Application of matrix-assisted laser desorption/ionization Fourier transform mass spectrometry to the analysis of planar porphyrins and highly substituted nonplanar porphyrins

The applicability of matrix-assisted laser desorption/ionization (MALDI) Fourier transform mass spectrometry to the analysis of porphyrins has been examined. High resolution spectra were readily obtained with good sensitivity and a detection limit as low as 2 fmol. A mixed solvent system of toluene and ethanol (1:1 by volume) proved to be compatible with the 2,5-dihydroxybenzoic acid (DHB) matrix and solubilized most of the porphyrins examined in this study. Porphyrins which were insoluble in this solvent mixture could be dissolved in an appropriate solvent and deposited on a layer of DHB (layered MALDI). The parent ion was generally the largest peak in the spectrum, although for some metalloporphyrins the peak corresponding to (M - metal + 3H)⁺ was dominant. The extent of demetallation was found to depend on factors such as sample preparation, the metal ion and the laser intensity. Addition of more than one hydrogen occurred in many cases but was a minor process. Spectra of compounds ionized by different methods showed increasing fragmentation in the order MALDI <layered MALDI < fast-atom bombardment <laser desorption.