Capillary electrochromatography using continuous-bed columns of sol–gel bonded silica particles with mixed-mode octadecyl and propylsulfonic acid functional groups

Continuous-bed columns containing sol–gel bonded 3 μm silica particles with mixed-mode octadecyl and propylsulfonic acid functional groups (ODS/SCX) were prepared by first packing the ODS/SCX particles into a fused-silica capillary, then filling the packed capillary with a siliceous sol–gel, curing the sol–gel, and finally drying the column with supercritical carbon dioxide. The performance of the sol–gel bonded ODS/SCX columns was evaluated for capillary electrochromatography using acetonitrile aqueous mobile phase containing phosphate buffer. The columns were mechanically strong and permeable. Both EOF velocity and current increased linearly with elevation of the applied electric field strength. The EOF velocity was high at low pH and nearly constant over a range of pH 2–9. Higher buffer concentration resulted in higher current and lower EOF velocity. The acetonitrile content had no significant effect on the EOF. Without thermosetting the column, no bubble formation was noticed with currents up to 2.5 μA. The minimum plate height of a 25/34 cm×75 μm I.D. sol–gel bonded 3 μm ODS/SCX column was 5.7 μm (1.75×105 plates per meter) at an optimum EOF velocity of 0.92 mm s−1. Mixtures of test aromatic compounds and aromatic hydrocarbon homologues gave symmetrical peaks when using a low pH mobile phase. The retention and elution order of aromatic compounds represented a typical reversed-phase separation mechanism similar to conventional ODS columns. The run-to-run and column-to-column retention factor reproducibility was better than 2.5% and 8.0% RSD, respectively.