ANALYTICAL CHEMISTRY, VOL. 50, NO. 11, SEPTEMBER 1978 (8) H. A. van der Sloot and H. A. Das,
Reactor Centrum Nederland report
RCN-75-001 (1974). (9) M. Levstek, L. Kosta, M. Dermelj, and A. R. Byrne, “Nuclear Activation
(13) E. Steinnes, ref. 12 p 149 (14) T. Ishimori and E . Nakamura, Jpn JAERI-1047 (1963).
1555
A t . Energy Res. Inst. Rep.,
Techniques in the Life Sciences”,International Atomic Energy Agency,
Vienna, 1972, p 111 110) . . K. Hevdorn and H. R. Lukens. Danish Atomic Enerav -. Commission. Riso Repoi No. 138 (1966). (11) E. Damsgaard, K. Heydorn, and B. Rietz, ref. 9, p 119. (12) J . 0. Pierce, A . Abu-Samra, D. Fehlauer, T. Clevenger, and J. Vogt, I. A . E . A . Rep., IAEA-157, 103 (1973).
3, 1978. Accepted J u n e 19, 1978. The fellowship support for R.O.A. from the Royal Norwegian Council for Scientific and Industrial Research and from the University of Virginia is gratefully acknowledged. RECEIVED for review May
Determination of Trace Elements on Small Geological Samples Fused in Lithium Tetraborate with X-Ray Fluorescence Spectrometry Emil Jagoutz and Christ1 Palme” Max-Planck-Institut fur Chemie (Otto-Hahn-Institut), Abteilung Kosmochemie, Saarstrasse 23, 6500 Mainr, Federal Republic of Germany
The determination of the trace elements Rb, Sr, Y, Zr, and Nb on small geologlcal samples (100 mg) by X-ray spectrometry is dlscussed and results are shown. For prevlous major element analyses, the samples have been fused in Li2B407(900 mg) and NaNO, (200 mg) to form a glass disk with a thickness of approximately 1 mm. These relatively thin samples require a thickness correction for the X-ray intensities for energies higher than Fe K a . Synthetic standards were used for calibration. Accuracy, estimated by comparison with international standards, is about 10 % for concentrations between 20 and 100 ppm and 5 % for concentrations of more than 100 ppm.
X-ray fluorescence analysis is an appropriate method for t h e determination of many trace elements. Some of these elements, like Zr, Y, and Nb, are difficult to determine with other methods. Since we are primarily interested in analyses of lunar and meteoritic samples, we have developed our own method. In contrast t o the majority of t h e analysts who prepare fused glass disks for major element analyses and pressed powder pellets for trace element analyses ( 1 - 4 , we determine major and trace elements on the same samples. We work with very small sample sizes (100 mg) diluted in a Li2B407a n d NaN03 flux, with a sample to flux ratio of approximately 1:lO. T h e application of our method for major elements has already been published ( 5 ) . In this work we will discuss the possibility of determining trace elements on the same fused glass disks, and we shall give accuracy and detection limits for the determination of Rb, Sr, Y, Zr, and Nb.
