Anal. Chem. 1904, 56,407-411
407
Natl. Bur. Stand., Sect. A 1986, 70A 453-458. (14) Garner, E. L.; Machlan, L. A,; Gramlich, J. W.; Moore, L. J.; Murphy, T. J.; Barnes, I.L. NBS Spec. fu6l. ( U . S . ) 1978, 422, 951-960. (15) Wlttmaack, K.; Schuiz, F.; Werner, E. I n “Secondary Ion Mass Spectrometry SIMS 11”; Benninghoven, C. A,, Evans, C. A., Jr., Powell, R. A., Shimlzu, R., Storms, H. A,, Eds.; Springer-Veriag: New York, 1979; pp 245-247. (16) Burns, M. S. I n ”Microbeam Analysis 1982”; Heinrlch, K. F. J., Ed.; San Francisco Press: San Franclsco, CA, 1982; p 138. (17) Schwartz, R.; Spencer, H.; Wentworth, R. A. Clln. Chim. Acta 1978, 87, 265-273. (18) Burdo, R. A.; Morrlson, G. H. “Tables of Atomic and Molecular Lines for Spark Source Mass Spectrometry of Complex Sample and Graphite Mixes”; Report 1670 Materials Science Center, Cornell University, Ithaca, NY, 1970. Wlttmeack, K. Apply. fhys. Left. 1976, 29, 552-554. Snedecor, G. W.; Cochran, W. G. “Statlstical Methods”, 7th ed.; Iowa State Unlversity Press: Ames, IA, 1980; Chapter 12. Schwartz, R.; Giesecke, C. C. Clin. Chim. Acta 1979, 97, 1-8. Ramseyer, G. 0.; Morrison, G. H. Anal. Chern. 1983, 55, 1963-1970. Slodzian, G., Lorln, J. C.; Havette, A.; Dennebouy, R. “Isotopic Measurements with High Mass Resolution by ElectrostatlcPeak Swltchlng”; SIMS IV, Japan, Nov 1983.
LITERATURE CITED (1) Shlmlzu, N.; Hart, S. R. J . Appl. fhj’S. 1982, 53, 1303-1311. (2) Jull, A. J. T. Int. J. Mass Spechorn. Ion Phys. 1982. 41, 135-141. (3) Zlnner, E.; Grasserbauer, M. I n ”Secondary Ion Mass Spectrometry SIMS 111”; Bennlnghoven. A., Giber, J., Laszlo, J., Riedel, M., Werner, H. W., Eds.; Springer-Verlag: New York, 1982; pp 292-296. (4) Armstrong, J. T.; Huneke, J. C.; Shaw, H. F.; Flnnerty, T. A.; Wasserberg, G. J. I n “Microbeam Analysls 1982”; Heinrich, K. F. J., Ed.; San Francisco Press: San Francisco, CA, 1982; pp 202-204. (5) Russell, W. A.; Papanastassiou, D. A.; Tombrelio, T. A. Radiat. EM. 1980. 52, 41-52. (6) SMzlan, G.; Lorin, J.C.; Havette, A. J. mys. Left. (Orsay, Fr.) 1980, 4 7 , L555-L558. (7) Lorln, J. C.; Havette, A.; Slodzlan, G. I n “Secondary Ion Mass Spectrometry SIMS 111”; Benninghoven, A., Glber, J., Laszlo, J., Rledel, M., Werner, H. W., Eds.; Sprlnger-Verlag: New York, 1982; pp 140-1 50. (8) Armstrong, J. T.; Huneke, J. C.; Shaw, H. F.; Finnerty, T. A,; Wasserburg. G. J. I n “Mlcrobeam Analysis 1982”; Heinrich, K. F. J., Ed.; San Francisco Press: San Franclsco, CA, 1982; pp 205-209. (9) Christie, W. H.; Eby. R. E.; Warmack, R. J.; Landau, L. Anal. Chern. 1981, 53, 13-17. (IO) “Separated Isotopes: Vital Tools for Science and Medicine”; Natlonal Research Council, Natlonal Academy Press: Washington, DC, 1982. (11) Janghorbanl, M.; Young, V. R. I n “Advances in Nutritlonal Research”: Draper, H. H., Ed.; Plenum Publishing Corp.: New York, 1980: pp 127-154. (12) Schwartz, R. fed. R o c . 1982, 41, 2709-2713. (13) Cantanzaro, E. J.; Murphy, T. J.; Garner, E. L.; Shields, W. R. J. Res.
