An approach for the dynamic modification of the separations of dansyl amino acids (DAAs) both prior to and during elution using a novel electrochemically modulated liquid chromatographic (EMLC) system is described. The EMLC system consists of polypyrrole (PPy)-coated glassy carbon spheres which are connected as the working electrode in a three-electrode electrochemical cell arrangement. Electrochemical switching of PPy between its oxidized (cationic) and reduced (neutral) forms allowed for alteration of the anion-exchange capacity of the stationary phase over a wide range. Modification of the voltage applied to the PPy prior to injection was used to alter the capacity factors of the DAAs by at least an order of magnitude. Voltage steps applied to the column during elution were used to optimize the separation of a mixture of three DAAs. In a manner directly analogous to gradient elution in conventional liquid chromatography, both band half-widths and retention times could be significantly decreased using this technique. A detailed characterization of the ion-transport properties of the PPy stationary phase is also provided to aid in the development of an electrochemical retention mechanism. The ion-transport properties were investigated through analysis of the PPy films using both energy-dispersive X-ray analysis and quartz crystal microbalance techniques. Both the ion-transport and chromatographic studies provided evidence for the preferential interaction of PPy in its fully oxidized, partially oxidized and reduced forms with hydrophobic, pi-electron containing anions via hydrophobic and donor-acceptor interactions. The application of EMLC to the preconcentration of dilute (ca.1 ppm) solutions of the DAAs is also briefly discussed.