Sensitivity vs Competing Proton Transfer Reactions: Addressing Key Parameters of Ion Chemistry in Ion Mobility Spectrometry.

Ion mobility spectrometers (IMS) are widely used in various gas sensing applications due to their high sensitivity and rapid analysis times. However, in complex gas mixtures, reactions between the protonated target analyte and interfering species can lead to discrimination of analytes with low gas basicity, reducing sensitivity or even making detection impossible. Operating IMS at low pressure and high reduced electric field strengths has been shown to mitigate these competing ion-molecule reactions. Therefore, in this work, we present a kinetic model to evaluate the effect of key operating parameters on the ion suppression caused by competing ion-molecule reactions, guiding the instrumental design of IMS. The results demonstrate that measures to reduce competing ion-molecule reactions, such as lowering the operating pressure or reaction time, also reduce sensitivity due to fewer ion-neutral collisions. However, in scenarios with high concentrations of interferents, the reduced effect of competing ion-molecule reactions is critical for detecting target analytes with low gas basicity, thereby enhancing sensitivity under such conditions. Based on these findings, decreasing operating pressure and reaction time or increasing reduced electric field strength are the most promising strategies to minimize competing reactions and, in complex chemical backgrounds, increase sensitivity.