Ionic liquids as mobile phase additives for the high-performance liquid chromatographic analysis of fluoroquinolone antibiotics in water samples

In this work, four ionic liquids differing in the length of the alkyl chain on the imidazolium cation and one ionic liquid containing tetraethylammonium, all with the same counterion, (i.e. 1-ethyl-3-methylimidazolium tetrafluoroborate (EMIm-BF4), 1-butyl-3-methylimidazolium tetrafluoroborate (BMIm-BF4), 1-hexyl-3-methylimidazolium tetrafluoroborate (HMIm-BF4), 1-methyl-3-octylimidazolium tetrafluoroborate (MOIm-BF4), and tetraethylammonium tetrafluroborate (Et4N-BF4)) were tested as mobile phase additives for HPLC separation of a group of seven basic fluoroquinolone (FQ) antibiotics for human and veterinary use (i.e. fleroxacin, ciprofloxacin, lomefloxacin, danofloxacin, enrofloxacin, sarafloxacin, and difloxacin) using a conventional reversed-phase Nova-Pak C-18 Column. Fluorescence detection was used. Among the ionic liquids selected, use of BMIm-BF4 enabled effective separation of these compounds with relatively low analysis time (14 min). The best separation was achieved by isocratic elution at 1 mL min(-1) with 5 mmol L-1 BMIm-BF4 and 10 mmol L-1 ammonium acetate at pH 3.0 with 13% (v/v) acetonitrile. Limits of detection (LODs) for fluorescence detection were in the range 0.5-11 mu g L-1. The method was tested by analyzing several water samples after the optimization of a suitable solid-phase extraction (SPE) procedure using Oasis HLB cartridges. Mean recovery values were above 84% for all analytes with LODs in the range 1-29 ng L-1.