Critical micellar concentration chromatography

In order to separate neutral compounds, Terabe et al. [13] added surfactants to the buffer electrolyte. Above their critical micellar concentration (cmc), these surfactants form micelles in the aqueous solution of the buffer electrolyte. The technique is then called Micellar electrokinetic capillary chromatography, abbreviated as MECC or MEKC. Micelles are dynamic structures consisting of aggregates of surfactant molecules. They are highly hydrophobic in their inner structure and hydrophilic at the outer part. The micelles are usually... 

Micellar electrokinetic capillary chromatography (MECC), in contrast to capillary electrophoresis (CE) and capillary zone electrophoresis (CZE), is useful for the separation of neutral and partially charged species [266,267]. In MECC, a surfactant, usually sodium dodecyl sulfate (SDS), is added to the buffer solution above its critical micellar concentration to form micelles. Although SDS is certainly the most popular anionic surfactant in MECC, other surfactants such as bile salts have proved to be very effective in separating nonpolar analytes that could not be resolved using SDS [268]. 

Micellar Liquid Chromatography (MLC) uses surfactant solutions as mobile phases for reversed phase liquid chromatography. The two main properties of surfactant molecules, as related to chromatography, are micelle formation and adsorption at interfaces. The micelles play the role of the organic modifier, so their influence on retention has been extensively studied (1). At surfactant concentrations above the critical micellar concentration (CMC), micelles are present and the amount of free surfactant is essentially... 

The first report on the analytical use of an aqueous solution of a surfactant, above its critical micellar concentration (CMC), as mobile phase in reversed-phase liquid chromatography (RPLC) was published in 1980. The technique, named micellar liquid chromatography (MLC), is an interesting example of the modification of the chromatographic behavior taking advantage of secondary equilibria to vary both retention and selectivity. 

Monomeric nucleosides and nucleotides, which appear as impurities in oligonucleotide synthesis, may be analyzed using the method of micellar electrokinetic chromatography (MEKC). With addition of SDS to the buffer solution, the surfactant concentration is above the critical micellar concentration. Movement of the micelles towards the anode against the EOF involves distribution processes, in which also nucleosides, nucleobases and nucleotides participate. Typical conditions are 50 mM phosphate buffer 40 mM SDS pH 6.5 20 kV (116 mA) 21 °C [186]. 

Reversed-Phase Liquid Chromatography (RPLC) is an important tool in protein chemistry. Examination of sorption isotherms revealed that alcohohc buffers did not desorb proteins near physiological pH in RPLC systems, while buffers containing a poly(ethoxy alcohol) surfactant did not desorb protein at pH 2, but they did at pH 7 with concentrations of surfactant apparently well above the critical micellar concentration (cmc) [2]. It has been proposed that a necessary condition for the desorption of a protein from a surface is that the surface tension of the solvent falls between that of the protein and the surface [6]. This condition is fiilfilled for many proteins with surfactant solutions near conditions of physiological pH and ionic strength. Therefore, it was expected that separations of proteins could be thieved in these conditions. 

In the late 1970s, ionic surfactants were added for the first time to polar aqueous-organic mobile phases in reversed-phase liquid chromatography (RPLC) to form ion pairs (IPs). In IP chromatography, surfactant monomers adsorbed on a bonded silica-based stationary phase associate with neutral and ionic solutes, modifying their retention. The concentration of surfactant in the mobile phases is kept below the critical micellar concentration (CMC). In 1980, Daniel W. Armstrong reported the possibility of using... 

Another possible way to minimize the interaction of collagen with the capillary wall is the presence of a high concentration of surfactant (above the critical micellar concentration), i.e., by micellar electrokinetic chromatography (MEKC). A usefiil system consists of a 50 mM phosphate buffer (pH 2.5) with 50 mM SDS this system has to be run in negative polarity mode. At low, submicel-lar concentrations, the separations are different and only reflect interactions between the peptides and with the capillary wall, but not the presence of SDS micelles in the background electrolyte. 

Lucangioli, S., Carducci, C., Tripodi, V., Kenndler, E. and 2001, Retention of bile salts in micellar electrokinetic chromatography Relation of capacity factor to octanol-water partition coefficient and critical micellar concentration. J. Chromatogr. B 765, 113. 

Many pharmaceutical preparations contain multiple components with a wide array of physico-chemical properties. Although CZE is a very effective means of separation for ionic species, an additional selectivity factor is required to discriminate neutral analytes in CE. Terabe first introduced the concept of micellar electrokinetic capillary chromatography (MEKC) in which ionic surfactants were included in the running buffer at a concentration above the critical micelle concentration (CMC) [17], Micelles, which have hydrophobic interiors and anionic exteriors, serve as a pseudostation-ary phase, which is pumped electrophoretically. Separations are based on the differential association of analytes with the micelle. Interactions between the analyte and micelles may be due to any one or a combination of the following electrostatic interactions, hydrogen bonding, and/or hydro-phobic interactions. The applicability of MEKC is limited in some cases to small molecules and peptides due to the physical size of macromolecules... 

Temperature has a large effect on the mass transfer between the micelle and the stationary phase and can therefore be used to improve the efficiency of the separation. Micellar chromatography should be carried out at elevated temperature, typically around 40°C. At elevated temperatures, the effects of flow rate and surfactant concentration on the efficiency of the separation are minimized. For optimum efficiency, however, the flow rate should be minimized while still maintaining a reasonable elution time.33 Likewise, a surfactant concentration close to but above the critical micelle concentration should be used.33... 

Aqueous micellar solutions were first used as mobile phases in LC in 1980. A micellar solution is one that contains a surfactant at a concentration above the critical micelle concentration, about 10-2 M. The nature of micelles and their use in analytical chemistry, including chromatography, has been recently described.82... 

Increasing the HR above its critical micelle concentration leads into the field of micellar chromatography in which analyte may partition between the mobile phase and both the stationary phase and the micelle. 

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