Separation conditions micellar electrokinetic

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. 

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. 

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. 

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

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. 

Figure 28 Electrokinetic separation of the niacin derivatives in the presence of SDS. Conditions applied voltage, 15 kV sample injection, injected by raising the positive end of the capillary about 4 cm higher than the other end capillary, a polyimide-coated fused-silica (70 cm X 50 xm i.d.) micellar solution, 0.15 M SDS in 0.02 M borate-0.01 M KOH (pH 9.1) detection wavelength, 210 nm. 1, Isonicotinic acid hydrazide 2, Nam 3, pyri-dine-3-methanol 4, 6-AN 5, MNA 6, pyridine-3-aldehyde 7, pyridine 8, 3-acetylpyri-dine 9, thionicotinamide 10, NiA 11, pyridine-3-sulfonic acid 12, P-picoline 13, nicotinic acid ethyl ester. (From Ref. 71.)...

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