Trace Elemental Analysis by Heavy Ion Induced X-ray Emission R. Zelsler,’ J. B. Cross, and E. A. Schweikert* Center for Trace Characterization,Department of Chemistry, Texas,A& M University, College Station, Texas 77843
The parameters governing x-ray emlsslon and background productlon resultlng from bombardment wlth hlgh energy oxygen beams have been Investigated. Experlmental K and L x-ray yields from thick targets are given for elements of 13 IZ I56, and ‘:On+ energies of 8, 16,33,54, and 113 MeV, respectlvely. The method features a slmultaneous multlelement capablllty for trace elements at levels as low as g In small samples 9). Comparison with experimental data from proton Induced x-ray emisslon (1.6 MeV IE,, 5 7.0 MeV), shows that the sensltivlty Is Improved with ’:On+ bombardment. These capabilitles are illustrated with analyses of NBS standard materlais (orchard leaves, bovine liver, glass) and of a single human hair.
A considerable amount of work has been devoted in recent years to charged particle induced x-ray emission as a multielement trace analysis method. Most studies thus far have involved proton or a excitation and have focused on the optimization of experimental parameters for maximum measurement sensitivity and the application of the technique to a variety of samples including air filters and biological specimens (e.g., 1-6). In this context proton excitation applied on thin samples is considered to provide the best analytical results, i.e., high sensitivity (as low as g/g for E , -2-4 MeV) coupled with broad elemental coverage (1-6). However, x-ray excitation via heavy ion bombardment should provide still much improved prospects for simultaneous multielement trace analysis. This observation is derived from the model describing atomic collisions and considering bombarding ions of equal velocity (7, 8). Efforts of experimental exploration in this direction have been very fragmentary so far, covering only a limited range of both energy and type of bombarding ions and a limited number of target elements (9,19).In particular, only limited attempts have been made to evaluate and compare the analytical potential of a given bombarding particle at different incident energies on a wide range of elements. The objective of this study was to provide an experimental assessment of heavy ion induced x-ray emission. More specifically, efforts were focused on a case study utilizing oxygen ion beams for x-ray excitation in thick samples. Topics examined include: a) evaluation of the parameters affecting sensitivity and detection limits for a single element; b) feasibility of multielement analysis; c) comparison of oxygen with proton excitation; d) application of the technique on selected samples.
2 1 = atomic number of projectile, 22 = atomic number of target atom. The above equation is derived from the work of Merzbacher et al. (7)and of Garcia (8). For analytical applications, the magnitude of ax will affect the sensitivity. In this respect one should note that the effect of the bombarding energy must be assessed in terms of the energy per atomic mass units ( E l amu); any increase in this term will lead to considerably higher x-ray yields. Further, for equal velocity projectiles (equal should Elamu), the particle of higher atomic number (21) yield a larger value for UX. The 2 2 dependence on the other hand outlines limitations as to the target elements accessible by this technique. Background. For an appraisal of the analytical potential, the preceding considerations must be expanded into an examination of signal-to-noise ratios. This requires first that the sources of background be identified. Most of the background radiation is due to the production of secondary electrons which occurs as a part of the excitation process. Thus higher x-ray yields lead also to proportionately higher secondary electron production which cause Bremsstrahlung within the target. The spectrum of the Bremsstrahlung background is limited to the low photon energy region, and, according to Folkmann et al. (5),its maximum energy, T,, is proportional to the maximum velocity which can be transferred from a projectile with the mass AIM to a free electron of mass m , (A4= mass of one nucleon). This relationship has been formulated as follows:
T , = ( 4 m / M )(ElA1)
[in keV]
Viewing this equation in the context of multielement trace analysis, several comments can be made: a) severe limitations can be expected in the detection of elements with x-ray energies close or below T,; b) as T , varies linearly with Elamu of the incident particle, the optimization of the signal-to-noise ratio is critically dependent on the proper selection of the bombarding energy; c) the value of T, is for all practical purposes independent of the chemical composition of the sample. It is clear though that production of characteristic x-rays from major and minor constituents will impose limitations on the trace elements that can be detected, these will vary with the sample makeup. Other sources of background, Bremsstrahlung generated by the projectiles themselves and Compton scattering of y rays generated by nuclear excitation during the particle-target impact must be assessed experimentally for a given projectile-matrix combination. EXPERIMENTAL
THEORETICAL CONSIDERATIONS X-ray Production Cross Section. The parameters which determine the K-shell x-ray production cross section, UX, may be described as follows:
with A1 = mass number of projectile, E = bombarding energy, 1 On leave of absence from Institut fur Radiochemie, Technische Universitat, Munchen.
