Capillary electrophoresis micelle formation

MA Schwarz, K Raith, G Dongowski, R Neubert. Effect on the partition equilibrium of various drugs by the formation of mixed bile salt/phosphatidylcho-line/fatty acid micelles. A characterization by micellar affinity capillary electrophoresis. Part IV. J Chromatogr A 809 219-229, 1998. 

The separation of phospholipids by micellar electrokinetic capillary electrophoresis (MEKC) has been described (17-19). In this technique, solutes are separated based on their distribution between a mobile (usually aqueous) and a pseudostationary (micellar) phase. Szucs et al. found that the major soybean phospholipids were fully resolved in only 7 minutes using deox ycholic acid for micelle formation in combination with 30% n-propanol at 50°C (18). However, quantification of the separated compounds remains troublesome. This is due first of all to the fact that only UV detection can be used, thus making the response highly dependent on the degree of unsaturation of the phospholipids. Besides, the comparison of peak areas in MEKC is more complicated than in HPLC, because all compounds are moving with different velocities. 

CE provides a complementary approach to HPLC separation. It is performed in several different formats, including capillary zone electrophoresis, miceller electrokinetic chromatography, capillary gel electrophoresis, capillary isoelectric focusing, isotachophoresis, and capillary electrochromatography. Of these formats, capillary zone electrophoresis is the most popular separation technique. The most successful coupling of CE with mass spectrometry is achieved via an ESI interface. The three most practical designs are sheathless interface, sheath-flow interface, and liquid-junction interface. 

Nakamura, H., Sano, A., and Matsuura, K. 1998, Determination of critical micelle concentration of anionic surfactants by capillary electrophoresis using 2-naphthale-nemethanol as a marker for micelle formation. Anal. Sci. 14, 379. 

Most published capillary electrophoresis (CE) methods use buffers containing an organic solvent such as acetonitrile in order to prevent micelle formation. The micelles have different electrophoretic mobility than the isolated surfactant molecules (1). Since individual surfactant molecules are also present, the presence of micelles causes severe tailing of chromatographic peaks (2). Organic eluents also minimize the adsorption of the surfactant to the walls of the capillary. 

---