RECEIVED for review August 9,1983. Accepted November 14, 1983. This work was funded by the National Institues of Health under Grant No. R01 GM-24314 and AM 16985.
Microanalysis in Galena by Secondary Ion Mass Spectrometry for Determination of Sulfur Isotopes Michael Pimminger and Manfred Grasserbauer* Institut fur Analytische Chemie, Technische Universitat Wien, Getreidemarkt 9,A-1060 Wien, Austria Erich Schroll Geotechnisches Institut der Bundesversuchs- und Forschungsanstalt Arsenal, Franz-Grill-Strasse 3, A-1031 Wien, Austria Immo Cerny Bleiberger Bergwerks- Union AG, A-9530 Bad Bleiberg, Austria
The posslbllltles and llmltatlons of lsotoplc analysls as one maln fleld of appllcatlon of SIMS for analytlcal geochemlstry were lnvestlgated systematlcally. For the problem of accurate determlnatlon of sulfur losotopic ratlos, optlmum operatlonal condltlons were developed to ellmlnate the influence of Interferences; also several other effects, llke nonllnearlty of the countlng system and Instrumental mass dlscrlmlnatlon whlch llmlt preclslon and accuracy of results, were studied. By use of a serles of lead sulfide speclmens with known Isotopic ratlos, lt could be demonstrated that even at high mass resolutlon ( m / A m 2 5000) a preclslon and accuracy of 23% for the uS/s2S ratio can be obtalned In an analyzed area of 8 hm dlameter. I n practlcal appllcatlon a varlatlon of the sulfur lsotoplc ratlo In zonal growth structures of slngle galena octahedrons could be observed.
In nature variations in the isotopic ratios are caused either by radioactive decomposition processes or by a fractionation in chemical and physical reactions. Abundance ratio determinations, especially with sulfur, lead, carbon, or oxygen, have proved to be of great value to questions of earth sciences (1). In isotopic geochemistry mass spectrometry is used as the 0003-2700/84/0356-0407$0 1.50/0
common analytical method which yields results with high precision and accuracy. However, besides the necessity for a particular sample preparation, the most crucial disadvantage of this technique is that it is not applicable for in situ microanalysis. For this reason secondary ion mass spectrometryhas already been employed for isotopic microanalysis,particularly in those cases in which tiny inclusions of valuable samples like extraterrestrial material should be analyzed (2, 3 ) . However, most applications have been carried out at low mass resolution, when interferences from molecular ions were negligible ( 2 , 4 , 5 ) )or could be eliminated by energy filtering of secondary ions (6) or by a peak-stripping routine ( 3 ) . In the current investigations the applicability and reliability of isotopic analysis by SIMS were studied also at high mass resolving power with the example of sulfur abundance ratios in galena samples. In many cases sulfur isotopes can supply additional information about ore genesis (7). In nature four stable isotopes with the following mass numbers and ranges of relative abundances are found (8): M = 32,95.253-94.638%; M = 33, 0.780-0.731%; M = 34, 4.562-4.001%; M = 36, 0.0199-0.0153%. In general the ratio Rij of the isotopes i and j which are influenced by chemical fractionation processes are stated as 0 1984 American Chemlcal Society
408
ANALYTICAL CHEMISTRY, VOL. 56, NO. 3, MARCH 1984
deviations 6, in parts per mil from a reference ratio RLJ*,given by the formula 6 , = 1000(R,/Ri,* - 1)
I
1
(1)
In first approximation a linear relationship between chemical fractionation and mass difference can be assumed (9)
(2)
= a.Ah4i1
'3-
where a is the fractionation factor per atomic mass unit in %o mu-' and AMLJis the mass number difference between the isotope i and the reference isotope j . It should be mentioned that in literature different definitions for a fractionation factor a can be found.
