Comparison of detection selectivity in ion mobility spectrometry: proton-attachment versus electron exchange ionization

Abstract

Two types of ionization have been compared for ion mobility spectrometry. The first uses a β-emitter to initiate proton exchange between an amine reactive gas and the analyte, whereas the second is based on electron exchange between photo-generated radical cations and the analyte. The latter method uses a 266-nm microchip laser in combination with naphthalene vapor to form naphthalene radical cations via two-photon ionization. Under ideal conditions, the laser-based method selectively ionizes analyte molecules on the basis of their ionization potential whereas the former does so on the basis of their proton affinity. Analysis of both pure and complex mixtures of compounds, including a variety of controlled substances such as cocaine, result in different response characteristics that are dependent on the given type of ionization employed. This gives rise to the possibility that a single instrument package can be equipped with two drift tubes to provide simultaneous analysis via the two types of ionization and thereby improve the overall detection selectivity.