Metalloporphyrin contaminations in purified porphyrin preparations as demonstrated by mass spectrometry.

Mass spectrometric studies were performed on selected specimens from my collection of over 700 crystalline porphyrin esters prepared over 20 years as well as on similar preparations from other laboratories. Mass spectrometry was performed by J Moller (Department of Chemistry, Odense University) and Elfinn Larsen (Department of Chemistry, The Experimental Division, Risoe, of the Danish Atomic Energy Commission). Nearly all typical crystalline porphyrins contained Cu- and Ni- and minor amounts of Zn- and Fe2-porphyrins, most often above one molar per cent, sometimes 5--10% and occasionally more, up to 35%. There was no close correlation between the metal content and the melting point. A method is proposed for preparation of metal-free ester preparations: Dissolution of either free or esterified porphyrin in conc. H2SO4; mixing of the solution with 19 volumes of methanol; after at least 12 hours adding of 2 vol. CHCl3 and 2 vol. H2O, and washing the CHCl3, phase with 1/2 vol. distilled water until washwater becomes neutral (8--10 times). Preparations made in this way showed below 0.5% metalloporphyrin contamination. The method does not work with protoporphyrin, which is oxidized by conc. H2SO4. The acid extraction leaves some porphyrin partially esterified, which can be removed by preparative TLC. A TLC-method for detection of metalloporphyrin in porphyrin esters is described.