DYNAMICAL PARAMETERS OF ION EJECTION AND ION FORMATION IN MATRIX-ASSISTED LASER DESORPTION/IONIZATION

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.