The effect of matrix evaporation on secondary ion formation in fast atom bombardment mass spectrometry has been investigated for samples dissolved in glycerol with and without the need of a co-solvent. Measurements of the evaporation rate of co-solvent-containing sample solutions exposed to vacuum show that supersaturation in the near surface layer is achieved by the loss of a significant fraction of the co-solvent in less than 1 min. Particle bombardment hampers the formation of a solid layer by precipitation of sample molecules. The physical state of viscous supersaturated solutions, amenable to molecular ion formation during continuous erosion of the layer, is considered as a metastable colloid state with precipitated microparticles. The time variation of the molecular ion intensity during erosion of a layer is mainly determined by the analyte concentration profile across the solution at the onset of sputtering and by the loss of matrix during sputtering which raises the arialyre concentration at the surface and worsens the sputtering and ionization conditions. For raffinose dissolved in glycerol molecular ions could be recorded under dynamic bombardment conditions even after a residence time of the solution in vacuum for more than 2 days.

In liquid SIMS (or fast atom bombardment mass spectrometry) a liquid sample solution is exposed to an incident keV particle beam and molecular ions are recorded during continuous erosion of the layer. We have investigated this mode of molecular ion formation under dynamic bombarding conditions regarding the effect of concentration, solid particle formation by cristallization of solutes from supersaturated solutions and the formation of a closed solid surface layer prior to the onset of particle bombardment. The experiments were performed with peptides and organic salts applying mass spectrometry and optical microscopy. They showed that molecular ions and/or cluster ions are obtained under dynamic bombarding conditions without accumulation of radiation damage from supersaturated solutions, even in the presence of small precipitated particles and after erosion of thin solid surface layers from the remaining solution. Furthermore it was found that the incident particle beam hampers crystallization of the solute from a supersaturated solution.