DYNAMIC MODIFICATION OF SEPARATIONS USING ELECTROCHEMICALLY MODULATEDLIQUID-CHROMATOGRAPHY

A new method for modifying and fine-tuning liquid chromatographic sepa rations without manipulating the mobile phase composition is discussed . This method, termed electrochemically modulated liquid chromatograph y (EMLC), is based on the electrochemical manipulation of the capacity factors (k' values) of analytes both prior to and during their elutio n from a column packed with nonporous glassy carbon (GC) spheres. The GC spheres are connected as the working electrode in the three-electro de electrochemical cell. Improvements in the separation of a mixture o f aromatic sulfonates (ASFs) obtained at open circuit are demonstrated by the application of several fixed voltages (E(app)) as well as volt age and charge ramps to the column. The latter two approaches resulted in an improved resolution of the mixture, a marked decrease in the wi dths of the elution bands, and a large decrease in the total elution t ime. A comparison of these separations to those obtained at various mo bile phase combinations suggests that EMLC offers some potentially use ful differences in retention that are not easily gained through compos itional alterations of the mobile phase. Investigation of the electroc hemical retention mechanism revealed that changes in E(app) altered th e ability of the carbon surface to participate in donor-acceptor and s olvophobic interactions with the ASFs, leading to changes in their k' values. In general, the k' values increased as the value of E(app) bec ame more positive. The largest changes in k' values were found for the ASFs having the largest ionic charge and the largest overall donor st rengths. The results of preliminary studies using porous graphitic car bon spheres as a high surface area packing material to affect EMLC sep arations of more complex mixtures are also reported.