Interpretation of Ion-Mobility Experiments in Reacting Systems

Abstract

The Tyndall drift-tube experiment with a pulsed ion source has been analyzed for the case of two ions that interconvert in the drift space. Exact solutions for the relevant differential equations have been obtained. It is shown that when the time constants of the interconversion reactions become commensurate with the ion drift times, the ion velocities are given by the initial and final bounds of the ion-arrival curves for the fast and slow ion, respectively. Peaks which may occur at intermediate times are a consequence of the reacting system and must not be misinterpreted as an ion transit time. It is also shown that the reaction time constants may be extracted from the experimental data, if the analysis is extended to include diffusion and effects of the finite time resolution of the experiment. For this purpose a numerical simulation of the system is required. The relationship between this numerical approach and the analytical solution is demonstrated.