Study of Coulombic broadening in stand-alone ion mobility spectrometry

This paper proposes a method for investigating the Coulombic broadening, t_cou, in an ambient pressure ion mobility spectrometer based on the analysis of peaks originating from O₂ ^-(H₂O)_n and electrons. It showed that the full width at half maximum (FWHM) of electrons was independent of electron density and remained at a constant contribution from the initial width of ion packets, t_g, and amplifier broadening, t_amp; in contrast, the FWHM of O₂ ^-(H₂O)_n increased with the rise of O₂ ^-(H₂O)_n density due to additional contributions from t_cou and diffusion broadening t_diff. The t_cou of O₂ ^-(H₂O)_n was extracted from the FWHM by subtracting t_g, t_amp, and t_diff. The t_cou of O₂ ^-(H₂O)_n was found to increase from 0.14 ms to 0.24 ms as the O₂ ^-(H₂O)_n density increased from 7.35 × 10⁵ to 1.22 × 10⁷ cm^-3. The percentage of t_cou in the FWHM was in the range of 45%-80%, and the Coulomb-limited resolving power decreased from 70 to 40 as the ion density increased, which indicated that the Coulomb effect was dramatic.