Micellar electrokinetic capillary aqueous phase

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. 

The phenomena just described are quite similar to what occurs in a liquid partition chromatographic column except that the stationary phase is moving along the length of the column at a much slower rate than the mobile phase. The mechanism of separations is identical in the two cases and depends on differences in distribution constants for analytes between the mobile aqueous phase the hydrocarbon pseudostationary phase. The process is thus true chromatography hence, the name micellar electrokinetic capillary chromatography. Figure 33-15 illustrates two typical separations by MECC. 

Ph Morin, JC Archambault, P Andre, M Dreux, E Gaydou. Separation of hydroxyl-ated and methoxylated flavonoids by micellar electrokinetic capillary chromatography. Determination of analyte partition coefficients between aqueous and sodium dodecyl sulfate micellar phases. J Chromatogr A 791 289-297, 1997. 

Micellar Electrokinetic Capillary Chromatography (MEKC). MEKC separation is based on the partition of analytes between micelles and the surrounding aqueous phase [38]. This technique can be considered a type of chromatography where the stationary phase is essentially mobile and the mobile phase is electro-osmotically... 

Laskaridou-Monnerville developed micellar electrokinetic capillary chromatography, which is a modified capillary chromatography technique and was applied to determine capsaicin and dihydrocapsaicin in different varieties of C. frutescens [19]. Using their method, they had separated the samples by differential partition between micelles (pseudo stationary stage) and the mobile phase (aqueous buffer at pH 9). Both of the capsaicinoid compounds, capsaicin and dihydrocapsaicin, were detected within 11 min with an excellent resolution [19]. 

The more soluble the neutral molecule is in the micelle, the more time it spends inside the micelle and the longer is its migration time. The nonpolar interior of a sodium dodecyl sulfate micelle dissolves nonpolar solutes best. Polar solutes are not as soluble in the micelles and have a shorter retention time than nonpolar solutes do. Migration times of cations and anions also can be affected by micelles because ions might associate with micelles. Micellar electrokinetic capillary chromatography is truly a form of chromatography because micelles behave like a pseudostationary phase. Solutes partition between the mobile phase (the aqueous solution) and the pseudostationary micelles. 

A number of other separation mechanisms can be realized by simply altering the nature of the separation medium. Micellar electrokinetic capillary chromatography (MECC) involves the use of charged surfactants in the separation media where analytes partition between aqueous and micellar phases (Terabe etal., 1984 Vinde-vogel and Sandra, 1992 Terabe, 1992). In this mode, highly polar analytes elute... 

Separation in Micellar Electrokinetic Chromatography (MEKC) is based on partitioning of the analyte molecules between the aqueous run buffer and the core of micelles, which are contained in the run buffer. The technique is essentially a hybrid between CE and liquid chromatography (LC). The run buffer and micelles are moved through the capillary by an applied electric field. The analytes are dragged with the bulk solution. Similar to LC, the analytes partition between two phases, in this case two mobile phases, the hydrophilic run buffer and the hydrophobic micelles. Unlike other electrophoresis modes, MEKC can distinguish between different neutral compounds according to their hydrophobicity. 

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