ANALYTICAL CHEMISTRY, VOL. 51, N O . 2, FEBRUARY 1979
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Solvent Extraction Method for Determination of Thorium in Soft Tissues Narayani P. Singh," Shawki Amin Ibrahim, Norman Cohen, and McDonald E. Wrenn Institute of Environmental Medicine, New York University Medical Center, 550 First Avenue, New York, New York
A simple, precise and accurate analytical technique has been developed for the determination of thorium isotopes in soft tissues. The method consists of preliminary nitric acid digestion of tissues after adding 229Thtracer, followed by a mixture of nitric and sulfuric acid with occasional addition of hydrogen peroxide; thorium is then coprecipitated with iron carrier by ammonium hydroxide. The precipitate is washed until free of sulfate ions, dissolved in 1:l HNO, and finally adjusted to 4 M HNO,. Thorium is extracted twice into 25% trilaurylamine (TLA) in xylene (pre-equilibrated with 4 M HNO,) and backwashed twice with 10 M HCI. The aqueous phase is evaporated to almost dryness, treated with H2S04with frequent addition of a few drops of HNO,, and electrodeposited onto a platinum planchet prior to a spectrometry with a surfacebarrier silicon detector. The final total recovery ranged from 24-93% with a mean of 6 5 % in 28 samples. Yield appeared to be independent of total iron when 10 to 100 mg Fe were added, and independent of the amount of added tracer. The natural 22&Thcontent of three different beef liver samples was 1.3,1.4, and 3.0 pCi/kg wet weight.
T h e radiological impact of plutonium contamination in t h e environment has been discussed widely but thorium, another actinide element, chemically a n d biologically similar t o plutonium, has been largely ignored. This may be due t o t h e fact that t h e radiation dose t o human tissues from natural thorium is significantly lower t h a n t h a t from other natural a-emitting nuclides. Natural thorium is distributed widely in our environment; it is essential t o q u a n t i t a t e t h e thorium content of human tissues because of our need t o be able t o evaluate t h e accumulation of this element by man from his environment. T o 230Th,a n d assess t h e content of t h e thorium nuclides 228Th, 232Thin human tissues, a new analytical technique has been developed. T h e methods available for t h e determination of thorium in tissues are limited. Petrow e t al. ( 1 ) described a n indirect technique for determining 228Th which takes a considerable amount of time and is limited to only one isotope of thorium. Another procedure described by Petrow a n d Strehlow (2) is a colorimetric method for total thorium a n d again does not measure t h e isotopic composition. Sill ( 3 , 4 ) has reported techniques for determining thorium in soil a n d ore samples a n d Percival and Martin ( 5 ) developed a method for assay of thorium isotopes in environmental a n d process waste samples. N o method has been available, however, for t h e multiple isotopic determination of thorium in biological tissues. Therefore, a simple, precise a n d sensitive analytical technique h a s been developed in this laboratory for t h e simultaneous determination of all a-emitting isotopes in tissues. Trilauryl amine is chosen as t h e extracting agent because of its successful use for t h e determination of plutonium in soft tissues (6). EXPERIMENTAL Reagents a n d A p p a r a t u s . All the reagents used are of analytical grade. Trilauryl amine (TLA), Matheson, Coleman and 0003-2700/79/0351-0207$01 .OO/O
10016
Bell manufacturing chemicals (MC/B); a 25%' TLA solution is prepared in xylene and shaken with '/ volume of 4 M HNO, for 10 min. The organic phase is separated and centrifuged before use. A stock solution of TLA cannot be stored since it begins deteriorating after 24 h. Once it is equilibrated with 4 M HNO,, it must be used the same day. Dilute acids such as 2 M H2S04, 3 M HNO?, and 10 M HC1 are prepared by appropriate dilution of concentrated acids with deionized, distilled water. Thorium-229 tracer (this tracer has 8 . 