CRYSTALLITE SIZE DEPENDENCE OF VOLATILIZATION IN MATRIX-ASSISTED LASER-DESORPTION IONIZATION

During the desorption and ionization processes in matrix-assisted lase r desorption ionization (MALDI), the laser beam interacts with differe nt areas of the polycrystalline sample surface varying in crystal size and structure. CCD imaging of dried-droplet and electrospray deposite d MALDI samples was used to explore the effect of laser exposure on su rface morphology at atmospheric pressure. Crystal size distributions o f the sinapinic acid target were examined prior to and after laser irr adiation. Image processing and analysis of the difference images revea led the morphological changes in the sample. The results indicate that near-threshold irradiance smaller crystals (similar to 2 mu m) are co mpletely volatilized by the laser shot, whereas larger crystals underg o layer-by-layer evaporation (peeling). Heat conduction simulations in a finite slab have demonstrated that under similar conditions, the su rface temperature of small crystallites increases markedly compared to their larger counterparts. These higher surface temperatures can lead to the selective volatilization of smaller crystallites. This study p oints to the influence of sample preparation on the crystal size distr ibution and its consequence on volatilization in MALDI-mass spectromet ry (MS). (C) 1998 Elsevier Science B.V.