EXPERIMENTAL SECTION Samples, Standards, and Preparation. For calibration and application,galena (PbS) crystals were prepared in the usual way by cutting, embedding in a low melting point alloy or in a vacuum proof synthetic resin, grinding, and polishing. Then after ultrasonic cleaning a conductive film of 20-30 nm Au was deposited on the plane surface. The ratio 34S/32Sof the troilite phase of the meteorite from Caiion Diablo, AZ (&/32* = 0.045005), is accepted worldwide as a standard value ( 7 , l O ) . In present investigations a series of to -26.9%0 were PbS working standards with 6% values from -1%0 employed which were analyzed with respect to the meterorite standard with sufficient accuracy (*0.2%0)by mass spectrometry
Magnetic field
Flgure 1. Hlgh resolution mass spectra in PbS with an Ar+ primary beam (E, = 10 keV, I , = 150 nA, m l A m = 5100).
resolving power, with a peak-stripping routine. Through combination of eq 1to 4 the hydride fraction can be calculated.
(A
H =-
2R34/32* R33/32*
.)
+
(11).
Though the abundance ratios of the isotopes % and 32Swould be sufficient for characterization of the sulfur (12), the fractionation of 33Swas determined in most cases to get an additional check for instrumental effects. Since in mass spectrometry only 634Svalues are determined, no standard ratio 33S/32Sis known. Therefore for &/32* a value of 0.007 99 was extrapolated from measurements of the working standards assuming linear fractionation after eq 2. Due to a very low abundance 36Susually was not detected. Instrumentation. Experiments were carried out with the CAMECA IMS 3f secondary ion mass analyzer, a direct imaging, double focusing instrument, of which the analytical principle is described in detail elsewhere (13-15). Art and Oz+primary ions could be used without any precautions for charge compensation, because no sample charging could be observed during ion bombardment, if a conductive connection between sample mounting and PbS crystal was maintained. After sample precleaning by ion etching, for analysis the focused beam (10-30 pm diameter) was rastered over areas of about 25 X 25 pm to obtain a more constant ion emission. For analysis the negative ions were measured, because of their larger intensity than the positive mass spectrum. The analyzed area was limited to a diameter of 60 fim or 8 pm by a mechanical diaphragm. The energy slit was centered to the maximum of intensity. At high mass resolving power (m/Am 5000) energy acceptance was in the range of 20 to 50 eV, whereas at low mass resolution ( m / A m 300) the slit was kept open providing an energy acceptance of 190 eV. The secondary ions were detected in the pulse counting mode with an electron multiplier.
-
-
RESULTS AND DISCUSSION Optimization of Analytical Parameters. By use of an unfiltered Ar+ primary beam the interference of is found to be less than and only hydride ions of sulfur appear in the spectrum (Figure 1). Assuming a constant portion, H, of hydrides, as expressed by
[32SH-] -=-=H
ps-]
[33SH-]
ps-]
(3)
and a linear mass fractionation according to
(4) one can correct the isotopic ratios a (=(33S- + 32SH-)/32S-) and b (=(%S- + 33SH-)/32S-), which are measured at low mass 634s
= 2633s
Neglecting the low contribution of 33SH-,an approximate calculation of H can be performed by the simple formula
With eq 6 the errors for the 6's values are less than 0.5% for usually occurring hydride contributions of 5 x to 5 X W4. Sulfur isotopic analysis a t low mass resolution with peakstripping correction involves the advantages of better counting statistics due to higher secondary ion intensities, of a lower demand for the stability of electrostatic and magnetic analyzer and of more ease in a reproducible adjustment of the ion probe. However, practical measurements have shown that tiny calcite and dolomite inclusions are often present in sulfides increasing the contribution of 02a t mass 32. This can also happen in the analysis of sulfide phases of only a few micrometers due to sputtering of adjacent oxidic compounds by neutral particles. In such cases peak-stripping techniques may yield irreproducible results, and elimination of those interferences by application of high mass resolution spectrometry is necessary. Therefore sulfur abundance ratio determination in lead sulfide was studied at high mass resolving power with an 02' primary ion beam. Use of 02+ was advantageous for its higher primary ion current stability (0.2%/10 min). In Figure 2 observed interferences are presented. With an appropriate adjustment (entrance slit, field aperture, energy slit, exit slit) only the LSpeaks are measured. To achieve the required high stability of the spectrometer system, a peak jumping sequence (to overcome the hysteresis effect of the magnetic analyzer) and an automatic high resolution adjustment routine for compensation of the inconstancy of the magnetic field was implemented into the computer measuring program (16). Fluctuations in the secondary ion emissions are compensated by averaging over a large number of measurements cycles (usually 10 x 10 cycles) with low counting times of 1to 2 s. Outlying results (1% - f I > 2 s) of single isotopic ratio determinations which partly must be traced back to short time instabilities of the magnetic field were discarded.
