Although infrared spectra were also obtained for peaks 2 , 7 to 11,14, and 15, the characteristics were not sufficiently distinct to claim tentative identification.
(2) Johnstone, R. A. W., Quan, P. M., J . Chem. SOC.1963, p. 2221. (3) Johnstone, R. A. W., Quan, P. M., Carruthers, W., Nature 195, 1267 (1962). (4) Kelliher, J. M., Brown, R. A., Abstracts, p. 28, Eastern Analytical Sym-
posium, New York City, November
LITERATURE CITED
(1) Cook, J. W., Johnst'one, R. A. W., Quan, P. M., Israel J. Chem. 1 , 356 (1963).
1964.
(5)-0sman, S., Barson, J., Tobacco Sci.
8, 158 (1964). (6) Stedman, R. L., Miller, R. L., J . Chromalog. 11, 409 (1963).
(7) Wilks, P. A., Jr., Brown, R. A., ANAL. CHEM.36, 1896 (1964).
IRWIN SCHMELTZ C. D. STILLS W. J. CHAMBERLAIN R. L. STEDMAN Eastern Utilization Research and Development Division Agricultural Research Service U. S. Department of Agriculture Philadelphia, Pa. 19118
Determination of Mercury in Wheat and Tobacco Leaf by Neutron Activation Analysis Using Mercury-197 and a Simple Exchange Separation SIR: We report a method for determination of mercury by neutron activation analysis which is more rapid and potentially more sensitive than methods previously described (3, 4,6, 9-11). None of the other methods appears suitable for a rapid and routine analytical procedure. In this study, a mercury exchange method was developed and applied to the separation of mercury from irradiated samples of wheat and tobacco. The method is highly selective so that further purification is not required. EXPERIMENTAL
Apparatus. Samples were counted using a R I D L 200 channel analyzer equipped with a 2- X 2-inch NaI(T1) well type detector (1- X 11/2-inch well size). The reflux condenser used was a Friedrichs Drip Tip type T 24/40; total length, 350 mm. A standard solution of mercury was prepared by dissolving a known weight of mercuric oxide in reagent grade concentrated nitric acid and diluting with demineralized water to give 48.2 pg. per ml. of mercury in a final concentration of O.1N nitric acid. The ground wheat used was supplied by the U. S. Food and Drug Administration. Tobacco leaf was obtained from the College of Agriculture, University of Maryland; the leaf was dried in an oven a t 45' C. for two weeks prior to analysis. Sample Irradiation. A weighed sample was transferred to a polyethylene vial in accordance with the procedure of Kim and Meinke (5). Mercury evaporation (9, 11) was minimized by coating the vials with paraffin, and the temperature of the reactor a t the loading site was maintained a t less than 30' C. ( 1 ) . The vials were positioned side by side within a polyethylene bottle along bith a vial containing the standard mercuric nitrate solution and irradiated in the University of Maryland reactor. The peak thermal neutron flux at the sample position was 1.4 X 10" neutron cm.-* set.-'
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ANALYTICAL CHEMISTRY
Rocedure. The irradiated sample was transferred to a 50-ml. round bottomed distilling flask containing 3 ml. of concentrated nitric acid. About 50 pg. of mercury carrier were added and the flask was connected to a reflux condenser. The sample was refluxed for 20 minutes using a Glas-Col heater. A 1- to 1.5-ml. portion of concentrated sulfuric acid was added through the side arm of the condenser, and heating was continued for an additional 25 minutes. The flask was cooled with an ice bath, and disconnected from the condenser. The outside of the flask was warmed to about 80" C. to eliminate nitrogen dioxide and was then replaced in the ice bat#h. The solution was diluted with 2 ml. of water and about 5 ml. of concentrated ammonium hydroxide were added dropwise until a solution with a pH of 1 to 2 was obtained. The solution was transferred to a 30-ml. Boston bottle and 0.5 gram of ammonium bromide was added; the bottle was shaken to dissolve the salt. A 0.050-ml. portion of triple-distilled mercury was added with a microliter pipet; the bottle was capped and shaken for 5 miputes. The mercury droplet was separated from the solution by passing the mixture through a fritted glass disk. The mercury was washed with water and acetone, and the droplet was transferred to a Lusteroid centrifuge tube (1 X 3.5 inches) for counting purposes. The mercury droplet was dissolved by adding 6 to 7 drops of concentrated nitric acid; volume was made to 10 ml. with water and counted in the spectrometer. The 68- to 77-k.e.v. photopeak region was measured to determine the mercury con tent .
Table 1. Activation Analysis of Tobacco Sample for Mercury
Wt. of sample, gram
Mercury found, gram
0.543 0.560 0.602
0.21 X 0.26 X loeB 0.27 X lod6
0
0.39 0.46 0.45 0.43 f 0,03"
Standard deviation.
Table 11. Activation Analysis of Wheat Sample for Mercury Wt. of
sample irradiated, gram
Level of Mercury Hg in found, gram sample, p.p.m.
0.634 0.779 0.872 0.738 0.872
5.9 X 5 . 5 X loT8 5.8 X 4.8 X 6 . 0 X lod8
5
0.093 0.071 0.066 0.065 0.069 0.073 zt 0.010"
Standard deviation.
