Ionization mechanisms in matrix-assisted laser desorption ionization mass spectrometry: Contribution of pre-formed ions

The mechanisms of ion formation in matrix-assisted laser desorption/ionization mass spectrometry are still largely unknown, Ions may exist as pre-formed species in the solid state, or may form by ion-molecule reactions initiated by the laser shot, or, most probably, they may originate from a combination of both processes, The contribution of pre-formed ions was approached from the standpoint of coordination chemistry and studied in different transition metal ion/matrix systems as well as in the copper/polystyrene system, The solution, the solid and the gas phases were investigated by means of infrared, visible and proton nuclear magnetic resonance spectroscopy, as well as by matrix-assisted laser desorption/ionization (MALDI) mass spectrometry. It was found that the contribution of pre-formed ions can be predicted on the basis of condensed phase thermodynamics, MALDI mass spectra qualitatively reflect the amount of pre-formed complexes in the solid target. (C) 1997 by John Wiley & Sons, Ltd.