ANALYTICAL CHEMISTRY, VOL. 56, NO. 3, MARCH 1984
409
,O.Wamu,
J
-4-
-6-
I
IO!
A\
I I
I
II
I
I
I
-8-
1""-
1
I
0
0.5
1
1.5
AT Magnetic field
Flgure 2. High resolutlon mass spectra in PbS wlth an 02+ primary beam ( E , = 10 keV, I, = 100 nA, m/Am = 6500).
Figure 3. Error of sulfur isotope determination A6'S in consequence of uncertainty of dead time AT after eq 9 (T = 25 ns and Ry= Rg*).
Detection and Counting System. For observation of isotopic deviations in the range of per mil (as in isotopic analysis), it is necessary that the detection system shows a constant and linearized response. In this case of application the dependence of quantum efficiency (number of output pulses per incoming ion) upon mass and identity of the secondary ion (17) is not so disturbing due to comparable masses and chemical identity of the isotopes of a given element, as the time resolution of the detection system. The counting loss caused by the dead time, T, the minimum time interval between two resolved pulses, is expressed by the formula (18) n' = ne-"?
(ns)
,
, /'
, '
(7)
where n'is the measured counting rate in counts per second (cps) and n is the true counting rate. Equation 7 can be simplified by a power series expansion (breaking off a t the second term) to
n = - n' 1 - n'r The best way for the dead time determination of the total counting system has proved to be the measurement of isotopic ratios. Through the isotopic ratio determination of pure Ti samples at low mass resolution with correction for hydride ions, the dead time can be determined in a fast and convenient way (16). The error in isotopic measurements Afiij caused by the uncertainty in dead time determination AT can be deduced from a combination of eq 1 and 8 after differentiation to
where ni'and nj'are the measured counting rates of the isotope
i and the reference isotope j in cps. In Figure 3 the error for sulfur isotopic measurements is presented as a function of error in dead time determination with different counting rates nj' of the reference isotope 32S. The conclusions to be drawn for practical measurements are the use of comparable secondary ion intensities for standard and sample and the choice of a countingrate of about 5 X lo5 cps for the most abundant isotope. The typical dead time of the pulse counting system was about 25 ns of which the error is expected to be less than fl ns. Instrumental Mass Discrimination. In addition to a chemical fractionation in nature an instrumental mass discrimination effect primarily caused by the sputtering and ionization process occurs.
-30 \
\
\
\
\
\
4
Flgure 4. Results of isotopic analysis in three calibration specimens with theoretical fractionatlon line (6% = 2634S)and calibration line (634~,,,, = 634~,s 2a,)(full spots represent mean values, O,+, E ,
= 10 keV, m/Am
+
N
5000).
