Precision and Detection Limits of Certain Minor and Trace Elements in Silicates by Electron Microprobe Analysis SIR: Values as low as 50 ppm and lower have been suggested as the minimum detectability limit (CnL)for electron probe analysis ( I ) and such results are possible under certain analytical conditions (2). Titanium, chromium, manganese, copper, nickel, vanadium, cobalt, and zinc are commonly of geologic significance when they occur as minor and trace components in silicates in concentrations as low as 200-500 ppm. We are carrying on a precision and detection-limit study of these elements in silicates and would like t o report preliminary results of our work at this time. We used an Applied Research Laboratories (ARL-EMXSM) microprobe operated at 15 and 20 kV with a beam diameter of 5 pm and a specimen current set t o 2 x 10-8 ampere on the mineral benitoite (BaTiSiaO,). Benitoite luminesces under the electron beam and is, therefore, a useful material for location and diameter adjustment of the beam. Spectrometers equipped with LiF crystals diffract the K a lines of the above elements. Fixed-beam current termination for a counting time of approximately 10 seconds for each observation was used. Observations were made on ten separate areas of each sample. Three synthetic-glass standards prepared by Corning Glass Works for the U S . Geological Survey (USGS Standards) used in this analytical study contain about 500, 50, and 10 ppm each of the elements of interest (3). These standards also contain about 30 additional elements in minor and trace quantities. They were procured initially for spectrochemical analysis but have been found useful also as electron probe standards. The concentrations of major elements in the standards are similar to those in common silicate rocks containing the following components as oxides in weight per cent: Si -61-62, AI -14-15, Fe (as FeO) -6, Mg -4, Ca -5, N a -4.5, and K -3.5. National Bureau of Standards (NBS) Standard Reference Materials (SRM 610 and 612) Trace Elements in a Glass Matrix ( 4 t a l s o prepared by Corning Glass Works-and USGS Standards were compared with respect t o trace-elements concentration and homogeneity. Thus our study is concerned with detectability limits for the determination of minor or trace concentrations referenced against standards also of low elemental content. Since the background assumes a considerable portion of the total count for the X-ray intensity at low concentrations, errors in background readings could markedly affect the accuracy of determinations. It is therefore important t o obtain the best possible estimate of its intensity. As pointed out by Heinrich and Yakowitz (5), more research needs t o be devoted to the problem of background correction. Keil ( I ) listed sources of background in electron probe analysis and two approaches commonly used to obtain an estimated value. Background readings can be obtained a t (1) K. Keil, Forrschr. Mineral., 44, 4 (1967). (2) J. I. Goldstein J . Geophys. Res., 72,4689 (1967). (3) A. T. Myers, U.S. Geological Survey, Denver, Colorado, personal communication, 1969. Values reported in Table I1 are provisional. (4) “Price and Availability Listing of Standard Reference Materials,” Nat. Bur. Stand. Spec. Publ. 260-Supplement, July 1970. (5) K. F. J. Heinrich and H. Yakowitz, “Fifth International Congress on X-ray Optics and Microanalysis,” G. Mollenstedt and K. H. Gaulker, Ed., Springer-Verlag, Berlin, 1969, p 152.
