these studies, the existence of tracer amounts of Sb(0) was not experimentally verified. It is very unlikely that Sb(0) would bc, formed or exist in the H2S04 solution used here. Since a clean separstion of antimony from iodine was essential for our needs, various traps, such as heated copper and other metals, w r e tried. The desiccant Drierite, a ?hydrous calcium sulfate, did by far the best job in removing the iodine aztivity, which was actually volatilized a< antimony iodide. It holds back other volatile fission product activities a!: well. A glass wool plug just befcre the desiccant stops most of the spray and enables one to make several runs before changing the desiccant. Decontamination :Factors. I n the determination of decontamination factors, a measured quantity of the nuclide to be studied was added to the acid and carriers, and arsenic or antimony mas separ,ited. The ratio of activity added to that collected on the arsenic or antimony recovered is the decontamination factor. Table I lists a number of these which were of interest
Table 1.
Decontamination Factors
Contaminant Te I Sn LIixed fission products ( 2 weeks old) As Sb
Arsenic >lo’ 107 2
x
Antimony 4 x 104 3 x 104 > 106 106
20-50 io8
...
in this work. Generally, decontamination factors for arsenic are better than those for antimony, as expected, since arsenic is collected after antimony in the train. Reasons for High Yield. The speed and high yield attained with this method compared to older separations using arsine and stibine are felt to be primarily due to the heating of the zinc and the consequent rapid removal of the hydrides from the solution before they decompose, and to the reduction in volume of acid reacting with the zinc. With a large volume
of acid the stibine map be oxidized to the metal before it escapes from the solution. With a volume of solution just large enough to wet the zinc, the stibine escapes easily, along with the voluminous amount of H ? produced by the reaction of the hot zinc with the sulfuric acid. LITERATURE CITED
(1) Beard, H. C.,“The Radiochemistry of Arsenic,” Natl. Acad. Sciences, Nuclear Science Series, Monograph 3002 (1960). ( 2 ) Greendale, A. E., Love, D. L., “A System for Rapidly Handling an Irradiated Solution,” U. S.Naval Radiological Research Lab., USNRDL-TR601 (Nov. 13, 1962). (3) Hildebrand, I. H., Latimer, mi. M., “Refyrence Book of Inorganic Chemistry, p. 85, Macmillan, New York, 1936. (4) Maeck, W. J., “The Radiochemistry of Antimony,” Natl. Acad. Sciences, Nuclear Science Series, Monograph 3033 (1961). (5) Van Aman, R. E., Hollibaugh, F. D., Kanzelmeyer, J. H., ANAL.CHEM.31, 1783 (1959). RECEIVEDfor review March 12, 1962. Resubmitted January 21, 1963. Accepted February 25,1963.
Estimatioci of Mercury in Biological Material by Neutron Activation Analysis HAMILTON SMITH Deparfmenf o f Forensic Medicine, The University, Glasgow, Scotland, and Wesfern Regional Hospital Board, Regional Physics Deparfmenf, Glasgow, Scotland
b Neutron activation analysis combined with chemical separation is a quick, accurate method for the estimation of mercury in small samples of biological materials. After nitric-sulfuric acid digestion of the activated sample a precipitatilm separation is combined with a gravimetric yield determination. The activity is detected by scintillation counting.
