Influence of Gas Flow on Ion Transport in PTR-MS Drift Tubes Using an Electro-Hydrodynamic Coupled Model

Proton transfer reaction mass spectrometry (PTR-MS) is widely used for the real-time detection of volatile organic compounds (VOCs). Previous efforts to improve PTR-MS sensitivity have mainly focused on electric-field optimization, whereas the influence of macroscopic gas flow on ion transport has received much less attention, despite the inevitable gas flow induced by pressure gradients within the drift tube. In this study, a coupled CFD-ion trajectory simulation approach was employed to systematically investigate the influence of gas flow on ion transport and the underlying mechanisms. The results show that, under the combined effects of the electric field and gas flow, ion transport is mainly influenced by three mechanisms: collision focusing, aerodynamic focusing, and recirculation loss. At low to intermediate pressures, collision focusing and aerodynamic focusing jointly enhance ion transport, resulting in an increase in ion transmission efficiency with increasing pressure. For m/z 100, the transmission efficiency increased from 33% at 100 Pa to 65% at 1000 Pa. At higher pressures, the recirculation region gradually developed and extended toward the main ion transport pathway. The resulting recirculation loss increasingly competed with aerodynamic focusing, leading to a gradual decrease in ion transmission efficiency; for m/z 100, the transmission efficiency decreased to 4% at 8000 Pa. A symmetric inlet sampling structure was further proposed to regulate the flow field. At 6000 Pa, this design substantially reduced the recirculation region and increased the ion transmission efficiency of m/z 100 from 30% to 75%, further supporting the proposed interpretation of recirculation-induced ion loss. These results reveal how gas flow modulates ion transport in PTR-MS drift tubes and provide a physical basis for selecting operating conditions, optimizing flow fields, and improving drift-tube design.