The ionization in ultraviolet laser desorption was investigated for a large number of small polar organic molecules which have a strong resonance absorption at the laser wavelength. In many cases, both positive-and/or negative-ion mass spectra show strong signals of ion species which deviate from the simple scheme of even-electron quasi-mo\ecular and fragment ion formation commonly expected for desorption techniques. These are radical cations and ion species formed by single and multiple hydrogen cleavage or addition. A model is proposed and discussed which explains these features assuming photoionization as the common initial ionization step followed by ion-molecule reactions to the final product ions. The mass spectra of all compounds proved to function well in matrix-assisted ultraviolet laser desorptionfionization show characteristic features indicative of their photochemical reactivity. This observation substantiates the hypothesis of the essential role of the matrix in analyte ionization

Selected benzoic acid derivatives and related substances were used as additives to 2,5-dihydroxybenzoic acid (2,5DHB) and the performance of the mixtures in matrix-assisted laser desorption/ionization mass spectrometry was investigated. Using benzoic acid derivatives substituted at position 2 and/or 5 or related substances as a co-matrix in the 1-10% range with 2,5DHB results in improved ion yields and signal-to-noise ratio of analyte molecules, especially for the high-mass range. The enhanced performance is prominent for 2-hydroxy-5metboxybenzoic acid and exists for both proteins and oligosaccharides. It is suggested that the improvement is caused by a disorder in the 2,5DHB crystal lattice allowing 'softer' desorption. Charge transfer from matrix ions to additive molecules at the expense of analyte ionization gives a simple explanation for the deteriorating effects of some tested additives.

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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 fluences and the kinetics of the luminescence were investigated. Measurements of 2,5dihydroxybenzoic 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. Timeresolved 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 , S , 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 , s dihydroxybenzoic acid and ferulic acid, whereas 3-hydroxypicolinic acid emits photons up to 100 ps after the laser pulse.

A new strategy for the characterization of Coomassie Brilliant Blue stained SDS-PAGE separated proteins by UV-MALDI-MS is reported. The proteins are extracted directly from the polyacrylamide gel by treatment with an organic solvent mixture consisting of formic acid, acetonitrile, isopropanol and water in an ultrasonic bath. A fraction of the supernatant is then mixed directly with the matrix solution and measured by MALDI-MS. High quality spectra could be obtained from gels which were loaded with 6 pmol of myoglobin. Compared to other methods based on electroblotting or electroelution this method is much simpler and less time consuming. The sensitivity is higher than or comparable to the Coomassie Blue staining procedure for proteins up to about 25 kDa. Another advantage is that mass shifts due to charging effects of the membranes, which are common if membranes are mounted directly on the sample target, can be avoided. However, all proteins studied showed slightly higher masses than expected which reduces mass accuracy to 0.2-0.3%. This is presumably partly due to formylation of serine or threonine residues during incubation in formic acid. Gel electrophoresis induced modifications can contribute as well. The possibility of further characterizing the remaining part of the supernatant after extraction by means of proteolytic digestion is also demonstrated. The knowledge of both molecular weight of the whole protein and of the proteolytic fragments increases specificity for protein identification by searching in sequence databases.