Reversed-phase and weak anion-exchange mixed-mode silica-based monolithic column for capillary electrochromatography

A silica-based CEC monolithic column with mixed modes of RP and weak anion-exchange (WAX) was successfully prepared by using the sol-gel technique at mild temperature. The synthesizing procedure was optimized by changing the ratios of tetraethoxysilane (TEOS), aminopropyltriethoxysilane (APTES), and octyltriethoxysilane (C(8)-TEOS) in the mixture. While serving as WAX group, the amino group dominated the charge on the surface of the capillary column and generated an EOF from cathode to anode at low pH. At pH above 7.5, a cathodic EOF was observed due to the full ionization of silanol group and the suppression in the ionization of amino group. The morphology of monolithic columns was examined by SEM, and the performance of column was evaluated in detail by separating different kinds of compounds. As expected, the monolithic column exhibited RP chromatographic behavior for neutral solutes. Fast and efficient separation of six aromatic acids was obtained using acidic mobile phase with column efficiency up to 160,000 plates/m. Symmetrical peaks can be obtained for aromatic amines because positively charged amino groups on the surface can effectively minimize the adsorption of positively charged analytes to the stationary phase.