A bipolar rate equation model of MALDI primary and secondary ionization processes, with application to positive/negative analyte ion ratios and suppression effects

The rate equation model for MALDI ion formation and reaction [R. Knochenmuss, J. Mass Spectrom. 37 (2002) 867; R. Knochenmuss, Anal. Chem. 75 (2003) 2199], including both chemical and physical dynamic aspects of MALDI, is extended to explicitly include both positive and negative ions of matrix and analyte. The resulting positive/negative ratios of secondary analyte ions show that a recent static equilibrium approach is not adequate for quantitative analysis of MALDI experiments. In particular, the ion ratios remain close to unity whenever the reaction free energies are at least moderately favorable. This is the case for high and low analyte concentrations and also for a range of experimental conditions. The results are consistent with the available experimental data, and show once again that the dynamic aspects of MALDI cannot be neglected. In addition, the extent of matrix and analyte suppression effects is found to depend on reactions in the opposite polarity. (C) 2009 Elsevier B.V. All rights reserved.