T
HE PRoRLEhIs in the quantitative determination of mercury in biological tissue were: the cestruction of the tissue while retaining mercury in the reaction medium; the development of a simple and specific s e p m ~ t i o nprocedure; a yield recovery eqtiination and final mercury determination. The following outline covered these points and acted as B basis for the invet.tigation. After neutron activation samples were placed in flasks with some inactive carrier and digested with a mixture of nitric and sulfuric acids, a preliminary precipitation was macle using ascorbic acid ‘and the rema ning interfering
materials were removed by a silver precipitation. Finally the mercury was precipitated as mercury iodidecopper ethylene diamine complex. A yield determination was made and the activity was measured and compared with a standard. EXPERIMENTAL
Preparation and Irradiation of Samples. The samples, preferably about 20 mg., were weighed into aluminium or silica tubes which were then sealed. A sample of high purity metallic mercury (about 1 mg.) was weighed into a silica tube which was then sealed. The samples and standard were packed into a standard aluminium can and irradiated in a reactor a t a thermal neutron flux of 1012 neutrons per square centimeter per second for a week. The unit was returned and processed as described below. The standard was dissolved in nitric acid and diluted as necessary. Mercury Isotopes. Two isotopes were available for study by activation analysis. Both mercury-196 and mercury-202 after irradiation for 1
week became reasonably active by neutron capture as shown below. Irn
Hg1g6 (tl/s - 65 hours) and Hg197m (tl/g - 24 hours) gave out rather low energy x-rays which were not so easy to detect as the 0.279-m.e.v. y-rays from Hg203 ( t 1 / 2 - 47 days), so the latter was used. Westermark and Sjostrand ( d ) , however, have used Hg197 with y-ray spectrometry. It was possible to have interference from T1203(n,p) Hg203 with a TI matrix and from Pb206 (n,cr) Hg203 with a Pb matrix. Reagents. Where possible, high purity reagents were used. The complexing agent for mercury was prepared by mixing one part of lOyo copper sulfate solution (w./v.) with 10 parts of 10% 1,2-ethylenediamine (v./y.) * Digestion of Samples. For the success of the separation procedure it was necessary to make sure t h a t any organic mercury compounds were destroyed and yet keep the mercury in solution as a simple compound. This VOL 35, NO. 6, MAY 1963
0
635
Table 1.
Relation of Mercury Precipitation to Acid Concentration
Sulfuric acid,
Nitric acid,
36N
16N
... 2 drops110 drops 1.O ml. over 1 . 5 ml. ... Table II.
Mercury precipitate wet wet acetone dried acetone dried acetone dried
.
Precipitation yes slight no Yes slight
...
...
... ...
...
any volume
no
no
Solution of Mercury in Nitric Acid
Strength of HNOI used 16N 24N < 16N 16A7 24N
was accomplished by a wet digestion technique. The sample and some carrier mercury (10 mg. in 1 ml. of water) with 2 ml. of a mixture of 16N nitric and 36N sulfuric acids in the ratio of 1:1 gave recoveries agreeing within 1% when digested in 25-ml. conicalbottomed flasks with 6-inch necks. If beakers were used the recoveries often fell below 90%. Attempts to digest the sample with 16.V nitric acid alone or with only traces of sulfuric acid present resulted in similar losses, and there was often a greater loss from the active sample than from the added inactive mercury. Initial Separation. The digestion solution was transferred to a 50-ml. centrifuge tube and neutralized with 5 S sodium hydroxide solution. Thereafter it was made acid by adding 3 drops of 1 6 N nitric acid and diluted to 10 ml. After heating in a water bath for a few minutes, the mercury was precipitated as the metal by adding 2 ml. of ascorbic acid solution (1% y./v.). The precipitate was then centrifuged and washed twice with water and once with acetone. The free acid concentration a t this stage was critical as precipitation could be seriously retarded or stopped by too much or the wrong acid. Table I shows the results of adding acids to 10 mg. of mercury in 10 ml. of water, heating, and precipitating with 2 ml. of ascorbic acid solution. Interfering substances were tested for by precipitating in the above conditions. Silver and gold gave precipitates, the following did not: Ba, Be, Bi, Ca, Cd, Co, Cr, Cu, Fe, Xi, Th, T1, Sb, Sn. It was necessary to wash the mercury precipitate carefully and dry after the acetone wash because even small amounts of ascorbic acid or solvent allowed the formation of mercurous salts (Table 11) which caused losses of over 10% in the next separation step. It lvas useful to remove the last traces of acetone by heating for a few seconds on a boiling water bath.
