Micellar electrokinetic chromatography critical micelle concentration

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... 

Micellar electrokinetic capillary chromatography uses a totally aqueous buffer into which a surfactant in excess of its critical micelle concentration is introduced [18,35—37]. This approach makes use of solute separation via partitioning into... 

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]. 

The general theory of micellar electrokinetic chromatography represents a confluence of chromatographic and electrophoretic principles. The expressions for electrophoretic mobility under different separation conditions are summarized in Table 8.4 [161,162]. These relationships allow the determination of the critical micelle concentration and equilibrium distribution constants for solute-micelle association complexes under typical conditions for micellar electrokinetic chromatography [60-64,161-164]. These properties change significantly with the composition of the electrolyte solution, and are generally different to common reference values for pure water. 

Polymeric micelles form stable pseudostationary phases with a critical micelle concentration of virtually zero (aggregation number of 1), and are tolerant of high organic solvent concentrations in the electrolyte solution. Mass transfer kinetics are slow compared with conventional surfactant micelles, and peak distortion from mass overloading is a problem for some polymer compositions. Preliminary studies indicate that polymeric surfactants are effective pseudostationary phases in micellar electrokinetic chromatography, but only a limited number of practical applications have been demonstrated, and uptake has been slow. 

Micellar electrokinetic chromatography uses ionic surfactants at a concentration above the critical micelle concentration (CMC) as a component of the run buffer chosen to separate compounds. This generates a pseudo-stationary phase that performs the separation. This technique is therefore optimal for separating neutral and charged compounds from each other. In addition compounds that are very hydrophobic, and those typically insoluble in traditional capillary electrophoresis run separate buffers under these conditions. Neutral compounds elute in the order of their hydrophobicity. 

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. 

Owing to its outstanding high-performance separation potential also capillary electrophoresis has been employed in different modes and with different detection methods for separation and determination of MC. Both capillary zone electrophoresis (CZE) and micellar electrokinetic chromatography (MEKC), which employs a surfactant above the critical micelle concentration to separate neutral and uncharged molecules based on electromigration principle, have been employed for determination of... 

Micellar electrokinetic chromatography (MEKC). The electrolyte contains a surfactant [e.g., sodium dodecyl sulfate (SDS)] above its critical micelle concentration. Micelles present in the capillary interact with the sample components. Differences in the interactions between micelles and various analytes give rise to separation. Some non-ionogenic species can also interact with the surfactant micelles, thus promoting separation. Due to its resemblance to chromatography, the micellar solution in MEKC is sometimes referred to as pseudo stationary phase. 

Micellar electrokinetic capillary chromatography (MECC or MEKC) Partitioning of solutes between micellar phase and solution phase Detergent added to buffer above critical micelle concentration Neutral molecules... 

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]. 

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