An Ionization Method Based on Photoelectron Induced Thermal Electron Generation: capillary Atmospheric Pressure Electron Capture Ionization (cAPECI)

摘要

A novel method for atmospheric pressure ionization of compounds with high electron affinity (e.g., nitroaromatic compounds) or gas phase acidity (e.g., phenols) is reported. The method is based on the generation of thermal electrons by the photoelectric effect, followed by electron capture of oxygen in air or, within pure nitrogen, of the analyte itself. In the presence of oxygen, ionization of the analyte is accomplished via charge transfer or proton abstraction by the strong gas phase base O_2~-. In terms of least invasive sample structure, the interaction of UV-light with metals represents a very clean method for the generation of thermal electrons at atmospheric pressure. This leads to a soft and selective ionization method, generating exclusively negative ions. The implementation of the ionization stage within a fast flowing gas system additionally reduces the retention time of the ionized sample within the high pressure region of the mass spectrometer. Therefore ion transformation processes are reduced and the mass spectrum corresponds more closely to the neutral analyte distribution than for ionization methods operating in conventional ion sources.