Influence of zwitterionic liquid chemical structure on separation of volatile carboxylic acids when used as stationary phases in gas chromatography

Abstract

Volatile carboxylic acids (VCAs) are important chemicals and synthetic building blocks in various industrial fields ranging from foods to polymers and pharmaceuticals. The analysis of VCAs using gas chromatography (GC) is challenging due to poor peak shapes on many traditional polar stationary phases. Zwitterionic liquids (ZILs) are promising GC stationary phase materials, featuring improved inertness for acidic molecules. In this study, imidazolium-based ZILs with sulfonate and triflimide anions are evaluated to understand the influence of ZIL chemical structure on the separation of polar analytes. Five VCAs were used as probe molecules to evaluate chromatographic retention, peak shapes, and selectivity. Higher VCA retention was observed on imidazolium sulfonate ZIL stationary phases compared to a commercially available wax-type or polyethylene glycol-based columns. ZILs containing either octyl or decyl substituents provided higher VCA retention than oligoether functionalized ZILs, but incorporation of a phenyl moiety to the imidazolium ring increased retention of oligoether functionalized ZILs. VCA retention also decreased when either the anion was exchanged or linker length connecting the cation and anion groups was decreased. The chemical nature of substituent groups and chain length appears to play important roles in controlling VCA selectivity, where alkyl-substituted ZILs offered higher selectivity than the oligoether ZILs. Excellent peak asymmetry factors (< 2) were observed for most ZIL stationary phases; however, oligoether functionalized ZILs exhibited limited analyte loading capacity for less polar VCAs. When ZIL stationary phases were coated on a salt-deactivated capillary, an increase in VCA retention was observed along with slightly higher peak asymmetry factors. For an oligoether ZIL stationary phase on salt-deactivated capillary, more prominent fronting peak shapes were observed due to reduced solubility of less polar VCAs in the stationary phase.

Keywords: Gas chromatography; Partition coefficients; Peak asymmetry; Selectivity; Volatile carboxylic acids; Zwitterionic liquids.