The matrix suppression effect in matrix-assisted laser desorption/ionization: application to negative ions and further characteristics

Abstract

As recently shown for positive ions (Rapid Commun. Mass Spectrom. 10, 871 (1996)), small to moderate-sized analyte ions (<20 000 Da) can fully suppress all matrix signals in matrix-assisted laser desorption/ionization (MALDI) mass spectra at appropriate matrix:analyte mixing ratios. The technique is useful for removing matrix interferences and led to a model for primary ion formation mechanisms. We here extend the method to negative ions, and investigate the effects of analyte type and size on the matrix suppression concentration range. For a given matrix/analyte pair, suppression was observed in either positive- or negative-ion mode, but not both. Some matrices do not show the effect at all, which is attributed to unfavourable thermodynamics. The analyte concentration where suppression appears is correlated with the analyte molecular weight and supports the proximity requirement of the model. The dependence of matrix signals on analyte concentration is very similar in both modes, and may have implications for the role of plume reactions at low analyte concentrations. Delayed extraction was found to extend matrix suppression to lower analyte concentrations. Lack of homogenous matrix/analyte co-crystallization at low analyte concentrations is shown to prevent the appearance of matrix suppression or to modify the shape of the concentration curves in some cases. Evidence is presented for the activity of electronic excitations in IR as well as in UV MALDI. © 1998 John Wiley & Sons, Ltd.