A NEW SPECTROPHOTOMETRIC METHOD FOR DETERMINATION OF CATIONIC PORPHYRINS

A new method based on difference spectrophotometry has been developed for the determination of cationic porphyrins. In the reference compart ment the porphyrin is transformed to a zinc complex in situ with a kno wn excess of zinc at pH approximately 12. The measuring compartment co ntains the same amount of zinc as the reference compartment but no por phyrin. The difference in free zinc concentration of the two solutions , measured with the zincon reagent at 622 nm in a pH 9.0 borate buffer , is equal to the concentration of porphyrin. The measured A622 has to be corrected by that of the zinc-porphyrin determined under identical conditions. Beer's law is obeyed between 15 and 45 muM porphyrin. The method was utilized for determination of five meso-substituted cation ic porphyrins; the standard deviation was found to be within 3%. (C) 1 994 Academic Press, Inc.