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

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 RP = 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

Negative Reactant Ion Formation in High Kinetic Energy Ion Mobility Spectrometry (HiKE-IMS)

Due to the operation at background pressures between 10-40 mbar and high reduced electric field strengths of up to 120 Td, the ion–molecule reactions in High Kinetic Energy Ion Mobility Spectrometers (HiKE-IMS) differ from those in classical ambient pressure IMS. In the positive ion polarity mode, the reactant ions H+(H2O)n, O2+(H2O)n, and NO+(H2O)n are observed in the HiKE-IMS. The relative abundances of these reactant ion species significantly depend on the reduced electric field strength in the reaction region, the operating pressure, and the water concentration in the reaction region. In this work, the formation of negative reactant ions in HiKE-IMS is investigated in detail. On the basis of kinetic and thermodynamic data from the literature, the processes resulting in the formation of negative reactant ions are kinetically modeled. To verify the model, we present measurements of the negative reactant ion population in the HiKE-IMS and its dependence on the reduced electric field strength as well as the water and carbon dioxide concentrations in the reaction region. The ion species underlying individual peaks in the ion mobility spectrum are identified by coupling the HiKE-IMS to a time-of-flight mass spectrometer (TOF-MS) using a simple gated interface that enables the transfer of selected peaks of the ion mobility spectrum into the TOF-MS. Both the theoretical model as well as the experimental data suggest the predominant generation of the oxygen-based ions O–, OH–, O2–, and O3– in purified air containing 70 ppmv of water and 30 ppmv of carbon dioxide. Additionally, small amounts of NO2– and CO3– are observed. Their relative abundances highly depend on the reduced electric field strength as well as the water and carbon dioxide concentration. An increase of the water concentration in the reaction region results in the generation of OH– ions, whereas increasing the carbon dioxide concentration favors the generation of CO3– ions, as expected

Positive Reactant Ion Formation in High Kinetic Energy Ion Mobility Spectrometry (HiKE-IMS)

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 of between 10 and 40 mbar and higher reduced electric field strengths of up to 120 Td. Thus, the ion–molecule reactions occurring in the HiKE-IMS can significantly differ from those in classical ambient pressure IMS. In order to predict the ionization pathways of specific analyte molecules, profound knowledge of the reactant ion species generated in HiKE-IMS and their dependence on the ionization conditions is essential. In this work, the formation of positive reactant ions in HiKE-IMS is investigated in detail. On the basis of kinetic and thermodynamic data from the literature, the ion–molecule reactions are kinetically modeled. To verify the model, we present measurements of the reactant ion population and its dependence on the reduced electric field strength, the operating pressure, and the water concentration in the sample gas. All of these parameters significantly affect the reactant ion population formed in HiKE-IMS

The role of ion-bound cluster formation in negative ion mass spectrometry

RATIONALEThe ionization mechanisms operative in negative ion atmospheric pressure mass spectrometry are far from being properly understood. In an excess of oxygen superoxide (O2–) is generally the primary charge-carrying species that is generated. However, subsequent reactions leading to the finally detected ion signals remain obscure.METHODSSince adiabatic expansion induced cluster growth and collision-induced dissociation (CID) processes rendered a representative sampling of ion distributions present in the source difficult, a custom-built thermally sampling time-of-flight mass spectrometer was used for the investigations. Using atmospheric pressure laser ionization of toluene as the reagent gas, high yields of thermal electrons were observed, but only negligible amounts of by-products. Ab initio calculations for individual ion/molecule reaction pathways were performed.RESULTSElectron capture by molecular oxygen resulted in the formation of subsequent superoxide water clusters as well as distinct analyte-adduct ions. By adjusting the extent of CID within the ion optical stages of the mass spectrometer, the cluster distribution changes to smaller cluster sizes and the analyte signals strongly shifted towards M– or [M–H]–. The observed superoxide water cluster distribution was close to thermal. The theoretical results confirmed the experimental findings.CONCLUSIONSIn negative atmospheric pressure mass spectrometry the water concentration in the ion source (determining the ionization efficiency) and the CID energy provided through electrical fields (determining the ion distribution) are primary, critical parameters for the observed overall ionization mechanism and efficiency