Crystallite size dependence of volatilization in matrix-assisted laser desorption ionization

Abstract

During the desorption and ionization processes in matrix-assisted laser desorption ionization (MALDI), the laser beam interacts with different areas of the polycrystalline sample surface varying in crystal size and structure. CCD imaging of dried-droplet and electrospray deposited MALDI samples was used to explore the effect of laser exposure on surface morphology at atmospheric pressure. Crystal size distributions of the sinapinic acid target were examined prior to and after laser irradiation. Image processing and analysis of the difference images revealed the morphological changes in the sample. The results indicate that near-threshold irradiance smaller crystals (∼2 μm) are completely volatilized by the laser shot, whereas larger crystals undergo layer-by-layer evaporation (peeling). Heat conduction simulations in a finite slab have demonstrated that under similar conditions, the surface 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 points to the influence of sample preparation on the crystal size distribution and its consequence on volatilization in MALDI-mass spectrometry (MS).