Investigation of MALDI-TOF Mass Spectrometry of Diverse Synthetic Metalloporphyrins, Phthalocyanines and Multiporphyrin Arrays

Abstract

We investigated the utility of matrix-assisted laser desorption/ionization time-of-flight mass spectrometry (MALDI-TOF-MS) for analyzing porphyrinic compounds using a variety of different synthetic porphyrins, azaporphyrins, phthalocyanines and multiporphyrin arrays. Comparisons of spectra obtained from these analytes deposited either as neat samples or codeposited with neutral or acidic matrices have been made with the goal of identifying conditions that yield minimal demetalation, transmetalation, adduct formation and fragmentation. It was found that the molecular masses of many porphyrins can be successfully measured from neat sample preparations and do not require a matrix to facilitate desorption and ionization, although the measurement of large multiporphyrin arrays was facilitated by the use of matrices. Demetalation of magnesium porphyrins occurred in the presence of acidic matrices, but not with neutral matrices such as 1,4-benzoquinone. Positive ion spectra were obtained for each compound and negative ion spectra were also collected for the azaporphyrins and phthalocyanines. Examination of selected samples (prepared neat, with 1,4-benzoquinone, 2,3,5,6-tetrachloro-1,4-benzoquinone or α-cyano-4-hydroxycinnamic acid) showed that the dominant process of ionization involved oxidation yielding the radical cation M+· rather than the protonated molecule [M+H]+. MALDI-TOF-MS is shown to be a powerful analytical tool for the characterization of diverse synthetic porphyrinic compounds.