The Distribution of Ion Acceptance in Atmospheric Pressure Ion Sources: Spatially Resolved APLI Measurements

It is demonstrated that spatially resolved mass selected analysis using atmospheric pressure laser ionization mass spectrometry (APLI MS) represents a new powerful tool for mechanistic studies of ion-molecule chemistry occurring within atmospheric pressure (AP) ion sources as well as for evaluation and optimization of ion source performance. A focused low-energy UV laser beam is positioned computer controlled orthogonally on a two-dimensional grid in the ion source enclosure. Resonance enhanced multiphoton ionization (REMPI) of selected analytes occurs only within the confined volume of the laser beam. Depending on the experimental conditions and the reactivity of the primary photo-generated ions, specific signal patterns become visible after data treatment, as visualized in, e.g., contour or pseudo-color plots. The resulting spatial dependence of sensitivity is defined in this context as the distribution of ion acceptance (DIA) of the source/analyzer combination. This approach provides a much more detailed analysis of the diverse processes occurring in AP ion sources compared with conventional bulk signal response measurements.