Liquid-Scintillation Counting of 14C-Labelled Animal Tissues

Developments of liquid-scintillation counting as a practical method for the assay of radioisotopes in biological samples followed work by Reynolds, Harrison & Salvini (1950) and Kailmann & Furst (1950). Reviews of progress and developments in liquid-scintillation counting have been given by Davidson & Fiegelson (1957) and Bell & Hayes (1958). Guinn (1958) and Stitch (1959) have reviewed the advantages and disadvantages of liquidscintillation counting. The advantages cited include freedom from self-absorption and window-absorption, simplicity of sample preparation, high sensitivity and reproducibility and ease of absolute calibration. Amongst the disadvantages are the high cost of the complex apparatus, the lowering of efficiency caused by phosphorescence quenching and the limitation of its use to samples soluble (or readily suspended) in the liquid scintillator. The mechanism ofphosphorescence-quenching processes is discussed by Kallmann & Furst (1958). The liquid-scintillation counting of tissues, proteins and amino acids has been accomplished in several ways. Counting of suspensions has been adopted for tissues and proteins (Hayes, Rodgers & Langham, 1956; Funt, 1956; White & Helf, 1956; Funt & Hetherington, 1957). Water-soluble substances have been blended with scintillator with 1:4-dioxan-naphthalene systems (Furst, Kallmann & Brown, 1955). The 14CO2 from digested tissues and proteins has been trapped in methanolic solutions of the free base of the quaternary ammonium compound Hyamine 10-X. Such solutions are completely miscible with scintillator (Passmann, Radin & Cooper, 1956; Eisenberg, 1958). A solution of this free base has also been used to dissolve amino acids, and even proteins are soluble if incubated for 2 hr. at 600 (Radin, 1958; Passmann, Radin & Cooper, 1957; Steinberg, Vaughan, Anfinsen & Gorry, 1957; Vaughan, Steinberg & Logan, 1957). The chemical procedures required for the preparation of suspensions, solutions of the free base of Hyamine 10-X and digestion of tissues are lengthy. It was for this reason that the authors sought to dissolve dried tissues and proteins directly in a solution of the chloride form of Hyamine 10-X in methanol. The high efficiencies attained (90% for xylenesoluble sources and 60% for sources soluble in aqueous solutions), and the method of blending sources insoluble in scintillator, together with the considerable number of variables which affect the efficiency and reproducibility of both blending and counting, should be of general application and of interest to workers in this and other fields.