Ionic liquids as stationary phase solvents for methylated cyclodextrins in gas chromatography

Summary

Room temperature ionic liquids (RTIL) are molten salts with melting points well below room temperature. 1-butyl-3-methylimidazolium chloride is a typical example of such RTIL. It was used as a solvent to dissolve permethylated-β-cyclodextrin (BPM) and dimethylated-ßcyclodextrin (BDM) to prepare stationary phases for capillary columns in gas chromatography for chiral separation. The RTIL containing columns were compared to commercial columns containing the same chiral selectors. A set of 64 chiral compounds separated by the commercial BPM column was tested on the RTIL BPM column. Only 21 were enantioresolved. Similarly, a set of 80 compounds separated by the commercial BDM column was passed on the RTIL BDM column with only 16 positive separations. It is proposed that the imidazolium ion pair could make an inclusion complex with the cyclodextrin cavity, blocking it for chiral recognition. All the chiral compounds recognized by the RTIL columns had their asymmetric carbon that was part of a ring structure. The retention factors of the derivatized solutes were lower on the RTIL columns than those obtained on the commercial equivalent column. The peak efficiencies obtained with the RTIL capillary were significantly higher than that obtained with the commercial column. These observations may contribute to the knowledge of the mechanism of cyclodextrin-based GC enantioselective separations.