thumb|300px|Distribution of analytes (A) in micellar electrokinetic chromatography based on their hydrophobicity.
Micellar electrokinetic chromatography (MEKC) is a chromatography technique used in analytical chemistry. It is a modification of capillary electrophoresis (CE), extending its functionality to neutral analytes, where the samples are separated by differential partitioning between micelles (pseudo-stationary phase) and a surrounding aqueous buffer solution (mobile phase).
The basic set-up and detection methods used for MEKC are the same as those used in CE. The difference is that the solution contains a surfactant at a concentration that is greater than the critical micelle concentration (CMC). Above this concentration, surfactant monomers are in equilibrium with micelles.
In most applications, MEKC is performed in open capillaries under alkaline conditions to generate a strong electroosmotic flow. Sodium dodecyl sulfate (SDS) is the most commonly used surfactant in MEKC applications. The anionic character of the sulfate groups of SDS causes the surfactant and micelles to have electrophoretic mobility that is counter to the direction of the strong electroosmotic flow. As a result, the surfactant monomers and micelles migrate quite slowly, though their net movement is still toward the cathode. During a MEKC separation, analytes distribute themselves between the hydrophobic interior of the micelle and hydrophilic buffer solution as shown in figure 1.
Analytes that are insoluble in the interior of micelles should migrate at the electroosmotic flow velocity, <math>u_o</math>, and be detected at the retention time of the buffer, <math>t_M</math>. Analytes that solubilize completely within the micelles (analytes that are highly hydrophobic) should migrate at the micelle velocity, <math>u_c</math>, and elute at the final elution time, <math>t_c</math>.
Theory
The micelle velocity is defined by:
:<math>u_c= u_p+u_o</math>
where <math>u_p</math> is the electrophoretic velocity of a micelle.
The fraction of the sample in the aqueous phase, <math>R</math>, is given by:
:<math> R= \frac{u_s-u_c}{u_o-u_c}</math>
where <math>u_s</math> is the migration velocity of the solute.
:<math>t_r =\left ( \frac{1+k^1}{1+(t_M/t_c)k^1} \right )t_M</math>
From this equation it can be seen that all analytes that partition strongly into the micellar phase (where <math>k^1</math> is essentially ∞) migrate at the same time, <math>t_c</math>. In conventional chromatography, separation of similar compounds can be improved by gradient elution. In MEKC, however, techniques must be used to extend the elution range to separate strongly retained analytes.
Recent applications of MEKC include the analysis of uncharged pesticides, essential and branched-chain amino acids in nutraceutical products, hydrocarbon and alcohol contents of the marjoram herb.
MEKC has also been targeted for its potential to be used in combinatorial chemical analysis. The advent of combinatorial chemistry has enabled medicinal chemists to synthesize and identify large numbers of potential drugs in relatively short periods of time. Small sample and solvent requirements and the high resolving power of MEKC have enabled this technique to be used to quickly analyze a large number of compounds with good resolution.
Traditional methods of analysis, like high-performance liquid chromatography (HPLC), can be used to identify the purity of a combinatorial library, but assays need to be rapid with good resolution for all components to provide useful information for the chemist. The introduction of surfactant to traditional capillary electrophoresis instrumentation has dramatically expanded the scope of analytes that can be separated by capillary electrophoresis.
MEKC can also be used in routine quality control of antibiotics in pharmaceuticals or feedstuffs.
References
Sources
- Kealey, D.;Haines P.J.; instant notes, Analytical Chemistry page 182-188