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Irradiations were performed at the Texas A&M University 88-inch variable energy cyclotron. Beams of protons (1.6 to 7 MeV) and I6On+ ions (8 to 113 MeV) have been utilized. The beams were focused on targets by quadrapole electrostatic lenses to a spot of 2-5 mm diameter. The current on target ranged from 0.1 to 100nA. X-ray spectra were measured with a Si(Li) x-ray spectrometer system providing an energy resolution of 190 eV (FWHM) at 6.4 keV. The target chamber is illustrated in Figure 1.The chamber contains a sample holder for up to 12 samples positioned at 45’ with respect to the center for the beam line and the detector. Further components include a rotating disk for different absorber foils, collimators, and a Faraday cup. The detector is isolated from the chamber itself only
ANALYTICAL CHEMISTRY, VOL. 48, NO. 14, DECEMBER 1976
Table I. Samples Pure element targets metal foils (2O.l-mm thickness) Ti Ni
cu
Zr Ag
Sn W Au Graphite pellet
Multielement graphite mixtures, elemental concentration in mol % 2
1
3
4
Ag
0.39 0.39 0.44 0.42 0.43 0.42 0.37
C1 K V Fe Cu Zn Ge Nb
0.32 0.32 0.34 0.38 0.36 0.34 0.35 0.35
C 0 N
91.94 4.83 0.37
C 0
94.35 2.89
Si S K Ti Cr Fe Ni
0.35 0.54 0.38 0.36 0.38 0.35 0.41
P Ca Ti Mn Ni Ge
C 0
94.33 2.90
K Cr Co Ni Ga As Br Nb Mo C 0
"Application" samples NBS Standard Reference Materials SRM618 500ppmGlass SRM1571 OrchardLeaves SRM 1577 BovineLiver Hair
5
Ni 0.48 Se 0.49 Rb 0.47 Nb 0.50
0.30 0.33 0.35 0.32 0.31 0.30 0.30 0.30 0.31
Sn 0.47 Ba 0.48 Gd 0.50 W 0.48 C 74.84 0 20.29 N 0.50
92.94 4.24
5r-l-
A Faraday Cups B Stepping Motor C Target Wheel
0.50
Ag
I
I
1-14
I
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I I L-----J
,, I
T
- -I6
E F G H
Graphite Beam Collimator Target X-ray Collimator Absorber Wheel I Detector (Cooled Si (Li) 1 J 1 Mil Be Window K X-ray Path
-s=
- -I7 2
0
c = mo 0 Ni X-ray
Countrate
b
6
E,
-19
Cross Section Brondt (9)
D O
J
-
D A
Irradiation Chamber
Ag X-ray Production
-2
Figure 1. Irradiation chamber
(10)
-3
0
1
2
3
Energy ';On+
by its 0.025-mm beryllium window. The vacuum in the chamber was typically 4 X low5Torr. The different materials used as thick targets
are listed in Table I. Pellets were obtained by pressing substances like NBS orchard leaves or grinding and mixing suitable amounts of the compounds containing the elements of interest (oxides, nitrates, ammonium salts) with reactor-grade graphite and pressing these mixtures. The -1-mm thick pellets were mounted on 0.1-mm thick aluminum foils which were fitted on the sample wheel. The composition of the mixtures was chosen in a way that no overlapping of induced x-rays, K a with KO of lower 2 element or Ko! with any L-line of higher 2 elements, occurred within the given resolutions of the detector.
RESULTS AND DISCUSSION Signal-to-Noise Ratios. T o optimize the excitation and detection conditions a clear understanding of the parameters affecting the x-ray signal ( N x )and the related background ( N B )is required. Experimental x-ray data were obtained from single element targets bombarded with ':On+ions of different energies. For each specific beam used, the ion charges are indicated below. They refer to the characteristics of the beam prior to striking the target. X-ray yields ( N x )measured for Ag and Ni are presented in Figure 2. These were compared with published cross-section values (11,12);there is good agreement between the respective excitation curves. The trends for N B are illustrated in Figure 3 with experimental backgrounds observed when bombarding a graphite target with oxygen ions of different energies. Data for 2.6-MeV protons obtained under the same conditions are shown for comparison. Ratios of N ~ I N as B a function of the bombarding oxygen ion energy are shown for three typical target elements in Figure 4.The Nx/NB ratio for the low 2 element, Ti, is a t a maximum in the low energy region, and remains close to this
4
5
6
7
-22
( MeWamu 1
Figure 2. K x-ray production cross sections and experimental countrates for Ni and Ag vs. oxygen bombarding energies
; I
I.
