Influence of Reduced Field Strength on Product Ion Formation in High Kinetic Energy Ion Mobility Spectrometry (HiKE-IMS)

Abstract

Classical ion mobility spectrometers (IMS) operated at ambient pressure, often use atmospheric pressure chemical ionization (APCI) sources to ionize organic compounds. In APCI, reactant ions ionize neutral analyte molecules via gas-phase ion–molecule reactions. The positively charged reactant ions in purified, dry air are H3O+, NO+, and O2+•. However, the hydration of reactant ions in classical IMS operated at ambient pressure renders ionization of certain analytes difficult. In contrast to classical IMS operated at ambient pressure, High Kinetic Energy Ion Mobility Spectrometers (HiKE-IMS) are operated at a decreased pressure of 10–40 mbar, allowing operation at high reduced electric field strengths of up to 120 Td. At such high reduced field strengths, ions reach high effective temperatures causing collision-induced cluster dissociation of the hydrated gas-phase ions, allowing ionization of nonpolar and low proton affinity analytes. The reactant ion population, consisting of H3O+(H2O)n, NO+(H2O)m, and O2+•(H2O)p with an individual abundance that strongly depends on the reduced field strength, differs from the reactant ion population in IMS operated at ambient pressure, which affects the ionization of analyte molecules. In this work, we investigate the influence of reduced field strength on the product ion formation of aromatic hydrocarbons used as model substances. A HiKE-IMS-MS coupling was used to identify the detected ion species. The results show that the analytes form parent cations via charge transfer with NO+(H2O)m and O2+•(H2O)p depending on ionization energy and protonated parent molecules via proton transfer and ligand switching with H3O+(H2O)n mainly depending on proton affinity.