Molecular ejection and ionization processes in matrix assisted laser desorption/ionization (MALDI) mass spectrometry are described. Angular resolved initial velocity distributions of matrix and analyte ions, obtained both from MALDI samples prepared normally and from peptide doped single crystals of dihydroxybenzoic acid, are presented as ion density plots. The ejection distributions have been found to be bimodal, consisting of a slow, less forward peaked fraction and a fast fraction, which is directed more towards the surface normal. The ion plots show a significant quenching of matrix ions in areas of high peptide ion density. This effect is interpreted as the result of ion-molecule reactions in the gas phase leading to proton transfer from matrix ions to analyte molecules and is described by a simple kinetic overlap model. Additional effects of solid phase absorption, electronic excitation life times and fluorescence yield are discussed.

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