Biomolecules in the gas phase. III. Multiphoton ionization mass spectra of phenylthiohydantoin amino acids and free amino acids

The multiphoton ionization (MPI) mass spectra of some phenylthiohydantoin (PTH) amino acids and free amino acids are measured and discussed. It is shown that the coupling of laser desorption with MPI allows even highly polar substances to be measured without degradation before ionization. The spatial and temporal separation of the desorption and the ionization step, together with adiabatic cooling of the desorbed neutrals, are main features of the method. As a result, this method yields mass spectra which show the molecular ion as a prominent signal in every case. In some cases, it was found essential to vary the laser wavelength to get better results. One outstanding effect of this feature was the detection of the molecular ion (M^+.) of L-arginine for the first time. The influence of the wavelength used in the multiphoton ionization step is discussed, as well as the influence of the laser intensity. The consequences for analytical purposes are reviewed.