Humidity Effect on the Drift Times of the Reactant Ions in Ion Mobility Spectrometry

The effect of moisture content on the drift times of NH₄⁺ and H₃O⁺ reactant ions at different temperatures was experimentally and theoretically studied using an ion mobility spectrometer (IMS). The peak positions of the ions shifted to higher drift times as the humidity of the drift gas increased. The peak displacements were attributed to the consecutive formation of hydrated ion clusters, RI⁺(H₂O). Using chemical equilibrium relations and thermodynamic data derived from DFT calculation, a model was proposed to formulate the change in the drift times as 1/t_d = 1/t_d^Θ - βT log[H₂O], where β is a constant and T is temperature. [H₂O] is the concentration of water in ppm and, t_d^Θ is the drift time at the standard condition of [H₂O] = 1 ppm. The proposed equation perfectly predicted the change in the drift times of the reactant ions as a function of the moisture in the drift gas. Accordingly, standard mobility, K = K^Θ - γT ln[H₂O], was defined, which is independent of the moisture level of the drift gas and reflects the chemical reactivity. In this work, it is proposed to correct the reported reduced mobilities for the moisture to a standard condition of 1 ppm water concentration, in a similar manner to the corrections to the standard temperature and pressure of 273 K and 760 mbar, respectively. Finally, the likelihood that different hydrates forms of the reactant ion exist is discussed based on the entropy concept.