric values, whereas in the ERCO TSP, the sodium/phosphorus ratio is closer to values calculated for disodium phosphate. The data suggest tha t the sur- face of the ERCO TSP is not trisodium phosphate.

The major applications of quantitative E D A X analysis are: (1) to determine the composition of an unknown, homogeneous microsample, generally a contaminant or

22 Volume 37, Number 1, 1983

T A B L E III. P r e c i s i o n a n d a c c u r a c y o f s u l f u r a n a l y s i s . "

Point Accelerat ing voltage pairs, kv

10.8-13.6 10.8-18 10.8-26 13.6-18 13,6-26 18-26 Average S.D. Relat ive

error

Wt. % sulfur 1 13.9 13.8 14.2 14.0 14,1 2 13.7 14.0 14.1 14.3 14.2 3 13.0 14.0 14,2 14.5 14.4 4 15.4 15.1 14.7 15.3 14.1 5 14.1 13.4 13.0 14.0 14.4

14.1 14.0 0.147 1.1 13.9 14.0 0.216 1.5 14.2 14.1 0.543 3.8 14.3 14.8 0.538 3.6 14.2 13.9 0,536 3.9

Average 14.0 14.1 14.0 14.4 14.2 14.1 S.D. 0.876 0.631 0.627 0.536 0.15 0.15 Relative error 6,3 4.4 4.5 3.7 1.1 1.1

" A t × 380 in Kal (SO4)2.12H20; calculated: 13.5 wt, %.

T A B L E IV. P r e c i s i o n a n d a c c u r a c y o f p o t a s s i u m a n a l y s i s . "

Point No. Accelerat ing voltage pairs, kv

10.8-13.6 10.8-18 10.8-26 13.6-18 13.6-26 18-26 Average S.D. Relat ive

error

W t . % p o t a s s i u m 1 7.9 7.7 7.9 8.3 8.1 2 7.7 7.9 8.1 8.5 8.6 3 5.6 7.7 7.8 8.2 8.0 4 7.7 8.8 7.8 8.6 4.8 5 10.2 8.6 7.4 8.3 12.7

8.0 8.0 0.204 2.5 7.4 8.0 0,463 5.8 8.0 7.6 0.971 12.8 8.0 7.6 1.448 19.1

12.7 10.0 2.290 22.9

Average 7.82 8.14 7.8 8.4 8.4 8.82 S.D. 1,630 0.522 0.255 0,164 2.816 2,184 Relative error 20.8 6.4 3.2 2.0 33.5 24.8

"A t x 380 in KAI (SO~)2.12H20; calculated 8.2 wt. %.

26

M ¢I.

Q © 18 0

E o 0

m i ii 13.6

I I I 10,8 13.6 16

V 1 = Low-Voltage Comp. of Voltage-Pair KV

FIG. 3. Dependence of pooled s t andard deviat ion on voltage pairs at X 380 KAI(SO4)2.12H20.

inclusion; and (2) to establish the variation of composi- tion within a nonhomogeneous sample.

From Table VI, it is obvious that the average experi- mental concentrations based on 5 to 10 points are prac- tically identical to theoretical valu£s; thus, highly accu- rate analysis is feasible within 15 to 30 min, applying a statistical approach.

When the purpose of the analysis is to establish the spatial variation of the composition within the sample,

T A B L E V. E f f e c t o f m a g n i f i c a t i o n o n t h e r e l a t i v e e r r o r f o r t h e 13.6-18 k V v o l t a g e pa ir .

% relative error

S K A1

x 130 23.5 46.9 71.7 X 380 3.7 2.0 16.8 × 1000 5.2 15.5 25.4 X 2000 3.6 18.4 27.8

the method is semiquantitative for Na and A1, but quan- titative for the heavier elements.

The table also shows that accuracy and precision are very good when the energy of the analyzed X-rays is above about 1.5 kV. When the energy of the X-radiation is lower than this value, absorption is high and the relative error is in the 10 to 15% range. This error was established for the K lines of the light elements, but similar errors are anticipated for the low energy L and M lines of heavy elements. Consequently, for case 2, the analysis is to be considered semiquantitative rather than quantitative when it is based on X-rays with energies below 1.5 kV. When the analysis is based on higher energy X-rays, it can be considered quantitative in this application as well.

