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

Maria Allers,Ansgar T Kirk,Christoph Schaefer,Florian Schlottmann,Stefan Zimmermann

doi:10.1002/rcm.8998

Abstract

RationaleIon mobility spectrometry (IMS) instruments are typically equipped with atmospheric pressure chemical ionization (APCI) sources operated at ambient pressure. However, classical APCI‐IMS suffers from a limited ionization yield for nonpolar substances with low proton affinity (PA). This is mainly due to ion clustering processes, especially those that involve water molecules, inhibiting the ionization of these substances.MethodsHigh Kinetic Energy (HiKE)‐IMS instruments are operated at decreased pressures and high reduced electric field strengths. As most clustering reactions are inhibited under these conditions, the ionization yield for nonpolar substances with low PA in HiKE‐IMS should differ from that in classical APCI‐IMS. To gain first insights into the ionization capabilities and limitations of HiKE‐IMS, we investigated the ionization of four model substances with low PA in HiKE‐IMS using HiKE‐IMS‐MS as a function of the reduced electric field strength.ResultsThe four model substances all have proton affinities between those of H2O and (H2O)2 but exhibit different ionization energies, dipole moments, and polarizabilities. As expected, the results show that the ionization yield for these substances differs considerably at low reduced electric field strengths due to ion cluster formation. In contrast, at high reduced electric field strengths, all substances can be ionized via charge and/or proton transfer in HiKE‐IMS.ConclusionsConsidering the detection of polar substances with high PAs, classical ambient pressure IMS should reach better detection limits than HiKE‐IMS. However, considering the detection of nonpolar substances with low PA that are not detected, or only difficult to detect, at ambient pressure, HiKE‐IMS would be beneficial.

Highlights

Due to their high sensitivity, fast response times, and compact design, ion mobility spectrometers are commonly used in safety and security applications such as the detection of chemical warfare agents,[1,2] toxic industrial chemicals,[3,4] drugs,[5,6] and explosives.[7,8,9] Basically, ion mobility spectrometry (IMS) instruments can be divided by their principle of ion separation
In DT-IMS, ions are separated by their motion along the axis of a drift tube driven by a homogeneous static electric field
The ionization pathways occurring in High Kinetic Energy (HiKE)-IMS significantly depend on the reduced electric field strength in the reaction region, affecting both the ion's residence time in the reaction region and its kinetic energy

Summary

Introduction

Due to their high sensitivity, fast response times, and compact design, ion mobility spectrometers are commonly used in safety and security applications such as the detection of chemical warfare agents,[1,2] toxic industrial chemicals,[3,4] drugs,[5,6] and explosives.[7,8,9] Basically, ion mobility spectrometry (IMS) instruments can be divided by their principle of ion separation. A drift tube (DT) ion mobility spectrometer is used. In DT-IMS, ions are separated by their motion along the axis of a drift tube driven by a homogeneous static electric field. An ion packet is injected into the drift tube. During their motion, the ions are separated based on the absolute value of their ion mobility in the present drift gas. At the end of the drift tube, the ions are captured by a detector that converts

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