Spatially Confined Photoinduced Penning Ionization for Highly Sensitive Mass Spectrometry

In vacuum ultraviolet (VUV) lamp-based photoionization mass spectrometry (PI-MS), the axially mounted configuration exhibits higher sensitivity due to a longer light absorption path. However, VUV light irradiation of the metal surfaces within the transmission chamber induces photoelectron emission; excessive background gas ions are generated via photoelectron ionization under the radio frequency (RF) electric field, which affects the quantitative performance and repeatability of PI-MS, especially for deuterium lamps with a higher photon flux. In this study, a size- and position-adjustable baffle electrode was integrated into the ionization chamber to reduce the light leakage into the transmission chamber. For a 3 mm-diameter baffle positioned 5.4 mm from the skimmer, the intensity of background gas ions generated in the transmission chamber decreased to 1/506, while the intensity of the trichloroethylene ion was maintained at half that without the baffle. The intensities of chlorinated hydrocarbon ions were enhanced by 9-fold compared with the SPI mode when an RF voltage was further applied to the baffle electrode, under which the dominant ionization process was Penning ionization (PenI). Surprisingly, the intensities of chlorinated hydrocarbon ions did not decrease but increased by up to 126% compared to those without the baffle. Simulation of the electric field distribution and electron kinetic energy highlights that the precise overlap of the ion generation region and efficient transmission region plays a pivotal role in the regulation of spatially confined PenI toward sensitivity improvement. This approach has achieved limits of detection down to sub-pptv, even for molecules with ionization energies higher than the photon energy.