Influence of ionization energy on charge-transfer ionization in matrix-assisted laser desorption/ionization mass spectrometry

In this study, non-polar matrices are used in matrix-assisted laser desorption/ionization time-of-flight mass spectrometry (MALDI-TOFMS) to analyze selected non-polar analytes. Our hypothesis is that gas-phase charge-transfer reactions between matrix and analyte are responsible for the generation of analyte radical molecular ions. Following this hypothesis, the ionization energies of the matrices and analytes should have a direct influence on the production of radical molecular cations of the analytes. Several non-polar analytes, including ferrocene and ferrocene derivatives, trans-stilbene, triphenylphosphine, 2,2'-methylenebis(6-tert-butyl-4-methylphenol), biphenyl and 1,4-bis(methylthio)benzene were studied using positive-ion mode MALDI-TOFMS. The results of these studies demonstrate that formation of the radical molecular cation depends on the difference in ionization energies between the matrix and the analyte. The propensity for charge-transfer ionization, as opposed to proton-transfer ionization, for these analytes, was confirmed using atmospheric pressure chemical ionization mass spectrometry. Charge-transfer ionization using non-polar matrices in MALDI-MS is a suitable method for the characterization of a number of non-polar, thermally labile analytes. (C) 1999 Elsevier Science B.V. All rights reserved.