Field-Dependent Reduced Ion Mobilities of Positive and Negative Ions in Air and Nitrogen in High Kinetic Energy Ion Mobility Spectrometry (HiKE-IMS)

Abstract

In High Kinetic Energy Ion Mobility Spectrometry (HiKE-IMS), ions are formed in a reaction region and separated in a drift region, which is similar to classical drift tube ion mobility spectrometers (IMS) operated at ambient pressure. However, in contrast to the latter, the HiKE-IMS is operated at a decreased background pressure of 10-40 mbar and achieves high reduced electric field strengths of up to 120 Td in both the reaction and the drift region. Thus, the HiKE-IMS allows insights into the chemical kinetics of ion-bound water cluster systems at effective ion temperatures exceeding 1000 K, although it is operated at the low absolute temperature of 45 °C. In this work, a HiKE-IMS with a high resolving power of <i>R</i>_<i>P</i> = 140 is used to study the dependence of reduced ion mobilities on the drift gas humidity and the effective ion temperature for the positive reactant ions H₃O⁺(H₂O)_n, O₂⁺(H₂O)_n, NO⁺(H₂O)_n, NO₂⁺(H₂O)_n, and NH₄⁺(H₂O)_n, as well as the negative reactant ions O₂^-(H₂O)_n, O₃^-(H₂O)_n, CO₃^-(H₂O)_n, HCO₃^-(H₂O)_n, and NO₂^-(H₂O)_n. By varying the reduced electric field strength in the drift region, cluster transitions are observed in the ion mobility spectra. This is demonstrated for the cluster systems H₃O⁺(H₂O)_n and NO⁺(H₂O)_n.