When109Cd is placed in a liquid scintillation counter, 146±1 counts are obtained for every 100109Cd nuclides which undergo decay. Thus, the average efficiency for each of the two steps in the decay process is 73%. In examining the distribution of energy among the windows available, it was noted that the distribution pattern is quite similar to that of14C. An average Cherenkov counting efficiency, in water, was found to be 0.0258±0.0005%. Both 0.010M K2Cr2O7 and CHCl3 quench the count rate, particularly in the lower energy channel, where count rates are suppressed by up to a factor of 20.
Using as eluent a sequence of 3M HCl, 12M HCl, and 8M HNO3, a mixture of210Pb,210Bi, and210Po may be clearly separated on a column of Dowex 1×2−100 anion exchange resin. A Cherenkov count in H2O and the variation in count rate with time confirm that the nuclides emerge in the order210Pb→210Bi→210Po. If 12M HCl is replaced by 1.5M H2SO4/2.3 M Na2SO4, a clean separation also results, but recovery of210Po becomes considerably more difficult. All three nuclides are readily detectable by liquid scintillation counting, with the
efficiency for210Pb in the 60–70% range. The Cherenkov aqueous counting efficiency for210Bi is ∼14–15%.
109Cd may be used as a radiolabel in tracer studies of Cd(II) because of the ease and efficiency with which it may be counted by liquid scintillation counting. It has been found that ppm solutions of Cd2+, spiked with109Cd, may be stored for at least 200 d, in either amber glass or polypropylene containers, without measurable container adsorption, at pH's of 5 or less. At pH's above 5, there is some evidence of container adsorption, particularly in the case of polypropylene containers. Amber glass is recommended if pH's between 5 and 7 must be used for storage.
Aqueous Pb(II) samples, spiked with210Pb/210Bi in secular equilibrium to study container adsorption, can provide some helpful insights if one takes advantage of the Cherenkov effect. While both radionuclides give high efficiencies when counted in a scintillator cocktail medium, only210Bi has a sufficiently energetic -particle to yield an appreciable count rate in a water medium. Thus, monitoring the cocktail-to-water count rate ratio can provide important clues as to what happens during adsorption. A significant change in the ratio indicates not only disturbance of the secular equilibrium relationship, but indicates which nuclide is preferentially adsorbed.
Solutions of Pb2+ at ppb to ppm levels were tagged with210Pb and then counted from time to time over a period of one year to measure the extent of container adsorption. All count rates for samples which were acidified with HNO3 were at least 90% of their initial values, even after one year of storage. Amber glass containers proved to be superior to polypropylene for 0.1 and 1.0 ppm Pb2+ solutions; but below 0.1 ppm, polypropylene was superior. For non-acidified samples, the count rate dropped rapidly within the first two months, and then declined more gradually thereafter, with polypropylene generally superior.
A radioanalytical procedure is described in which Hg2+, in an aqueous solution, is brought into contact with109Cd-radiolabeled diethyldithiocarbamate (DDTC), in an organic phase. Because Hg2+ forms a much more stable chelate with DDTC than does Cd2+, it displaces (radioreleases)109Cd into the aqueous phase. This latter activity is, linearly related to the concentration of Hg(II) in the sample solution,
with excellent linearity down to ≈5ppb. Response factors are reported for a number of ions which may interfere with the determination.
Major interferents include Cu2+, Ag+, and Pb2+.
A method for the separation of210Pb,210Bi and210Po using spontaneous deposition has been developed. The210Bi and210Po are simultaneously removed by deposition onto nickel foil (copper and tin could also be used but less effectively) while the210Po is separated from210Bi, after dissolution of the nickel foil, by deposition onto silver foil. The effectiveness of each separation was evaluated by adding aliquot portions of each solution to a cocktail and counting with a liquid scintillation counter. Water was used as the medium to observe the Cherenkov count of the sample solution.
A radioisotope dilution procedure has been developed which permits the determination of lead over a concentration range of 0.1 to 10 ppm. Samples in an ammoniacal cyanide/sulfite medium are labelled with210Pb, followed by addition of substoichiometric amount of dithizone. The Pb/HDz/2 formed is extracted into CH2Cl2, and aliquot portions are taken for liquid scintillation counting. Count rate is plotted vs. the reciprocal of the lead concentration, yielding a straight line. An average error of ±3.5% is estimated, based on a linear least squares fit.