An automatic gas counter for quantitative micro-determinations of tritium in biological material

An automatic microcounter for quantitative determinations of tritium has been developed. It has a background of 0.5 cpm and a constant and quench-free efficiency of 57.4 ± 2.1% ($\text{E}{}^2\text{b}$ 6600). Radioactivities of 2 pCi can be measured with a standard deviation of ±10% using a 1-hr counting time. It is particularly adapted for tritium microassay from biological material in which tritium is converted to a gaseous state by a single step combustion and reduction method, utilizing a mixture of metallic zinc and anhydrous sodium carbonate. Samples weighing up to 2 mg can be measured using sample ampules made of Pyrex glass with outer diameter of 5 mm and length of 30 mm. The material costs are extremely low and one technician can load and scal over 100 samples in a day. The apparatus may have significance as an additional instrument beside liquid scintillation counters in the region of radioactivities below 20 pCi. The method has been successfully used in investigations concerning steroid metabolism in rat adrenal cortex and testis. Specific activities have been determined from tissue samples weighing 1 μg and less.