NOTES
Cadmium Determination in Biological Tissue by Neutron Activation Analysis K. W. Lieberman’ and H. H. Kramer Union Carbide Corporation, Sterling Forest Research Center, Tuxedo, N . Y. 10987 CADMIUM is present in most human organs o n the order of one ppm (dry weight basis); but, according to Tipton and Cook ( I ) , the concentrations in the human liver and kidney on a dry basis are 6.7 and 130 ppm, respectively. Others have reported 0.08 ppm in the lung ( 2 ) and 0.05 ppm in the heart (3). Cadmium has been determined by neutron activation analysis in various biological materials (4-7). However, only one (7) described a specific radiochemical procedure for cadmium, which is somewhat time-consuming and does not yield radiochemically pure gamma-ray spectra. This note describes a more rapid post irradiation chemical procedure for the determination of cadmium in biological tissue. EXPERIMENTAL
Preparation and Irradiation of Samples and Standards. Frozen-dry human tissue, ranging in weight from 50-100 mg were weighed into high-purity quartz vials (i.d. = 7 mm). The vials were heat sealed. Cadmium standards were prepared by pipetting 100-X aliquots of a standard cadmium solution (8.43 pg CdjlOO X of solution) into high-purity quartz vials. The solution was evaporated to dryness and the vials were heat sealed. The samples and standards were packed into a single standard aluminum can and irradiated for 72 to 100 hours at a thermal neutron flux of 1013 n/cm2 second. The irradiated materials were allowed t o decay for 48-72 hours t o let the radioactivity reach a safe working level. Cadmium Radionuclides. A number of nuclear reactions involving stable cadmium nuclides and thermal-energy neutrons are suitable for the analysis of cadmium (8). From the viewpoints of sensitivity of analysis, time or irradiation, and presence of acceptable gamma rays for measurement, the nuclear reaction l14Cd (n,r) l l C d proved t o be the most satisfactory for biological matrices. Reagents. Dithizone(dipheny1thiocarbazone) solution is prepared by mixing 50 mg of dithizone with 250 ml of chloroform and storing in cool darkness to prevent decomposition. The l W d , used for determining chemical yield, is 1.3 X microcurie/100 X of solution. 1 Present address, Psychiatric Institute, Department of Internal Medicine, 722 West 168th Street, New York, N. Y . 10032
(1) I. H. Tipton and M. J. Cook, Heulth Phys., 9, 103 (1963). (2) S . R. Stitch, Biochem. J., 67,97 (1957). (3) P. 0. Wester, Biochim. Biophys. Acta, 109, 268 (1965). (4) T. Westermark and B. Sjostrand, Znt. J. Appl. Radiat. Isotopes, 9, 78 (1960). (5) H. D. Livingston, H. Smith, and N. Stojanovic, Talanta, 14, 505 (1967). (6) H. J. M. Bowen, Analyst (London), 92, 118 (1967). (7) K. Fritze and R. Robertson, J. Radioanal. Chem., 1, 463 (1968). (8) C. M. Lederer, J. M. Hollander, and I. Perlman, “Table of Isotopes,” 6th Ed., Wiley and Sons, New York, N. Y . , 1967, pp 55-57. 266
Dissolution of Samples. The soft tissue samples were destroyed by wet ashing with concentrated H2S04in a charring step and solid (NH4)&08 in a n oxidation step. In detail, 25 pg of cadmium and zinc and 100 X of lo9Cdtracer were introduced into a 50-ml beaker containing 8 ml of concentrated H2S04. The acid was swirled gently (to assure homogeneous distribution of the carriers and the tracer) and then heated on a hot plate until SOz fumes emanated. An irradiated tissue sample was added to the fuming acid. Solid (NH4)&Os were added slowly, in approximately 0.5 gm increments, to the dark solution until it became colorless or tinged slightly yellow (depending on the kind of tissue). The time for dissolution ranged from six to eight minutes, It is not known in what chemical combination cadmium enters into tissue; however, it is probably as CdCl?, the chloride being the most common anion in human tissue. The melting point of CdCl, is 868 “C and the boiling point 980 O C (9). The maximum temperature attained during irradiation was 66 “C, during wet ashing, 313 “C; both temperatures are well below the