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, u o {\displaystyle u_{o}} , and be detected at the retention time of the buffer, t M {\displaystyle t_{M}} . Analytes that solubilize completely within the micelles (analytes that are highly hydrophobic) should migrate at the micelle velocity, u c {\displaystyle u_{c}} , and elute at the final elution time, t c {\displaystyle t_{c}} .
Theory The micelle velocity is defined by:
u c = u p + u o {\displaystyle u_{c}=u_{p}+u_{o}}
where u p {\displaystyle u_{p}} is the electrophoretic velocity of a micelle. The retention time of a given sample should depend on the capacity factor, k 1 {\displaystyle k^{1}} :
k 1 = n c n w {\displaystyle k^{1}={\frac {n_{c}}{n_{w}}}}
where n c {\displaystyle n_{c}} is the total number of moles of solute in the micelle and n w {\displaystyle n_{w}} is the total moles in the aqueous phase. The retention time of a solute should then be within the range:
t M ≤ t r ≤ t c {\displaystyle t_{M}\leq t_{r}\leq t_{c}}
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. The fraction of the sample in the aqueous phase, R {\displaystyle R} , is given by:
R = u s − u c u o − u c {\displaystyle R={\frac {u_{s}-u_{c}}{u_{o}-u_{c}}}}
where u s {\displaystyle u_{s}} is the migration velocity of the solute. The value R {\displaystyle R} can also be expressed in terms of the capacity factor:
R = 1 1 + k 1 {\displaystyle R={\frac {1}{1+k^{1}}}}
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