Special feature: Perspective - The initial ion velocity and its dependenceon matrix, analyte and preparation method in ultraviolet matrix-assisted laser desorption ionization

Since the early days of matrix-assisted laser desorption/ionization (MALDI) , measurements shelving that MALDI ions and neutrals have high initial velo cities have led to wide acceptance of the idea that a jet of released mater ial entrains analyte ions. The initial velocity, which could previously be determined only with large uncertainty, can be measured today with high rel iability in a delayed-extraction MALDI/time-of-flight system by following t he linear dependence of ion flight time vs the applied extraction delay. Th e detection of different initial velocities for different matrices, with an d without additives, for various preparation protocols and for different cl asses of analytes proves that the magnitude of the initial velocity can ind eed be regarded as a valuable and meaningful characteristic of the MALDI pr ocess. Based on the results reported here, it is postulated that a high ini tial velocity results from incorporation of the analyte into the matrix cry stals and that cooling upon expansion is effective at high initial velociti es and responsible for reduced fragmentation observed in such cases compare d with 'slow' matrices. Copyright (C) 1999 John Wiley & Sons, Ltd.