Immobilized ionic liquids as high-selectivity/high-temperature/high-stability gas chromatography stationary phases

Ionic liquids (ILs) are a class of nonmolecular solvents in which the cation/anion combination can be easily tuned to provide desired chemical and physical properties. When used as stationary phases in gas-liquid chromatography, ionic liquids exhibit dual nature retention selectivity. That is, they are able to separate polar molecules such as a polar stationary phase and nonpolar molecules such as a nonpolar stationary phase. However, issues such as optimization of the wetting ability of the ionic liquid on fused-silica capillaries, the maximum operating temperatures of the stationary phases, and nonuniform film thickness on the wall of the capillary at high temperatures have limited their use in gas chromatography. As described in this paper, these limitations are overcome by cross-linking a new class of ionic liquid monomers by free radical reactions to provide a more durable and robust stationary phase. By lightly cross-linking the ionic liquid stationary phase using a small amount of free radical initiator, high-efficiency capillary columns were produced that are able to endure high temperatures with little column bleed. Two types of cross-linked IL stationary phases are developed. A partially cross-linked stationary phase allows for high-efficiency separations up to temperatures of approximately 280 degrees C. However, by creating a more highly cross-linked stationary phase of geminal dicationic ILs, exclusively, an increase in efficiency is observed at high temperatures allowing for its use over 350 degrees C. In addition, through the use of solvation thermodynamics and interaction parameters, it was shown that the cross-linking/immobilization of the ionic liquid does not affect the selectivity of the stationary phase thereby preserving its dual nature retention behavior.