title: Rapid quantification of imidazolium-based ionic liquids by hydrophilic interaction liquid chromatography: Methodology and an investigation of the retention mechanisms.
abstract: The separation of nine N,N'-dialkylimidazolium-based ionic liquids (ILs) by an isocratic hydrophilic interaction high-performance liquid chromatographic method using an unmodified silica column was investigated. The chosen analytical conditions using a 90:10 acetonitrile-ammonium formate buffer mobile phase on a high-purity, unmodified silica column were found to be efficient, robust, and sensitive for the determination of ILs in a variety of solutions. The retention window (k' = 2-11) was narrower than that of previous methods, resulting in a 7-min runtime for the nine IL homologues. The lower limit of quantification of the method, 2-3 μmol L(-1), was significantly lower than those reported previously for HPLC-UV methods. The effects of systematically modifying the IL cation alkyl chain length, column temperature, and mobile-phase water and buffer content on solute retention were examined. Cation exchange was identified as the dominant retention mechanism for most of the solutes, with a distinct (single methylene group) transition to a dominant partitioning mode at the highest solute polarity.
authors.affiliation: Department of Chemistry and Biochemistry, University of Wisconsin-Milwaukee Milwaukee, WI 53211, USA. Electronic address: cahawkin@uci.edu.
authors.forename: Cory A
authors.initials: CA
authors.lastname: Hawkins
authors.affiliation: Department of Chemistry and Biochemistry, University of Wisconsin-Milwaukee Milwaukee, WI 53211, USA.
authors.forename: Anna
authors.initials: A
authors.lastname: Rud
authors.affiliation: Department of Chemistry and Biochemistry, University of Wisconsin-Milwaukee Milwaukee, WI 53211, USA.
authors.forename: Margaret L
authors.initials: ML
authors.lastname: Guthrie
authors.affiliation: Department of Chemistry and Biochemistry, University of Wisconsin-Milwaukee Milwaukee, WI 53211, USA.
authors.forename: Mark L
authors.initials: ML
authors.lastname: Dietz
mesh_terms: D000097:Acetonitriles / Q000737:chemistry; D002021:Buffers; D002412:Cations / Q000737:chemistry; D002623:Chemistry Techniques, Analytical / Q000379:methods*; D002853:Chromatography, Liquid; D057927:Hydrophobic and Hydrophilic Interactions; D052578:Ionic Liquids / Q000032:analysis*; D012822:Silicon Dioxide / Q000737:chemistry; D013696:Temperature; D014867:Water / Q000737:chemistry
chemical_list: D000097:Acetonitriles; D002021:Buffers; D002412:Cations; D052578:Ionic Liquids; D014867:Water; D012822:Silicon Dioxide; C032159:acetonitrile
keywords: Aqueous normal phase; HPLC; Ionic liquid determination; Mixed-mode retention; Rapid separation