EXPERIMENTAL S a m p l e P r e p a r a t i o n a n d Fabrication of Synthetic Standards. Samples were prepared in the same way as described previously ( 5 ) . For the purpose of calibration, sets of synthetic standards were fabricated for each element. The standards were prepared from standard solutions; a few microliters of the solutions of the different elements were pipetted into a Pt-Au crucible and dried, 100 mg SiOz were added to form the matrix, and 200 mg NaN03 and 900 mg Li2B407for the flux. This mixture was fused and a glass disk was formed. The concentrations of the elements in the standard solutions were checked either by wet chemical analysis or by neutron activation analysis. Since it was impossible to dissolve Nb, solid Nb205was diluted with Li2B407. 0003-2700/78/0350-1555$01,00/0
Instrumental Settings. The measurements were taken with a Philips PW 1410 sequential spectrometer with additional equipment as earlier described (5). An X-ray tube with a silver anode, operated at 50 kV and 40 mA, was used with a fine collimator, a LiF (200) crystal, and a scintillation counter. The counting time was 400 s for the peak positions and 200 s for each background position. All measurements were repeated three times. Determination of t h e Background. Four interference-free background points were selected for the background determination of all five elements. A polynomial of second order was then fitted through these points, and the background intensities below the K a peaks were determined. The selected background positions, taken from the Sr K a peak were: As,(28): 5.62, 1.85, -4.80, and -6.45 degrees. Thickness a n d Matrix Correction. The thickness of our glass disks is only about 1 mm. The thickness is not the same for all disks since it depends on the viscosity of the melt. For trace element determinations, this thickness cannot be considered as “infinite” for all energies. Figure 1 shows how the intensity for different energies varies with the thickness of the disks. For all wavelengths longer than Fe K a the full intensity is obtained, whereas for energies between Rb K a and Nb K a the intensity is only about half of that of an infinitely thick sample. The exact form of the curves of Figure 1 also depends on the matrix composition of the samples. That means the thickness correction and the matrix correction are not independent. For the matrix correction, the fundamental parameter method has been applied for the determination of major elements ( 5 ) . Contributions for the intensities of different volume elements of the sample are summed up in the correction factor of the fundamental parameter method (6). For samples which can be considered as infinitely thick, this integral is taken from zero to infinity. For samples with a finite thickness the integration is performed only over the sample thickness, d. Using this method, one obtains Equation 1,which has been given by R. 0. Muller (7,8) for thin samples.
hmin. hedee
A
with
The definition of the symbols is: I,, intensity of the measured element; k,, constant to be determined (includes fluorescence yield and geometric factors) from calibration line; c,, concentration of
0 1978 American Chemical Society
1556
ANALYTICAL CHEMISTRY, VOL. 50, NO. 11, SEPTEMBER 1978 INTENSITY
2 L mm SAMPLE THICKNESS
Figure 1. Variation of the X-ray intensity with the thickness of the glass disks
the measured element; I ( A k ) . l A k , spectral distribution of the mass absorption coefficient of the matrix primary radiation; ~,,,(i), for wavelength A; pl(Aj, mass absorption coefficient of element i for wavelength A; A,, wavelength of the characteristic radiation of the measured element; a , B, angles between the incident and emergent beams with the sample surface; f, density of the sample; and d, thickness of the sample. Equation 1is only a more general form of the correction formula of the fundamental parameter method (6). In this method the simplifying assumption of parallel incident and emergent X-rays is made (9). Since the major element compositions of our samples are already known, we can calculate their total absorption coefficients. The new variables to be determined are the density of the glass, f, and the thickness, d. For {we take the same value for all beads, though the density of a glass depends slightly on its composition (10). The thickness, d, is determined by measurements with a micrometer, and the mean thickness determined by repeated measurements over the central part of the disk, excluding the rim, is used, since the thickness of a disk is not constant over its area. Examples for the errors of the thickness measurements will he discussed later. Interfering Elements. The intensity of the measured peak (I,)is the sum of the intensities of the unknown element and the interfering element. We accounted for the case of an interfering elements by the following Equation 2:
I , = kiclPi+ kkCkPk
-k k l
10
71
