Counting of Beta and Gamma Emitters Used in Clinical Radioimmunoassays and Competitive-Binding Assays by Liquid Scintillation Counters

Abstract

Abstract Although liquid scintillation (LS) counters were originally designed to count low-energy beta emitting isotopes, they can also be used to count gamma emitters used in radioimmunoassay. This is made possible by placing the gamma emitter inside a "Microfuge" tube suspended from a LS vial cap into a scintillator fluid containing an electron density increaser. Efficiencies of 71% are obtained for 125I by this method, comparable to that obtained with Nal (TI) crystal gamma counters. Efficiencies for other clinically important isotopes used in radioimmunoassay—131I, 57Co, 60Co, 51Cr, 59Fe—by the Microfuge method are 15-19%; therefore, for these isotopes a LS "Mini-Vial" method is suggested, in which smaller volumes may be used. The dual capability of LS counters to count both beta and gamma emitters extends its usefulness in the clinical laboratory.