the “on-peak” spectrometer setting for element A, but with other elements or compounds free of A, in the electron beam and preferably having average or “effective” atomic numbers
(z = i 5 C‘,Zi) similar or identical t o 2 of matrices in which =1 A is measured. These requirements are not always easy t o fulfill. An alternative is to take “off-peak” measurements of the continuous spectrum on the high and low wavelength sides of the element of interest-in our case =tO.O7 A from the peaks of the respective K a lines. Using the sample itself has the advantage of an identical 2 and actual intensity contribution of all of the constituents in the sample. Since magnesium, aluminum, and silicon could be present in some mineral-oxide form in silicates in wide concentration ranges, and could make a significant background contribution, an estimate of their background intensities was obtained from on-peak readings for pure SiO, (quartz), MgO (periclase), and A1203(corundum). On-peak intensities for these three compounds in most instances were less or only slightly higher, than intensities from off-peak measurements on the glass standards. The generally higher off-peak readings of the various glass standards may be due to small background contributions of the numerous elements present in minor or trace quantities. X-Ray intensities obtained on the two lowest concentration standards (-50 and -10 ppm) are consistently though not significantly higher statistically than the on-peak intensity for S O 2 , MgO, or Al,O8. The data for off-peak background readings of the standards and samples for 15 and 20 kV operating voltage are summarized in Table I as well as the intensities for the lowest concentration USGS standards. Backgrounds could also be estimated by graphical extrapolation on the calibration curve t o obtain its intensity at zero concentration. Several definitions have been proposed for reporting a value for the CDL(6-10). We have calculated CDL’sin terms of a line intensity 3 standard deviations (3 U-99 confidence level) above background as suggested by Birks (6). To convert intensities t o weight per cent composition, we have determined the sensitivity S in terms of the slope of a calibration curve in the interval between the estimated background counts at zero concentration and counts of the 500-ppm-level standard for the various elements. The coefficients of variation [a = i ( N 1 Nb)l/*/(N1 - Nb)] are implied t o be k50z and turn out t o be approximately this value on calculation for each C=Lreported. These data are also summarized in Table I. The results on NBS-SRM-610 standard referenced t o the USGS standards are summarized in Table 11. The values reported were obtained graphically from a plot of the curves based on the averaged intensities for the ten observations on the 500- and 50-ppm standards. The X-ray intensity data were also reduced t o weight-per cent composition by use of the
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(6) L. S. Birks, “X-ray Spectrochemical Analysis,” Interscience, New York, N.Y., 1959, p 54. (7) J. Colby, Adcan. X-ray Anal., 11, 287 (1968). (8) R. Theisen, “Quantitative Electron Microprobe Analysis,” Springer-Verlag, New York, N.Y., 1965. (9) T. 0. Ziebold, ANAL.CHEM.,39, 859 (1967). (10) T. 0. Ziebold and R. E. Ogilvie, ibid., 36, 322 (1964).
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Table I. Sensitivity and Minimum Detection Limits by Electron Microprobe of Certain Period 4 Elements in Silicate Matrix Intensity, counts/lO sec K Av-off-peak Lowest concn excitation K~ on stds & samples USGS std potential line, A 15 kV 20 kV- 15 kV 20 kV Element -2 kV 1.436 222 372 218 401 Zn - 30 9.66 171 305 190 295 1.542 CU - 29 8.90 168 229 154 248 1.659 8.33 Ni - 28 136 150 145 169 7.71 CO - 27 1.789 100 157 .,. ... 2.101 6.54 Mn - 25 82 117 85 115 2.289 Cr - 24 5.99 72 95 65 103 V - 23 5.46 2.504 59 83 60 83 Ti - 22 4.97 2.749
Sensitivity, s (cti %) for 5Wppm std 15 kV 20 kV 930 1710 1100 1690 1270 2020 1070 1210 1070 1890 1280 2130 850 1730 1180 1630
Precision, f 2 ~ , 5Wppm std, ppm 15kV 20kV f330 f320 f420 f280 f210 f90 f180 f210 f100 f150 f100 f40 f150 f1270 f290 i200
Minimum detectability limit (CDL)at 3 u above background, ppm 15 kV 20 kV 480 340 360 310 310 220 330 300 280 200 210 150 300 170 190 170
