Evaluation of carrier gases for use in high-field asymmetric waveform ion mobility spectrometry.

Effects of carrier gas type (N2, O2, CO2, N2O, and SF6) on changes in the ratio of high- to low-field ion mobility, Kh/K, of cesium, gramicidin S, tetrahexylammonium, heptadecanoic acid, and aspartic acid in fields of up to 67 Td are presented. The theory of the mobility of ions at high E/N in different gases is discussed. Plots of Kh/K as a function of the ionic energy parameter, E/N, for the five ions in each of the gases were derived from experimental data collected using a high-field asymmetric waveform ion mobility spectrometer. The change in the ratio of high- to low-field ion mobility of cesium in carrier gases of O2 and N2 showed excellent agreement with literature values. The behavior of cesium in O2 and N2 is used to illustrate that the ratio Kh/K as a function of effective temperature is invariant with gas type as long as the well depth of the interaction potential significantly exceeds thermal energy. From these results, it appears that the well depth of the interaction potential of the heavier ions studied here, including gramicidin S, tetrahexylammonium, and heptadecanoic acid, with bath gases such as N2 and O2, is shallow relative to thermal energy.