Analyzing Positive Reactant Ions in High Kinetic Energy Ion Mobility Spectrometry (HiKE-IMS) by HiKE-IMS–MS

Abstract

In contrast to classical ion mobility spectrometers (IMS) operating at ambient pressure, the high kinetic energy ion mobility spectrometer (HiKE-IMS) is operated at reduced pressures between 10–40 mbar. In HiKE-IMS, ions are generated in a reaction region before they are separated in a drift region. Due to the operation at reduced pressure, it is possible to reach high reduced electric field strengths up to 120 Td in both the reaction as well as drift region, resulting in a pronounced decrease in chemical cross sensitivities and a significant enhancement of the dynamic range. Until now though, only limited knowledge about the ionization pathways in HiKE-IMS is available. Typically, proton bound water clusters, H+(H2O) n , are the most abundant positive reactant ion species in classical IMS with atmospheric chemical ionization sources. However, at reduced pressure and increased effective ion temperature, the reactant ion population significantly changes. As the ionization efficiency of analyte molecules in HiKE-IMS strongly depends on the reactant ion population, a detailed knowledge of the reactant ion population generated in HiKE-IMS is essential. Here, we present a coupling stage of the HiKE-IMS to a mass spectrometer enabling the identification of ion species and the investigation of ion molecule reactions prevailing in HiKE-IMS. In the present study, the HiKE-IMS–MS is used to identify positive reactant ion populations in both, purified air and nitrogen, respectively. The experimental data suggest the generation of systems of clustered primary ions (H+(H2O) n , NO+(H2O) m , and O2 +(H2O) p ), which most probably serve as reactant ions. Their relative abundances highly depend on the reduced electric field strength in the reaction region. Furthermore, their effective mobilities are studied as a function of the reduced electric field strength in the drift region.

Formation of positive product ions from substances with low proton affinity in high kinetic energy ion mobility spectrometry

“…However, when evaluating the ionization pathways in HiKE‐IMS, the reactant ion population inside the reaction region is relevant. As known from previous studies, 21,22 field‐ and moisture‐dependent conversion reactions of O 2 + (H 2 O) n and NO + (H 2 O) n into protonated water clusters H 3 O + (H 2 O) n proceed in the reaction region of the HiKE‐IMS instrument. Thus, the reactant ion population inside the reaction region significantly differs from the population at the end of the reaction region, shown in Figure 1.…”

“…In previous studies, the prevailing positive reactant ion population in HiKE‐IMS was identified by coupling the HiKE‐IMS instrument to a mass spectrometer, 21 and H 3 O + (H 2 O) n , NO + (H 2 O) n , and O 2 + (H 2 O) n ions were observed. At thermal ion–molecule interaction energies, extensive information is available from selected ion flow tube mass spectrometry studies concerning the reactions of H 3 O + , NO + , and O 2 + with various analyte molecules 25–39 .…”

“…In previous works, the ion species underlying these peaks have been identified as NO + (H 2 O) n , H 3 O + (H 2 O) n , and O 2 + (H 2 O) n . 21,22 Increasing the reduced electric field strength in the reaction region, the total charge in the spectrum increases due to reduced Coulomb repulsion 58 . However, when investigating chemical reactions leading to the conversion of ion species, it is more useful to consider relative abundances instead of absolute abundances.…”

“…A detailed description of the HiKE‐IMS setup was previously provided by Kirk et al 24 The operating parameters used in this work are presented in Table 1. To identify individual ion species associated with signals in the ion mobility spectrum, the HiKE‐IMS‐MS coupling described in a previous work 21 is used. It should be noted that the HiKE‐IMS instrument may generally be operated at pressures between 10 and 40 mbar.…”

“…However, until now, only limited knowledge about the ionization processes occurring in HiKE‐IMS has been available. In previous studies, the prevailing reactant ion population in HiKE‐IMS has been identified by coupling a HiKE‐IMS instrument to a mass spectrometer 21–23 . Due to operation at decreased pressure and high reduced electric field strengths, the reactant ion population in HiKE‐IMS can significantly differ from that in classical ambient pressure IMS.…”