THE MATRIX SUPPRESSION EFFECT IN MATRIX-ASSISTED-LASER-DESORPTION IONIZATION - APPLICATION TO NEGATIVE-IONS AND FURTHER CHARACTERISTICS/

As recently shown for positive ions (Rapid Commun, Mass Spectrom, 10, 871 (1996)), small to moderate-sized analyte ions (<20000 Da) can full y suppress all matrix signals in matrix-assisted laser desorption/ioni zation (MALDI) mass spectra at appropriate matrix:analyte mixing ratio s. The technique is useful for removing matrix interferences and led t o a model for primary ion formation mechanisms. We here extend the met hod 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 negativ e-ion mode, but not both. Some matrices do not show the effect at all, which is attributed to unfavourable thermodynamics. The analyte conce ntration where suppression appears is correlated with the analyte mole cular 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 reactio ns at low analyte concentrations, Delayed extraction was found to exte nd matrix suppression to lower analyte concentrations. Lack of homogen ous matrix/analyte co-crystallization at low analyte concentrations is shown to prevent the appearance of matrix suppression or to modify th e shape of the concentration curves in some cases. Evidence is present ed for the activity of electronic excitations in IR as well as in UV M ALDI. (C) 1998 John Wiley & Sons, Ltd.