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The process of matrix-assisted laser desorption/ionization (MALDI) has been investigated by studying the luminescence of the matrix molecules after UV-laser irradiation. A grating spectrograph equipped with an optical multichannel analyzer (OMA) has been used for the measurements. The broad shape of the spectrum and the fact that the molecules form hydrogen bonds with neighbouring molecules in the solid phase support the hypothesis that the molecules form excimers after laser irradiation. The increase of the decay time and the intensity with decreasing temperature suggests that backbone vibrations play an important role and that photon emission occurs simultaneously with the emission of a phonon. The activation energy for this process has been estimated to be 0.023 eV. At laser fluences above 2 mJ/cm 2 the luminescence is quenched, presumably by S 1 -S 1 interactions leading to the excitation of higher excited states S n or to electron emission. Both S 1 -S 1 -annihilation and electron emission can influence the MALDI-process.

In 1974 Macfarlane discovered that fast heavy ions from a 252-Cf source can desorb and ionize molecules from a solid surface. The mass of the molecules was determined by time-of-flight technique. It has been shown that the desorption mechanism is associated with the electronic part of the stopping power of the primary ion and the name ‘electronic sputtering’ has been adopted for the phenomenon to distinguish it from the well-known sputtering process with ions of keV energy. A review of electronic sputtering of biomolecules will be given as well as recent measurements on Langmuir-Blodgett films. One important application of electronic sputtering is in the field of mass spectrometry. With this technique large and nonvolatile molecules can be studied. Particularly adsorption of biomolecules to a nitrocellulose backing has proven to be very useful. Examples will be given of mass spectra from peptides with a molecular weight above 20,000 u.