Micellar electrokinetic chromatography
It is a modification of capillary electrophoresis (CE), extending its functionality to neutral analytes,[1] where the samples are separated by differential partitioning between micelles (pseudo-stationary phase) and a surrounding aqueous buffer solution (mobile phase).
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.
[3] 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,
[4] The retention time of a solute should then be within the range: Charged analytes have a more complex interaction in the capillary because they exhibit electrophoretic mobility, engage in electrostatic interactions with the micelle, and participate in hydrophobic partitioning.
can also be expressed in terms of the capacity factor: Using the relationship between velocity, tube length from the injection end to the detector cell (
, a relationship between the capacity factor and retention times can be formulated:[5] The extra term enclosed in parentheses accounts for the partial mobility of the hydrophobic phase in MEKC.
in conventional packed bed chromatography: A rearrangement of the previous equation can be used to write an expression for the retention factor:[6] From this equation it can be seen that all analytes that partition strongly into the micellar phase (where
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.
[5] Elution ranges can be extended by several techniques including the use of organic modifiers, cyclodextrins, and mixed micelle systems.
Cyclodextrins are cyclic polysaccharides that form inclusion complexes that can cause competitive hydrophobic partitioning of the analyte.
Since analyte-cyclodextrin complexes are neutral, they will migrate toward the cathode at a higher velocity than that of the negatively charged micelles.
Mixed micelle systems, such as the one formed by combining SDS with the non-ionic surfactant Brij-35, can also be used to alter the selectivity of MEKC.
[5] The simplicity and efficiency of MEKC have made it an attractive technique for a variety of applications.
Unfortunately, this technique is not suitable for protein analysis because proteins are generally too large to partition into a surfactant micelle and tend to bind to surfactant monomers to form SDS-protein complexes.
[7] Recent applications of MEKC include the analysis of uncharged pesticides,[8] essential and branched-chain amino acids in nutraceutical products,[9] hydrocarbon and alcohol contents of the marjoram herb.
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.
MEKC can also be used in routine quality control of antibiotics in pharmaceuticals or feedstuffs.
