Abstract :
[en] In 1984, Terabe and co-workers introduced a modified version of CZE, called micellar electrokinetic chromatography (MEKC), in which surfactant-formed micelles are included in the running buffer providing a two phases chromatographic system for the separation of neutral compounds. Depending on their hydrophobicity, compounds can interact with the core of the micelles.
Many MEKC methods have been described for the separation of neutral and basic compounds and for various applications. However, the migration behaviour of cationic, anionic and neutral analytes mixture is not well established regarding to the surfactant concentration and to the proportion of organic solvent in the background electrolyte (BGE). With such a mixture, it is important to remember that the separation of charged solutes in MEKC involves a combination of chromatographic and electrophoretic separation mechanisms. Moreover, it is worth noting that with charged analytes, two kinds of interactions with the micelles may occur; not only with the hydrophobic core but also with the head groups of the micelles through electrostatic interactions. In systems in which the surfactant has an opposite charge to that of the solute, ion pairing may occur. On the contrary, when a solute and the surfactant have similar charges, coulombic forces may cause the repulsion of these molecules.
The goal of this study is to improve our understanding of the migration behaviour of charged and uncharged analytes in MEKC systems. With this aim in view, effective mobility (electrophoretic mobility under the influence of micelles) of neutrals, cations and anions were mesured at neutral, basic and acidic pH with BGE containing different SDS concentrations and ACN proportions. This study is also focused on the changes of migration order between CZE and MEKC systems using different BGE compositions.
In the MEKC systems investigated in the study, SDS concentration and ACN proportion show a tremendous effect on the effective mobilities and migration order of the model compounds. While anions interact very weakly with SDS micelles, neutrals and cations interact with SDS through hydrophobic and ionic bonds. These interactions become stronger with the increase of SDS concentration and weaker with high ACN proportion. With 20 mM of SDS in the BGE, CZE behaviour is observed till 40% of ACN. But, when the SDS concentration is high and the ACN proportion is low, the migration order of analytes is reversed compared to CZE: EOF first, then the anions, followed by the neutrals and finally the cations. The migration order inside each group (cations, neutrals and anions) depends on the hydrophobicity of the analytes. Different organic solvents were also investigated to study the ion-pair formation.
Those observations confirm the interest of using MEKC not only for the separation of neutral compounds but also variously charged analytes.
