Micellar liquid chromatography separation technique

The first intentional use of surfactants in chromatographic mobile phases at concentrations above the CMC was proposed in 1977 by Armstrong and co-workers (1,86-100). Since the initial reports, the general method, dubbed pseudophase liquid chromatography (PLC) or micellar liquid chromatography (MLC), has moved from the realm of an academic novelty to a demonstrated practical separation technique. 

The majority of enantioseparations are performed by pressure-driven liquid chromatography. However, in the last decade other liquid-phase separation techniques have evolved and demonstrated their usefulness for enantioseparations, including supercritical fluid chromatography (SFC), capillary electrophoresis (CE), micellar electrokinetic chromatography (MEKC), and open-tubular and packed-bed electrochromatography (OT-EC and CEC). 

Lovgren U, Johansson M, Kronlcvist K, Edhohn LE. Sample pretreatment for immunochemical techniques using micellar liquid chromatography for separation of corticosteroids. J Chromatogr B Biomed Appl 1995 672 33-44. 

This is a particularly opportune time for this book. The field is sufficiently mature that one is able to examine it with a proper perspective and to see how it gave rise and spread into other areas of separation science and analytical chemistry. Micellar Liquid Chromatography is a thorough scholarly and practical presentation of all areas of this special separation technique. It is likely that it will be the definitive reference volume in this area of research and technology for years to come. 

Micellar electrokinetic chromatography performed in capillaries is a separation technique combining some of the operational principles of micellar liquid chromatography and capillary zone electrophoresis. This technique was termed micellar electrokinetic capillary chromatography (MECC) by Burton et al. [79]. MECC uses the addition of a surface-active agent in the working electrolyte, which creates new possibilities for electrophetic separations. 

MEKC is usually used as a separation technique in which the basic properties of micellar liquid chromatography and CE are combined. MEKC was first described by Terabe in 1984 for the separation of nonionic aromatic compounds and is a powerful separation technique for lipophilic and nonionic species. By addition of surfactants to the background electrolyte, new options for solving electrophoretic separation problems are opened, but it is also possible to apply this technique to study the affinities of drug molecules to surface-active compounds. The term micellar affinity capillary electrophoresis (MACE) is used... 

Caruso and co-workers [7] separated alkyltin compounds using micellar liquid chromatography and tested the compatibility of this separation technique with inductively coupled plasma mass spectrometric detection (ICP-MS). This study was undertaken because many of the HPLC separations developed for these compounds involve the use of hydro-organic mobile phases, and use of organic solvents with ICP-MS results in a decrease in sensitivity due to excessive solvent loading of the plasma. Separations for many of these species were shown using 5-/im Ci8 Spherisorb silica-bonded columns, 50 mm x 4.6 mm i.d., and mobile phases containing sodium dodecyl sulfate, acetic acid, propanol, and potassium fluoride. 

Recently a new method was developed for the complete liquid chromatographic separation and diode array detection of standard mixtures of the 14 most frequently used synthetic colorants. Protocols for RP-HPLC - " and IP-HPLC techniques have been extensively described and the techniques were compared with micellar electrokinetic capillary chromatography, - which has been shown to be suitable for the analysis of synthetic colorants. 

This phenomenon can be exploited for separation and concentration of solutes. If one solute has certain affinity for the micellar entity in solution then, by altering the conditions of the solution to ensure separation of the micellar solution into two phases, it is possible to separate and concentrate the solute in the surfactant-rich phase. This technique is known as cloud point extraction (CPE) or micelle-mediated extraction (ME). The ratio of the concentrations of the solute in the surfactant-rich phase to that in the dilute phase can exceed 500 with phase volume ratios exceeding 20, which indicates the high efficiency of this technique. Moreover, the surfactant-rich phase is compatible with the micellar and aqueous-organic mobile phases in liquid chromatography and thus facilitates the determination of chemical species by different analytical methods [104]. 

Capillary electrochromatography (CEC) is a rapidly emerging technique that adds a new dimension to current separation science. The major "news" in this method is that the hydrodynamic flow of the eluting liquid, which is typical of HPLC, is replaced by a flow driven by electro-endoosmosis. This increases considerably the selection of available separation mechanisms. For example, combinations of traditional processes such as reversed-phase- or ion-exchange- separations with electromigration techniques are now possible. Also, CEC is opening new horizons in the separation of non-polar compounds, and thus represents an alternative to the widely used micellar electrokinetic chromatography. 

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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