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Attenuation of I-125 Radiation by Chloroform and Other Dense Solvents, and Its Relevance to Radioimmunoassays: Concise Communication

William S. Middleton Memorial Veterans Hospital and the University of Wisconsin, Madison, Wisconsin

Correspondence: For reprints contact: Theodore L. Goodfriend, MD, William S. Middleton Memorial Veterans Hospital, 2500 Overlook Terrace, Madison, WI 53705.

ABSTRACT

The counting efficiency of iodine-125 in well-type scintillation counters is reduced when the labeled compound is dissolved or suspended in chlorinated hydrocarbons or potassium iodide solution. The reduction is probably caused by absorption of the weak gamma and x-rays of I-125 by the halogen atoms in the solvent or solute molecules. This phenomenon may introduce artifacts into procedures involving radioiodinated compounds and organic solvents and KI, but it could also be useful in differentiating bound from free labeled ligands by their differential solubility (and attenuation) in dense solvents. The reduction in counting efficiency can be overcome by evaporation of the solvent, or by the use of emitters with higher energy radiation.