as used by Quass ( 8 ) ,oil was added to a slurry of powdered oil shale and water. However, no separation could be obtained unless the oil shale was wet by oil before exposure to water. Preparation of a paste of oil and oil s h d e is a necessary initial step. The carbonate minerals in Green River oil shale inhibit mineral removal by this method and must be destroyed chemically before the concentration procedure is begun. The mechanism of the concentration action is based on preferential wetting of the organic and mineral components by oil and water, respectively. As new surfaces are exposed in agitating the paste, water breaks the oil film on mineral particles, wets them, and incorporates them into suspension in the water phase. Since shearing of the paste is the action that exposes new mineral particles to the water phase, the mixing equipment should be chosen to increase this action. A laboratory model of the Szegvari Attritor, manufactured by the Union Process Co., Akron, Ohio, was used in these tests, Exact proportion of oil and shale in the paste is not critical as long as the
paste is plastic but will not slump or flow. The type of oil used is not critical. Low viscosity, nonvolatile oils will afford separation. Cetane was used in this experimental work because i t has appropriate viscosity and volatility characteristics. It provided satisfactory control of the oil effects in development of this procedure. Progress of the enrichment can be checked approximately by weighing the charged shale and then obtaining a cumulative weight of the mineral matter removed. Comparison of this weight with an amount of mineral estimated from the ash content of the carbonatefree starting material provides an estimate of the remaining mineral. I n Green River oil shale, this progress can also be folloaed by the diminishing and eventual disappearance of the x-ray diffraction peak at 28.0" 28 in the concentrate. This is the major peak of soda feldspar. a removable mineral constituent of oil shale not found in ordinary porcelains. Mineral removal by this method is not complete for two reasons. First, when particles are sufficiently small, water cannot break the oil film to wet
and remove such particles. Second, particles such as pyrite, which are readily wetted by oil, are retained in the paste and are concentrated. For this reason metal components in the mixing equipment should be avoided as a source of contamination. Material lost from porcelain equipment readily enters the water phase and is removed. This organic concentrate represents well the organic material in the raw shale. Two possible alterations in its composition are the extraction of benzcne-soluble portions and the addition of acetic acid. Since this organic concentrate can be produced in quantity and can be analyzed easily by routine methods, it provides a starting material for further study of the nature of the organic matter in Green River oil shale. LITERATURE CITED (1) Himus, G. W., Basak, G. C., Fuel 28, 5 7 4 4 (1949). (2) Quass, F. W., J. Inst. Petrol. 25, 813-19 (1939).
JOHN WARDSMITH L. WARRENHICBY Laramie Petroleum Research Center U. S. Bureau of Mines Laramie, Wyo.
Fluorescent X-Ray Determination of Selenium in Plant Material SIR: Rapid reliable determinations of selenium in plant material are important in areas where the selenium content of forage may be deleterious to grazing animals. Wet chemical methods (8, 5-7) are sensitive enough but too slow for such survey purposes. The method reported here is similar in rationale to that suggested by Brandt and Lazar ( 1 ) but in our experience yielded somewhat more consistent results. Selenium has a K a peak at a wave length of 1.106 -4. Arsenic and bromine with KCYemissions at 1.177 and 1.041 .4.,respectively, are possible interferences, through spectral coincidence. However, with a lithium fluoride anal ~ z i n gcrystal (2d = 4.0269 A.) resolution of the three peaks is satisfactory, 3 e differences of nearly two degrees between selenium and either bromine or arsenic being obtained. The importance of matris effects in the analysis of plant material by sray fluorescent spectrography is apparrJnt in the data of Figure 1 in which the fluorcscent and scattered h-radiation from samples of t a o species, sugar beet .tnd the selenium-accumulating need, \t:inleys p i n a t a , is ompnri.6. Each -'mpli. contained 15 p.p.m. oi selenium. '!?F greater background emission obiuined with the Stank ya sampie is ac-
companied by a proportionally higher emission of selenium K a radiation. The following correction procedure was effective : The scattered primary radiation is measured at two values of 2 8, one on either side of the selenium K a peak. The estimated background radiation at the selenium K a peak is obtained from these values by linear interpolation. This background is then subtracted from the total radiation measured a t the selenium KCYpeak. The difference is due to the presence of selenium in the sample. This value must be corrected
30
further since differences in background due to differences in internal shielding of the sample will be accompanied by a nearly proportional difference in the fluorescent radiation measured. To do this one of the standards of known selenium content is selected for the reference background. The value obtained after subtraction of the estimated background is then multiplied by the ratio of the reference background to the estimated background. The procedure may be represented symbolically as follows:
1
b Figure 1 . Plot of total counts per second as function of 2 0 for plant samples, each containing 15 p.p.m. of Se
+ - + Stanleya
0 - 0 Sugar
beet
VOL. 32, NO. 12, NOVEMBER 1960
1719
Table 1. Selenium Content of Plant Samples Comparison of Analytical Results Obtained by X-Ray Spectrography with Those Obtained by Chemical Method
Sninpic
Se Content, X-Ray Method (P.P.M.)