e
0 t
In0
- = I 5 6-11
82
6
-3
level with increasing projectile energy even though the x-ray production cross section is still increasing. With increasing beam energy, T , becomes larger and the background specB starts trum exceeds the x ray of interest; thus the N ~ I N ratio to decrease. For medium 2 elements such as Ni and Ag, the Nx/NB ratios increase with increasing beam energy because of the increase in x-ray production cross section. For Ni, T ,
ANALYTICAL CHEMISTRY, VOL. 48, NO. 14, DECEMBER 1976
2125
Table 11. Sensitivity Calculation for the Case of Traces of Ni in Graphite Using Oxygen Ions Relative detection Minimum detectable Minimum detectable limit NDINT signal SD" atoms N D ~ Oxygen ion Sample size NT ppm (atomic) (atoms carbon) (countsls) (atoms Ni) energy, MeVIamu 16 1.2 (14) 923 0.50 1.3 (17) 27 1.2 (13) 29 1.00 4.1 (17) 19 9.8 (12) 8 2.06 1.2 (18) 3.37 1.9 (18) 19 1.9 (12) 1 1 7.06 4.6 (18) 85 3.1 (12) a These values are based on a beam intensity that yielded a total count rate of 500 countsls with an irradiation time of 1000 s.
limited to date to literature data on Ni and Ag. As already noted, these have been listed together with our experimental yields in Figure 2 with good agreement between both sets of data. Based on this experimental verification, values for NT were derived for each element from the normalized x-ray yields. It should be emphasized here that the sparsity of cross section measurements prevented cross checking between Nx, NT, and OX;consequently the values obtained for NT were only considered to be estimates (Le., within a factor of 2 of the true values). The detection limit of the method was estimated from the experimental data by using the definition for the "lowest detectable signal", given by Currie (13):
Ni 0
Ti
0
Ag
I
I
I
I
I
I
I
1
2
3
4
5
6
7
E('~O"+)/amu Figure 4. Signal-to-noise ratios for single elements in graphite ma-
trix
exceeds Ni K a at an energy of 3.4 MeVIamu; hence, Nx/NB will drop off for higher ion velocities, whereas for Ag, the Nx/Ng ratio should be improving up to the highest beam energy used in this work. These trends are already apparent in the data for 7.06 MeV/amu (Figure 4). Maximum sensitivity for a given element should thus be obtained by increasing the beam energy as long as the value of T , is kept below the energy of the x ray of interest. However, the expected gains in Nx/NB are limited by increasing background from other sources. This background, in the spectral region well above T,, is due to Compton scattering of y rays generated by nuclear excitation and nuclear reactions of high energy beams (>4MeV/amu) with the target. Clearly then, boundaries are imposed on what can be obtained in terms of sensitivity even if T, is below the x ray of interest. Sensitivity a n d Detection Limits. Proceeding from the x-ray yield to the corresponding number of atoms subjected to excitation (NT)requires knowledge of the x-ray production cross section (OX).This can be expressed as follows: (3) where N1 = integrated number of incident particles, NT = number of target atoms, NX,E= x-ray yield of a given energy, E , ax = x-ray production cross section, and O d N ~ l h r= experimentally determined absolute efficiency for the targetdetector assembly (Figure 1). Cross-section information of interest in our case has been 2126
where SD= Number of counts which can be detected and T N B = Standard deviation of a background of Poisson distribution. NB was obtained from the irradiation of thick graphite pellets at the ion bombarding energies of interest (Figure 3). As pointed out earlier, the width of the background is a function of Elamu of the bombarding particle. The minimum number of atoms NDthat could be detected may be calculated using Equation 3 relating x-ray counts (SDderived from Equation 3) with the number of atoms subjected to excitation. NTvalues for a graphite matrix were estimated by comparing the x-ray yields from a graphite sample doped with 1%atomic nickel to the x-ray yield from a nickel foil. A numerical illustration of a sensitivity calculation is given in Table 11, showing values for NT,SDand NDfrom experimental data for the case of determining nickel in graphite with different energy oxygen ion beams. The relative sensitivity improves with increasing ';On+energies despite the larger number of atoms needed to get the lowest detectable signal because the sample analyzed (NT)increases in comparison somewhat more rapidly. Feasibility of Multielement Analysis. The monotonic trend of inner shell ionization as 2 of the target atom varies (see Equation 1)and the inherently simple nature of K and L x-ray spectra translate into a potential for simultaneous multielement excitation and detection. The actual behavior of OX as a function of 2 number was tested with a series of samples each containing from 7 to 9 elements (Table I). The experimental x-ray yields measured as a function of the emitted x-ray energies, i.e., corresponding to a broad range of elements and for different oxygen ion energies, are presented in Figure 5. Before further examining these data, a comment on x-ray absorption needs to be made. This effect was considered negligible for elements of 2 > 30 (Le., E K 2~8 keV), given the nature of the samples used (>go% graphite) and the shallow penetration of the bombarding ions. This was confirmed by calculated absorption coefficients 1/10, assuming that all x rays originate at a depth equal to half the total penetration of the bombarding ion. Considering target elements with 2 < 30, similar calculations of Ill0 shows a nonnegligible (>lo%) x-ray
ANALYTICAL CHEMISTRY, VOL. 48, NO. 14, DECEMBER 1976
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A 3.37
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0
X 0.50 MeVIarnu
L-Data 8
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I
2.06 MeVIamu ':02'
0
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'lot 'io3*
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o)
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c 0-25-
a?
