SIMULTANEOUS LIQUID SCINTILLATION DETERMINATION OF TRITIUM AND SULFUR-35 IN BIOLOGICAL LOW-LEVEL SAMPLES USING OXYGEN FLASK METHOD

The combustion of biological samples for the simultaneous determination of 3H and 35S requires two trapping systems. Tritium is generally trapped as tritiated water by simple cooling while sulfur-35 has to be fixed by a chemical trapping agent (e.g., 2-phenylethylamine). When biological low-level samples have to be assayed by the oxygen flask method, as occurs very often in human tracer studies, large amounts of material have to be burned and consequently large amounts of the chemical trapping agent are used, which leads to an important increase of the quenching in the counting mixture. This increase can be avoided by adequate dilutions but, in the case of low activities, the counting rate must be easily distinguished from the background rate. In this last case a compromise between counting rate and quenching level (i.e., the dilution) has to be found. We tried to find the most accurate and reliable experimental conditions that may be expected with the method described. Experiments were carried out on blood. Five radioactive levels (ranging from 9600 3H dpm/ml and 3200 35S dpm/ml to 600 3H dpm/ml and 200 35S dpm/ml) at four different quenchings (i.e., at different dilutions) for each level were assayed. The accuracy, the precision and the sensitivity of the method are discussed on a statistical basis and it appears to be essential that the best determinations for very low level samples are obtained at the lowest dilutions. © 1968.