Micellar electrokinetic chromatography modifiers

Nishi, H. and Matsuo, M., Separation of corticosteroids and aromatic hydrocarbons by cyclodextrin-modified micellar electrokinetic chromatography, J. Liq. Chromatogr., 14, 973, 1991. 

Lucangioli, S. E., Rodriguez, V. G., Otero, G. C. E, and Carducci, C. N. (1998). Determination of related impurities of bile acids in bulk drugs by cyclodextrin-modified micellar electrokinetic chromatography./. Capillary Electrophor. 5, 139-142. 

Hansen, S. H., Gabel-Jensen, C., El-Sherbiny, D. T. M., and Pedersen-Bjergaard, S. (2001). Microemulsion electrokinetic chromatography - or solvent-modified micellar electrokinetic chromatography TrAC, Trends Anal. Chem. 20, 614—619. 

The scope of this review is limited to the hop-borne a- and /3-acids and to products derived from them that have appreciable commercial relevance. Thus, the liquid chromatography of the iso-a-acids and their chemically modified counterparts as well as of the hulupones will be considered. By far the major focus here will be HPLC analyses, but other methodologies that have been applied (countercurrent distribution, gas chromatography, supercritical-fluid chromatography, thin-layer chromatography, and micellar electrokinetic chromatography) will also be briefly considered. 

Micellar electrokinetic chromatography was used as a fast screening method for the determination of 1,4-dihydropyridines, and was used for the separation of six drugs of this group including nimodipine [38]. A fused silica tube (58.5 cm x 75 pm i.d., 50 cm to detector) was operated at 25 kV with detection at 200-236 nm. The optimized buffer (pH 8.2) contained 20 mM SDS in 50 mM borate buffer modified with acetonitrile. Linear calibration graphs extended upto 70 pg/mL from lower limits of 6.7 pg/mL. 

Terabe S, Miyashita Y, Shibata O, Barnhart ER, Alexander LR, Patterson DG, Karger BL, Hosoyo K, Tanaka N. Separation of highly hydrophobic compounds by cyclodex-trin-modified micellar electrokinetic chromatography. J Chromatogr 1990 516 23. 

The FITC labeling method was also applied to chiral separations of amino acids on a microchip to determine the enantiomeric ratios of amino acids found on a meteorite [27], Since biotic amino acids are normally single enantiomers, chiral separations of amino acids are not truly clinical in nature, but illustrate the potential for chiral separations of small molecules of clinical interest. Ma-thies and co-workers used this technique to search for evidence of life in extraterrestrial environments. Enantiomeric forms of Val, Ala, Glu, and Asp could be discriminated by addition of a-, (3-, or y-cyclodextrin (CD) to the run buffer. Improved resolution with faster separations was found with respect to conventional CE. This method has been modified, by addition of SDS to the buffer, to perform cyclodextrin-modified micellar electrokinetic chromatography (CD-MEKC) [28]. Increasing the SDS concentration decreased the magnitude of elec-troosmotic flow (EOF), increasing the effective migration distance, and therefore the resolution on the microchips. 

Mwongela, S.M., Numan, A., Gill, N.L., Agbaria, R.A., and Warner, I.M. 2003. Separation of achiral and chiral analytes using polymeric surfactants with ionic liquids as modifiers in micellar electrokinetic chromatography. Analytical Chemistry, 75 6089-96. 

As shown in Eq. (2) together with the chiral recognition Kk + Kg), the other necessary requirement for enantioseparations in CE is a mobility difference between the free and the complexed analyte fXf - fjL, 0). Otherwise, it will be impossible to transfer a chiral recognition into a chiral separation. This requirement does not hold when neutral analytes are analyzed with neutral chiral selectors. In such a case, an additional buffer component is required that will assist in generating a difference between the mobilities of an analyte in its free and complexed forms with a chiral selector. This is achieved by an achiral micellar phase in cyclodextrin-modified micellar electrokinetic chromatography (CD-MEKC) [9]. However, a charged CD or a chiral micellar phase can combine the... 

Furuta R, Doi T. 1994. Chiral separation of diniconazole, uniconazole and structurally related compounds by cyclodextrin-modified micellar electrokinetic chromatography. Electrophoresis 15 1322-1325. 

Marina, M.L., I. Benito, J.C. Dfez-Masa, and M.J. Gonzalez. 1996. Chiral separation of polychlorinated biphenyls by micellar electrokinetic chromatography with y-cyclodextrin as modifier in the separation buffer. Chromatographia 42 269-272. 

Terabe, S., Y. Miyashita, Y. Ishama, and O. Shibata. 1993. Cyclodextrin-modified micellar electrokinetic chromatography separation of hydrophobic and enantiomeric compounds. J. Chromatogr. 636 47-55. 

SE Lucangioli, LG Hermida, VP Tripodi, VPT Rodriguez, EE Lopez, PD Rouge, CN Carducci. Analysis of cis-trans isomers and enantiomers of sertraline by cyclo-dextrin-modified micellar electrokinetic chromatography. J Chromatogr A 871 207-215, 2000. 

Gotti, R., Fiori, J., Mancini, F., and Cavrini, V. 2005. Modified micellar electrokinetic chromatography in the analysis of catechins and xanthines in chocolate. Electrophoresis 25 3282-91. 

