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 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 high resolving power of R P = 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 H 3 O + (H 2 O) n , O 2 + (H 2 O) n , NO + (H 2 O) n , NO 2 + (H 2 O) n , and NH 4 + (H 2 O) n , as well as the negative reactant ions O 2 -(H 2 O) n , O 3 -(H 2 O) n , CO 3 -(H 2 O) n , HCO 3 -(H 2 O) n , and NO 2 -(H 2 O) 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 H 3 O + (H 2 O) n and NO + (H 2 O) n .

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

“…Because of the high E DT /N of 120 Td used in this work, the hydrates formed in the reaction region quickly dissociate within the drift region. 53 As a result, the ions travel through the drift region at the same drift velocity regardless of their cluster size within the reaction region. For this reason, the cluster size of the reactant ions within the reaction region, significantly affecting the ionization of analyte molecules, can only be investigated by kinetic modeling.…”

“…After the ions generated in the reaction region are injected into the drift region, the equilibrium of the association reactions and shifts depending on the reduced drift field strength and water concentration inside the drift region. Because of the high E DT / N of 120 Td used in this work, the hydrates formed in the reaction region quickly dissociate within the drift region . As a result, the ions travel through the drift region at the same drift velocity regardless of their cluster size within the reaction region.…”

Simulation of Cluster Dynamics of Proton-Bound Water Clusters in a High Kinetic Energy Ion-Mobility Spectrometer

“…This is caused by the very fast cluster reactions: At atmospheric pressure, chemical equilibrium within the water cluster system is reached within a few microseconds under common operation conditions, whereas the drift time is in the range of few milliseconds. Thus, the very fast clustering/declustering reactions increasingly decorrelate the chemical state of the individual cluster ions from their initial chemical state when entering the drift region; this holds true even in the HiKE-IMS …”

“…Thus, the very fast clustering/declustering reactions increasingly decorrelate the chemical state of the individual cluster ions from their initial chemical state when entering the drift region; this holds true even in the HiKE-IMS. 10 The RIP is one of the most common ion signals in IMS, as proton-bound water clusters are generally formed in atmospheric pressure chemical ionization (APCI) sources, e.g., by corona discharge 11−14 or β radiation from, e.g., 63 Ni. 15 Proton-bound water clusters are widely used to generate protonated analyte molecules.…”

“…In combination, these characteristics lead to a situation where the water cluster system does not necessarily reach full equilibration in the drift region anymore. 10 While in an AP-IMS instrument this system is essentially always equilibrated and thus thermodynamically controlled, a transient state and therefore kinetically controlled concentrations can be achieved in the HiKE-IMS: The time to reach equilibrium becomes longer than the residence time in the HiKE-IMS at low water mixing ratios. Thus, the chemical reaction system is in a transient state during the drift through the HiKE-IMS.…”