Theoretical exploration of separations and experimental investigation of micellar isotherm of benzene by headspace gas chromatography

Statistical-overlap theory (SOT) was verified by separation of a simple 38-component mixture in micellar electrokinetic chromatography (MEKC) in a pH = 7.0 phosphate buffer containing 50 mM sodium dodecyl sulfate (SDS). These 38 compounds are representative of the whole migration-time range of MEKC and the observed peaks are well predicted by SOT.;Another exploration of SOT was carried out for two-dimensional separations, which predicted the dependence of average minimum resolution R*s on the saturation factor to separate two clusters that are composed of single-component peaks (SCPs). The resolution to separate the two nearest SCPs in two neighboring clusters (each in one cluster) varies with the number of SCPs in both clusters. These resolutions are weighted by the probability of the two clusters containing the postulated number of SCPs and summed as R*s . With consideration of boundary effects, the theory was in good agreement with Monte Carlo simulations on a reduced rectangular plane at low and high saturation.;Analyte adsorption onto the separation column is a long-standing problem, and adsorption often deteriorates the separation efficiency. There is a very interesting phenomenon in MEKC. When the pH of background electrolyte is near 2.5, the plate numbers of analytes drop instead of increase with increase of the electric field. This efficiency drop is presumably due to the adsorption of analytes. Two experimental modes were proposed to track the adsorption by frontal elution of analyte decanophenone (C16) and reference thiourea, which did not adsorb onto the capillary wall. Gravity was used to transport analytes in these experiments. One was in a small interlock box with a height difference of 15.8 cm, and the other was in a tower with a height difference of 2.18 m. Both of them did not differentiate between the elution times of C16 and thiourea. An indirect UV detection method was employed to pursue the significant elution time difference. In this method, C16 and SDS displaced a strong UV background to produce negative peaks corresponding to the elution times of C16 and SDS. Again this method led to non-conclusive results.;Peak shape in MEKC is an interesting topic in that it concerns the separation efficiency. The relationship between the isotherm and the peak shapes is of concern because it enables one to quantitatively study the tailing or fronting of peaks. Asymmetric peaks have been thought to originate from dynamic sources such as the electric field effect. However, according to recent studies, these asymmetric profiles can be interpreted in terms of thermodynamics. To verify this point of view, the nonlinear concave upward isotherm of benzene in a 3 mM NaH2PO4/3 mM Na2HPO4 buffer containing 50 mM SDS was measured by headspace gas chromatography, and compared to that measured by vacancy MEKC, an electrokinetic means to measure micellar isotherms. No substantive difference was found between the isotherms, including the nonlinear regions. Therefore these findings are the proof of thermodynamics dominating effect upon the peak shapes in MEKC.