Micellar electrokinetic capillary chromatography MECC

The next important milestone in CE was achieved in 1984, when Terabe et al. described the method of micellar electrokinetic capillary chromatography (MECC or MEKC). By simply adding a surfactant to the separation buffer electrolyte, it was possible to separate both charged and neutral compounds simultaneously in CE. Erom this point on, the technique developed rapidly with many applications resulting in a demand for identification information. Coupling of CE to mass spectrometry was a next challenge and the... 

Shah, P. A., and Quinones, L. (1995). Validation of a micellar electrokinetic capillary chromatography (MECC) method for the determination of p-toluenesulfonic acid impurity in a pharmaceutical intermediate./. Liq. Chromatogr. 18, 1349 — 1362. 

Suomi, J., Wiedmer, S. K., Jussila, M., and Riekkola, M. L. (2002). Analysis of eleven iridoid glycosides by micellar electrokinetic capillary chromatography (MECC) and screening of plant samples by partial filling (MECC)-electrospray ionization mass spectrometry.. Chromatogr. A 970, 287-296. 

Micellar electrokinetic capillary chromatography (MECC), in contrast to capillary electrophoresis (CE) and capillary zone electrophoresis (CZE), is useful for the separation of neutral and partially charged species [266,267]. In MECC, a surfactant, usually sodium dodecyl sulfate (SDS), is added to the buffer solution above its critical micellar concentration to form micelles. Although SDS is certainly the most popular anionic surfactant in MECC, other surfactants such as bile salts have proved to be very effective in separating nonpolar analytes that could not be resolved using SDS [268]. 

Micellar electrokinetic chromatography (MEKC) is a modality of liquid chromatography having a surfactant molecule in the form of a micelle, which was introduced by Terabe et al. in 1984 [38]. The formation and separation occur in the capillary and, hence, it is also called micellar electrokinetic capillary chromatography (MECC). This modality is useful for some specific molecules having solubilities in micelles and, therefore, utilized for the separation and identification of such compounds with great efficiency, reproducibility, and low levels of detections. The most commonly used compounds for micelle formation are sodium dodecyl sulfate (SDS), sodium tetradecyl sulfate, sodium decanesulfonate, sodium /V-lauryl-/V-mcthyllauratc, sodium... 

Another format of chromatography which has been performed on electrophoresis chips is micellar electrokinetic capillary chromatography (MECC). This technique represents a very powerful extension of CE for the separation of both neutral species and ionic compounds, and has been originally developed by Terabe et al. [76]. Transfer of MECC to CE microchips has been first demonstrated by Moore et al. [77]. The experimental procedure used is identical to high speed CE, apart from the fact that a MECC buffer with 50 mM SDS and 10% methanol was used. Again three coumarin dyes were used as a model sample. Separation was achieved within a few minutes. At low applied field strengths (< 400 V/cm), the reproducibility was found to be excellent (below 1 %). 

The first and most often encountered separation mechanism in CE is based on mobility differences of the analytes in an electric field these differences are dependent on the size and charge-to-mass ratio of the analyte ion. Analyte ions are separated into distinct zones when the mobility of one analyte differs sufficiently from the mobility of the next. This mechanism is exemplified by capillary zone electrophoresis (CZE) which is the simplest CE mode. A number of other recognized CE modes are variations of CZE. These are micellar electrokinetic capillary chromatography (MECC), capillary gel electrophoresis (CGE), capillary electrochromatography (CEC), and chiral CE. In MECC the separation is similar to CZE, but an additional mechanism is in effect that is based on differences in the partition coefficients of the solutes between the buffer and micelles present in the buffer. In CGE the additional mechanism is based on solute size, as the capillary is filled with a gel or a polymer network that inhibits the passage of larger molecules. In chiral CE the additional separation mechanism is based on chiral selectivity. Finally, in CEC the capillary is packed with a stationary phase that can retain solutes on basis of the same distribution equilibria found in chromatography. 

Micellar electrokinetic capillary chromatography (MECC) is a mode of CE similar to CZE, in which surfactants (micelles) are added to the buffer system. Micellar solutions can be used to solubilize hydrophobic compounds that would otherwise be insoluble in water. In MECC the micelles are used to provide a reversed-phase character to the separation mechanism. Although MECC was originally developed for the separation of neutral species by capillary electrophoresis, it has also been shown to enhance resolution in the analysis of a variety of charged species.16... 

Boyce et al. (2003) reported the use of the mixed micellar electrokinetic capillary chromatography (MECC) method for the... 

TABLE XII. Summary of Successful Applications employing Micellar Electrokinetic Capillary Chromatography (MECC) in Separations... 

The first reports of micellar electrokinetic capillary chromatography (MECC or MEKC) appeared in the literature in 1984 [16]. Through April 2002, there have been over 1100 English-language papers published in the field. The subject is covered in all general textbooks on capillary electrophoresis. While a vast number of surfactants and related reagents can be employed, most separations can be accomplished with a few simple recipes. 

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

A range of electrophoretic techniques use the capillary prindple capillary zone electrophoresis (CZE), capillary isotachophoresis (CITP), capillary gel electrophoresis (CGE), capillary isoelectric focusing (CIEF) and micellar electrokinetic capillary chromatography (MECC). Some of these techniques, particularly CZE and CGE, have already established themselves as important analytical tools others, notably MECC, may open new approaches in analytical biochemistry. Table 4-11 summarises the areas of application of the various techniques in what follows we shall focus on CZE, CGE and MECC. 

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. 

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