Oxygen Determinations in Silicates and Total Maior Elemental Analysis of Rocks by Soft X-Ray Spectrometry A. K. BAIRD and B. L. HENKE Departments o f Geology and Physics, Pomona College, Claremonf, Calif.
b N e w developments in forming thinfilm x-ray tube and detector windows and building large d-spacing analyzing crystals have resulted in 30- to 50-fold increases in the counting rates for oxygen K a in silicates using a vacuum-path x-ray spectrometer. A test of the precision of oxygen determinations over the range 44 to 5470 shows that it i s now possible to achieve better than 1% relative standard deviation in routine rock analyses. This improved precision allows the detection of significant biases in calibration which seem to b e caused by an inadequate knowledge of the concentrations of the other elements present and by the lack of adequate standards.
S
of high intensity soft x-rays, applied to vacuum-path spectrometers in 1961 (79, extended the x-ray method of analysis of silicates to all elements of atomic number 11 and higher. The precisions (Table I) and speed of the method are superior to wet-chemical techniques ( 2 , 3, 8 ) . New advances (9, 10) in casting thinfilm window materials for x-ray sources and detectors, and “building” analyzing crystals which combine large 2d-spacings with high reflectivity, have further extended the method to boron, oxygen, carbon, nitrogen, and fluorine. I n bulk analyses of common silicates, the x-ray spectrometer can now be used for all major and minor element determinations, often with a single preparation of the sample. The possibility of independent analysis for the major element oxygen meqns that accuracy checks, through summations approaching 100% by weight, assume new importance. Other advantages of direct determination of oxygen have been stated clearly by Volborth and t3anta (14). This paper describes precision tests of oxygen determinations over the range 44 to 5497, osygen in plutonic igneous rocks of diverse elemental and mineralogical composition. The results are discussed in terms of resolution, analytical bias, and the importance of standards for x-ray analysis. OURCES
EXPERIMENTAL
Apparatus. A Philips Electronic Instruments vacuum-path spectrometer was equipped with a demountable soft x-ray source using a copper target structure and operated a t low voltage and high current to emit C u L x-radiation. Conditions of operation, including window material, collimation, analyzing crystal, settings for the detector a n d pulse-amplitude analyzer, are summarized in Table 11. Detailed descriptions of the equipment and discussions of the requirements for attainment of optimum signal have been published elsewhere (9, 10). Samples. Seventeen rock samples, including t h e U. S. Geological Survey Standard Rock Diabase R-1, were used along with optically clear quartz. Prior knowledge of the oxygen contents of all rock samples was based upon difference calculations only (10097, minus the sum of all cations previously determined). There are no direct standards available for oxygen in silicate rocks. The quartz was assumed to contain stoichiometric oxygen and used as a reference. Except for the Diabase W-1, for which much information is available (6, IS) on major, minor, and trace constituents (but not osygen), calculations for oxygen by difference for the rocks accounted for nine other major and minor elements ( 1 5 ) . The nine elements, and their approximate concentrations over all the samples in weight yo ranges are: Na, 1.3 to 4.1; Mg, 0.0 to 4.3; -21, 7.0 to 9.7;
Table I.