This effect led to false conclusionsin some earlier isotopic investigations with SIMS (9). Since that time more systematical studies were carried out with following results (9, 19, 20,21): In principle an enrichment of the lighter isotope is found which can be described with a theoretical fractionation line after eq 2. The extent depends on the relative mass difference and amounts to 60-70%0 amu-' for B and only about 5% amu-' for Pb. Instrumental mass discrimination depends on the matrix, on the initial energy of the secondary ions, and also to some extent on energy and identity of the primary ions. Moreover, a time dependence and a spatial inhomogeneityof instrumental mass discrimination within the sputtering area could be observed. This instrumental isotope effect is the most serious limiting factor in isotopic abundance analysis with SIMS. It causes more problems in the study of physicochemical fractionations of light elements than in cases of isotopic deviations caused by radioactive processes (9, 16). The conclusionsfor practical measurements are the use of standards of chemical identity and an adjustment of the instrument with the greatest possible reproducibility. Especially at high mass resolution one has to pay special attention to the latter because a severe change in the adjustment of the ion optical parameters could result in a variation of instrument induced discrimination (21). In Figure 4 results of single
410
ANALYTICAL CHEMISTRY, VOL. 56, NO. 3, MARCH 1984
Table I. Comparison of Measured (SIMS) with “True” (MS) Isotopic Ratios for Determination of Instrumental Mass Fractionation ai and Evaluation of Precision and Accuracya sample VRI-S-1078b VRI-S-763 VRI-S-766 VRI-S-1063 VRI-S-1074 VRI-S-1068 VRI-S-1066 VRI-S-767
-1.0 -5.9 -6.5 -12.4 -15.6 -19.4 -21.0 -26.9
-25.6 -30.1 -33.1 -29.1 -35.4 -35.1 -38.7 -37.2
t
4.4
5.3 i: 5.2 k 5.8 * 5.8 t 5.3 i 5.6 t 4.9 i:
s = k5.3
-50.9 -56.1 -59.6 -65.5 -66.4 -68.0
1.0 2.9 2 2.5 i 3.7 i: 1.5 i: 3.6 -72.7 t 4.0 -74.8 +. 2.8 t
i:
S = k2.8
-25.0 -25.8
0.6 0.2
-27.7
2.1
-25.3 -26.1 -24.7 -26.6 -23.9
0.3 0.5 0.9 1.o 1.7
a . = - 25.6
,
lai - zi I =
0.9
a Measurement conditions: O;, E , = 1 0 keV, I , = 10-20 nA. Rastering: 20 X 20 pm, m / A m = 5000. The samples are registrated and determined mass spectrometrically by E. Pak (Institute for Radium Research and Nuclear Physics, Austrian Academy of Science, Vienna). 6 3 4 Svalues and descriptions of the samples shall be published in the reports of this institute; appearing in “ Anzeiger der Osterreichischen Akademie der Wissenschaften, mathem.-naturwissen. Klasse” (Vienna).
measurements in three PbS crystals with their mean values and the calibration line established with the “true” values from mass spectrometry (MS) are presented. Between individual measurements sample changing and therefore a slight readjustment of the primary beam (deflection and focus) had to be carried out. The deviation of the SIMS measurement points from the theoretical fractionation line are of statistical nature (typically 1%0)while the distribution along the fractionation line represents the mass discrimination effect (typically 10%). Precision and Accuracy. Andersen et al. stated that accuracy can match precision, if suitable standard specimens are available (22). For analysis at low mass resolution precision and accuracy values from 1% to lo’% for isotopic ratios are reported (e.g., precision of 5% for B and 10%0 for Li (4), accuracy of 1-2%0 (5) and 5%0( 6 ) ,respectively, for Pb). The statistical uncertainty for the counting of single events can be separated from the total error, since the relative standard deviation sr(n) in percent is given by (23, 24)
s,(n) = 100/(nt)”2
(10)
where n is the counting rate in cps and t the counting time. Due to error propagation the relative standard deviation s,(R,) of the isotopic ratio RLjis determined by s,(Rij) = (sr(nJ2 + Sr(nj)2)1’2
(11)
Differentiation of eq 1and combination with eq 11lead to the following expression for the standard deviation of the 6 values: 10RiJ s(6ij) = T s r ( R i j ) (12) RLJ
Thus with a counting rate nJ of 5 X lo6 for 32Sand a measurement time t of 100 s (or 100 repeated measurements with t = 1s) for R , = R,j* standard deviations of 1.6 %o for 633S and 0.7 %O for 6% can be calculated according to the counting statistics. In addition to the statistical uncertainty, systematicalerrors caused by instrumental mass fractionation, have to be determined. Since in a given measurement series the instrumental parameters are not changed severely, in a first approximation also a Gaussian distribution of instrumental mass discrimination can be assumed and therefore a statistical treatment of total error is permissible. Also in mass spectrometry a so-called “internal standard deviation” which is calculated from one set of isotopic ratio determinations and an “external standard deviation”, which is determined from mean values of a number of separate determinations with
