An ambient detection system for visualization of charged particles generated with ionization methods at atmospheric pressure

RATIONALE: With the current state-of-the-art detection of ions only taking place under vacuum conditions, active pixel detectors that operate under ambient conditions are of particular interest. These detectors are ideally suited to study and characterize the charge distributions generated by ambient ionization sources. METHODS: The direct imaging capabilities of the active pixel detector are used to investigate the spatial distributions of charged droplets generated by three ionization sources, named electrospray ionization (ESI), paper spray ionization (PSI) and surface acoustic wave nebulization (SAWN). The ionization spray (ESI/PSI) and ionization plume (SAWN) originating from each source are directly imaged. The effect of source parameters such as spray voltage for ESI and PSI, and the angle of the paper spray tip on the charge distributions, is investigated. Two types of SAWN liquid interface, progressive wave (PW) and standing wave (SW), are studied. RESULTS: Direct charge detection under ambient conditions is demonstrated using an active pixel detector. Direct charge distributions are obtained of weak, homogeneous/focused and dispersed spray plumes by applying low, intermediate and high spray potentials, respectively, for ESI. Spray plume footprints obtained for various angles of PSI shows the possibility to focus the ion beam as a function of the paper angle. Differences between two designs of the SAWN interface are determined. Droplet charge flux changes are illustrated in a way similar to a total ion chromatogram. CONCLUSIONS: The use of this active pixel detector allows the rapid characterization and optimization of different ambient ionization sources without the actual use of a mass spectrometer. Valuable illustrations are obtained of changes in spatial distribution and number of charges detected for ESI, PSI and SAWN ion plumes.