Development of an Atmospheric Pressure Penning Ionization Source for Gas Analysis

Abstract

A direct gas introduction type atmospheric pressure Penning ionization source for on-site analysis of environmental organic pollutants was developed. The long-lived metastable argon atoms Ar^* (³P₂ and ³P₀) were utilized for Penning ionization. As the excitation energies of Ar^* are 11.5 eV (³P₂) and 11.8 eV (³P₀), the major components of air such as N₂, O₂, CO₂, CO, and H₂O are not ionized with Ar^* but most of the organic compounds can be ionized. Atmospheric pressure Penning ionization mass spectra of all samples studied in our experiment showed abundant molecular-related ions (M⁺, [M+H]⁺, and [M-H]⁺) as base peaks. In the mass spectra of sample gases (M) mixed with methanol, intense ion [M+CH₃]⁺ was observed in addition to [M+H]⁺ and [2M+H]⁺. The appearance of [M+CH₃]⁺ is due to the occurrence of the methyl cation transfer reaction, CH₃OH₂⁺+M→CH₃⁺M+H₂O.