A combustion method for the simultaneous determination of 3H, 14C, and 35S in triply labeled organic samples by liquid scintillation counting.

The organic samples triply labeled with 3H, 14C, and 35S were combusted in a flow-type combustion flask under a continuous oxygen flow. The oxygen gas containing the combustion products, which were mainly 3H2O, 14CO2, and 35SO2, was introduced into successive 0.5 M H2O2 and Oxisorb (2-methoxyethylamine). The 35SO2 and 3H2O that had been partially adsorbed onto the inner wall of the combustion flask were recovered by rinsing the wall with an H2O2 solution. 3H and 35S were quantitatively recovered from the H2O2 solutions. 14C was not found in the H2O2 solutions and was recovered from Oxisorb quantitatively. The amounts of 3H, 35S, and 14C were determined by measuring the radioactivity of the H2O2 solutions and of Oxisorb with a liquid scintillation counter. Thus the separate determination of 14C and 35S in a sample was achieved. Through this combustion method, the materials including fresh animal tissues up to ca. 300 mg were conveniently combusted with quantitative recovery of 3H, 14C, and 35S.