Micellar electrokinetic chromatography separation conditions

OPTIMIZATION OF MICELLAR ELECTROKINETIC CHROMATOGRAPHY SEPARATION CONDITIONS BY CHEMOMETRIC METHODS... 

Figure 13.9 Microchip-based micellar electrokinetic chromatography (MEKC) electro-pherogram of a mixture of nitroaromatics and nitramines. Analytes 20 ppm of each (1) TNB, (2) DNB, (3) NB, (4) TNT, (5) tetryl, (6) 2,4-DNT, (7) 2,6-DNT, (8) 2-, 3-, and 4-NT, (9) 2-Am-4,6-DNT, (10) 4-Am-2,6-DNT. Conditions MEKC buffer, 50 mM borate, pH 8.5, 50 mM SDS, 5 M Cy7, separation voltage 4 kV, separation distance 65 mm. (Reprinted in part with permission from [37]. Copyright 2000 American Chemical Society.)...

Nitroaromatic explosives and other nitrated organic explosives are under the normal conditions neutral compounds and therefore cannot be separated directly by capillary zone electrophoresis (CZE) technique. Another separation vector must be introduced in order to achieve the resolution between the solutes. Micellar electrokinetic chromatography (MEKC) is typically employed on microchip scene for separation of nitroaromatic explosives. 

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. 

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. 

Felhofer, J., Hanrahan, G., and Garcia, C.D. (2009) Multivariate versus univariate optimization of separation conditions in micellar electrokinetic chromatography. Talanta, , 1172-1178. 

As the name implies, micellar electrokinetic chromatography (MEKC) is a chromatographic technique in which samples are separated by differential partitioning between two phases (Fig. 9). The technique is usually performed in uncoated capillaries under alkaline conditions to generate a high electroosmotic flow. The background electrolyte contains a surfactant at a concentration above its critical... 

Different separation mechanisms, which determine selectivity, can be exploited in HPCE by appropriate choice of operating conditions. There are four principal modes of operation (Table 4.22) and it should be noted that in only one, micellar electrokinetic capillary chromatography (MEKC), is it possible to separate neutral species from one another. 

Micelles and cyclodextrins are the most common reagents used for this technique. Micellar electrokinetic capillary chromatography (MECC or MEKC) is generally used for the separation of small molecules [6], Sodium dodecyl sulfate at concentrations from 20 to 150 mM in conjunction with 20 mM borate buffer (pH 9.3) or phosphate buffer (pH 7.0) represent the most common operating conditions. The mechanism of separation is related to reversed-phase liquid chromatography, at least for neutral solutes. Organic solvents such as 5-20% methanol or acetonitrile are useful to modify selectivity when there is too much retention in the system. Alternative surfactants such as bile salts (sodium cholate), cationic surfactants (cetyltrimethy-lammonium bromide), nonionic surfactants (poly-oxyethylene-23-lauryl ether), and alkyl glucosides can be used as well. 

Capillary electrophoresis (CE) is an emerging analytical technique for determination of catechins. The majority of CE studies involve the analysis of catechins in tea infusion, extracts as well as supplements. The three variants of CE suitable for the analysis of catechins include capillary zone electrophoresis (CZE), micellar electro-kinetic chromatography (MEKC), and microemulsion electrokinetic chromatography (MEEKC) with UV detection. In general, the resolution of MEKC was found to be superior to CZE for separation of catechins. MEEKC is a relatively new technique, and the few reports available suggest that it offers a performance similar to MEKC. CE conditions are often quite complex, and many factors, such as buffer composition, pH, presence of surfactants, and column temperature, can all affect the quality of separation and should be optimized individually. On the other hand, CE offers several advantages over HPLC. The short analysis time (<20 minutes), low running costs, and reduced use of solvents make it an attractive alternative for routine analysis of catechins. 

---