The Protective Effects of CO₂ on Fragile Ions in Differential Mobility Spectrometry

We explore the protective effects of adding CO₂ to the N₂ carrier gas when we conduct differential mobility spectrometry (DMS) analysis of fragile ions. A selection of fragile analytes of varying chemistries were chosen from our lab inventory and include protonated glycine, methylbenzyl ammonium, methoxybenzylpyridinium, the protonated 2-pentanone dimer, deprotonated GenX (a perfluoroalkyl substance; PFAS), and deprotonated trifluoroacetic acid. By raising the separation voltage or the carrier gas temperature, conditions were set to induce fragmentation of the analyte ions within the DMS collision cell. Subsequently introducing CO₂ into the N₂ carrier gas at concentrations ranging from 10 - 70% mitigated ion fragmentation and resulted in signal intensity gains of multiple orders of magnitude. Interestingly, stabilization of the fragile ions sometimes occurred without introducing significant ionogram peak shifts (i.e., shifts of less than 1 V), indicating that these ions exhibit relatively weak interactions with the CO₂ modifier. Electronic structure calculations yield Gibbs binding energies of ca. - 1 kJ mol^-1 under the DMS conditions employed, further supporting the hypothesis that dynamic ion-CO₂ clustering is not the root cause of the observed protective effect. The addition of CO₂ was also found to stabilize noncovalently bound dimers, presumably generated at the ionization source. These results indicate that, in these examples, CO₂ cools the ions in the energetic DMS environment via momentum transfer and energy partitioning, and that introducing CO₂ into DMS gas mixtures could enable the stabilization, separation, and analysis of fragile analytes.