ELECTROSPRAY IONIZATION OF PORPHYRINS USING A QUADRUPOLE ION TRAP FOR MASS ANALYSIS

The utility of electrospray (ES) ionization combined with ion trap mass spectrometry for the analysis of a wide variety of free-base porphyrins and metalloporphyrins is examined. A single molecular ionic species is observed in most cases for the porphyrins under study. The free bases are detected as the protonated molecules, (M + H)+, the trivalent metalloporphyrins as (M - L)+, where L- is a counteranion, and the divalent metalloporphyrins as M.+, (M + H)+, and/or (M + Na)+, depending upon both the metal complexed and the composition of the electrospray solvent. This marks the first reported observation of radical cations formed by ES ionization. Detection of the porphyrins studied by using continuous infusion required sampling as little as 1 fmol of material, while, with flow injection analysis, detection of 18 fmol of porphyrin is demonstrated. ES ionization is shown to be well suited for porphyrin molecular weight determination and for the analysis of porphyrin mixtures because no fragment ions are produced. Fragmentation of porphyrin ions via collision-induced dissociation (CID) and fragmentation induced by injection of ions into the trap are both shown to provide structure information. The degree of fragmentation upon injection can be varied via adjustment of ion injection parameters, either manually or automatically under computer control, providing a nonmass-selective means to induce fragmentation in the ion trap. The potential utility of automated control of ion injection parameters to provide both molecular weight and structure information when an on-line separation method is used with the ES/ion trap system is demonstrated.