Mean.
Std. Dev.
Confidence Limits 95%*
Se Content, Chemical Method
6 34 17b 38 44 458 26 31 40 14
8 4, 7 . 3 , 7 . 5 14.3, 14.4, 1 3 . 1 4 2 . 1 , 42.3, 4 4 . 1 71.5, 77.8, 7 4 . 1 86.5, 86.6, 81.7 120.0, 130.8, 125.9 256.5, 255.0, 261.0 267.5, 263.0, 256.5 327.0, 311.0, 316.9 970.0, 910.0, 9 5 8 . a
7.73 13.9 42.8 74.8 84.9 125.6 257.5 262.3 318.3 946.0
0.586 0.724 1.10 3.16 2.80 5.40 3.05 4.75 3.93 31.8
6.279 19 1 2 . 1 - 15.7 40.1-45.5 66.6 - 82.4 7 7 . 9 - 91.9 112.2 - 139.0 249.9 - 265.1 250.5 - 274.1 308.5 - 328.1 867.0 -1025.0
7.2, 8 . 3 , 7.4 11.9, 9 . 6 , 10.5 41.7,39.0,38.2 68.2, 6 7 . 7 , 68.9 78.9, 7 6 . 1 , 72.8 113.3, 114.8 230.2, 233.3, 245.7 245.4, 268.2, 264.1 313.1, 324.6, 327.0 902.3, 970.5, 938.8
a
Mean* Std. Dev. 7.63 10.7 39.6 68.3 75.9 114.0 236.4 259.2 321.6 937.2
0.586 1.16 1.52 0.19 3.05 1.06 8.20 12.2 7.43 34.1
Confidence Limits 950/0b 7.487.78 7.80- 13.6 3 5 . 8 - 43.4 67.8 - 68.8 6 8 . 3 - 83.5 104.4 - 123.6 216.0 - 256.8 228.9 - 289.5 303.2 - 340.0 952.5 -1021.9
Mean values for 3 replications.
* Terminology of Brownlee ( 2 ) .
