Effect of ionic liquid additives to mobile phase on separation and system efficiency for HPLC of selected alkaloids on different stationary phases

The silica-based stationary phases with favorable physical characteristics are the most popular in liquid chromatography. However, there are several problems with silica-based materials: severe peak tailing in the chromatography of basic compounds, non-reproducibility for the same chemistry columns, and limited pH stability. Ionic liquids (ILs) as mobile phase components can reduce peak tailing by masking residual free silanol groups. The chromatographic behavior of some alkaloids from different classes was studied on C18, phenyl, and pentafluorophenyl columns with different kinds and concentrations of ionic liquids as additives to aqueous mobile phases. Ionic liquids with different alkyl substituents on different cations or with different counterions as eluent additives were investigated. The addition of ionic liquids has great effects on the separation of alkaloids: decrease in band tailing, increase in system efficiency, and improved resolution. The retention, separation selectivity, and sequence of alkaloid elution were different when using eluents containing various ILs. The increase of IL concentration caused an increase in silanol blocking, thus conducted to decrease the interaction between alkaloid cations and free silanol groups, and caused a decrease of alkaloids retention, improvement of peak symmetry, and increase of theoretical plate number in most cases. The effect of ILs on stationary phases with different properties was also examined. The different properties of stationary phases resulted in differences in analyte retention, separation selectivity, peak shape, and system efficiency. The best shape of peaks and the highest theoretical plate number for most investigated alkaloids in mobile phases containing IL was obtained on pentafluorophenyl (PFP) phase.