The two-voltage method has several limitations. (1) Elements below fluorine cannot be analyzed. Efforts to determine carbon, nitrogen, and oxygen were unsuccess- ful.

(2) Oxygen is obtained by difference. Thus, the tech- nique presumes a knowledge of the oxygen content, either on the basis of sample history or through another ana- lytical technique.

APPLIED SPECTROSCOPY 23

I m

TABLE VI. S u m m a r y of analyt ical results .

Theoret ica l

1

Exper imenta l

Analyt ical reagent grade K2Cr207, × 380 K 26.6

Cr 35.3 0 38.1

CuSO4, × 2000 Cu 41.53 S 18.79 0 39.67

KA1 (SO4)2.12H20, × 380 K 8.2 A1 5.7 S 13.5 0 72.6

Commerc ia l grade "SALP 32B"

NaaAI2Hi~(PO4)s, × 2000

4 (NaaPO4.12H20)NaOH, x 1.70

Tr i sod ium phospha t e (ERCO), x 74

NA 7.7 A1 6.O P 27.5 O 57.0 Na 19.2 P 7.9 0 66.7[ H 6.2J 72.9

Na P ? O

Av- % rela- erage S.D. rive wt. % error

24.8 1.69 6.8 37.3 1.88 5.0 37.9 1.14 3.0 41.57 0.25 0.6 18.78 0.2 1.1 39.65 0.06 0.2

8.4 0.16 2.0 5.9 0.99 16.8

14.4 0.54 3.7 71.2 0.34 0.5

24 Volume 37, Number 1, 1983

7.6 1.1 17.3 6.0 1.0 12.7

27.5 0.6 2.2 57.0 1.4 2.4 20.6 2.75 13.3

7.8 1.82 23.0 71.5 O.92 1.3

9.7 1.08 11.1 6.7 1.34 20.1

83.9 0.62 0.74

FIG. 4. Crystals in commercia l products , r andomly selected for analysis. A, Monsan to SALP 3:2:8 = Na3A12H~5(PO4)8, × 2000; B, Stauffer T S P = 4 (Na:~PO4.12H20).NaOH, × 170; C, ERCO TSP, x 74.

(3) It cannot be used when the sample is coated with a layer of a different composition. The X-ray intensity of a certain element from the surface layer would be differ- ent from that produced in the bulk. The excitation vol- ume and escape depth of X-rays depends on the accel- erating voltage; therefore, the results from the two-volt- age technique would be in error.

The error depends on the thickness of the layer. It is negligible for very thin surface films (e.g., 100 to 500/~) because, compared to the approximately 10,000 to 30,000:~ (1 to 3 ttm) X-ray excitation depth of the bulk, their contribution would be minimal. The error would be considerable, however, when the surface layer is rela- tively thick.

This study shows that it is advantageous to select optimal conditions on a model compound with similar composition to that of the analyte when the analysis is expected to be frequently repeated. For one-of-a-kind analysis, such an optimization would be prohibitively expensive, and the voltage pair should be selected by careful and intelligent consideration of the composition of the sample.

1. S. Moll, N. Baumigarten, and W. Donnelly, "Geometrical considerations for ZAF corrections in the SEM," in Proceedings of the Annual Microanalysis Conference, Boston, Mass., Aug. 18-24, 1977, p. 33.

2. J. C. Russ and T. M. Hare, Can. J. Spectrosc. 25, No. 4, 1980, pp. 98-105. 3. P. B. DeGroot, "Method of Z-A-F analysis in the SEM in cases in which the

take-off angle is unknown," in Proceedings of the 16th Annual Conference of the Microbeam Analysis Society, Vail, Colorado, July 13-17, 1981, pp. 195-198. (Editor: Roy H. Geiss, San Francisco Press, Inc.).

4. D. Seaman and L. Solosky, Anal. Chem. 567 (1973) Vol. 45.

5. J. C. Russ, "A fast, self-contained, no-standards quantitative program for EDS," Proceedings of the 13th Annual Microanalysis Conference, of the Microbeam Analysis Society, Ann Arbol; Mich., 1978, p. 46 (San Francisco Press).

