RETENTION CHARACTERISTICS OF POLYPYRROLE AS A STATIONARY-PHASE FOR THE ELECTROCHEMICALLY MODULATED LIQUID-CHROMATOGRAPHIC (EMLC) SEPARATIONS OF DANSYL AMINO-ACIDS

An approach for the dynamic modification of the separations of dansyl amino acids (DAAs) both prior to and during elution using a novel elec trochemically modulated liquid chromatographic (EMLC) system is descri bed. The EMLC system consists of polypyrrole (PPy)-coated glassy carbo n spheres which are connected as the working electrode in a three-elec trode electrochemical cell arrangement. Electrochemical switching of P Py 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 pri or 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 du ring elution were used to optimize the separation of a mixture of thre e DAAs. In a manner directly analogous to gradient elution in conventi onal liquid chromatography, both band half-widths and retention times could be significantly decreased using this technique. A detailed char acterization of the ion-transport properties of the PPy stationary pha se is also provided to aid in the development of an electrochemical re tention mechanism. The ion-transport properties were investigated thro ugh analysis of the PPy films using both energy-dispersive X-ray analy sis and quartz crystal microbalance techniques. Both the ion-transport and chromatographic studies provided evidence for the preferential in teraction 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 preco ncentration of dilute (ca.1 ppm) solutions of the DAAs is also briefly discussed.