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.