DYNAMIC MODIFICATION OF SEPARATIONS USING ELECTROCHEMICALLY MODULATED LIQUID-CHROMATOGRAPHY

A new method for modifying and fine-tuning liquid chromatographic separations without manipulating the mobile phase composition is discussed. This method, termed electrochemically modulated liquid chromatography (EMLC), is based on the electrochemical manipulation of the capacity factors (k' values) of analytes both prior to and during their elution from a column packed with nonporous glassy carbon (GC) spheres. The GC spheres are connected as the working electrode in the three-electrode electrochemical cell. Improvements in the separation of a mixture of aromatic sulfonates (ASFs) obtained at open circuit are demonstrated by the application of several fixed voltages (E(app)) as well as voltage and charge ramps to the column. The latter two approaches resulted in an improved resolution of the mixture, a marked decrease in the widths of the elution bands, and a large decrease in the total elution time. A comparison of these separations to those obtained at various mobile phase combinations suggests that EMLC offers some potentially useful differences in retention that are not easily gained through compositional alterations of the mobile phase. Investigation of the electrochemical retention mechanism revealed that changes in E(app) altered the ability of the carbon surface to participate in donor-acceptor and solvophobic interactions with the ASFs, leading to changes in their k' values. In general, the k' values increased as the value of E(app) became more positive. The largest changes in k' values were found for the ASFs having the largest ionic charge and the largest overall donor strengths. The results of preliminary studies using porous graphitic carbon spheres as a high surface area packing material to affect EMLC separations of more complex mixtures are also reported.