Incorporation of thermal rotation of drifting ions into mobility calculations: Drastic effect for heavier buffer gases

Ion mobility spectrometry (IMS) assumes increasing prominence among the too ls for characterization of gas-phase ions and analysis of complex mixtures. The assignment of features observed in IMS experiments to specific structu res necessitates the calculation of mobilities for plausible candidate geom etries. All previous methods for these calculations have assumed that the i on-buffer gas collisions are fully elastic and that the drifting ions canno t rotate during a collisional event. This paradigm does not mesh well with the fact that the measured quantity is the orientationally averaged collisi on integral. Here we model the effect of the thermal rotation of drifting i ons on their mobility. Simulations show that the cross sections for rotatin g objects are greater than those for static ones because a molecular image "blurs out" over the duration of collision. This increase is particularly s ignificant for light and elongated ions. For a given ion, the effect is dra matically larger in heavy buffer gases, in some cases exceeding 20%. Presen t findings reveal the importance of accounting for the nonelasticity of sca ttering in ion mobility calculations.