Table 111. Activation Analysis of Ground Wheat Samples (0.1 p.p.m. of Hg added) Wt. of
sample irradiated, gram
0.6798 0.7102 0.9010
RESULTS AND DISCUSSION a
Results obtained from the neutron activat,ion analysis of tobacco and wheat samples are summarized in Tables I and 11, respectively. Table I11 shows analytical data for the wheat sample (original Hg content, 0.073 p.p.m.) with 0.10 p.p.m. of Hg added as N-(ethylmercury) p-toluene sulfonanilide; the
Level of Hg in sample, p.p.m.
Level of Hg in sample found 0.16 0.17 0.18 0.17 f O.0lo
Standard deviation.
expected value of 0.17 p.p.m. was obtained. Other investigators suggested that the loss of mercury during acid digestion could be considerable (8, 9). This
Table IV. Mercury Recovery in Acid Digestion and Overall Radiochemical Separation
Recovery
of mercury
Recovery in acid of mercury digestion, % exchange, % ’ 93 94 91 91 93
98 95 98 97 97
Overall chemical yield 91 89 89 89 90
>
50
5a 40
9 30
5
20
W
IO 0 0
study showed that the mercury recovery was incomplete when an open vessel was employed, but that recovery was consistently about 90% when the acid digestion was carried out in the reflux system (data are shown in Table IV). All results in Tables I to I11 include this correction. The essence of the separation procedure is the use of an insoluble mercury droplet to remove dissolved radioactive mercury from an aqueous, mildly acid solution. The principles of this heterogeneous exchange reaction are similar to those applied by DeVoe, Kim, and Meinke (g) in their use of amalgams of various metals to remove radioactive isotopes of the metals from solution. The amount of mercury used (0.050 ml.) was large enough to swamp the trivial quantity of radioactive mercury in the solution, but small enough so that self-absorption of the soft photon emissions of Hg197 are negligible. Several problems were encountered in the development of a rapid, efficient exchange technique. The choice of conditions is restricted by the tendency of mercury to be attacked by acidic oxidizing media, and the tendency for its dissolved salts to precipitate in alkaline solutions. The problem was investigated with Hgzostracer solutions. The mercury attack is not significant provided that nitrogen dioxide removal following the digestion step is complete, and the pH during the exchange step is greater than 0.5. Furthermore, the addition of 0.5 gram of ammonium bromide accelerates the exchange. The effect of pH was studied and, as a result,
50
100 150 ENERGY (KeV)
200
250
Figure 1. Combined gamma-ray spectrum of 65-hour Hglg7 separated from wheat sample 0 Top curve was taken immediately after separation A Middle curve 87.8 hours later Bottom curve taken 6 9 hours after middle curve
pH 1.5 was established as the optimum. The effect of shaking time was studied using a 15-ml. solution a t p H 1.5 containing ammonium bromide and a 0.050-ml. mercury droplet; under these conditions a shaking period of 5 minutes was adequate. Hg197 was chosen for counting because of the large cross section (3000 barns) (7) for its production and its convenient half-life. These advantages far outweighed the rather small abundance (0.146y0) of the target isotope, Hglg6. Hg197decays by electron capture; consequently, a photopeak a t 68 k.e.v. resulting from the Au K-x-ray is in the spectrum, but it cannot be resolved from the 77-k.e.v. y-ray photopeak. The composite peak, however, forms the basis of a reliable method for counting Hglg7. Figure 1 shows a typical Hg197 spectrum obtained after separation from a wheat sample. Because of the high absorption coefficient of liquid mercury for the soft emissions of Hglg7,the transmission of photons from the 0.050-ml. mercury droplet is only 13%. To improve counting efficiency by increasing the surface-to-volume ratio, the sample was dissolved in concentrated nitric acid and diluted with water to 10.0 ml. The dissolution in nitric acid improves the counting efficiency by a factor of 3.2.
LITERATURE CITED
(1) Allis-Chalmers Manufacturing
Co., Safeguards Evaluation of University of Maryland Reactor, UMNE-1, Washington, D. C., Feb. 7, 1960. (2) DeVoe, J. R., Kim, C. K., Meinke, W. W., Talanta 3, 298 (1960). (3) Hamagauchi, H., Kuroda, R., Hosohara, K., J. Atomic Energy SOC.,Japan 2 (6), 317 (1960). ( 4 ) Jacobs, M. S., Yamaguchi, S., Gold-
water, L. T., Gilbert, H., Am. Ind. Hyg.Assoc. J . 21, 475 (1960). (5) Kim, C., Meinke, W. W., Talanta 10, 83 (1963). (6) . . Lindstrom., 0.. , ANAL.CHEM.31. 461 (1959). (7) Mangal, S. K., Gill, P. S., Nucl. Phys. 41, 372 (1963). (8) Roesmer, J., Kruger, P., NAS-NS 3026 (1960). (9) Sjostrand, B., ANAL. CHEM.36, 814 (1964). (10) Smith, H., Ibid., 35, 635 (1963). (11) Westermark, T., Sjostrand, B., J. Appl. Rad. Isotope 9, 1 (1960).
CHONG K. KIM JOSEPH SILVERMAN Department of Chemical Engineering University of Maryland College Park, Md. WORKsupported by the Division of Isotopes Development of the U. S. Atomic Energy Commission and the National Bureau of Standards.
VOL. 37, NO. 12, NOVEMBER 1965
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