Dependence of polyatomic ion mobilities on ionic size

The ion mobilities of 11 negative ions in nitrogen have been measured using a plasma chromatograph. The ions range from small negative ions such as ${\mathrm{NO}}_2^{-}$ to large organic ions such as the parent negative ion of 1‐naphthaldehyde, (C₁₁H₈O)⁻. The reduced ion mobilities for these negative ions, in addition to previously published ion mobilities of protonated hydrocarbons, were compared with the Langevin theory of ion mobility. The hard sphere average collision cross section between the ion and the neutral carrier gas N₂ was estimated using two models: (1) static model where the ion is nonrotating and (2) rotational model where the ion is assumed to be rotating rapidly. Comparison of the calculated reduced ion mobilities with the experimental values reveals that the experimental collision cross sections are intermediate between those calculated using these two models. The experimental and calculated ion mobilities gave the best agreement using the mean collision cross section for the static and rotational models. The average deviation between the experimental and calculated ion mobilities was 4% for a total of 21 positive ions and 14 negative ions. These results suggest (1) the ion size is the dominant parameter affecting the ion mobility of large molecular ions and (2) the rotation of nonspherical ions must be considered.