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Since the early days of matrix-assisted laser desorption/ionization (MALDI), measurements showing that MALDI ions and neutrals have high initial velocities have led to wide acceptance of the idea that a jet of released material entrains analyte ions. The initial velocity, which could previously be determined only with large uncertainty, can be measured today with high reliability in a delayed-extraction MALDI/time-of-flight system by following the linear dependence of ion flight time vs the applied extraction delay. The detection of different initial velocities for different matrices, with and without additives, for various preparation protocols and for different classes of analytes proves that the magnitude of the initial velocity can indeed be regarded as a valuable and meaningful characteristic of the MALDI process. Based on the results reported here, it is postulated that a high initial velocity results from incorporation of the analyte into the matrix crystals and that cooling upon expansion is effective at high initial velocities and responsible for reduced fragmentation observed in such cases compared with ‘slow’ matrices. Copyright © 1999 John Wiley & Sons, Ltd.
Glückmann et al. (Sat,) studied this question.