Abstract: An electrophoretic separation apparatus includes a capillary tube having a length, a cross section, an inlet and an outlet. A first reservoir containing a solvent and, upon injection, a solute is in fluid-flow communication with the inlet and a second reservoir containing at least a solvent is also in fluid flow communication with the outlet, the capillary thereby being filled at least with the solvent. A first power supply means applies a separation potential between the first and second reservoirs and along the length of the capillary to thereby establish an electrophoretic flow of the solute therethrough. An electrically isolated conductor is juxtaposed to an external surface of the capillary tube and is connected to a second power supply. An electrostatic field is thereby applied across the cross section of the capillary tube to control the electroosmotic flow therein. The electrical isolation of the conductor prevents current flow between the power supply and the conductor.