Examination of Organic Vapor Adsorption onto Alkali Metal and Halide Atomic Ions by using Ion Mobility Mass Spectrometry

We utilize ion mobility mass spectrometry with an atmospheric pressure differential mobility analyzer coupled to a time-of-flight mass spectrometer (DMA-MS) to examine the formation of ion-vapor molecule complexes with seed ions of K⁺, Rb⁺, Cs⁺, Br⁻, and I⁻ exposed to n-butanol and n-nonane vapor under subsaturated conditions. Ion-vapor molecule complex formation is indicated by a shift in the apparent mobility of each ion. Measurement results are compared to predicted mobility shifts based upon the Kelvin–Thomson equation, which is commonly used in predicting rates of ion-induced nucleation. We find that n-butanol at saturation ratios as low as 0.03 readily binds to all seed ions, leading to mobility shifts in excess of 35 %. Conversely, the binding of n-nonane is not detectable for any ion for saturation ratios in the 0–0.27 range. An inverse correlation between the ionic radius of the initial seed and the extent of n-butanol uptake is observed, such that at elevated n-butanol concentrations, the smallest ion (K⁺) has the smallest apparent mobility and the largest (I⁻) has the largest apparent mobility. Though the differences in behavior of the two vapor molecules types examined and the observed effect of ionic seed radius are not accounted for by the Kelvin–Thomson equation, its predictions are in good agreement with measured mobility shifts for Rb⁺, Cs⁺, and Br⁻ in the presence of n-butanol (typically within 10 % of measurements).