Element 0 Na
Precisions of Silicate Analyses by Several Methods
Approx. concn., wt. % 48 2 6 0.5 9.3 30.0
Si, 21.7 to 36.4; P, 0.02 to 0.22; K, 0.4t03.6; Ca,0.4to8.2; T i , O . l t o l . l ; Fe, 0.4 to 11.4. They were determined by x-ray spectrometry from repeated comparisons with G-1, W-1 and other wet-analyzed rocks, and summations (calculated as oxides) exceeded 987, in all cases. Cnfortunately, only G-1 and W-1 are reasonably accurate standards for the cations (13). Samples were ground for 8 minutes in a pica mill (Pit’chford 3800) using tool steel vials and three balls. Resulting powders (about 5 to 10 microns, mean particle size) were compressed int’o coherent pellets for x-ray analysis a t 30,000 p.s.i. without an internal binder ( 1 ) . T o achieve a specimen working-surface as smooth as possible, the pellets were pressed against a chrome-plated disk which had been fine ground and a silk screen lapped with ‘/,-micron diamond paste. Before pressing, powders were dried a t 110’ C . in a vacuum oven; after pressing, the pellets were stored in a vacuum desiccator. Each rock sample was duplicated, and for one rock, an additional eight replicate preparations were made. Procedure. T h e x-ray spectrometer used for this test has a standard 4-sample holder supplied commercially with the instrument. T o minimize possible biases introduced by the positioning of specimens, by variations in optic path vacuum levels between batches of four samples, a n d by differences in windows replaced during the run, the following analytical procedures were used after establishing
Intralaboratory analytical precision, std. dev., wt. % Wet KeutroT X-ray0 chemistry Emission Flameb activationc 0 4 0 02 0.01 0.04 0.10 0.03 0.01 0,003 0.02
0 3d 0 l5C 0.15c 0.19c 0 . 14c 0.21c 0.07c
0 9 0 11 0 07 0.12 0.53 Si 1.1 , . . ... K 3.7 0.15 0.07 . . . Ca 2.5 0.14 . . . . . Ti 0.22 ... ... . . . ... Fe 3.1 0.21c 0.14 . . ... a Ref. 16; except for oxygen and recent improvements in Si. Oxygen value from this test. Values assume two replicate determinations. * Ref. 6. c Ref. 14. d Ref. il; average of 92 determinations, converted to s.
2
VOL. 37, NO. 6, M A Y 1965
727
II
DIABASE W-l 44.0% OXYGEN
6300
1
QUARTZ (SiO,)
I N
I
Figure 1. Pulse amplitude distributions and line profiles for oxygen Kcu in U. W- 1 and optically clear quartz. Pulse amplitude curves obtained with a 1 -volt window:
T'acuum X-ray tube Power Emission Window
Collimation Primary Secondary Crystal Composition 2d 28
Detector Gas Flow rate Window
Voltage Pre-amplifier Pulse height analyzer Base line Window
Conditions Philips Electronic Instruments, vacuumpath 50 microns, liquid air trapped Demountable, Cu target 6 kv., 330 ma. Cu L (13.3 A . ) 100 pg./sq. cm each of A1 and Formvar (1micron A1 foil dipped in Formvar) Philips blade-type Spacers and 1 blade (3.50) Every other blade removed (2.7') Lead stearate decanoate 100 5 A. 27.3" Flow proportional LTethane 0.8 c.f.h. (air) 2 double layers, 30 pg. of Formvar each supported on Buckbee llears screen of 200mesh and 707, transmission 2340 Tennelec l00B 6 volts (Figure 1) 34 volts
appropriate 20 and pulse-analyzer settings. .I single specimen of quartz was used in each batch of four samples placed in the spectrometer. The intensity ratio of each of three rock preparations, per batch, to quartz was taken. In each successive batch the quartz specimen was advanced to the next holder position.
728 *
ANALYTICAL CHEMISTRY
RESULTS
,
Survey Standard Rock Diabase
Line profiles obtained with base line a t 6 volts and window set to acceptfrom 6 volts to 40 volts.
The 42 replicate preparations of rocks were randomized and then numbered consecutively. Sixteen total batches of four preparations each were run in the following arrangements: batches 1 through 4 each contained quartz, replicate number 5 and two other replicates selected in consecutive order beginning with number 1 . Batches 5 through 12 each contained quartz and three other replicates selected in consecutive order beginning with number 9. Batches 13 through 16 each contained quartz, replicate number 5 , and two other replicates selected in consecutive order. Within each batch vacuum level, x-ray tube and detector window thicknesses were constant. Counts on specimens were accumulated €or 100 seconds on peak 20 position, then €or 10 seconds on each of two background 20 settings on both sides of the peak (Figure 1 ) . The resulting ratios of net signal per rock to net signal of quartz yielded the following measures: eight determinations of replicate readings on a single preparation of a single rock; 10 determinations on replicate preparations of a single rock; and 34 determinations on duplicate preparations of 17 rocks. To discuss the results in terms of weight % oxygen, predictions from a linear regression of the intensity ratios (rock sample to quartz) us. calculated oxygen are used in the text to follow. The scatter of points from this line ( T = 0.82) is too great to demonstrate anything but a linear correlation. Possible reasons for this scatter, and problems encountered in using direct calculations of oxygen from the ratios to quartz, are discussed below.