636
...
...
4 drops-1 ml.
1.25 ml. 1 . 5 ml. or more 4 drops 4 drops 4 drops 4 drops
Hydrochloric acid, 12N
ANALYTICAL CHEMISTRY
Solution of mercury yes Yes yes Ye? white solid dissolves on dilution
Mercurous present yes
yes yes no no
Intermediate Precipitation. d b o u t 10 drops of 16N nitric acid (24N may be used) were added to the dry mercury which dissolved and was then diluted to about 10 ml. One milliliter of a 4% (w./v.) silver nitrate solution was added to the mercury solution and well mixed. This was next precipitated by 2 ml. of 10% (w./v.) sodium iodide solution. The precipitate was spun down in a centrifuge and the supernatant was retained. To remove the last traces of silver the solution was filtered and then neutralized with 10% (v./v.) ammonia to congo red end point in preparation for the final precipitation. In this step any silver or gold which came through the primary separation was removed. Sodium iodide was used to precipitate the silver because the excess iodide was required for the formation of the complex mercury compound in the final step. Final Precipitation. Three milliliters of the copper ethylenediamine complex prepared as described earlier were added t o the solution containing mercury iodide ions and the purple copper ethylenediamine-mercury iodide complex separated as a fine crystalline solid ( I ) . The complex was centrifuged then washed twice with water and once with isopropyl alcohol. The complex dissolved in acetone and methyl alcohol, less in ethyl alcohol and very slightly in isopropyl alcohol so the latter was used for washing and for transferring. RESULTS
Recovery and Activity Estimation.
It was possible to calculate the recovery gravimetrically from the 10 mg. of mercury carrier. After washing with isopropyl alcohol the complex precipitate was slurried onto a weighed, stainless steel planchet with isopropyl alcohol and dried under infrared lamps a t 90” C.
The activities of the samples \vere estimated by counting on a scintillation counter. The mercury content was calculated by comparing the recovery weights and count rates with the standard sample. Radiochemical purity was determined by ?-scintillation y’ectrometry and half life considerations. Stability of Complex. The qtability of solid copper ethylenediamine-mercury iodide a t high temperature was investigated by keeping it a t 95” C. for some time and weighing a t intervals. S o physical alteration was noted, the weight remained unchanged for 1 hour. It was possible, therefore, to free the complex of isopropyl alcohol by drying under infrared lamps a t about 90’ C. Sensitivity. The application of activation analysis to mercury microestimations allowed amounts of the order of 10-lC gram to be detected. -1. the half life \vas relatively long, the samples could be proce-.;ed in large batches and their activity detected a day or two later nhen all t l i P qzmples were completed. Eight qeries of trials ivere made on solutions of known niercurl- content, gram to covering the range 2 X 10-7 gram. The relative error \?as between 0.7 and 0.1% varying 11-ith each series, The maximum difference found between an experimental value and the true value v a s 1V0. In a similar set of trials on biological samples of kn0JT-n mercury content the relative error was between 1 and 0.2% varying with each serieq. The maximum difference found between an experimental value and the true value waq 27,. CONCLUSION
This method eliminated blanks other than the standard sample which gave the specific activity of the mercury. Microseparations were avoided by adding inactive mercury carrier in convenient amount.. ACKNOWLEDGMENT
The author thanks John Galister, J. 11.A. Lenihan, and Edgar Rentoul for support and laboratory facilities during the investigation. LITERATURE CITED
(1) Vogel, A. I., “Textbook of Micro and
Semi Micro Qualitative Inorganic -4nalLongmans, Green, and Co., London and New York, 1954. (2) Westermark, T., Sjostrand, B., 1111. J. A p p l . Rad. Isotopes 9 , l (1960). ysis,” 4th ed., p. 220,
RECEIVEDfor review riugust 24, 1962. -4ccepted January 2, 1963. The author thanks the Medical Research Council for the grants during the tenure of which the work was performed.
