Influence of the Coulomb repulsion on the ion cloud propagation in atmospheric pressure laser ionization (APLI) ion mobility spectrometry

Ion Mobility Spectrometers are versatile gas analyzers. Their advantages are a small portable size, simple set up and thus robustness, fast response time and very good sensitivity in the ppb range. Thus they are typical instruments for diverse applications including environmental monitoring, work place safety (e.g. at industrial sites handling hazardous materials, but also e.g. first-responders like fire fighters etc.) and military applications. The selectivity is, however limited, so that the application for the identification of unknown substances is problematic. Since the spectrometers work with ions, one way to overcome the stated problem is to use laser radiation for analyte ionization. The ionization efficiency of lasers can be very high. If the concentration of ions becomes too large, however, Coulomb-Effects become challenging since the signals obtained when detecting the ions become distorted. In this paper we have analyzed the ion cloud propagation under the influence of Coulombic Effects for different concentrations and the related cloud shape distortion with help of finite elements method simulations.