Determination of Phosphorus-32 and -33 in Aqueous Solution L. C. Brown Oak Ridge National Laboratory, P.O. Box X,Oak Ridge, Tenn. 37830
THEACCLTRATE AND RAPID DETERMINATION of small amounts of 32P (14.3-day half-life) or 33P (25.2-day half-life) in the presence of larger amounts of the other phosphorus radioisotope is necessary in agricultural, biological, or ecological double-labeling experiments and for the production of high-purity 33P-i.e., 32P activity 2 0.05 33P activity (1). The usual analytical methods for the analysis of 32P-33P mixtures take advantage of the large beta decay energy differences-Le., 32P(Ep-,,, = 1.709 MeV) us. 33P(Ep-max = 0.248 MeV). One such method has been to determine the 32Pcontent of a mixture via the planchet counting of samples using a G M counter and aluminum absorbers to discriminate against the lower energy 33Pbeta in conjunction with liquid scintillation counting to determine the combined azP-3sP content of a sample. Although this method offers accurate results, it has several disadvantages: the analyses are relatively time-consuming ; two separate and entirely different types of counting systems (which must be intercalibrated) are required; and the method does not lend itself readily to automatic counting if a large number of samples must be assayed. Analysis of 32P-33Pmixtures by liquid scintillation counting alone offers automatic operation, but does not offer a high degree of sensitivity or accuracy because of the relatively low energy resolution inherent in this counting method. In order to overcome the disadvantages of these other analytical methods, a routine analytical procedure based on a combination of Cerenkov radiation counting and liquid scintillation counting has been devised and tested. This procedure is rapid (requiring less than 30 minutes to prepare a moderate number of samples and perform the required final calculations), requires only one counting system, can be easily automated for routine multiple analyses, and yields high accuracy. Cerenkov radiation is produced when a charged particle, e.g., beta particle, passes through a transparent medium at a velocity greater than the speed of light in the same medium. The theoretical lower energy limit or threshold for the stimulation of Cerenkov radiation in water by beta particles is 263 keV. Phosphorus-33, which lies below the Cerenkov energy threshold, should not induce measurable Cerenkov emission, whereas 32P,which lies well above this threshold, will induce Cerenkov radiation. Therefore, any counts arising from an aqueous 32P-33Pmixture should be due only to 32P, with no contribution from the 33Ppresent. Kisielexhibits a 0.53 Cerenkov eski (2), however, states that counting efficiency which is contrary to theory and could introduce appreciable errors in samples containing large amounts of 33P, e.g., approximately 20% errors in highpurity 33Pproduction samples with > 9 5 x 33P. The Cerenkov radiation counting efficiency of 33Phas been determined to be
