Researchers Hualei Hua and Haiyang Li, from the Dalian Institute of Chemical Physics, Chinese Academy of Sciences, have uncovered a new phenomenon of switching between pressure-driven photoelectron ionization and Penning ionization using their independently developed photoionization time-of-flight mass spectrometry. They have proposed a novel method to broaden the ionization range and enhance sensitivity by coupling with radiofrequency fields and adjusting pressure and carrier gas. Their findings have been published in Analytical Chemistry.
In this study, it was observed that in a coupled radiofrequency field photoionization source, as the pressure increased from 50 Pa to 500 Pa, the contribution of photoelectrons gradually shifted from collisional ionization, leading to reactant ions, initiating ion-molecule reactions, to collisional excitation resulting in metastable atoms/molecules, and eventually triggering Penning ionization. Penning ionization efficiently ionizes analytes with ionization energies higher than the photon energy, exhibiting sensitivity comparable to single-photon ionization. For analytes with ionization energies lower than the photon energy, such as toluene, sensitivity was enhanced by over 5 times compared to single-photon ionization. The team further investigated mixtures of different ionization energy analytes and background gases with different excitation energies, revealing the contribution of metastable atoms/molecules to the ionization enhancement process. They established the kinetic energy balance equation for electrons in the electric and flow fields, explored the relationship between electron energy distribution and pressure, and elucidated the physical mechanism of switching between photoelectron ionization and Penning ionization.
This research provides new insights for developing novel, highly selective, and sensitive photoionization sources, as well as understanding the ionization mechanisms of other discharge ionization sources.
Related paper: https://doi.org/10.1021/acs.analchem.4c00686
