Atmospheric Pressure Sampling for Laser Ablation Based Nanoscale Imaging Mass Spectrometry: Ions or Neutrals?

Although the ratio of neutrals-to-ions in a typical laser ablation event was reported to be on the order of 1000 or greater, most imaging mass spectrometry (IMS) studies collect the minor ionic component instead of the abundant neutrals for subsequent mass analysis. In this report, we present a fundamentally different strategy, sampling neutrals from atmospheric pressure laser ablation into the vacuum of the mass spectrometer, followed by postionization, and mass spectrometric analysis. We compare its overall sampling efficiency (transfer efficiency + ionization efficiency) with that of ion sampling. The products from many single ablation events (creating a crater of ∼1 μm in diameter each) were deposited on a collection plate placed on the vacuum side of the sampling capillary. The sample surface and the collection plate were carefully examined both prior to and after laser ablation with scanning electron microscopy (SEM) and scanning probe microscopy (SPM). Volumetric measurements gave an estimate of the sampling efficiency of (1 ± 0.7) × 10−4 overall. It was found that with a proper collection geometry, ablated neutral molecules can be efficiently directed to the inlet of the sampling capillary: several % are available for further MS analysis. We observed that the fraction of the ablated mass that leaves the sample in the form of particles was not sampled into the vacuum but was instead deposited between the ablation site and the capillary inlet. By comparing our results with those of other IMS techniques using ion sampling, we conclude that the overall sampling efficiencies are similar. Advantages provided by the neutral sampling approach include a large amount of analyte available for collection, the potential for improving the ionization efficiency, and the elimination of sample pretreatment steps.