The Effect of Metal Atoms on Electron Ionization and Ammonia Chemical Ionization Mass Spectra of Metalloporphyrins: Implications for Geoporphyrin Analysis

Eight metal complexes of octaethylporphyrin were analyzed by electron ionization (EI) and ammonia chemical ionization (NH₃ CI) mass spectrometry. In both modes, features of the mass spectra of the metal complexes correlated well with the stability index of the metalloporphyrin, S_i = [100(PE) (V)]/R, where PE is the Pauling Electronegativity of the metal, V is the valence of the metal, and R is the ionic radius of the metal. Metalloporphyrins with high stability indices produced abundant doubly charged fragments in the EI mode, and gave very little fragmentation with NH₃ CI. Metalloporphyrins with low stability indices gave less intense doubly charged ions in EI, but gave abundant monopyrrolic fragments, and some dipyrrolic fragments under NH₃ CI conditions. These results will be very valuable in geoporphyrin analysis, since it will now be possible to control the degree of fragmentation by inserting an appropriate metal into demetalated geoporphyrins.