Optimization of the resolution of PTH‐amino acids through control of surfactant concentration in micellar electrokinetic capillary chromatography: SDS vs. brij 35/SDS micellar systems

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Abstract
The effect of surfactant concentration on resolution in micellar electrokinetic capillary chromatography (MECC) was investigated using several phenylthiohydantoin (PTH) derivatized amino acids as test solutes. Two aqueous micellar systems were employed: sodium dodecyl sulfate (SDS) and a mixed micellar system consisting of SDS and polyoxyethylene(23)dodecanol (Brij® 35). In the SDS system, the tmc/to ratio (elution range) increased significantly with increasing surfactant concentration, making the determination of the optimal concentration, which is dependent on tmc/to, difficult. In contrast, in the Brij 35/SDS system the tmc/to ratio decreased only slightly with increasing concentration of Brij 35. Whereas the SDS and Brij 35/SDS systems are better suited for moderately hydrophobic and hydrophilic solutes, respectively, neither is well‐suited for hydrophobic solutes without organic modifiers. The narrower elution range (lower tmc/to) observed for the Brij 35/SDS micellar system was compensated by several potentially important advantages over the pure SDS system, including significantly higher efficiencies (2–3 times) and lower operating currents. Differences in selectivity and retention mechanisms among solutes and surfactant systems are also discussed.