Dynamics of capillary electrochromatography experimental study on the electrosmotic flow and conductance in open and packed capillaries

Abstract

For capillary electrochromatography (CEC) to become an analytical separation technique of high speed and resolution the factors determining the conductivity of the column as well as the generation and control of electrosmotic flow (EOF) in porous media have to be understood. In the present study the conductance packed with a variety of stationary phases was evaluated with respect to the conductance of the open capillary and the data were interpreted in the light of the Tobias equation. However, the consistently observed reduction of the EOF when a capillary having a charged inner wall is packed with particles having charges of the same sign and the dependence of the EOF velocity on the particle size needs further explanation. The data suggests that, due to the employment of relatively long columns packed with small particles, CEC may offer peak capacities much higher than HPLC or micro-HPLC. The CEC columns are unique as they consist of a packed and an open capillary segment having different conductances and consequently different voltage gradients and electrical field strengths. Therefore, any sufficiently detailed study on CEC systems requires also the characterization of the individual column segments. EOF velocities of 6–7 mm/s could be realized at 60 kV applied voltage with a 23/32 cm×50 μm raw fused-silica capillary packed with 6μm Zorbax ODS particles. The current was a linear function of the field strength up to 1.8 kV/cm, but at high field strengths the EOF increased with squared field strength. Data on band spreading indicate that with a given column the plate height at high EOF velocities is smaller in CEC than in micro-HPLC and it is weakly dependent on the velocity.