RESULTS AND DISCUSSION E x a m p l e for the T h i c k n e s s Correction. Although the quantity of material (flux and sample) used for the preparation of single glass disks is nearly constant, there is some variation in their thicknesses. This depends on the angle of contact between the melt and the crucible because the angle of contact determines the height of the rim and therefore the amount
14
13
15
16
Figure 2. Three ghss disks; 100-mg sample fused with 900 mg Li2B407 cwntsk pprr
1.1
SLOPE OF THE CALIBRATION LINE
1/
‘ o u n ~ ~ ~
BACKGROUND
500 DETECTION LIMIT
PPm
12a level with LCOsec c a L n I l n ~t , m e I
3
2
1
1L
16
KeV
ENERGY
Figure 3. Sensitivity, background intensity, and theoretical detection limit ( 2 u ) for X-ray energies between Rb and Nb Table I. XRF Concentrations (ppm) Measured on the Glass Disks of Two Lunar Samples with Identical Major Element Composition
(2)
where k,, kk, k l are constants; c, and c k , the concentrations of the two elements; and P, and P k , their correction factors. For all elements discussed here, only one interfering element contributes significantly to the measured intensities. The calibration constants k,, k k . and k l can be determined by a set of at least three standards containing the two elements in variable and uncorrelated concentrations. In our case, the strongest line interferences were Rb K/3 with Y KO and Sr KP with Zr K a . Computer Program. A Fortran program was written to calculate the matrix correction, the thickness correction, and the correction for the interfering element. The program calculates linear calibration lines from the corrected intensities of standards. The elemental concentrations are calculated from the corrected intensities of the unknown samples. Calibration with Synthetic Standards. The calibration constants k,, kk, k , were determined with synthetic standards. The concentrations for these standards covered the range from 20 ppm to 400 ppm in the undiluted sample (corresponding to an approximate concentration range in the diluted fusion of 2 t o 40 ppmj. A minimum of five different concentration levels within this concentration range was produced for each element and a duplicate was made of each composition. The standards were prepared in such a way that for each element the corresponding interfering element was also present in different concentrations.
12
sample 15028
15027
thickness, mm 0.96 1.23
Rb
Sr
Y
Zr
Nb
16
148 145
154 158
666 662
48.2 47.5
13
of material concentrated near the rim of the disk. Figure 2 shows some of our disks. Since the angle of contact depends on the composition of the melt, on slight differences in the cooling rate, and on the conditions of the crucible surface, one may get disks with similar compositions, but different thicknesses. Table I shows an example. Two Apollo 15 breccias (15028 and 15027)>with identical major element compositions, show a considerable difference in the thickness of their glass disks. The disk of 15027 is about 28% thicker than the one of 15028. Therefore the calculated correction factors for 15027 are 1 2 % and 18% higher for Rb and Nb, respectively, than those of 15028. Table I shows that, in spite of the difference in thickness of the disks, the results of the two analyses coincide very well and there are no systematic differences between the results for the two samples. Further confirmation that the thickness correction works quite well is seen in the examples for the reproducibility given in the corresponding section. S e n s i t i v i t y a n d B a c k g r o u n d I n t e n s i t y . Another problem for the determination of trace elements on fused glass disks is their lower net-peak intensity and their higher background in comparison with powder pellets. Both effects raise the detection limit. The reasons for the loss of intensity are: (1)the dilution of the sample by a factor of ten, which produces a loss of intensity by a factor of approximately three
ANALYTICAL CHEMISTRY, VOL. 50, NO. 11, SEPTEMBER 1978
Table 111. Results of the Regression Analysis for the Element Y
Table 11. Coefficients of Variation of the Thickness Measurements
sample Jonz Malv Bere Pasa BCR
w-1
MlOl M102
1557
Y concentration c [ppm]
mean of eight thickness determinations d , mm
CV_= 100
1.054 0.953 1.057 1.056 1.288 1.215 0.973 1.245
0.8 1.4 0.6 1.4 0.7 1.0 1.8 0.7
s / d , ?4
compared to an undiluted sample of the same thickness, ( 2 ) t h e finite thickness of the disks, which lowers the intensity by a factor of two compared to infinitely thick ones. An exact sensitivity (counts/s ppm in the undiluted sample) cannot be given; it depends on the matrix composition, the thickness, and the exact dilution factor. Nevertheless, values for the sensitivity are shown in Figure 3. They were calculated from the uncorrected intensities of the synthetic standards. T h e background intensities in Figure 3 are for one special standard and show the order of magnitude and the variation with the energy of the detected radiation. The background intensity also depends on t h e matrix composition and the thickness. T h e data in the lowest part of Figure 3 are the lower limits of detection calculated from the sensitivity and the background