Variation of anionic moieties of dicationic ionic liquid GC stationary phases: Effect on stability and selectivity

Dicationic ionic liquids (DILs) are more and more accepted as a new class of high temperature and polar stationary phases for gas chromatography (GC). This study deals with the effect of seven different fluorosulfonyl derivatized anions associated with two dications: 1,3-di(3-methylimidazolium)-2-methylpropane [2mC₃(mim)₂], and 1,3-di(3-methylimidazolium)-isobutene [i-eneC₄(mim)₂]. Thermophysical properties of the 14 synthesized DILs were evaluated in terms of melting point, viscosity, and thermal stability. The optimal physicochemical properties of 13 DILs allowed preparing 13 GC capillary columns. Accordingly, the polarity and selectivity of the DILs were evaluated by determining the Rohrschneider-McReynolds constants and the equivalent chain lengths of C18 fatty acid methyl esters (FAMEs). The symmetrical fluoroalkylsulfonyl and the trifluorosulfonate anions seem to produce the most polar DILs. Compared to classical polyethyleneglycol phases, the DILs showed substantially decreased retention of apolar compounds and a much stronger retention of the polar ones. Unique selectivities were observed with unsaturated FAMEs, polyaromatic hydrocarbons, and bacterial specific FAMEs. The two applications presented included a biodiesel and bacterial FAME analyses.