6. O. C. Wells, Scanning Electron Microscopy (McGraw/Hill, New York, 1974).

Volatile Species in Inductively Coupled Plasma Atomic Emission Spectroscopy: Implications for Enhanced Sensitivity

K. D. SUMMERHAYS,* P. J. LAMOTHE, and T. L. FRIES U.S. Geological Survey, Menlo Park, California 94025

W e p r o p o s e a sample introduction method involving the for- m a t i o n of volatile species in solution and subsequent pneumatic nebulization for inductively coupled plasma atomic emission spectrometry. A study of osmium oxidized to OsO4 a n d prelim- inary results for mercury reduced to the elemental form suggest that enhancements in sensitivity b y a factor of 10 to 100 a r e achievable without degradation in analytical precision, in com- parison with pneumatic nebulization of solutions containing the element of interest in a nonvolatile form. We discuss the phys- ical basis for the method and support it b y a brief theoretical treatment. The technique promises a b r o a d applicability to elements capable of existing in a volatile form in solution. F o r the elements studied, no modifications of a c o m m e r c i a l ICP spectrometer or sample introduction system are r e q u i r e d .

Index Headings: Emission spectroscopy; Inductively coupled plasma; Sample introduction.

INTRODUCTION

During the past several years, atomic emission spec- troscopy, using inductively coupled plasma (ICP) exci- tation, has risen to a position of prominence in the field of trace analysis. 1-4 Today, with several commercial in- struments available, reports of new ICP applications are appearing in the literature at an ever increasing fre- quency. Also appearing with increasing regularity are papers concerning sample preparation and introduction techniques. Whereas the standard method of sample introduction has involved pneumatic or ultrasonic ne- bulization of solutions, several alternative approaches also have been presented, including electrothermal-atom- ization methods, '%6 direct sample placement in the plasma, 7 hydride generation, s and chromatographic tech- niques.,, ,0 In most cases, the impetus for developing these alternative methods of sample introduction has been the

Received 8 March 1982; revision received 24 J u n e 1982. * On leave from the D e p a r t m e n t of Chemist ry , Univers i ty of San

Francisco, San Francisco, CA 94117.

need to cope with problems associated with small sample size and (or) low concentrations of the species to be determined. Although the widely used pneumatic nebu- lizer/spray chamber sample introduction systems are the simplest in design, maintenance, and usage, they are also notably inefficient in transporting analyte species to the plasma: ordinarily, only 1 to 5% of the analyte solution reaches the detection region. 1l''2 Use of alternative modes of sample introduction commonly involves exten- sive instrument modifications and, in some cases, inter- feres with the multielement nature of routine ICP anal- ysis. Wolnik e t a l . '~ have recently shown that this inter- ference can be overcome in the case of hydride generation through use of a tandem nebulization system. The pur- pose of our study was to investigate means of enhancing the percentage of analyte species reaching the plasma without any alteration of a standard commercial instru- mental system. Our procedure is based on the formation in solution of a volatile chemical form of the element of interest and on the direct nebulization of the solution containing this volatile species. Vaporization of the vol- atile species from solution droplets in the spray chamber and subsequent sweeping of the vapors into the plasma lead to significant enhancements in signal intensities relative to the results for solutions containing the same element at the same concentration but in a nonvolatile f o r m .

To illustrate the phenomenon of enhanced emission intensity from solutions in which the element of interest is present in a volatile form, we here report relative intensity data for solutions containing the osmium com- pound OsO4 in comparison with solutions of osmium in various nonvolatile forms. We show this phenomenon to be independent of the osmium emission line observed, of rf power levels, and of the position of observation in the plasma. A calculation supporting the concept that the phenomenon has its basis in vaporization from the aero- sol is likewise provided. We also discuss details of the analytical application of our method for osmium. Finally, we present preliminary data illustrating the results for another element (Hg).

Volume 37, Number 1, 1983 0003-7028/83/3701-002552.00/0 01983 Society for Applied Spectroscopy

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