intensity according to the statistical formula for the concentration equivalent to 2u of the background counts for a measuring time in the background points of 400 (11). The lower limit of detection determined in this way is not really significant for the practical purposes. However, in the next section we shall show that in practice this detection limit is not much higher. P r e c i s i o n of the Method. T h e precision of our method is influenced by statistical variations of the measured intensities of peak and background, by the way of background subtraction, by the absorption and thickness corrections, and by the uncertainties of the element concentrations in the standard rocks. From previous experience with major element analyses, we know t h a t the absorption correction is reproducible within a limit of approximately 1%. The contributions of thickness measurements to the error can be seen from Table 11. Mean thicknesses of eight disks were determined seven times and the coefficients of variation of each of these independent measurements were calculated. Only in one case is it larger than 1.5%. This error of the disk thickness directly influences the correction factor and the corrected intensity as one can see from Figure 1. Consequently, the precision of the determination of low concentration levels with small peak to background ratios is determined by statistical effects, and by the way of background interpolation. T o become independent from literature data for trace element concentrations of standard rocks, we made synthetic standards. From t h e measured intensities, I,, and the computed correction factors, P,, Pk,a multiple regression analysis was made to determine the calibration constants k,, k l , kk according to Equation 2. In the case of Sr, Rb, and Nh no interfering element was considered. (The interference of Y KP with N b K a was insignificant). Table I11 shows the results for the element Y . In column one, the Y concentrations calculated from multiple regression are listed. These are compared to the concentrations known from sample preparation in column two, AC is the difference between both. The standard deviation s of the regression analysis was also calculated (see Table IV). As R b is an interfering element for Y, three parameters had to be determined and therefore the number of the degrees of freedom is n - 3. The standard
sample BNKYl BNRY2 BNRY3 BNRY4 BNRY5 BNRY6 BNRY7 BNRY8 SY1 SY2 SYZl SYZ2 SYZ3 SYZ4 SYZ5 SYZ6 SYZ7 SYZ8
known from sample compu- preparation te d
139.4 133.5 32.4 32.5 72.3 75.2 16.9 14.0 260.0 272.0 68.3 61.5 32.0 34.0 131.5 132.3 20.9 19.3
137.9 137.7 34.8 34.1 71.6 70.9 13.9 13.7 263.3 266.8 65.3 64.9 33.9 35.8 133.0 132.6 19.7 19.8
AC
1.5 - 4.2 - 3.4
-1.6 0.7 4.3 3.0 0.3 --3.3 5.2 3.0 - 3.4 - 1.9 - 1.8 - 1.5 -0.3 1.2 -- 0.5
R b concentration, ppm
216.2 21 5.4 108.9 106.4 44.4 44.0 21.8 21.4
-
-
Table IV. Concentration Ranges and Standard Deviations of the Regression Analysis for Synthetic Standards
element Rb Sr Y Zr Nb
concen trations in synthetic standno. ards, ppm of intermini- maxi- fering standmum mum element ards 21 426 10 27 370 10 19 269 Rb 18 46 498 Sr 12 17 395 10
std dev of the regression analysis, PPm 4.8 3.1
3.0 8.3 2.4
deviation could be considered as a criterion for the precision of the method; it does not say anything about the accuracy, as the same standards were used for calibration and measurement. Neff proposed (12, 13) to use s as an indication for the practical lower limit of detection. T h e precision defined in this way includes t h e reproducibility of t h e preparation of the standards, the reproducibility of the measurements, the absorption correction, the thickness correction, and the correction for eventually interfering elements. The other elements Rb, Sr, Zr, and N b give similar results. In Table IV the concentration ranges of the synthetic standards and the standard deviations of the regression analysis for all elements are shown. Comparison of M e a s u r e m e n t s w i t h L i t e r a t u r e Data. In Table V we compared our measurements of some standard rocks with literature data. The literature data of the first row are all taken from Flanagan (14). Data from other sources are noted in the second row. The report of Govindaraju and de la Roche of CRPG (15) is a more r w e n t compilation of analyses of the standard rocks, B R and GA. T h e data from de Laeter et al. (16')for Kb and Sr, were obtained by isotopic dilution and should have an accuracy of 2-3%. The data from Willis e t al. (17) are X R F measurements on powder pellets. I t can be seen that our results generally agree very well with the literature data. T h e most significant deviations appear for Zr. Our value for GSP-1 is much higher than the one given by Flanagan (14) and even the one of Willis et al. (17). However, it should be noted that nondestructive fast neutron
1558
ANALYTICAL CHEMISTRY, VOL. 50, NO. 11, SEPTEMBER 1978
Table V. Comparison of Rb, Sr, Y, Nb, and Zr Data from Standard Rocks with Literature Data" Rb Sr Y Nb this work
this work
lit.
