1-Hexyl-3-methyl imidazolium tetrafluoroborate: an efficient column enhancer for the separation of basic drugs by reversed-phase liquid chromatography.

Ionic liquids are dual modifiers composed by a large anion and a large cation, which interact with both the hydrophobic alkyl-bonded phase and the anionic residual silanols in C18 columns. The deactivation of the silanol groups has important implications on the chromatographic analysis of basic drugs, being the improvement of peak profiles and shorter retention times the most noticeable features. However, other characteristics as selectivity or resolution are not usually considered, or are only examined for selected chromatographic conditions. In this work, the effect of the addition of the ionic liquid 1-hexyl-3-methyl imidazolium tetrafluoroborate to acetonitrile-water mixtures in a wide range of concentrations, using three C18 columns (Zorbax SB-C18, Nucleosil and Spherisorb) was studied for the separation of a group of β-adrenolytic drugs. The extremely poor chromatographic performance was highly improved in the presence of the ionic liquid, giving rise to shorter analysis times, smaller consumption of acetonitrile, significantly narrower and more symmetric peaks, and high resolution. The improvement was especially noteworthy for the Spherisorb column, which yielded the poorest results in the absence of additive.