On the formation of initial ion velocities in matrix-assisted laser desorption ionization: Virtual desorption time as an additional parameter describing ion ejection dynamics

Measurements of initial ion velocities prior to ion acceleration were performed using a special instrumental setup with a variable length of a first field-free stage in a two-stage MALDI ion source. Experimental results showed that drift times through the field-free stage cannot be extrapolated to zero when reducing the length of this stage, but indicate a condition-specific temporal offset. This offset can mathematically be interpreted as a delayed start of ions and is therefore called "virtual desorption time". Mechanistically these virtual desorption times have to be interpreted as being the result of acceleration processes of analyte ions in a molecular jet, leading to various dependencies of analyte ion velocities and virtual desorption times on the chosen experimental parameters, rather than being the result of a delayed desorption of molecular ions or clusters from the surface. Formation of analyte ions by cluster decay is in accordance with the reported observations, as is formation of analyte ions by charge exchange in the gas phase. In both cases, however, the presence of an efficient molecular jet is suggested by these experimental results, leading to a subsequent acceleration of entrained molecular ions after detachment of molecular entities from the solid phase. (C) 2002 Elsevier Science B.V. All rights reserved.