volatilization point. Previous workers (IO, 11) state that cadmium is lost by volatilization when biological materials are dry ashed, not when they are wet ashed. In a n attempt t o confirm their statement, three unirradiated tissue samples with known activities of lo9Cd tracer were subjected to the H2S01-(NH4),S20s dissolution procedure. The samples were cooled, NaOH added, and the 10gCd activities were determined. Cadmium recoveries were 99.9%, 101 %, and 98.5%, which indicated no loss. There is no reason to expect cadmium should be lost due to volatilization in the dissolution step. Separation of Gross Radioactive Interferences. The dissolved tissue sample was cooled in air for 1-2 minutes and placed in an ice bath for several minutes. Fifty milliliters of distilled water were added slowly to the solution, and the p H was adjusted t o between 1-2 with approximately 15 ml of 50% NaOH. The beaker was kept in the ice bath to minimize the heat produced by the exothermic interaction between the solution and the NaOH. The solution was transferred to a 250-ml separatory funnel and extracted with 30 ml of dithizone solution. The organic phase was drawn off and rejected, and the aqueous phase was washed with successive 20-ml aliquots of chloroform until the chloroform layer was colorless. Extraction at p H of 1-2 removes gross radioactive interferences. Isolation of Cadmium and Zinc. The aqueous phase was transferred to a 150-ml beaker and 10 ml of a 15% tartaric acid solution was added. The p H was adjusted to 13-14 with about 5 ml of 50% NaOH. A second extraction was done with 30 ml of the dithizone solution. The organic layer (a brilliant red color) was withdrawn and saved. Suc(9) F. A. Cotton and G. Wilkinson, “Advanced Inorganic Chemistry,” Interscience, New York, N. Y., 1962, p 476. (10) J. Cholak and D. M. Hubbard, IND.ENG.CHEM.,ANAL.ED., 16, 333 (1944). (11) A. K. Klein and H. J. Wichmann, J. Ass. Ofic. Agr. Chem., 28, 257 (1945).
ANALYTICAL CHEMISTRY, VOL. 42, NO. 2, FEBRUARY 1970
cessive extractions were made with 20-ml aliquots of dithizone solution until the organic layer was pale red, indicating the end of the extraction. All of the organic layers were combined, and the aqueous layer was discarded. The tartaric acid served as a masking agent fcr possible remnants of any interferences not removed by the earlier acid extractions with dithizone. The extraction with dithizone was done at pH 13-14 because of the stability of cadmium dithizonate and instability of zinc dithizonate in very basic solution. Separation of Cadmium from Zinc. The collected organic layers were transferred to a 250-ml separatory funnel and washed with 30 ml of distilled water to remove any traces of entrained 24N2. The cadmium and zinc were backextracted into the aqueous phase with 10 ml of 1 M HCI. Four grams of NaSCN were dissolved into the solution containing the cadmium and zinc, and zinc was separated from the cadmium by extraction into a 1:4 mixture of isoamyl alcohol and anhydrous diethyl ether. The extraction of the zinc into etheral isoamyl alcohol was done three times, using 20-ml, 10-ml, and 10-ml aliquots. One molar HC1 back-extracts cadmium and zinc dithizonates rapidly and quantitatively. Cadmium was separated frolm the zinc by the preferential solubility of what is theorized to be zinc thiocyanate in the etheral isoamyl alcohol. Preparation of Standards and Processed Samples for Counting. Standards were prepared for counting by rinsing the irradiated vial 4-5 times with 0.1N HC1 and decanting each time carefully into a counting vial. The volume of the decantate in the counting vial was brought up to 25 ml. The processed tissue samples were prepared for counting by transferring the aqueous solution (which contained the radiochemically pure 11Cd) from a separatory funnel to a counting vial and diluting up to a final volume of 25 ml. Measurement of the ll5Cd Activity. The l W d activities in the samples and standards were determined by counting the combined 0.53-0.49-MeV gamma-ray photopeak. Counting
