The influence of ion/molecule reactions on the evaluation of ion mobility and diffusion coefficients

This paper deals with the evaluation of the mean and the variance of the ion flux at the exit of a drift tube, from which the drift velocity, v(d), and the longitudinal diffusion coefficient, D-L, can be derived. Besides drift and diffusion, the presence of a primary ion conversion process through reactions with the gas is fully considered from the outset. Full expressions for the mean and variance of the ion flux are then approximated by resorting to experimental conditions in which low ionic reactivity, adequate drift tube geometry, and other experimental conditions are met, thus arriving at very simple expressions from which v(d) and D-L are derived. These simple expressions have been obtained previously from analyses ignoring ion/molecule reactions from the outset. The full expressions derived here and their approximations are used to provide a means of evaluating the errors incurred when very simple expressions are used in highly reacting ion/neutral systems.