Oxygen Determination in Rocks, Minerals, and Water by Neutron Activation ALEXIS VOLBORTH' and H. E. BANTA Oak Ridge Institute of Nuclear Studies, Oak Ridge, Jenn.
b Oxygen in rocks and minerals i s determined using the reaction 0lB (n,p)N16 in a 14-m.e.v. neutron flux of 7 X l o 8 n/cm.2/seccmd, produced by bombardment of a ti,itiated titanium target b y deuteron beam of 500 pa. accelerated at 150 kv. The gammaray activity of the 7.4-second nitrogen i s counted b y a NallTI) scintillation crystal and a multichcnnel analyzer. Two-gram samples are used. The method permits about 80 analyses per day and i s nondestructive. The relative standard deviation achieved on a water standard i s l).39y0. Using water and ignited oxides T i 0 2 and A1201 as standards, oxygen was determined in rocks G- 1 and ' N -1. Gamma activities used lie in the region of 6 to 7 m.e.v., whereas mosi other gamma activities of elements present in these rocks fall in the region of 0 to 3 m.e.v. The short half life of nitrogen-16 requires exact timing and a fast transfer system. In rocks w th F and B the reactions F1g(n,~)N'C and Bl1(n,p)Be11 have to be taken into account.
F
liavc! been used for analysis of trace amounts of oxygen, mainly in orgmic compounds, (3, 9, IO, using the reaction 016((n,p)S16 1 2 ) . Trace amounts of osygen in inorganic samples have also been determined (2, 3, 6). In 1;he cited work ( 2 , 3), sensitivity a,chiered mas about 30 p.p.m., and relative ;standard deviation varied from 10 t o 257& Winchester (16) used the reaction 018(d,2n)Fl8 to determine oxygen in quartz, kyaiiit'e. and galena, finding interference from E a and other elements. 13ccause of the rehtively tedious nature of all conventional met.hods for determination of os!.gen in organic and inorganic materials. a method was sought that would enable fast instrumental determination of this element in rock< and minerals, that would be compatible to the x-ray emission method presently under develclpment a t the Nev:tda Mining Analytical Laboratory (13, 14). In silicate rocks and minerals oxygen is the major component. To be of practical value, oxygen determination in these materials has tji be performed
described by Meinke (r), and built in the Oak Ridge Institute of Kuclear Studies is used. Sample transfer time is approximately 0.6 second, a t 25-p.s.i. pressure. Extra-thick, 10-curie, 1.3 c. per sq. cm., tritiated titanium target is bombarded by a deuteron beam of 500 pa. accelerated at 150 kv. to produce the reaction H3(d,n)He4. The initially generated flus of 14-m.e.v. neutrons was approximately 7 X lo9n sq. cm./second, with decays of more than two orders of magnitude during use. This neutron flux was measured using the SiB(n,p)Al% reaction and the known cross section for this reaction. The resulting 1.78-
with higher precision than previously achieved by similar neutron activation methods. EXPERIMENTAL
Apparatus. The apparatus consists of a Texas Kuclear neutron generator and control, equipped with the Oak Ridge ion source (8). A Radiation Counter Laboratories 256 channel analyzer and a 3- X 3-inch Harshaw Integral Line S a 1(TI) crystal are used for the detection. The spectra are printed out by an IBM typewriter. A fast transfer system, similar to that I
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7.3 5 0 . 2 seconds
AST NEUTROXS
Present address, Nevada hIining AnaIytic:rl I,aboratory, University of Sevada, Renu. Sev.
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Figure 1.
