How "Soft" Are Your Gas Mixtures? Effects of Modifier Gas Types on the Dissociation of Labile Ions in Differential...

We investigate the stabilizing or destabilizing effects of several carrier gas mixtures on fragile ions analyzed with differential mobility spectrometry (DMS). We employed two "thermometer ion" species, 4-methylbenzyl ammonium and 4-methoxybenzylpyridinium to estimate the effective temperature (Teff) ions experience inside the DMS in these mixed-gas environments, as well as protonated glycine, for an additional ion chemistry. Raising the separation voltage (SV) to induce substantial field heating of the thermometer ions, we found in each case that Teff was lowered by 5 K for every 10% CO2 added to the default N2 carrier gas. When holding the N2 fraction of the gas constant at 80%, combined with 20% of one of several modifier gases (He, Ne, Ar, CO2, CH4, SF6), then ramping the SV to induce ion dissociation, we found the slope of the Teff(SV) curve insensitive to modifier gas type, but that the inflection point of the survival yield curve, or critical SV (SVcrit, a proxy for the highest SV available for resolution enhancement), changed with gas type. A strong correlation was observed between SVcrit and the gas mixture viscosity. Relating this trend to theory, we propose an easy-to-calculate approximation of this correlation, including a mass correction. This model generally improved the linearity of the comparison, except for (the much more massive) SF6. These results provide empirical data on the (de)stabilizing effects of a wide range of modifier gases in DMS and provide a simple approach to predicting the impact of gas mixtures on ion stability using readily available gas viscosity data.