Sodium dodecyl sulfate-Tween 20 mixed micellar electrokinetic chromatography for separation of hydrophobic cations:...

Separation of hydrophobic cations in capillary electrophoresis under the mixed micellar system composed of SDS and Tween 20 was investigated from the viewpoints of thermodynamics and practical application. Hydrophobic cations interact strongly with the anionic SDS micelle, and this often lead to predominant dissolution of the analytes into the micellar phase, resulting in poor resolution. The ionic interaction was evaluated to be close in strength to the hydrophobic interaction between benzenes and the SDS micelle. Tween 20 as a component to the mixed micelle was found to weaken the attractive ionic interactions between the cationic solutes and SDS. In addition, the polyether chain of Tween 20 serves also as a hydrogen acceptor to cause the attractive hydrogen-bonding interactions with hydrogen-donating analytes. The two different functions of Tween 20 improve the separation of hydrophobic cations remarkably. Separation patterns are well controlled by varying the mixing ratio of the two surfactants. Adrenaline and its six precursors were successfully separated with this mixed micellar system. The improved separation efficiency was not affected by the presence of bovine serum matrix, realizing the direct MEKC analysis of serum sample without deproteination, in which proteins were comigrated with the micelle.