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 for the analysis of luminescence. The luminescence spectra of several MALDI matrices in the solid and liquid phase, the intensity and shape of the spectra at different temperatures, the intensity of the emitted light as a function of the laser flueuces and the kinetics of the luminescence were investigated. Measurements of 2,5-dihydroxybenzoic acid and ferulic acid at different temperatures allow the estimation of the average luminescence quantum yield, which is less than 0.2 at room temperature and the ionization threshold laser fluence. The low yield means that only a minor part of the absorbed energy is emitted. The major part of photon energy absorbed by the molecules relaxes by internal conversion and therefore contributes to the desorption/ionization process. At lower temperatures, the quantum yield increases significantly, which can explain the previously observed increase in the threshold fluence 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 threshold the luminescence is quenched, presumably by S-1-S-1 annihilation processes and/or phase transitions. The annihilation processes demonstrate 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 explain ion formation at laser fluences where two-photon ionization in the gas phase is improbable. At low temperatures the quenching starts at lower 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 phosphorescence, were detected for 2,5-dihydroxybenzoic acid and ferulic acid, whereas 3-hydroxypicolinic acid emits photons up to 100 mu s after the laser pulse.