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2-26 0
CI,
5
-27 -28
-
a
-29 0 -30
5
IO
15
20
25
30
35
X-Ray Energy (keV) Figure 5. X-ray yields using oxygen ion excitation
absorption effect for ion bombardment of higher energy. This is clearly apparent in the experimental data from 54 MeV and 113 MeV '!On+ beams, where the count rates for elements of 2 < 26 do not follow the general trend of increasing with decreasing 2. The feasibility of multielement analysis can be evaluated by comparing the K and L x-ray yields for the different projectile energies given in Figure 5 with the corresponding pattern for the background (Figure 3). Considering the data presented in Figure 5, the following features become apparent: for a given projectile energy the production of K and L x rays follows the same trend as 2 varies; still more interesting is the observation that the yields of K and L x rays of similar energies are about the same for identical excitation conditions (nature and energy of projectile), e.g., for a ':On+ beam of 2 MeV/amu, the K x-ray yield for P ( E K=~2 keV) is about the same as the L x-ray yield for 2r ( E L= 2.1 keV). For this reason, x-ray yields in Figure 5 are plotted on an energy scale. It follows that a low energy ion beam will provide a multielement capability limited to two "windows" on the scale of elements, namely those species emitting K or L x rays of energies comprised between a lower limit imposed by the background and an upper limit determined by the rapid decrease of the x-ray production with increasing 2 number. As an example, the elements that can be detected at or below the 5-ppm level with a 1 MeV/amu ';On+ beam would include chlorine to chro~ to 5.4 keV) and rhodium to cerium ( E L2.8 to mium ( E K2.7 5.4 keV). As the projectile energy increases ( E 2 2 MeVjamu), the background will progressively mask lower 2 species; conversely the upper limit of the elemental range that can be detected, will be extended. Eventually a t higher ion bombarding energies the upward extension of the spectrum of elements that can be detected via K x rays a t or below a given relative concentration level will come close to those detectable with similar sensitivity via their L x rays. This is actually the case with the 7 MeV/amu ':On+ beam. These observations can be further brought into focus on a quantitative and comprehensive scale. Following the approach already described in relation with Table 11, detection limits
!!I
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10, 5ppma
5,pprna
I0,ppma
50 p p m j
I
20
30
40
1
50
Atomic Number (KQ Detection) I I 50 75
Atomic Number (LQ Detection)
Figure 6. Detection limits: ppm (atomic)in graphite for different elements as a function of the oxygen bombarding energy were estimated by comparing the minimum number of detectable atoms, N D (Equation 2), derived from experimental background data for a graphite target, with the corresponding sample size. The results are presented in Figure 6. The contours outline the scope of the method, i.e., elements that can be detected at specified minimum levels of concentration with different oxygen ion energies. As already noted, for a given projectile energy, K and L x rays of similar energies are produced with essentially identical yields. Consequently, the data presented refers to both K and L x-ray emitters. The preceding considerations also delineate the type of matrix-trace element combination for which the technique can be applied. Clearly the method is well suited for the determination of trace elements with 2 higher than those of the
ANALYTiCAL CHEMISTRY, VOL. 48, NO. 14, DECEMBER 1976
2127
:i
+
-2t -31
1
1
1
1
1
'
7.06 M e V / o r n u ' ~ 0 4 ' 7.06 MeV/alru P ' 2.62 MeV/arnu p f 1.62 MeV/amu p*
v X
1 1
1
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1
1
20
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30
'
I
I J
35
X - R o y Energy (keVJ
Flgure 7. K x-ray yields obtained with different proton and oxygen
beams
X-Ray Energy (keV)
Figure 10. X-ray spectrum from NBS Bovine Liver, SRM 1577, irradiated with 113 MeV ':04+ beam of 1 nA;1000-s count with 1 0 0 - ~ AI absorber
N
d 6000
UI
