Electrochemically Modulated Liquid Chromatography in Fused Silica Capillary Columns

Electrochemically modulated liquid chromatography (EMLC) uses electrical potentials, applied to a conductive chromatographic stationary phase (e.g., porous graphitic carbon [PGC]), to manipulate analyte retention. This paper reports the design of a capillary EMLC column with a smaller internal diameter (ID; 250 μm) than that of the standard bore predecessor (3.3 mm ID). The new capillary EMLC columns are configured so that the PGC stationary phase serves as the working electrode in a two-electrode electrochemical cell and simplifies electrode placement by obviating the need for a counter electrode. This configuration also eliminates the internal Nafion sleeve that is critical to operation for the standard bore columns, thereby avoiding Nafion deformation as a source of chromatographic band broadening and rupturing as a mode of column failure. Indeed, values for chromatographic efficiency obtained on the capillary columns meet or exceed those measured for the standard columns (20 000-40 000 vs 14 000 plates/m, respectively) with near symmetric elution bands (asymmetry factors of 1.1-1.4 for well-packed capillaries) that surpass band symmetries observed in all prior studies. A test suite of aromatic sulfonates was used to characterize the chromatographic performance of the capillary EMLC columns. Separations of this test mixture showed that retention factors for individual analytes could be manipulated by as much as 21× by changing the applied potential at the PGC stationary phase. Changes in retention behavior at different potential ranges, hypothesized to result from differences in adsorption orientation, were also observed and are consistent with past work. Collectively, the retention behavior unique to EMLC is operative in this new capillary configuration and promises to open new avenues in tuning LC separations.