(x- B,)
X
3 = corrected reading BZ
11-here X
total radiation emitted by unknown sample at selenium KC^ peak (2 e = 31.88’ with LiF analyzing crystal) B , = estimated background radiation of unknown sample B , = background of reference sample =
Although differences in the scattering power of disks composed of the same material are very small and probably fortuitous, it is good practice to correct the values obtained for standard disks the same way. The value chosen for the reference background is wholly arbitrary. Any number would serve this purpose. I n practice, however, the estimated background of the standard containing no selenium was used. Since the true background of this sample can be measured directly, the accuracy of the interpolation procedure may thus be checked. I n selecting values of 2 e to be used in estimating the background, the angles chosen should be far enough removed from the selenium K a peak so that the radiation measured is entirely free of selenium fluorescence. Otherwise, spuriously high backgrounds will be recorded. For the same reason, the plant material must be essentially free of any elements (except selenium) represented by fluorescent peaks occurring in the portion of the spectrum included between the two background angles. Angles of 2 e of 30.6” and 33.4” satisfied these requirements. EXPERIMENTAL
Apparatus. A General Electric S R D - 5 x-ray unit 15ith an argonfilled Geiger tube (Korth American Phillips KO. 62019) and a lithium fluoride analyzing crystal were used. The x-ray tube was a Rfachlett -4.E.G. 50 T with a molybdenum target operated at 52.5 kv. and 45 ma. The Geiger tube was used in prefer1720
ANALYTICAL CHEMISTRY
ence t o a proportional counter because i t was found that with the latter interference caused by line-borne electrical transients resulted in very erratic data, particularly with samples low in selenium. Collimation of the fluorescent beam was accomplished by 0.005-inch Soller slits. Sample Preparation. Plant material dried for 24 hours at 60’ C. was ground t o pass the 40-mesh screen of the intermediate Wiley mill. One gram was pressed into a disk of 1-inch diameter using a n aluminum window shade roller cap to hold t h e sample and a laboratory hydraulic press. Pressure used was not critical within the limits of 2000 t o 16,000 p.s.i. Standards were prepared by adding known amounts of selenium as a solution of sodium selenite to accurately weighed quantities of ground beet material known to be originally free of selenium. After addition of the solution, the material was dried at 60’ C. in a vacuum oven, ground to uniformity in a mechanical mortar, mixed again by the Wig-L-Bug, and finally formed into disks containing 1 gram of plant material as described previously. Chemicsl estimation of selenium was made by the method of Williams and Lakin (6). RESULTS A N D DISCUSSION
I n the range of 0 to 300 p.p.m. a n essentially linear relationship was obtained between corrected counts per second and known selenium content, the C.P.S. being approximately 0.32. ratio D X Deviation from linearity occurred when the selenium content exceeded 300p.p.m. because of coincidence in the counting system. Statistical analysis ( 4 ) indicates that the probablc error will be less than 10% for a sample containing 10 p.p.m. selenium when 10,000 total counts are recorded. The time necessary for this determination including background estimation is less than 20 minutes.
Because of the relatively high background the probable error rapidly becomes very high as the concentration of selenium decreases below 10 p.p.m., excessively long periods of counting being required to reduce i t to a reasonable figure. A comparison of x-ray spectrographic and chemical determinations of the selenium content of field samples of Stanleya pinnata is shown in Table I. I n preparing this table chemical analyses of individual disks were made. The x-ray method yields somewhat higher results in nearly all cases, the discrepancy being greater than the selenium content is small. In investigating this i t was found that the chemical method yielded erroneously low results with samples low in selenium. Losses apparently occurred chiefly in the filtration of the precipitated elemental selenium rather than in volatilization during wet digestion. For this reason the x-ray values are believed to be the more reliable. Agreement was good a t the higher levels of selenium content.
LITERATURE CITED
(1) Brandt, C. S.,Lazar, V. A,, J . Agr. Food Chem. 6,306 (1958). (2) Brownlee, I(. A,, “Industrial Experimentation,” pp. 32-6, Chemical Publishing Co., Brooklyn, N. Y., 1949. (3) Handley, R., Johnson, C. M,,ANAL. CHEM.31,2105 (1959). (4) Klug, H. P., Alexander, L. E., “1-Ray Diffraction Procedures for Polycrystalline and Amorphous hlaterials,” pp. 270-274, Wiley, S e w York, 1954. (5) Robinson, W. O., Dudley, H. C.,
Williams, K. T., Byers, H. G., I ~ D . ENG.CHEM.,ANAL.ED.6, 274 (1934). (6) Trelease, Y. F., Beath, 0. A,, “Selenium,” pp. 253-62, publ. by the authors, New York, 1949. (7) Williams, K. T , Lakin, 13. W., I N U . ENG. CHEM. ANAL ED., ’7,409 (1935). RAYMOSD HANDLEY Department of Soils and Plant Nutritidn University of California Berkeley, Calif.