1 7 ~228Thon activity basis), methyl red indicator, platinum planchet, nickel disks, electrolytic cell, electrolb-tic analyzer (motor driven platinum electrode and a power supply) were used. Sample Preparation. Transfer 500-1000 g of tissue to a 4-L beaker and add 1-2 dpm ' q h tracer. The suitability of this tracer has been demonstrated in our earlier work ( 7 ) . Add just enough concentrated nitric acid to immerse the tissue, and cover the beaker with a watch glass. Heat gently on a hot plate with magnetic stirrer until frothing ceases. Raise the temperature slowly to approximately 100 "C and continue heating until the volume is reduced to approximately 100 mL. Heat a t a higher temperature while occasionally adding concentrated nitric acid (a few drops a t a time) until a clear solution is obtained. Add 200 mL 1:l HNO, and H2S0, mixture and heat vigorously until all the nitric acid is driven off. Add a few drops of "OB occasionally with constant heating until a clear colorless solution is obtained ensuring almost complete decomposition of organic materials. (Addition of a few drops of H 2 0 palong with nitric acid helps in faster decomposition of organic materials.) Remove most of the sulfuric acid by evaporation, without going to dryness, before proceeding further. In case of lung and lymph nodes, heat the tissue samples further with HF, after HN03-H2S04digestion, and then remove H F by continuous heating. P r o c e d u r e . Add 200 mL of 1:3 HCI to the clear solution of the tissue and boil for several minutes. Cool and add 10 mg of iron carrier (as 1 mL FeCI3) and swirl the beaker for proper mixing. Add concentrated ammonium hydroxide very gently until precipitation is complete. The precipitate must be allowed to settle completely overnight to avoid any loss of Fe(OH), precipitate. Remove the supernatent by centrifuging the precipitate in a 50-mL centrifuge tube. Dissolve the precipitate in 4-5 mL of concentrated HNO, and reprecipitate Fe(OH), with ammonia. Thorium is coprecipitated with iron along with some other metals present in the tissues. Repeat the precipitation and dissolution until the supernatent. after Fe(OH), precipitations, is free of sulfate ions. Dissolve the precipitate in a minimum volume of concentrated nitric acid and determine the acidity of the solution by titrating an aliquot (100 pL) against a standard sodium hydroxide (0.1 N) solution. Adjust the acidity to 4 M by adding a calculated volume of nitric acid of appropriate concentration. Solvent Extraction. Extract thorium from the solution, obtained after dissolving the Fe(OH)3precipitate in "OB (acidity adjusted to 4M), with an equal volume of 2570 TLA solution in xylene by shaking for 10 min in a 50-mL polyethylene tube (TLA solution was equilibrated with 4 M HY03 before use). Centrifuge for 10 min and remove the aqueous phase into another 50-mL polyethylene tube. Extract the aqueous phase once again with 25% TLA for 10 min and centrifuge for 10 min. Separate and discard the aqueous phase. Mix the organic phases from the first and second extraction and backwash thorium from the TLA phase by shaking it with 10 M HCI for 10 min (volume ratio 1:l). Centrifuge the tube and transfer the aqueous phase into a 100-mL beaker. Repeat the backwashing once again from the TLA phase with 10 M HCI and transfer the aqueous phase to the same beaker. Evaporate this aqueous solution to a smaller volume and then 'C 1979 American Chemical Society
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ANALYTICAL CHEMISTRY, VOL. 51, NO. 2, FEBRUARY 1979
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Table I. Recovery of 12'Th Added to Beef Liver as a Function of Iron Added to the Sample Amount of 12'Th, dpm sample amount of amount of no. tissue, g Fe added, mg added found 6 106 10 2.822 2.63 .: 0.12 9 140 25 2.822 2.54 i 0 . 1 3 10 140 50 2.822 1 . 3 9 I 0.09 11 140 75 2.822 2.20 :0 . 1 2 12 140 100 2.822 1.68 r 0.11
recovery, % 93. 90 i 49 2 78? 59-
4 6 3 4 4
Table 11. Recovery of 'IYThin Beef Liver with Variable Amounts of Added Tracer (""Th) sample no. 1
2 3 4 5 6 7 8
amount of tissue, g 130 130 130 130 106 106
106 106
amount of Fe added, mg 10 10 10 10 10 10
19 10
A4mountof 2 2 Y T hdpm ,
___ added 14.11 5.056 3.527 1.764 1.411 2.822 4.233 5.644
~~
found 3.39 x 0.14 3.51 i 0.14 2.69 :0.15 0 . 4 9 i 0.06 1.06 i 0.09 2.63 i 0.12 3.93 i 0.16 4.55 z 0 . 1 7
recovery, % 24 : 1 501 2
mean add 10 mL of 2 M sulfuric acid and heat. Once the black droplets are seen floating on the surface, add concentrated HKO.; and 3 0 7 ~ HzOz,drop by drop, along the side of the beaker. This addition enhances decomposition of the organic material floating on the surface. Add HNO, occasionally, until all the organics are completely decomposed. Continue heating to almost dryness. Electroplating. A p p a r a t u s . The plating apparatus (8) consists of an elongated 22-mm cap which holds a 1-oz polyethylene bottle with the bottom removed. The cap has space for an 18-mm diameter platinum plating disk and a nickel supporting disk. This may be firmly screwed into the polyethylene bottle forming a leak proof plating cell. A threaded brass brushing is molded into the cap