Influence of Sample Gas Humidity on Product Ion Formation in High Kinetic Energy Ion Mobility Spectrometry (HiKE-IMS)

Abstract

High Kinetic Energy Ion Mobility Spectrometers (HiKE-IMS) chemically ionize gaseous samples via reactant ions and separate the generated ions by their motion in a neutral gas under the influence of an electric field. Operation at reduced pressures of 10–40 mbar allows for reaching high reduced electric field strengths (E/N) of up to 120 Td. At these high E/N, the generated ions gain the namesake high kinetic energies, leading to a decrease in cluster size of the reactant ions by increasing the reaction rate of collision-induced cluster dissociation of hydrates. In positive ion polarity and in purified air, H3O+(H2O) n , NO+(H2O) n , and O2 +•(H2O) p are the most abundant reactant ions. In this work, we investigate the effect of varying sample gas humidity on product ion formation for several model substances. Results show that increasing the sample gas humidity at high E/N of 120 Td shifts product ion formation from a charge transfer dominated reaction system to a proton transfer dominated reaction system. For HiKE-IMS operated at high E/N, the reduction in cluster size of reactant ions allows ionization of analytes with low proton affinity even at high relative humidity in the sample gas of RH = 75% at 303.15 K and 1013.25 hPa. In contrast to conventional IMS, where increasing the sample gas humidity inhibits ionization for various analytes, increasing sample gas humidity in HiKE-IMS operated at 120 Td is actually beneficial for ionization yield of most analytes investigated in this work as it increases the number of H3O+(H2O)n.

The origin of isomerization of aniline revealed by high kinetic energy ion mobility spectrometry (HiKE-IMS)

“…Additionally, the lab-built mass spectrometer consists of an Einzel lens followed by an orthogonal time-of-flight with a spectrum acquisition rate of 50 Hz. 32,38…”

“…The HiKE-IMS with a tri-state ion shutter and the HiKE-IMS-MS used in these experiments have been described in detail previously. 31,32,38 In this work, three experiments were performed with regards to varying the reduced electric field strength in the drift region (E DR /N) and varying the reduced electric field strength in the reaction region (E RR /N) as detailed in Table 2. The HiKE-IMS is operated at a reduced pressure with an MVP-040 membrane pump controlled by an EVN-116 valve and monitored with a CMR-362 capacitance manometer (all from Pfeiffer Vacuum, Asslar Germany).…”

“…Additionally, the lab-built mass spectrometer consists of an Einzel lens followed by an orthogonal time-offlight with a spectrum acquisition rate of 50 Hz. 32,38 Data acquisition on both instruments was performed using custom software developed using LabView 2018 version 18.0f2 (National Instruments Corp., Austin, TX USA). Matlab R2020a version 9.8.0.1323502 for win64 (Mathworks Inc., Natick, MA USA) was used for data analysis (Fig.…”

Validation of Field-Dependent Ion–Solvent Cluster Modeling via Direct Measurement of Cluster Size Distributions

“…It should be noted that other mechanisms of ionization, like charge transfer reactions, can also occur in API. 7 While ion−solvent clusters still play a significant role in modern MS, 8−11 they are especially important in ion-mobility spectrometry (IMS) and related techniques. 12,13 In cases where solvent vapor aggregates with ions, their collisional cross section (CCS) and mass effectively increase, thereby diminishing the ion's mobility.…”

“…These species are bound predominantly by charge-dipole interactions at long-range and typically by hydrogen bonding at the site of protonation at short-range. It should be noted that other mechanisms of ionization, like charge transfer reactions, can also occur in API …”