The widely used With (1) modification of the Rimington and Sveinsson (2) spectrophotometric method for the assay of free erythrocyte porphyrins was designed to correct for impurit,ies that absorb in the region of the Soret band of the porphyrins. This absorbance was assumed to increase linearly from 430 t'o 380 n+.. Dresel and Falk (3) found this method inadequate for the assay of uroporphyrin isolated by their method and devised a correction factor based on the absorbance of uroporphyrin and the impurity at 397 and 405 mp. Van der Merwe and Findlay (4), in their analysis of the method of Rimington et al. ( 5) for the determination of free erythrocyte porphyrins, have shown that the impurity has a broad absorption peak between 390 and 400 mp. They found the With equations inadequate to compensate for this impurity and suggested purification of the contaminated porphyrin solutions before assay. The present author has found it more convenient to derive new correction factors for the porphyrin assays based on the true spectrum of the isolated impurity, under the conditions of each assay.
'All ethyl acetate used in the isolation of the porphyrins was saturated with disodium ethylenediaminetetrascetate (6).