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Laser evaporization of intact neutral molecules into a supersonic beam combined with multiphoton ionization (MUPI) is used to study the fragmentation behaviour of peptides. Owing to the separation of desorption and ionization, an optimization of these processes can be applied to the sample. The investigation of mixtures containing hydrophobic and hydrophilic peptides shows equal probabilities for detection of the two molecules, demonstrating that the neutral yield of both classes of compound is equal in the desorption process. A loss of sensitivity is not observed. By employing the feature of tunable fragmentation, it is possible to sequence peptides in the gas phase. At low laser intensities only the molecular ions are formed. By increasing photon intensities, fragmentation reactions are induced. Owing to the nature of the multiphoton ionization, these mass spectra (at moderate laser powers) contain few and only structurally dependent signals. The molecular ion of the sample investigated is detected in every case.

Details of the novel method of laser desorption with a low powered IR-laser and resonance-enhanced multiphoton ionization (REMPI) combined with a high-resolution Reflectron-Time-of-Flight (RETOF) mass spectrometer are explained. Different features of the method are discussed. Some results of mass spectrometric investigations of biomolecules are shown.