Table II. Instrumental Conditions for Oxygen Analyses Using Soft X-Ray Spectrometry
Component Spectrometer
S. Geological
Precision of Counting Statistics. Figure 1 shows pulse amplitude distributions and line profiles for oxygen K a in quartz and W-1. They represent the highest and lowest conditions, respectively, of counting rates and signal to noise ratios. Fixedtiming of peak measurements for 100
seconds yielded between 600,000 and 900,000 total counts on each specimen and the theoretical precision of counting ( l a ) is from 0.2 to 0.3y0 (C = relative standard deviation). The eight replicate readings of a single preparation from a single rock yielded: z = 48.1% oxygen, s = 0.4Yc, and C = 0.8%. The difference between the theoretical counting precision and measured counting precision is explained by the additional error introduced in repeated placements of the sample in different spectrometer holders. This additional error may be caused by specimen inhomogeneities and surface irregularities exposed in different orientation, or by real differences between the four holders used-e.g., nonuniform holder-heights above the x-ray tube. I n this test a standard deviation of 0.4% oxygen must be taken as the basic error. Precision of Specimen Preparation. T h e ten replicate preparations of a single rock yield one measure of our specimen preparation precision: z = 48.0% oxygen, s = 0.7yc,and C = 1.5%. One replicate, however, gave a value of 46.4yC oxygen, a figure more than two standard deviations less than the mean of all 10 replicates. Though possibly the result of a n error in preparation, neither inspection of this replicate for imperfections nor standard statistical tests for gross error permit us to reject the value. h more meaningful measure of preparation precision-i.e., with more degrees of freedom-comes from a single factor analysis of variance with replication of the 34 determinations of 17 rocks (Table 111). For this group the mean oxygen composition determined ranged from 44 to nearly 50Yc oxygen. Precision determined between duplicates of rocks is: s = 0.6% and C = 1.3% (assuming 2 = 47yC oxygen). This figure, which includes the counting and placement errors discussed above, rep-
resents our best estimate of the overall precision achieved in this test. If placement errors could be reduced to zero, the overall precision would be improved only slightly. Values presented above suggest the reduction in the standard deviation would be approximately O . l % , resulting in an overall precision of about 0.5%. DISCUSSION AND CONCLUSIONS
T h e tests reported above suggest t h a t the precision of a single oxygen determination by soft x-ray spectrometry can be about 0.5 t o 0.6 weight yGoxygen, standard deviation. I n routine rock analyses, where t h e oxygen content is about 48'%, t h e single determination would be 1.270 relative deviation. Involved in the single determination (as performed here) are 8 minutes of grinding, 3 minutes to press the sample, and about 1 minute for the reading. With the speed of the x-ray method, replication of analyses is relatively easy and has a twofold advantage: gross errors in preparation, placing in the spectrometer, or reading scalers can be detected; and confidence (standard deviation) in the resulting mean is improved by the square root of the number of replications. Thus, the routine analysis for oxygen in silicates should be better than 1% relative deviation if more than one preparation is used. I n Table I, a precision value is shown for oxygen determinations by x-ray spectrometry based on two replicates per determination; this approach is consistent with our routine procedures in x-ray analyses of several thousand rocks for 9 other elements to date (4, 5 , 15). I t may be compared in Table I with figures shown by Sharma and Clayton (11) for wet chemistry and Volborth and Banta (14) for neutron activation. Accuracy. I n the test reported here, oxygen was predicted from a linear regression of intensity ratios to quartz us. oxygen determined by calculation. This procedure raises two problems which require discussion: t h e possible matrix effects, and t h e inadequacy of our knowledge of t h e oxygen calculated by difference. Evidence of pronounced absorption of the fluorescent 0 Kcu line is found if the intensity ratios i(specimen/quartz) are used directly in a calculation with the stoichiometric oxygen content (53.25%) of quartz rather than using the regression analysis described above. W-1, known to have about 4470 oxygen by difference, has an intensity ratio to quartz averaging about 0.71, yielding an oxygen value of 37.8y0. This rock is lowest in oxygen and highest in the Precision and Speed.
