An evaluation of unbonded silica stationary phases for the separation of basic analytes using capillary electrochromatography

In this work, the use of three Hypersil unbonded silica phases of different purity with simple aqueous-organic mobile phase eluents has achieved excellent separations of a basic analyte mixture. LC determination of the ion-exchange capacity of the three silicas highlighted a significant difference in their acidic silanol activity i.e. Hypersil silica > Hypersil BDS silica > Hypurity silica. The total silanol activity for the three phases was very similar. Isocratic CEC analysis on the acidic Hypersil unbonded silica using RP-CEC mobile phase conditions (pH 7.8) generated excellent peak shape, good efficiencies (148 - 508,000 plates/metre) and baseline separation within 14 min for a mixture of six pharmaceutically relevant basic analytes. The less acidic unbonded silicas gave better peak shapes with reduced analysis times than the Hypersil silica, however baseline separation of the basic analytes was not achieved. It was postulated that the separation mechanism with unbonded silica is a balance between the analyte's electromobility and its interaction (i.e. ion-exchange and reversed-phase/adsorption mechanisms) with the silica surface. The magnitude of the interaction is dependent upon the proportion and distribution of acidic silanol groups present on the surface and the pH of the mobile phase. The silica materials exhibited a small, but significant, reversed-phase character thus enabling acid, base and neutral components to be separated within a single run. Excellent CEC retention predictions were achieved by taking into consideration the analyte's electromobilty in CE, its retention factor in LC and the EOF in CEC under identical experimental conditions. The technique has been shown to be truly orthogonal in nature to CE and LC and hence is a highly complementary and important technique for the analytical chemist. The use of unbonded silica in CEC, with reversed-phase mobile phase conditions, provides an excellent separation technique for the rapid analysis of basic analytes of wide structural diversity (i.e. differing pK(a)s and log P values).