this work
lit.
lit.
this work
Zr this work
lit.
lit.
1350, 132OC 27 27, 30C 104 looC 272 240, 25OC 330, 331.5L 14 13.5, 11.3w 188 190, 192w 39 37.1, 33.4" 190, 189.3L w-1 188 100 105, 95.1w 6 9.5, 6.BW 24 25.0, 20.3w G-1 11 13.0, 11.9" 250, 24klL 246 1 9 23.5, 21.7'' 218 210, 219" G-2 479, 474.8L 11 12.0, 472 1 2 13.5 349 300 GSP-1 233, 233.2L 24 29, 25.4w 578 500, 543" 226 24 30.4, 25.4w GA 305, 310" 13 13, 1 0 ' 1 5 8 140, 15OW 303 21 18, 21c AGV 657 657, 656.6L 21 21.3 13 15 232 225 BM 221 230 29 26.0 < 5 10 100 1 0 5 TB 184 1 7 8 162 163 34 39.0 18 1 2 194 1 7 5 " Literature data from: without symbol, Flanagan, 1972 ( 1 4 ) ; C, CRPG, 1977 (15);L, Laeter and Rosman, 1977 (16);W, \Yiilis et ai. ( 1 7). BK BCR
42 47 18 215 164 252 172 66
45,47c 46.6, 46.gL 21.0, 21.5L 220, 211.5L 168, 167.6L 254, 254.5L 175,175" 67, 66.6L
1360 328
Table VI. Trace Element Data (ppm) for Apollo 14 Soil 14163 author Rose et a1 ( 1 9 ) iViliis e t al. ( I 7) LSPET ( 2 0 ) / Hubbard e t al. ( 2 1 ) Strasheim et al. ( 3 ) Taylor et al. (22) Wanke et al. ( 2 3 ) H. Palme ( 2 4 ) mean value
methodD XRF XRF XRF
Sr
Rb 13 16
140
177 186
15
Y 290 209 21 3
Zr 8 20 1022 978
Nb
70 63 65
MS/XRF 12 235 23 5 825 62 SSMS/ES 13 180 190 850 46 ITNAA 23 180 IFNAA 208 1040 68 with standard 15.3 j 4.0 1 8 3 i 30 224 * 35 922 t 1 0 2 62.3 t 8.6 deviation this work XRF 18.9 i 2.0 182 t 9 229 i 11 998 t 50 69.7 r 3.5 a XRF, X-ray fluorescence; MS, mass spectrography; SSMS,spark source mass spectrography; ES, emission spectrography; ITNAA, instrumental thermal neutron activation analysis; IFNAA, instrumental fast neutron activation analvsis. ~
activation analysis yields a value of 590 ppm (18). This measurement was carried out in our department; therefore, the large spread in Zr values for GSP-1 could, in principle, be due to inhomogeneities in the standard rock. T h e 272 ppm Zr for BR measured by us may he slightly too high, as the Sr concentration of this rock is very high and the Sr K$ interference has not been tested for such high concentrations. In Table VI trace element data for the .4pollo 14 lunar soil 14163 are compared. This sample has been analyzed by various analysts who applied different methods.