times ranged from 10 to 30 minutes. The only gamma rays present in the sample were due to 115Cd-11jInm and 109Cd. N o other radionuclides were observed. Determination of Chemical Yield. The chemical yield of the cadmium was determined by comparing the net activities of the 88 keV gamma-ray photopeak of the lo9Cd standard and that of the lo9Cdtracer that had been introduced into the irradiated tissue sample at the beginning of the radiochemical separation procedure. Chemical yields of the cadmium ranged between 50-65 %. Cadmium-109 was a suitable radionuclide to serve as a chemical yield indicator because of its long half-life (453 days) and because it emits a gamma-ray at an energy not in conflict with any gamma-ray emission characteristics of lljCd. In addition, the level of lo9Cd activity introduced to the sample is high enough so that the induced I W d activity does not interfere with lo9Cdmeasurement. RESULTS
Twelve tissue samples from the same postmortem were analyzed for cadmium; specifically, five thigh-muscle, four kidney, and three lung samples. The five thigh muscle samples gave results of 0.35 f 0.03, 0.35 f 0.02, 1.00 f 0.06, 0.27 f 0.01, and 0.37 5 0.01 ppm cadmium (dry weight basis). The four kidney samples contained 450 i 10, 185 =t 5, 270 f 5, and 460 10 ppm cadmium; and the three lung samples 4.7 =t 0.4, 3.5 f 0.3, and 4.9 =t1.1 ppm cadmium. The associated errors are calculated from counting statistics only. The expected accuracy is about 5%. A practical lower limit of detection of the technique is 50 ppb cadmium when using a neutron flux of 1 X 1013m/cm2 sec, an irradiation period of 100 hours, and a decay period of 4 days. RECEIVED for review August 12, 1969. Accepted November 7,1969.
Neutron Activation Analysis of Thorium in Rocks and Ores by Multiple y-Ray Peak Ratio Determination Mariana Mantel, Propai Sung-Tung,’ and S a a d i a Amiel Nuclear Chemistry Department, Soreq Nuclear Research Centre, Yavne, Israel ACTIVATION ANALYSIS of thorium is mostly based on the nuclear reaction : P
232Th (n,y)Z33Th 9 233Pa 22.4min
6 27.0d
The determination of trace amounts of thorium as found in nature in rocks and ores usually involves chemical processing of the irradiated samples, with subsequent measurement of *33Th (1-3) or 233Pa(4-6) by beta counting or gamma scintillation spectrometry. However, in view of the inconvenience 1 Present address, Office of Atomic Energy for Peace, Bangkok, Thailand.
(1) E. N. Jen!cins, Analyst (London), 80, 301 (1955). Appl. . Radiat. Isotopes, (2) G. W. Smith and D. M. Morgan, Int. .I 16, 81 (1965). (3) H. Stark and C. Turkowsky, Radiochim. Acta, 5, 16 (1966). (4) Y.C. Schiltz and C. Coquema, BUN. Soc. Fr. Mitieral Crist., 87, 156 (1964); Anal. Abstr., 12, 2131 (1965). (5) G. L. Bate, Y . R. Huizenga, and H. A. Portratz, Geockim. Cosmochim. Acta, 16,88 (1969). (6) Wakita Hiroshi and Kigoshi Kunihiko, J. Chem. Soc. Japan, Pure Chem. Sect., 85, 476 (1964); Atial. Absfr., 12, 6589 (1965).
of performing chemical separations [the common procedures are paper chromatography ( I , 4, 7), solvent extraction (5,6) o r ion exchange (8, 2 , 311 in rock analysis, especially when handling many samples, a n instrumental analysis would be of great advantage. Nondestructive activation analysis techniques reported so far are delayed neutron counting from thorium fission by Amiel (9), and high resolution lithium-drifted-germanium y-ray spectrometry as mentioned recently by Gordon et a/. (10) and Olin and Sayre (11). (7) R. Coulomb and Y . C. Schiltz, “Radiochemical Methods of Analysis,” Vol. 11, Vienna, 1965, 177. (8) Y . W. Morgan and Y . F. Lovering, Atzal. Chim. Acta, 28, 405 (1963). (9) S. Amiel, A N ~ LCHEM., . 34, 1683 (1962). (10) E. G. Gordon, K. Randle, E. G. Coles, B. Y . Corliss, H. M. Beeson, and S. S . Oxley, Geoclzim. Cosmochim. Acta, 32, 369 (1968). (11) S. J. O h and V. E. Sayre, International Conference of Modern Trends in Activation Analysis, National Bureau of Standards, Gaithersburg, Md., October 1968, pp 207-21 3.
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