Determination of Radioiodide Contamination of Radio-hippuran and Other Radioiodinated Organic Salts.

Previously, 131iodide contamination of sodium o-'31iodohippurate (radio Hippuran) and other radioiodinated pharmaceuticals has been determined by time consuming chromatographic procedures, followed by strip scanning, strip segment counting, or autoradiographic procedures for quantitating the spots (1, 2). A simple technique has been developed for the assay of radioiodide con tamination of water-soluble, radioiodinated organic salts. A brief description of the preliminary development of this technique was made earlier (3) . Several modifications have been made subsequently to increase accuracy and speed. The new method is based upon the following physical and mathematical considerations: When a saturated solution of elemental iodine in carbon tetra chloride (Cd4 ) is allowed to come into equilibrium with water, only a very small per cent of the iodine dissolves in the water as iodide and hypoiodite ions. A continual exchange of ions occurs between the two layers. If a relatively very small amount of radioiodide ions are added to the aqueous layer, after equilibra tion, the radioactive ions will become evenly distributed throughout the system. However, since only a very small number of iodine atoms exist as iodide ions in the aqueous phase relative to the large number of iodine atoms present in the whole system, statistically almost all of the radioiodine atoms will have become distributed into the CC14 layer. Therefore, if these radioiodide ions were added to the aqueous phase of the system along with a water-soluble CC14-insoluble radioiodinated organic compound, the radioactivity due previously to radioiodide will pass into the CC14-L, layer, leaving the radioactive compound in the aqueous layer. The procedure becomes inaccurate when gross iodide concentrations in the aqueous phase become significant (> 0. 1%). The new test was checked quantitatively against a known chromatographic procedure (2), using autoradiographic techniques of spot localization.