which makes the electrical contact with the platinum disk cathode by clip leads. The entire cell is supported by a heavy brass base which is employed to fix the cell in the ice water bath. The anode is a 1.6-mm platinum-iridium rod, 4 inches long with a half-inch diameter platinum disk riveted at one end. (This dimension is not critical.) The disk is provided with a number of 0.3-mm holes. It is connected through a constant speed stirrer to the positive outlet of the power supply which furnishes a constant current ranging from e 1 0 A and a voltage ranging from 0-36 V. Procedure. After backwashing thorium from the TLA phase with 10 M HCI and decomposing the organic materials entrained with this HC1 solution, evaporate it to dryness. Add 1 mL 2 M HzSO, and heat gently at a low temperature on a hot plate. Transfer this solution to the plating cell. Wash two times with 1 mL 2 M H2S0, and transfer the solution to the plating cell. Add one drop of methyl red indicator and titrate with 1:l ammonia, drop by drop, to a yellow end point. Precautions should be taken not to add an excess of ammonia. Bring back to red by adding 2 M H2S0, dropwise and add 3-4 drops in excess to get the required pH of the plating solution. The total volume of the plating solution should be restricted to 3-4 mL as larger volumes were found to produce poor electroplating recovery. Electroplate thorium at an initial current of 1.2 A for 1 h. Quench the electrolyte with 3-4 drops of ammonium hydroxide at the end of 1 h. Dismantle the cell and rinse the platinum disk with water followed by alcohol. Flame the disk to red heat over a burner. Determine the recovery and the isotopic composition of thorium by counting the disk in an cy spectrometer with a surface barrier silicon diode.
RESULTS AND DISCUSSIONS Since not much information was available on t h e isotopic determination of thorium, a new method was developed in which thorium was extracted into TLA, backwashed with 10 M HC1, converted to sulfate with H2S04and electrodeposited. Accordingly, it was essential t o find t h e most favorable acid concentration for the extraction of thorium since every solvent
76- 4 28 i 4 75: 6 93- 4 93 i 4 81 i 3 65%
extraction system is greatly influenced by acidity. In this system '"Th tracer was extracted into 25% T L A solution in xylene from nitric acid ranging from 1-8 M. T h e best extraction efficiency occurred at 4 M. When the extraction was carried from the beef liver where thorium was coprecipitated with 100 mg Fe carrier, after the complete wet ashing of tissues t h e recovery of thorium was with a mixture of H2S04-HN0,1. reduced. I t was probably due to interference from iron, which was added into the tern for coprecipitation of thorium, or t h e trace metals a able in t h e tissues which are coprecipitated with iron along with thorium. or possibly both. A separate experiment was conducted to investigate whether iron interfered in the extraction a n d . hence, in t h e final recovery of thorium. About 2000 g of tissue were wet ashed until free of all organics a n d t h e volume was made u p t o 1000 mL t o give 100 g of tissuej50 m L of solution. Aliquots of 50 m L were placed into five different beakers a n d spiked with 2.8 d p m of regTh each. Iron was added in amounts varying from 10 t o 100 mg for coprecipitation of thorium, a n d extraction a n d electrodeposition were carried out as described earlier. Comparing the results in Tables I and I1 shows that t h e range of variability in yield with low iron content (10 mg added) exceeded t h e different in yield observed when a range of 10-100 mg F e is added. Hence, the extraction yield does not depend greatly on t h e iron content of t h e samples. One hundred mg of iron is t h e amount present in a b o u t 200 g of normal human blood (9). Accordingly, with tissue masses t o be analyzed u p t o 500 t o 600 g. it is unlikely t h a t t h e endogenous iron content will exceed t h e amount we have added. In all further experiments, therefore, the amount of iron added for coprecipitation of thorium was maintained a t 10 mg. It is also possible that some of the trace metals which might be coprecipitated with iron may interfere in t h e extraction, backwashing, a n d electroplating of thorium which would accordingly differ depending upon the quantity of t h e tissues. Thorium-232 can interfere with t h e electrodeposition process if present in amounts exceeding 100 pg (5). However, this large amount is unlikely in reasonable size samples of human or animal tissue. For example, it is estimated t h a t the total body content of thorium of a normal man is