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Decay curves for N16 in G-1, W - 1 , and H20; 0.73 sec. per channel VOL. 35, NO. 13, DECEMBER 1963
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ESCAPE PEAKS-
4 Figure 2. Superimposed NlGgarnma spectra of water, rocks G-1, W - 1 , and vein quartz, with residual spectra of same substances
1 MIN WAIT G-1, RESIDUAL IO
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B A C m
ENERGY, m , e v
m.e.v. gamma ray was measured with the equipment already described. Samples. The samples analyzed consisted of U. S. Geological Survey standards Granite G-1 and Diabase W-1 ( I I ) , distilled water, ignited anhydrous A1208 and Ti02 certified by Fisher, and natural quartz. Samples were packed tightly in cylindrical containers 2.8 cm. long and 1.5 cm. in diameter with two circular flanges to reduce friction, and a screw top. Volume of the sample was 1 cu. cm. in all cases. These "rabbits" were machined from Marlex, manufactured by Phillips Petroleum Co. of Bartlesville, Okla. The weight of samples varied from 1.2 to 2.1 grams and the oxygen content from 0.5 to 1.1 grams. As oxygen is the major element in these samples, the contribution of the atmospheric oxygen trapped in the container can be ignored. If this container were filled only with air, the contribution of atmospheric oxygen would be less than 0.3 mg. This is below the detection limit of the present method. Procedure. The activation procedure consisted of punch-button pneumatic transfer of the sample, 1-minute irradiation and return of the sample, and accumulative counting for 30 seconds. All samples were irradiated Table 1. Precision of Oxygen Determination in Water by Neutron Activation
Counts corrected for target
Counts/min. decay
124550 Std. dev. 123450 S = 480 counts 123800 123350 Rel. std. dev. 124250 C = 0.39% 124000 123800 Oxygen-S8.89% 123500 124650
127698 125751 125375 124115 124321 123331 122316 121231 121559 ~~~
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ANALYTICAL CHEMISTRY
four times, alternating each time with the same water standard. This way the activation of each sample was spread over a relatively wide range of target decay of 9 minutes. Int,ensity ratios were computed on both sides of the sample by comparison to the activity of distilled water. I n each case the mean of these two readings was used, and values given represent the mean of four such readings. The empty, air filled, Marlex rabbit showed no detectable activity in the oxygen spectrum region, with the counting apparatus used. RESULTS
Possible interference from other elements in the N16 spectrum region from 4 to 8 m.e.v. was investigated in G-1 and W-1 by taking residual spectra after a time lapse of 1, 2, and 3 minutes and comparing them with residual spectra of water. N o residual activity was detected. To assert this, half lives of the K14 activity in rocks G-1 and TV-1 were determined and decay curves compared with that of water. The half life was determined graphically as Ty, = 7.3 =I= 0.2 seconds. for both rocks (Figure 1). In addition, the oxygen spectra of water, G-1, W-1, and vein quartz were superimposed on a logarithmic scale to demonstrate their similarity (Figure 2). Background or residual spectrum for G-1, and the background for 1-second counting in neutron flux are also shown on this figure. Discrimination was set a t approximately the 4-m.e.v. energy level, or channel 55 of the multichannel analyzer. Under these instrumental conditions and with the 0.6-second delays caused by transfer mechanism, the only interfering reactions are F*9 (n, CU)N~S, and Bl1(n,p)Bel1 ( 9 , f O , l @ . All other gamma activities lie in the region of 0 to 3.0 m.e.v.
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SEPARATE R U N S ON WATER
Figure 3. Precision and tritium target decay demonstrated over short period with same water sample
Precision was determined by consecutive runs on distilled water, taking the target decay into account (Table I). From Figure 3 one can see that the target decay can be considered to be linear over a short period [see also Figure 1, in (IO)].The relative standard deviation of 0.39% so achieved, is to be regarded as the ultimate with the nonspecialized equipment used. The main difficulty in some runs was the fluctuating accelerator beam current. Microamperage had to be controlled manually and neutron flux variation could not be avoided. Because of this, deviations from the mean on rock and oxide powders varied from 0.0 to 3% of the amount present, and about 10% of all determinations had to be discarded. These fluctuationa were measured and recorded by an associated alpha particle counting system. In this system, alpha particles of approximately 3 m.e.v. energy are counted by a 0.8-mm.2 silicon surface barrier detector placed 26 inches from the target. This detector is followed by a Tennelec low-noise preamplifier Xodel 100A, and a power supply hIodel 900. The amplified pulses are fed to a Victoreen DD-2 amplifier Model 651A and a single channel analyzer. The amplifier gain is adjusted so that the alpha particle peak is about 50 P.H. units high, and the spectrum integrally counted above 30 P.H. units. I t was found that when the fluctuation occurred during the later stages of the irradiation, even the normalization of results based on the alpha particle count did not produce statistically satisfactory results, sometimes giving deviations of up to 47, from the mean. Some series of runs gave good precision as demonstrated in the case of water. Occasiunally, however, unstable periods of runs resulted, and therefore it was necessary to discard some of the results. This difficulty strongly suggests the use of a dual transfer and counting system to enable simultaneous activation of the standard and the unknown. The rock powders G-1 and W-1, quartz, AlzOa, TiOs, and H20 were packed into the Marlex containers so as to cover a reasonable range of oxygen content. To avoid dead time corrections and other statistical errors the sam-
Table II. Determination of Oxygen in Rocks G-1 and W-1 and Some Oxides by Neutron Activation
No.
Sample
1 20 34 6 51
Hz0 G-1
52
W-1
Quartz AlrO3
TiOs
Sample wt., mg.
Theoretical, mg.
Detd., mg.