separate sample loads, is known. For evaluation of precision and accuracy in SIMS the latter has to be taken into account because sample changing or adjustment of a new sample spot is unavoidable. In Table I results of eight standard specimens with known isotopic ratios are presented. The mean values in Table I consist of 10 different spot analyses, each with 10 times 10 measurement cycles with 1 s counting time per isotope peak. The instrumental fractionation factor aiis calculated in the way that contributions from 633Sand 634Sare weighed in the ratio of their statistical certainty of about 2:l. So for aiholds
For 634Sthe observed average standard deviation amounts to f2.8%0 while the contribution of the counting error should be 0.2%0due to eq 10-12. The deviation from the calibration line, that means the difference betwen “true” and measured values, is smaller than the calculated standard deviation in all cases. This fact confirms the statement that accuracy is equal to precision. For the evaluation of a significant difference for a given statistical certainty the well-known “t test” is applied (25). With a standard deviation of 2.8% a difference of 2.6% can clearly be distinguished with a statistical certainty of 95% and with 10 parallel determinations. So with a reasonable analysis time a precision and accuracy of 2-3%0 is obtained in sulfur isotopic analysis at high mass resolution. Application. This optimized and systematicallyevaluated method for sulfur isotopic analysis was applied to the study of abundance ratio variations in single galena octahedrons from the lead-zinc ore district of Bleiberg-Kreuth in Austria. This ore deposit is supposed to be of sedimentary genesis with bacterially reduced sulfur originating from seawater sulfate (26). Cross sections of a few crystals were repeatedly investigated in a period of more than 1year, in order to test the long-range reproducibility. A decrease of the 634Svalue of 10 to 15%0in the outermost region with a thickness of 100 to 150 pm could be observed with respect to the center (-18 to -20% vs. -8 to -1O%o absolute) (Figure 5). These results obtained at low and high mass resolution agree well also with the 633Svalues. This is an additional hint for the existence of bacterial activity in an open geochemical system during formation of the ore deposit.
CONCLUSIONS There is no possibility to specify a general precision and accuracy of abundance ratios determined with SIMS. These analytical figures of merit depend on kind and electrical
ANALYTICAL CHEMISTRY, VOL. 56, NO. 3, MARCH 1984
I
I
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411
ACKNOWLEDGMENT The authors are grateful to H. Malissa for support of this research project and to E. Zinner for cooperation in methodical development. Registry No. 32S,13981-57-2;33S,14257-58-0;34S, 13965-97-4; 14682-80-5;galena, 12179-39-4. LITERATURE CITED
Q 0
- 30 0
-41
I
200
600 800 Distance from edge ( N m )
400
Figure 5. Variation of sulfur isotopic ratio in cross section of a single galena octahedron (full spots represent mean values, 02+, E, = 12.5 keV, m / A m 5000).
-
conductivity of the sample, mass number, concentration, and secondary ion yield of the interesting element as well as abundance of interesting isotopes, necessary mass resolving power, presence of contaminations (e.g., C, 0, K, due to adsorption from residual gas), and the availability of standard specimens with identical matrix. Modern instrumental technology enables the study of isotope ratios even under conditions of severe interferences by application of high mass resolution. The example of the measurement of sulfur isotope ratios in PbS shows that even under such stringent analytical requirements as a mass resolution of 5000 a precision and accuracy of 2-3% can be achieved on the basis of extensive optimization of the technique and careful study of all effects influencing the result. For evaluation of the applicability of SIMS for sulfur isotopic analysis, a comparison with analytical features of mass spectrometry as reference method has to be taken into consideration. Due to the high accuracy of mass spectrometry (*0.2%0 for 6 %) the exact determination of average isotopic abundances will continue to be the domain of mass spectrometry. SIMS provides excellent analytical possibilities for isotopic investigations of microdomains. In such cases, e.g., for zonal structures, lateral resolution is often more important than ultimate precision and accuracy. Also for elements and matrices in which chemical separation of the isotopes of interest is difficult an in situ technique like SIMS provides superior possibilities.