CONCLUSIONS The results of the present work demonstrate the possibility of measuring trace elements on fused glass disks. Down to 20 ppm, the elements Rb, Sr, Y, Zr, and Nb can be determined with an accuracy of approximately 10%. For concentrations of more than 100 ppm, the accuracy should be better than 5%. T h e method described here for the determination of trace elements has the following advantages over t h t cmventionally used powder pellet method: (1)The glass disks do not show any mineralogical effects. (2) Major elements can be determined on the same samples. This is not possible for powder pellets, because of grain size effects for large wavelengths. (3) Only very small quantities of material are necessary for the analyses (-100 mg). (4) It is easy to prepare synthetic standards from standard solutions. Some disadvantages of the fused glass disks should be considered: (1)One has to be careful not to contaminate the samples by the flux. (2) The peak to background ratio is larger than in undiluted and infinitely thick powder pellets. (3) A mathematical thickness correction has to be applied to correct the intensities for the finite thickness of the disks.
ACKNOWLEDGMENT The authors thank H. Kruse for writing the computer programs and H. Baddenhausen for determining the standard solutions by wet chemistry. LITERATURE CITED W. Compston et al., Proc. Apoiio 7 7 Lunar Sci. Conf., Geochm. Cosmochim. Acta, Suppi. 7 , 2, 1007 (1970). J. P. Willis et al., Proc. 2ndLunar Sci. Conf., Geochim. Cosmochim. Acta, Suppl. 2. 2, 1123 (1971). A. Strasheim, Proc. 3rdLunar Sci. Conf., Geochim. Cosmochim. Acta. Suppl. 3 , 2, 1337 (1972). J. M. Rhodes et al., Proc. 5th Lunar Sci. Conf., Geochim. Cosrnochim Acta, Suppl. 5,2, 1097 (1974). C . Palme and E. Jagoutz, Anal. Chem., 49, 717 (1977). J. W. Criss and L. S. Birks, Anal. Chem., 40, 1080 (1966) R. 0. Muller, "Spektrochemische Analvsen mit Rontaenfluoreszenz". R. Oldenburg, 1967. D. Laguitton and M. Mantler, Adv. X-ray Anal., 20, 515 (1977). K. Weber. X-Rav Soecbom.. 3. 159 11974). H. Schoize, "Glas, Natur, Struktur and'Eigenschaften", Friedr. Vieweg & Sohn, Braunschweig, 1965. R. Jenkins and J. L. de Vries, "Practical X-ray Spectrometry", Springer Verlag New York, N.Y., 1970. H. Neff, XXth CSI and 7th ICAS in Prag, 1977, Discussion. R. Piesch, X-Ray Spectrom., 5, 204 (1976). F. J. Flanagan, Geochim. Cosmochim. Acta, 37, 1189 (1973). K. Govindaraju and H. de la Roche, Geosfandards Newsl., 1, 67 (1977). J. R. de Laeter and K. J. R. Rosman, Geostandards Newsl., 1, 35 (1977). J. P. Wiliis et al., ref. 3, p 1269. H. Palme, Max-Planck-Inst., Mainz, personal communication. H.J. Rose et al. ref. 3, p 1215. Lunar Sample Preliminary Examination Team, Apollo 15. Science, 175, 363 (1972). N. J. Hubbard et al., ref. 3, p 1161. S. R. Taylor et al.. ref. 3, p 1231. H. Wanke et al.. ref. 3, p 1251. H. Palme, in "Analyses extraterrestrischen Materials", W. Kiesl and H. Melissa, Jr., Ed., Springer Verlag, New York, N.Y., 1974.
RECEIVEDfor review March 17,1978. Accepted June 12, 1978. The financial support of the Deutsche Forschungsgemeinschaft is gratefully acknowledged.
