Thermally stable and polar zwitterionic liquid stationary phases for gas chromatography: Understanding the impact of chemical structure

The chemical structure of nine imidazolium sulfonate and triflimide zwitterionic liquids (ZILs) were systematically tuned to increase their thermal stability for gas chromatography (GC) separations. Substituents for imidazolium and 2-phenylimidazolium cation systems, comprised of alkyl, benzyl, and oligoether groups of varying chain lengths, were studied as stationary phases in GC. Propanesulfonate, ethanesulfonate, and propanetriflimide anions were examined to understand the effect of linker length and nucleophilicity on ZIL thermal stability. Studies were conducted to assess film stability and thermal lability of ZIL stationary phases on fused silica capillaries when exposed to elevated temperatures for prolonged time periods. All stationary phases exhibited relatively poor film stability on untreated capillary surfaces, but most showed repeatable chromatographic retention after stepwise heating from 100 to 200 °C. To understand the thermal degradation pathways of the ZILs, mass spectrometry (MS) was used to monitor the degradation/volatilization of the stationary phase when heated from 40 to 250 °C. Salt-deactivated surfaces were effective at mitigating stationary phase instability, but were observed to participate in the degradation of alkyl functionalized ZILs via nucleophilic attack of the alkyl substituent. This was not observed for oligoether substituted ZILs. Imidazolium propanesulfonate ZILs all underwent degradation through the detachment of the anion system, resulting in the reformation of 1,3-propanesultone. Most ZIL stationary phases degraded below 230 °C, but the cation substituent was observed to play a significant role in overall ZIL thermal stability. For the imidazolium propanetriflimide ZIL, degradation of the anion system occurred prior to the detachment of the entire anion system via elimination and occurred at around 245 °C.