Determination of Plutonium in Biological Materials by Extraction and Liquid Scintillation Counting R. F. Keough and G . J. Powers Battelle Memorial Institute, Pacific Northwest Laboratory, Richland. Wash. 99352 PLUTON~UM metabolism and toxicity studies in this laboratory require the analysis of numerous samples of animal origin containing concentrations of plutonium too low to be directly counted on planchets. Previously the plutonium in these samples was concentrated by a thenoyltrifluoroacetone (TTA) extraction after coprecipitation with lanthanum fluoride (I). A method has now been developed which provides significant improvements in speed, precision, and recoveries. Alkyl-substituted phosphoric acids have been extensively investigated as extractants for nuclear fuel processing. Recently they have found increased analytical applications. Horrocks and Studier (2) observed that the alpha radiation from Za6Pu could be counted with 100% efficiency by the liquid scintillation technique in a scintillation solution containing dibutyl phosphoric acid (DBP). Toribara and coworkers (3) have counted plutonium in a single phase system consisting of aqueous sample, ethanol, and toluene scintillator. Ludwick ( 4 ) has counted plutonium in a scintillator solution of DBP after multiple extractions from hydrochloric acid solution. Extensive chemical manipulations were required after ashing and dissolution in all cases where biological samples were involved. The method described here utilizes the direct extraction of plutonium from the acidic solution of sample ash into a toluene solution of di(2-ethylhexyl) phosphoric acid (D2EHPA) and scintillators. Carrying out the extraction in the counting vial, by virtue of its simplicity, increases precision and speed while reducing the opportunities for cross-contamination. Liquid scintillation counting is done with both phases present in the vial.
EXPERIMENTAL Apparatus. Liquid scintillation counting was done with Packard Tri-Carb Model 314EX and Nuclear-Chicago Unilux instruments. The latter was modified to permit external adjustment of the photomultiplier voltage. Typical backgrounds were 4 and 18 counts min-l, respectively. Both instruments count the plutonium alpha radiation with 100% efficiency. All counting was done in 20-1111 glass vials with polyeth ylene-lined caps. Reagents. Scintillation chemicals were purchased from Packard Instrument Co. and extractants were obtained from Eastman Kodak and Rohm and Haas. Preliminary experiments indicated that the extractive properties of the alkyl phosphoric acids were not significantly changed by purification so, with one exception, subsequent studies were done with commercial grade extractants. D2EHPA was passed through a column of charcoal to reduce colored impurities. The extractant-scintillation solution was made by dissolving 200 ml of D2EHPA in 800 ml of toluene containing 5 grams of terphenyl and 0.05 gram of 1,4-bis-2(5 phenyloxazoly1)(1) L. C. Schwendiman, J. W. Healy, and D. L. Reid, Hanford Works Report, HW-22680, Richland, Wash., Nov. 1951. (2) D. L. Horrocks and M. H. Studier, ANAL.CHEM., 30, 1747 (1958). (3) T. Y. Toribara, D. A. Morken, and C . Predrnore, Talanta, 10, 205 (1963). (4) J. D. Ludwick, Health Phys., 6, 63 (1961).
benzene (POPOP). This solution was stable for several months. Procedure. Reduce the sample to carbon-free ash in a new beaker by alternate muffling at 450°C and digestion with hot, concentrated nitric acid. Thoroughly soak the ash with a 6 M nitric-6M hydrofluoric acid solution and evaporate to dryness. Repeat the nitric-hydrofluoric acid digestion, then cover the residue with 8M nitric acid and evaporate to dryness at a low heat. Dissolve the salts in sufficient hot 2 M H N 0 3 containing 0.2M boric acid to produce a final solution containing 1-15% dissolved salts. Disregard the small amount of insoluble material which results from etching of the beaker and from soil or sand present in some types of samples. Transfer up to a 10-rnl aliquot to a 20-ml counting vial containing a drop of 4 M urea; if a smaller aliquot is taken, dilute with sufficient 2M nitric acid to bring the volume to 10 rnl. Add 4 ml of extractant-scintillator solution, cap, and shake vigorously for 30 seconds. Repeat the shaking 5 times at 5 minute intervals before counting. RESULTS AND DISCUSSION Preliminary screening of possible extractants was done with 0.1M extractants in toluene-scintillator solutions. Toluene was used as the diluent in all cases because of its superior characteristics for liquid scintillation counting. The aqueous phase consisted of 2M nitric acid, 0.5M in both sodium hydrogen sulfate and sodium dihydrogen phosphate, except when Primine JMT was examined. Primine JMT, a mixture of primary alkyl amines obtained from Rohm and Haas Co., was studied in 0.5 to 5M sulfuric acid containing 0.5Msodium dihydrogen phosphate. This system has been shown to be useful for biological samples wet-ashed in sulfuric acid (5). Screening results indicated that DBP, D2EHPA and Primine JMT were potentially useful as extractants, while tri-n-octylamine and tri-n-octylphosphine oxide (TOPO) were ineffective in the presence of sulfate and phosphate. Primine JMT was rejected because of problems associated with the wet-ashing of samples with sulfuric acid. D2EHPA was chosen over DBP because of its higher decontamination factor for trivalent metals, specifically iron and americium (6). Counting efficiencies of 100% were obtainable in scintillator solutions containing as high as 1.7M D2EHPA although shifts in photomultiplier voltage were required to compensate for weaker scintillation pulses. A D2EHPA concentration of 0.6M (207, v/v) gave high distribution coefficients with a minimum quenching effect, and was used for all further work. Variables were studied in scintillation vials by contacting 4 ml of extractant-scintillator solution with 10 ml of an aqueous phase containing 3000 disintegrations min-l of 23BPuwhich was stabilized in the tetravalent state with 0.01M sodium nitrite prior to use. Recovery was then determined by liquid scintillation counting of the vials. All counting was done at fixed instrument settings. (5) F. W. Bruenger, B. J. Stover, and D. R. Atherton, ANAL. CHEM., 35,1671 (1963). (6) E. S. Gureev, N. V. Kosyakov, and G. N. Yakovlev, Radiokhimya, 6,655 (1964), Chem. Abstr. 63, 1429f (1965). ANALYTICAL CHEMISTRY, VOL. 42, NO. 3, MARCH 1970
419
100
Table I. Limiting Amounts of Diverse Ions Ion mg/lO ml Ascorbate
2 al
8
>300
BO^ 3Ca 2+
>lo00
>400
c1-
60
0)
c104-
6 u
Cr2072-
+
k
