Modeling of anti-Langmuirian peaks in micellar electrokinetic chromatography: benzene and naphthalene

Peaks of benzene (bz) and naphthalene (np) having diffuse fronts and steep rears under overload conditions were studied quantitatively in MEKC with SDS surfactant. The retardation factors of these compounds, solubilized at microM to mM concentrations by either 10, 30, or 50 mM SDS, were determined by vacancy MEKC and frontal analysis MEKC. Isotherm coordinates were calculated from the retardation factors, and the equation for the concave upward anti-Langmuir isotherm was fit to them. Peak profiles were computed with the MacCormack algorithm from the isotherm fits and a simplified continuity equation appropriate to MEKC. These profiles were compared to ones generated in normal MEKC from samples of bz and np solubilized at muM to mM concentrations by either 10, 30, or 50 mM SDS. In all cases, the anti-Langmuir isotherm described the asymmetry of experimental peaks. For bz in 30 and 50 mM SDS and np in 10 and 50 mM SDS, good to excellent agreement was found between the experimental and predicted profiles. For bz in 10 mM SDS, the experimental profiles were more broadened than the predicted ones, although their asymmetries agreed. For np in 30 mM SDS, the experimental isotherm predicted greater peak asymmetry than was observed, and the correct anti-Langmuir isotherm for all sample concentrations and field strengths was calculated from the most asymmetrical peak by the inverse method. The relative decrease of zone velocity with increasing analyte concentration was calculated from the isotherm parameters, electrokinetic mobilities, retardation factors, surfactant concentrations, and CMC. The simplification of the continuity equation was justified.