Flgure 8. X-ray spectrum from NBS Glass, SRM 610, irradiated with 113 MeV 'to4+beam of 40 nA, 500-s count with 2-mm acryl absorber I
I
IO
I
I
I
20
I
I
30
X-Ray Energy (keW Figure 11. X-ray spectrum from single hair (-3-mm length),irradiated with 113 MeV 'to4+beam of 4 nA, 2004 count with 100-p AI ab-
sorber
Flgure 9. X-ray spectrum from NBS Orchard Leaves, SRM 1571, irradiated with 113 MeV ':04+beam of 100 nA, 850-s count with 2-mm acryl absorber
major sample component. Examples of trace analyses in low 2 matrices are presented below. Other samples where the method may be readily applied include those with major constituents of much higher Z than those of the trace elements sought. In these cases, the K x-ray yields from high Z elements can be kept low and the respective L x rays screened out without impairing the production excitation and detection of K x rays from trace species in the medium 2 range. Comparison of Oxygen with Proton Excitation. The comments that follow are made for thick targets. Based on Equation 1, ax is expected to increase for projectiles of equal 2128
velocity as their mass increases. This trend is illustrated in Figure 7 where experimental thick target x-ray yields from '!On+ and those obtained from proton bombardment have been plotted. A comparison of the respective data shows that the count rates corresponding to proton beams are lower than those obtained with oxygen beams even without correcting for the larger volume of samples irradiated with protons. High energy '!On+ beams (E > 3.3 MeV/amu) cover about the same range of elements (identical slopes of count rate vs. atomic number), but with better absolute detection limits ( N D ) . The trend that can be derived from a comparison of the x-ray yields and the corresponding backgrounds (Figures 7 and 3, respectively), indicates that the relative sensitivity of ]:On+ excitation (3 to 7 MeV/amu) exceeds what can be achieved with proton bombardment (2 to 3 MeV) by two to five times for trace elements with x-ray energies close to the value of T,, and improves steadily as the atomic number of the element sought increases. The proton energy taken as basis for comparison is considered to provide optimum analytical performance by numerous authors (e.g., 3, 5, 6 ) ; our work with protons of 2 to 7 MeV supports this evaluation. In particular for medium Z elements, increasing the proton energy above -2 MeV does not result in significant gains in x-ray production. N ~ I N deteriorates B rapidly for E, 2 2 MeV because of the increase of proton Bremsstrahlung, moreover higher proton energies will require rigorous correction of experimental x-ray data for absorption losses.
ANALYTICAL CHEMISTRY, VOL. 48, NO. 14, DECEMBER 1976
Table 111. Results of Bovine Liver Analysis (NBS SRM 1577) Element Values found K 9690,9600 Fe 411, 213, 255, 24'3, 295, 275 cu 309, 174,301,154,269,240 Zn 294,121,145,92,102,133 Rb 33,24
Table LV. Results of Orchard Leaves Analysis (NBS SRM 1571)
Element Fe cu Zn AS
Rb Sr
Values found 129,113 12 54,58 16,18 31,30 29,42
NBS data 300 f 20 12 f 1 25 f 3 14 f 2 12 f 1 37
Applications. Case studies were carried out to evaluate the sensitivity and range of elements detected in biological specimens and glass samples. The x-ray spectra obtained from three different NBS Standard Reference Materials are shown in Figures %lo. From a qualitative standpoint, they illustrate the capability of the method to determine simultaneously groups of trace elements at levels from 10 to several 100 ppm in very small samples (100-300 pg depending on the sample area analyzed). These spectra were obtained with short, low intensity beam exposures. Bombarding energies were selected to optimize analytical performance. In the case of glass and orchard leaves, a 7.06 MeV/amu '!04+beam was found to be well suited. In these samples the high calcium and potassium contents made the use of appropriate x-ray absorbers necessary. Consequently, bombarding energies were selected so that values as high as the accompanying Bremsstrahlung could be screened out by these absorbers. In the case of bovine liver, the matrix components did not impose any limits beyond those due to the detector