1250 2133 2040 1697 1708 1300
1111 1040 915 904 804 521
1110 1035 910 885 815 520
ple weight of rocks G-1 and W-1 was chosen to contain an amount of oxygen comparable with the mater sample. A linear calibration CUI ve was obtained on samples of different density over the range from 500 mg. to 1000 mg. of oxygen, based on intjnsity ratios of sample to water, (Figure 4). Numbers near calibration points indicate number of independent analyse.9. Milligrams of oxygen are given per 1c 1.cm. of powder. The fact that the s:tmples differ in weight and density indicates that the absorption effect of the high energy gammas in these materials can be ignored. This suggests negligible effects due to the density of the sample in this case. Each analysis is based on approximately 120,000 counts with no detectable background. The analytical results3 are compiled in Table 11. These resuks show that with the nonspecialized equipment used it is possible to determine oxygen directly from rock powders with an accuracy of about 5 to 10 p.p.t. The theoretical oxygen content was cdculated on the basis of major oxides on as-received basis as given by Stevens, page 78 (11). Recently Ingamells and Suhr (5) have partly reanalyzed and recalculated the rocks G-1 and W-1, taking into account all trace element dats, available. As expected, these data when converted to equivalent oxygen, do correspond better with the total oxygen d3termined by us,
Oxygen Theoretical,
%
88.88 48.76 44.85 53.27 47.07 40.07
Detd.,
%
88.80 48.52 44.61 52.15 47.72 40.00
giving for G-1 and W-1 48.51 and 44.81% oxygen, respectively. Considering the influence of the adsorbed water on the total oxygen of a rock powder, and the relatively poor precision with present nonspecialized equipment, the difference between these and the neutron activation data has to be considered insignificant. The cost of each determination in terms of the decaying target was about three dollars. The time required for one determination was 5 to 10 minutes. Improved targets, magnetic beam deflection, and operation a t lower yields but with larger crystals can undoubtedly bring the target cost down to 25 to 50 cents per determination. DISCUSSION AND CONCLUSIONS
The instrumental nondestructive neutron activation analysis of oxygen in rocks, minerals, and oxides is a fast and relatively interference-free analytical method. Because of insignificant interference it is comparable to absolute methods. It makes independent checks of totals of oxides in rock and mineral analyses possible. When precision and accuracy of this method are improved by designing a dual transfer and counting system, the determination of the ferrous/ferric ratio in rocks may become feasible. This will depend, however, on our ability to achieve higher accuracy
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Figure 4. Calibration curve for oxygen in rocks G-1 and W-1 by neutron activation
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in the determination of all other constituents. This neutron activation method for oxygen is compatible in speed and precision with the x-ray emission method for other constituents. Currently it appears to be superior in precision and sensitivity to the recently developed oxygen analysis by soft x-rays (1, 4 ) in rocks. The main difficulties of this method result from the short life of nitrogen-16. A fast transfer system and exact timing are therefore required. This is also the main cause for the sensitivity of this method to neutron flux fluctuations. I n geology and petrology the ability of fast total oxygen determination from rocks and minerals makes the comparison of oxygen content of similar rocks a t different depths feasible in connection with deep drilling programs. In difficult chemical analyses of meteoritic material, artificial minerals, or glasses, or other comples compounds, the ability to analyze for oxygen nondestructively brings an independent check that should enable the analyst to interpret the results better. In mineralogy and crystallography stoichiometry problems such as the deficiency of sulfur in certain sulfides or silicon in certain silicates and phosphorus in certain phosphates, for example, may be solved by the determination of total oxygen. LITERATURE CITED
(1) Baird, A. K.,
McIntyre, D. B., Welday, E. E., Geol. SOC. A m . Spec. Papers, in press. ( 2 ) Coleman, R. F., Perkin, J . L., Analyst
84,233 (1959). (3) Guinn, V. P., Johnson, R. A., Mull, G. C., Proc. Sixth World Petroleum Congress, FrankfurtlMain, Sec. 5 , Paper 20 (1963). (4) Henke, B. L., “Advances in X-ray Analysis,” 7, Plenum Press, New York, 1964: ( 5 ) Ingamells, C. O., Suhr, N. H., Geochim. et Cosmochim. Acta 27, KO.8,897 (1963). (6) Lbov, A. A,, Xaumova, I. I., At. Energ. U S S R 6,468 (1959). (7) Meinke, W. W., Univ. of Michigan, DeDt. Chemistry Progress Rept. 10 (lg60-1). (8) Moak, C. D., Reese, H., Jr., Good, W.M., Nucleonics 9,3,18 (1951). (9) Stallwood, R. A., Mott, W. E., Fanale, D. T., AKAL. CHEX. 35, 6 (1963). (10) Steele, E. L., Meinke, W.W., Ibzd., 34, 183 (1962). (11) Stevens. R. E., Geol. Surtev Bull. ‘ 1960, 1113: ( 1 2 ) Venl. n. J.. Cook. C. F., AXAL. ---CHEM.34, 178(i962). ‘ (13) Volborth, -4.,Xevada Bureau of Mines, Rept. 6, A and B , liniversity of h’evada, Reno, Nev., in press, 1963. (14) Volborth. A., Banta, H. E., Geol. . doc. Am. Spec. Papers, in press. (15) Winchester, J. W., Bottino, M. L., ANAL.CHEM.33,472 (1961). RECEIVED for review July 5 ) 1863. Accepted September 18, 1963. Work done under the auspices of the U. 9. Atomic Energy Commission, and supported in ’
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part, by the Sational Science Foundation, Grant No. GP-864. VOL. 35, NO. 13, DECEMBER 1963
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