Capillary electrophoresis (CE) has emerged as an efficient and rapid separation technique in recent years. Its high efficiency has been employed in many applications such as in the analysis of environmental pollutants.Different approaches have been adopted to enhance selectivity for the analysis of different types of compounds. There are two approaches most commonly used to improve CE separations the addition of modifiers into the electrophoretic medium and the modification of the column. Examples of the first approach include the addition of surfactants into the electrophoretic medium as in micellar electrokinetic chromatography (MEKC), and the use of organic solvents,cyclodextrines, " " or bile salts " as buffer modifiers. Examples of the second method include the use of gel-filled columns (capillary gel electrophoresis) " and the coating of the capillary wall surface. " " ... 

Macrocyclic compounds have been used as additives in micellar electrokinetic chromatography where their ability to modify selectivity by formation of inclusion complexes with a variety of compounds and ions of low water solubility is their most important property [55,243-245]. Neutral cyclodextrins in a buffered micelle solution are... 

Neutral cyclodextrins are also used in micellar electrokinetic chromatography with achiral surfactants to modify their enantioselectivity, particularly for the separation of hydrophobic analytes [53,55,185-187]. Enantioselectivity in this case results from differences in the distribution of enantiomers between the micellar pseudostation-ary phase and the cyclodextrin, as well as from the different migration velocities of the cyclodextrin and micelles. Neutral enantiomers can be separated based on differences in their equilibrium constants between the electrolyte solution and a charged chiral surfactant micellar phase, if the micelle has a different electrophoretic mobility to the free enantiomers. Suitable chiral surfactants include the bile salts (section 8.3.3), long alkyl-chain amino acid derivatives (e.g. sodium N-dodecanoyl-... 

Tonin, F. G, Jager, A. V., Micke, G A., Farah, J. R. S., and Tavares, M. F. M. Optimization of the separation of flavonoids using solvent-modified micellar electrokinetic chromatography. Electrophoresis, 26, 3387, 2005. 

Source Modified from Table 3 of Terabe, S., Micellar Electrokinetic Chromatography, Beckman, Fullerton, 1992. 

He, Y. and Lee, H.K., Orthogonal array design experiments for optimizing the separation of various pesticides by cyclodextrin-modified micellar electrokinetic chromatography, J. Chromatogr. A, 793, 331, 1998. 

As detailed in Chapter 3 by Terabe, micellar electrokinetic chromatography (MEKC) is a useful technique in the retention analysis of water-soluble compounds. The separation and analysis of lypophilic analytes, however, may be difficult in MEKC due to the strong affinity of lypophilic compounds to the micelle resulting in long separation times and poor resolution. An interesting approach for the simultaneous analysis of water- and fat-soluble vitamins by microemulsion electrokinetic chromatography (MEEKC) was proposed by Sanchez. The separation of both water- and fat-soluble vitamins (Bi, B2, B3, Be, B12, C, A palmitate, D, E acetate, and K) was obtained when the microemulsion was prepared with sodium dodecyl sulfate (SDS) as the surfactant, octane as the nonpolar modifier, butanol as the cosurfactant, and propanol as the second cosurfactant. Complete separation of all vitamins was carried out within 55 min however, this approach was tested only in multivitamin formulation. 

Nouadje, G, Nertz, M., andCourderc, F., Study of the racemization of serine by cyclodextrin-modified micellar electrokinetic chromatography and laser-induced fluorescence detection, J. Chromatogr. A, 716, 331, 1995. 

Kodama, S., Yamamoto, A., Saitoh, Y, Matsunaga, A., Okamura, K., Kizu, R., and Hayakawa, K., Enantioseparation of vinclozolin by P-cyclodextrin-modified micellar electrokinetic chromatography, J. Agric. Food Chem., 50, 1312, 2002. 

W.C. Lin, F.C. Chang and C.H. Kuei, Separation of atropisomeric polychlorinated biphenyls by cyclodextrin modified micellar electrokinetic chromatography, J. Microcolumn Sep., 11, 231-238,... 

Yu, L., Y. Xu, H. Feng, and S.F. Li. 2005. Separation and determination of toxic pyrrolizidine alkaloids in traditional Chinese herbal medicines by micellar electrokinetic chromatography with organic modifier. Electrophoresis 26(17) 3397-3404. 

Micellar electrokinetic chromatography (MEKC), a modified CE, enabled the separation of electrically neutral analytes. However, highly hydrophobic compounds including PAHs, corticosteroids, fat-soluble vitamins and polychlorinated biphenyl (PCB) congeners could not be separated by MEKC in electrophoretic medium of sodium dodecylsulfate (SDS) [59]. The use of CDs to the SDS solution can remarkably improve the resolution of analytes. For example, a mixture of water-soluble and fat-soluble vitamin or esterom can be successfully separated by MEKC by the addition of y-CD in the SDS electrophoretic medium [60]. 

Razak, JL, HJ Doyen and CE Lunte (2003). Cyclodextrin-modified micellar electrokinetic chromatography for the analysis of Esterom, a topical product consisting of hydrolyzed benzoylecgonine in propylene glycol. Electrophoresis, 24( 11), 1764-1769. 

Since micellar electrokinetic chromatography (MEKC) was first introduced in 1984, it has become one of major separation modes in capillary electrophoresis (CE), especially owing to its applicability to the separation of neutral compounds as well as charged ones. Chiral separation is one of the major objectives of CE, as well as MEKC, and a number of successful reports on enantiomer separations by CE and MEKC has been published. In chiral separations by MEKC, the following two modes are normally employed (a) MEKC using chiral micelles and (b) cyclodextrin (CD)-modified MEKC (CD/MEKC). 

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