Effect of 1-alkyl-3-methylimidazolium tetrafluoroborate on the adsorption behavior of amine and carboxylic compounds in reversed-phase liquid chromatography

Ionic liquids have recently been introduced as mobile phase additives to separate ionized solutes and to improve separation abilities in reversed-phase liquid chromatography. In this study, four ionic liquids that are composed of different lengths of alkyl chain (ethyl, butyl, hexyl, and octyl) in 1-alkyl-3-methylimidazolium cation and tetrafluoroborate anion were used as the mobile phase additives and five ionizable solutes (benzylamine, benzoic acid, 4-aminobenzoic acid, L-phenylalanine, and L-trypophan) that include carboxyl and/or amine were used as analytes. By changing the hydrophobic property of the ionic liquid with the length of alkyl chain of imidazolium cation, one can control the interaction between 1-alkyl-3-methytimida-zolium cation and the hydrophobic stationary phase. To explain the role of ionic liquids on the retention behaviors of ionized analytes, two adsorption mechanisms, the ion pairing forming mechanism and the dynamic ion-exchange mechanism, were presented. When 1-ethyl-3-methylimidazolium tetrafluoroborate was used as the additive, the retention behavior dominantly complied with the ion pairing forming mechanism and when other ionic liquids with long alkyl chain were used, the retention behavior can be explained by the dynamic ion-exchange mechanism.