Schroli, E. "Analytische Geochemie, Vol. 2: Grundlagen and Anwendungen"; F. Enke: Stuttgart, 1976. Okano, J.; Nishlmura, H. I n "Secondary Ion Mass Spectrometry SIMS 11"; Benninghoven, A., Evans, C. A., Jr., Powell, R. A., Shlmizu, R., Storms, H. A,, Eds.; Springer: Berlln, Heidelberg, New York, 1979; pp 216-218. Okano, J.; Nishimura, H. I n "Secondary Ion Mass Spectrometry SIMS 111"; Benninghoven, A., Giber, J., Laszlo, J., Riedel, M., Werner, H. W., Eds.; Springer: Berlin, Heidelberg, New York, 1982; pp 426-430. Christie, H. W.; Eby, R. E.; Warmack, R. J.; Landau, L. Anal. Chem. 1981,53, 13-17. Hart, S. R.; Shimizu, N.; Sverjensky, D. A. €con. Geol., in press. Shimizu, N.; Semet, M. P.; Allegre, C. J. Geochim. Cosmochim. Acta 1978,42, 1321-1334. Ohmoto, H.; Rye, R. 0. I n "Geochemistry of Hydrothermal Ore Deposits", 2nd ed.; Barnes, H. L., Ed.; Wiley-Interscience: New York, Chichester, Brisbane, Toronto, 1979; pp 509-567. Holden, N. E. Pure Appl. Chem. 1979,51, 405-433. Lorln, J. C.; Havette, A.; Siozdian, G. I n "Secondary Ion Mass Spectrometry SIMS 111"; Benninghoven, A., Evans, C. A,, Jr., Powell, R. A., Shimizu, R., Storms, H. A,, Eds.; Sprlnger: Berlln, Heidelberg, New York, 1982; pp 140-150. von Gehlen, K. Geol. Rundscb. 1966, 55, 178-197. Pak, E.; Felber, H. Sitzber. Ost. Ak. d . Wiss., math.-naturw. Kl., Abt. II1974, 183, 295-308. Birkenfekl, H.; Haase, G.; Zahn, M. I n "Physlkaiisch-chemlsche Trennmethoden, Vol. 5: Massenspektrometrische Isotopenanalyse", 2nd ed.; VEB Deutscher Verlag der Wissenschaften: Berlin, 1962. Morrison, G. H.; Slozdian. G. Anal. Chem. 1975, 4 7 , 932A-943A. Andersen, C. A. I n "Microprobe Analysis"; Andersen, C. A,, Ed.; Wiley-Interscience: New York, 1973; pp 531-553. McHugh, J. A. I n "Methods and Phenomena, Vol. 1: Methods of Surface Analysis"; Wolsky, S. P., Czanderna, A. W., Eds.; Elsevier: Amsterdam, 1975; pp 223-278. Zinner, E.; Grasserbauer, M. I n "Secondary Ion Mass Spectrometry SIMS 111"; Bennlnghaven, A., Evans, C. A., Jr., Powell, R. A,, Shimizu, R., Storms, H. A., Eds.; Swinger: Berlin, Heidelberg, New York, . 1982; pp 292-296. Wittmaack, K. Nucl. Instrum. Methods 1960, 168, 343-356. Bieyer, H. Thesis, Techn. Univ. Vienna, 1976. Shimizu, N.; Hart, R. S . J. Appl. fhys. 1982,53, 1303-1311. Siozdian, G. I n "Secondary Ion Mass Spectrometry SIMS 111" Benninghoven, A., Evans, C. A., Jr., Powell, R. A,, Shimizu, R., Storms, H. A., Eds.; Springer: Berlin, Heidelberg, New York, 1982; pp 115-123. Juli, A. J. T. Int. J. Mass Spectrom. Ion Phys. 1982,4 1 , 135-141. Andersen, C. A,; Hinthorne, J. R. Science 1972,853-860. Kaiser, H.; Specker, H. FreseniusZ. Anal. Chem. 1956, 149, 46-66. Werner, H. W. Surf. Interface Sci. 1980,2 , 56-74. Gottschalk, G. I n "Die chemische Analyse, Vol. 49: Statistik In der quantitativen chemlschen Analyse"; F. Enke: Stuttgart, 1962. Schroll, E.; Schuiz, 0.; Pak, E. Mlneral. Dep. 1983, 18, 17-25.
RECEIVED for review March 31, 1983. Accepted November 18,1983. The authors thank the Austrian Scientific Research Foundation for financial support (Project 3603).
