Ion Transport via a Traveling Wave in a Cylindrical Guide

Abstract

Commercial traveling-wave ion mobility (TWIM) devices require calibration for collisional cross section (CCS) measurements due to nonlinear effective ion mobilities in transport. Previous theoretical treatments of ion motion in TWIM devices have modeled the mean velocity of on-axis ions and have considered approximations to a real device via perturbation theory. This work extends previous on-axis analysis of the cylindrical guide to include three-dimensional effects. We provide a simplified theoretical treatment for ion motion in smoothly propagating traveling waves, wherefrom we derive the radially dependent axial mean drift speeds of the ions. We then investigate the different behaviors arising from the spatially and temporally varying electric field and how they affect the ion transit times. We also include a brief treatment of effects from anharmonic waveforms, which result in minimal corrections to axial transport, validated by previous work. We then consider perturbative inertial effects to account for the mass of ions, which lead to focusing effects that complicate axial transport speeds due to ions traveling radially within the cylindrical guide.