relatively heavy elements of those rocks used. This signal absorption effect decreases as the oxygen content rises in the other rocks tested until, at about 48% oxygen, it becomes impossible to detect the absorption as a consistent effect over the preparation and counting error. The second problem, that of inadequate knowledge of the oxygen content calculated by difference arises because no independent determinations of the major, minor, and trace elements are now available for the rocks used in this test (except W-1). Scatter of predicted points from the regression line follows a definite pattern (Table 111) : replicate pairs of rock preparations are relatively closely grouped, but pairs of results are relatively widely divergent from the calibration line. This suggests that the preparation techniques and the x-ray method are adequate for relatively high precision, and that either our knowledge of the oxygen contents of the rocks used in this test is relatively poor (because of calculation by difference), or an undetected matrix effect (possibly mineralogical) is influencing the linear regression correlation. S o significant correlation can be shown between this scatter of replicate pairs and the presumed concentration of any one of the other elements known to be present. Until independent determinations of the oxygen contents of these rocks are made and better cation standards are available, this problem is likely to remain unresolved. One independent indication of the accuracy of the x-ray method universally used by uet-chemical analysts is shown by a closer approach to the total of 100% by weight when including separately determined oxygen rather than summing the oxides computed from cation x-ray determinations. For example, values indicated for one rock, determined from five replicate preparations calibrated against the rocks used in the precision tests reported here, were oxides 98.48% and elements 98.9470. ACKNOWLEDGMENT
We thank E. E. Welday and E. S . Smith for help in the preparation and analyses of specimens. D. B. McIntyre aided us in the statistical tests reported here. LITERATURE CITED
( 1 ) Baird, A. K., Sorelco Replr. 7, 108 (1961). (2) Baird, A. K., RlcIntyre, D. B.,
MacColl, R. S., Advan. X-Ray Anal. 5, 412 (1962). (3) Baird, A. K., McIntyre, D. B., Welday, E. E., Ibid., 6, 377 (1963). ( 4 ) Baird, .4. K., McIntyre, D. B.,
Table 111. Comparison of Oxygen Values Determined by Difference Calculation and Predicted by Soft X-Ray Spectrometry
Specimen no. 1
Calcd
2
44.1
3
46.5
4
48.1
5
48.3
6
47.5
7
44.3
8
44.7
9
49.1
10
49.2
11
48.0
12
48.7
13
44.3
14
45.3
15
47.6
16
48.3
w-1
44.0
46.1
Wt. 76.02 predicted
Difference
46.8 46.8 44.5 44.6 48.7 48.5 47.5 47.5 47.4 48.5 48.2 46.3 44.4 43.6 45.4 46.4 47.7 47.5 48.2 49.2 47.2 46.4 47.6 47.5 45.2 46.6 46.3 46.2 47.2 47.5 49.1 49.3 44.3 44 3
+0.7 +0.7 -0.4 -0.5 +2.2 +2.0 -0.6 -0.6 -0.9 +O. 2 +0.7 -1.2 +O. 1 -0.7 +0.7 f1.7 -1.4 -1.6 -1.0 0.0 -0.8 -1.6 -1.1 -1 2 +0.9 $2.3 +1.0 -0.9 -0.4 -0.1 +0.8 +1.0 +0.3 +0.3
Welday, E. E., Geol. SOC.Am. Spec. Pavers 76. 187 (1964). (5) Baird, 'A. K., kcIntyre, D. B., Reelday, E. E., hIadlem, K. W., Science 146, 258 (1964). ( 6 ) Fairbairn, H. W., and others, U.S. Geological Survey Bull. 980, 71 pp. (1951). ( 7 ) Henke, B. L., Advan. X-Ray Anal. 5 , 285 (1962). (8) Ibid., 6, 361 (1963). (9) Ibid., 7, in press. (10) Ibid., 8, 460 (1964). ( 1 1 ) Sharma, T., Clayton. R. N.. ANAL. CHEM.36, 2002 (1964). (12) Stanley, R. C., British J . A p p l . Phys. 12, 503 (1961). (13) Stevens, R. E., and others, U. S. Geological Survey Bull. 1113, 126 pp. (1960). (14) Tolborth, A,, Banta, H. E., ANAL. CHEM.35, 2203 (1963). (15) Welday, E. E., Baird, A. K., M e Intyre, D. B., lladlem, K . w., A m . Minerabgist 49, 689 (1964).
RECEIVED for review December 3, 1964. Accepted February 16, 1965. Research supported by the Sational Science Foundation (GP-1336; grant to Baird and 1IcIntyre) and by the Ofice of Scientific Research, U.S.A.F. (grant to Henke).
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