STUDIES OF THE MALDI PROCESS BY LUMINESCENCE SPECTROSCOPY

The process of matrix-assisted laser desorption/ionization (MALDI) was investigated by studying the luminescence of matrix molecules induced by laser radiation. An optical multi-channel analyser was utilized fo r the analysis of luminescence. The luminescence spectra of several MA LDI matrices in the solid and liquid phase, the intensity and shape of the spectra at different temperatures, the intensity of the emitted l ight as a function of the laser flueuces and the kinetics of the lumin escence were investigated. Measurements of 2,5-dihydroxybenzoic acid a nd ferulic acid at different temperatures allow the estimation of the average luminescence quantum yield, which is less than 0.2 at room tem perature and the ionization threshold laser fluence. The low yield mea ns that only a minor part of the absorbed energy is emitted. The major part of photon energy absorbed by the molecules relaxes by internal c onversion and therefore contributes to the desorption/ionization proce ss. At lower temperatures, the quantum yield increases significantly, which can explain the previously observed increase in the threshold fl uence at lower temperatures. Time-resolved measurements and the shape of the spectra indicate that the molecules investigated form excimers by laser irradiation. At laser fluences around the desorption threshol d the luminescence is quenched, presumably by S-1-S-1 annihilation pro cesses and/or phase transitions. The annihilation processes demonstrat e that the electronically excited states are very mobile and interact with each other even at low exciton densities, i.e. laser fluences far below the ionization threshold laser fluence. The mobility can explai n ion formation at laser fluences where two-photon ionization in the g as phase is improbable. At low temperatures the quenching starts at lo wer laser fluences, which can be explained by the longer lifetimes of the excitons at these temperatures making interactions more probable. No long-lived excited states, i.e. delayed fluorescence or phosphoresc ence, were detected for 2,5-dihydroxybenzoic acid and ferulic acid, wh ereas 3-hydroxypicolinic acid emits photons up to 100 mu s after the l aser pulse.