Optimization of the ion cloud shape resulting from the direct coupling of an atmospheric ion mobility spectrometer to a mass spectrometer

Abstract

Ion Mobility Spectrometers are important devices whenever gases need to be analyzed, e.g. in environmental monitoring, to ensure workplace safety, but also in military applications. Their advantages are a small device footprint, robustness, and ease of use combined with very high sensitivity (in the ppb range) and speed of response (in the range of ms). The main disadvantage, however, is a quite limited selectivity, i.e. a lot of substances lead to the same response signal obtained from these devices. A typical approach to improve on that side is to combine an ion mobility spectrometer with a mass spectrometer, since then not only the mobility is known, but also the mass of the substance and thus two parameters. The main problem is that mass spectrometers need to operate in vacuum, while ion mobility spectrometers work in ambient pressure; the challenge is thus to transport the analytes through a set up where the pressure is lowered by several orders of magnitude. In this paper we analyzed and optimized with help of finite elements method simulations the influence of this coupling in form of a 100 μm wide tunnel on the ion cloud in the ion mobility spectrometer and thus the change of the signal response.