Simplified Procedure for Determining Amounts of Two Mutually Interfering Radioisotopes in Mixed Samples

The radioisotope cerium-144 (Ce 144 -Pr 144 ) was used by Chandler and Cragle (3) as a non-absorbed marker to investigate gastrointestinal sites of absorption and excretion of dietary calcium in dairy calves. This technique is also useful in studying radioisotope metabolism (1, 4). However, it becomes necessary to count two isotopes in a mixture. This is comparatively simple when one isotope has a much shorter half-life (2) or can be counted in a portion of the spectrum free from contamination by the other (1). Simultaneous equations must be used in cases where neither isotope can be counted without interference from the other. With a large number of samples, this becomes laborious without the use of a computer. The present report presents in greater detail a simplified and rapid procedure used earlier (4) for calculating concentrations of zinc-65 and cerium-144 in mixed samples. Evidence is also given in support of the accuracy of the method in determining zinc-65, strontium-85, and cesium-134, each in mixture with cerium-144.