itself. Detection of lower Z trace elements could thus be accompanied by lowering the bombarding energy. One current limitation of the technique is illustrated with the glass samples (Figure 8). This specimen is doped with 61 trace elements. Combining the large number of elements all present a t similar concentration levels with the broad multielement capability of the excitation technique results in such a density of peaks that their identification and the proper estimation of peak areas becomes difficult. Moreover, some of the peaks contain overlapping K and L lines which could not be resolved with the equipment on hand. As a consequence, no quantitative determinations were attempted in this case. An illustration of quantitative analysis is given with the cases of bovine liver (SRM 1577) and of orchard leaves (SRM 1571). The data presented in Tables I11 and IV are from single determinations on replicate samples and were obtained with different ':On+ bombarding energies; it thus relates also to different size samples. These are well below the minimum amounts of material recommended if average trace element
Av
283 241 148
NBS data 9700 f 600 270 f 20 193 f 10 130 f 10 18.3 k 1
contents are to be determined. Thus the results obtained cannot be strictly compared with the NBS data which are given as average values. They are intended to show that quantitative trace analysis in microsamples is feasible with this technique. The quality of the data depends critically on closely matched standards which must also be homogeneous on a microscale. Standards made of cellulose and doped at the 500-ppm level with a series of elements were found to be most appropriate for this work (14).Testing their homogeneity, the count rate-concentration dependence was found to be reproducible within 0.2%. The precision of the technique obtained on these cellulose specimens was found to be 5 f3% a t the 95%confidence level based on an average of seven determinations. These data support the view that the results obtained on microsamples of SRM's 1577 and 1571 are valid. A final example demonstrating the applicability of the technique to trace characterization in very small samples is that of the analysis of a 3-mm section of a single hair. An x-ray spectrum obtained with a 113-MeV *g60n+ beam of 4 nA and a 1000-sirradiation is presented in Figure 11.The sample was irradiated "as received"; for this reason some of the high trace element contents might be due to surface contamination.
ACKNOWLEDGMENT The assistance of the Texas A&M Cyclotron Operations Personnel is gratefully acknowledged. LITERATURE CITED (1) T. B. Johansson, R . Akselsson, and S. A. E. Johansson, Nucl. Instrum. Methods, 84, 141 (1970). (2) R. L. Watson, C. J. McNeal and F. E. Jenson, Adv. X-ray Anal., 18, 288 (1975). (3) J. L. Campbell, B. H. Orr. A. W. Herman, L. A. McNelles, J. A. Thomason, and W. B. Cook, Anal. Chem., 47, 1542 (1975). (4) R. G. Flocchini, P. J. Feeney, R. J. Sommerville, and T. A. Cahill, Nucl. lnstrum. Methods, 100, 397 (1972). (5) F. Folkmann, C. Gaarde, T. Huus, and K. Kemp, Nucl. Instrum. Metbods, 116, 487 (1974). (6) N. E. Whitehead, I.N.S. Report. No. 735, New Zealand, 1975. (7) E. Merzbacher and H. W. Lewis, "Encyclopedia of Physics", Vol. 34, S. Flugge, Ed., Springer-Verlag, Berlin, 1958, p 166. (8) J. D. Garcia, Pbys. Rev. A,, 1, 280(1970). (9) "Proceedings of the International Conference on Inner Shell Ionization Phenomena and Future Applications", Conf.-720404 (Vol. 2), R. W. Fink, S. T. Manson, J. W. Palms, and P.Venugopala Rao, Ed., NationalTechnical Information Service, US. Dept. of Commerce, Springfield, Va. 22151 (1972). (10) L. Shabason, B. L. Cohen, G. H. Wedberg, and K. C. Chan, J. Appl. Pbys., 44. 4749 (1973). (11) W. Brandt, Ref 9, p 948. (12) P. H. Nettles, G. A. Bissinger, S. W. Shafooth, and A. W. Walter, Ref. 9, p 1420
(13) L.'n:Currie, Anal. Cbem., 40, 586 (1968). (14) R . Giauque, F. S. Goulding, J. M. Jaklevic, andR. H. Pehl, Anal. Cbem., 45, 671 (1973).
RECEIVEDfor review June 14, 1976. Accepted August 27, 1976. Work supported by the National Science Foundation Grant MPS 75-17746.
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