Table 11. Electron Microprobe Results on National Bureau of Standards Standard Reference Material SRM-610-Trace Elements in a Glass Mixture Based on USGS Standards Concentrations - USGS standardization (ppm) Added to glass mixture, ppm 15 kV 20 kV Element USGS NBS Computer fu 500 ppm Computer f u 500 ppm Channel No. standard standard Graphical* calc& USGS std Chemicala calcdb USGS std Titanium-I 540 f 70 560 f 110 31140 440 f 30 430 f 50 dz loo Titanium I1 (520)“ (437)5 540 f 80 530 f 80 i50 550 f 70 550f100 f100 Vanadium-I 550 2~ 70 560 f 110 175 410 f 70 430 f 70 f60 . . .d 500 f 80 530 f 70 f80 460 f 40 450 f 100 f90 Vanadium I1 ( 540) Chromium I 440 f 40 440 f 90 fso 360 Z!Z 40 340 f 40 f20 Chromium-I1 (510) . . .d 500 f 120 510 f 60 f130 380 f 140 370 f 100 f140 Manganese-I 490 f 40 530 f 70 fso 490 f 70 480 f 100 f80 Manganese-I1 (650) 485 f 10 440 f 90 450 f 90 f100 530 f 80 550 f 100 f80 Cobalt-I 390 f 90 360 f 90 f90 390 f 110 360 f 60 f110 Cobalt-I1 (450) ( 390) 420 f 180 420 f 140 A100 390 f 100 400 f 130 f90 Nickel-I 480 f 80 470 f 70 f80 450 f 50 440 f 50 f40 Nickel-I1 (520) 450 f 8 550 f 180 550 f 140 +lo0 940 f 420 950 f 220 f90 Copper-I 270 f 150 230 f 210 i210 500f 100 5 1 0 f 110 f150 Copper-I1 (510) 444 f 4 360 f 130 350 f 200 f190 420 f 200 440 f 250 2200 Zinc-I 320 i 150 320 f 130 f170 590 f 170 500 f 140 f160 . . .d . . .d . . .d 6 0 0 k 190 6 5 0 f 140 A130 Zinc-I1 (500) (433) Values in parentheses are provisional. b Obtained from a plot of a curve based on averaged intensities for the 10 observations on the 500- and SGppm USGS standards. c Calculated using computer program “Calibration curve (linear) and analysis of unknowns.” See reference (11). Not reported.
computer program prepared by the US. Geological Survey (11) for the analysis of geologic materials based on suites of standards for calibration curves (12-15). Also shown is the standard deviation (fu ) in the 500-ppm-level standards. This gives a measure of the reliability of standardization. At this concentration level, about 2000 counts are accumulated for the 10-second counting periods, each on ten areas (100 seconds total time). The signal-to-background ratio is about 2, giving a calculated counting precision of f 5 at one CT. If a total of 2 X l o 4counts (counting period -17 minutes) were taken, the precision would be improved three-fold. With the resultant improvement in counting precision, the factor of sample inhomogeneity could be better evaluated. Since this
x
(11) Margaret R. Roberts, U. S. Geological Survey, Denver, Colo., private communication, 1971. (12) G. A. Desborough and R. H. Heidel, “Electron Microprobe Analysis of Silicon, Magnesium, and Aluminum at Low Operating Voltage,” Amer. Miner., in press. (13) G. A. Desborough and R. H. Heidel, “Improved Quantitative Electron Microprobe Analysis of Sulfur in Some Common Sulfides Using Low Operating Voltage,” Amer. Miner., in press. (14) R. H. Heidel, Amer. Lab., 3, 8 (1971). (15) G. A. Desborough, R. H. Heidel, W. H. Raymond, and Jacquie Tripp, “Primary Distribution of Silver and Copper in Native Gold for Six Deposits in the Western United States,” Mineralium Deposita, (Berlin), in press. 1908
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work is of a preliminary nature, we have chosen t o demonstrate what can be done with a rapid (10-second) routine traceelement determination by means of electron probe analysis. For the total counts in 10 seconds, the average of the coefficients of variation (u/M X 100) for all elements of the USGS and NBS glasses in terms of concentrations was 18.43 and 27.69x, respectively, at an operating voltage 20 kV. At 15 kV, the coefficient of variation for the USGS standard was 24.11 and for the NBS standard was 27.67 %. We have also started a study of precision and detection limits of cadmium, manganese, cobalt, and nickel in natural sulfides by electron microanalysis (16). ROBERTH. HEIDEL U S . Geological Survey Denver, Colo. 80225
RECEIVED for review May 12, 1971. Accepted August 26, 1971. Presented in part at the Sixth National Conference on Electron Probe Analysis, Pittsburgh, Pa., July 28-30, 1971. Publication authorized by the Director, U.S. Geological Survey. (16) R. H. Heidel, G. A. Desborough, and G. K. Czamanske, Abstract, program, Tenth National Meeting of the Society for Applied Spectroscopy, St. Louis, Mo., October 18-22, 1971
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