Electrokinetic chromatography electrophoretic mobility

Conceptually. CE enantioseparations are mainly applied to charged SAs. Micellar electrokinetic chromatography (MEKC) (introduced by Terabe et al. in 1984 488 ), in contrast, permits the separation of electrically neutral compounds. In enantiomer separation by MEKC. ionic pseudo-stationary phases, such as chiral micelles composed of chiral SO moieties, which migrate according to their electrophoretic mobility, may interact stereoselectively with the solutes to be separated. MEKC with synthetic (e.g. A-dodecoxycarbonylvalines, commercialized as SDVal by Waters) 1489.490) or naturally occurring chiral surfactants (e.g. bile salts) 1491-494). and cyclodextrin-moditied MEKC (most often SDS/CD combinations) 1495-498) are the mo.st widely used selector systems in MEKC. The topic of MEKC enantioseparation has been reviewed by Nishi )499). 

Capillary zone electrophoresis (CZE), micellar capillary electrokinetic chromatography (MECC), capillary gel electrophoresis (CGE), and affinity capillary electrophoresis (ACE) are CE modes using continuous electrolyte solution systems. In CZE, the velocity of migration is proportional to the electrophoretic mobilities of the analytes, which depends on their effective charge-to-hydrodynamic radius ratios. CZE appears to be the simplest and, probably, the most commonly employed mode of CE for the separation of amino acids, peptides, and proteins. Nevertheless, the molecular complexity of peptides and proteins and the multifunctional character of amino acids require particular attention in selecting the capillary tube and the composition of the electrolyte solution employed for the separations of these analytes by CZE. 

The first applications of CDs as chiral selectors in CE were reported in capillary isotachophoresis (CITP) [2] and capillary gel electrophoresis (CGE) [3]. Soon thereafter, Fanali described the application of CDs as chiral selectors in free-solution CE [4] and Terabe used the charged CD derivative for enantioseparations in the capillary electrokinetic chromatography (CEKC) mode [5]. It seems important to note that although the experiment in the CITP, CGE, CE, and CEKC is different, the enantiomers in all of these techniques are resolved based on the same (chromatographic) principle, which is a stereoselective distribution of enantiomers between two (pseudo) phases with different mobilities. Thus, enantioseparations in CE are commonly based on an electrophoretic migration principle and on a chromatographic separation principle [6]. 

Most CE work so far has been done using the capillary zone electrophoresis (CZE) mode, where analytes are separated on the basis of differences in electrophoretic mobility, which is related to charge density. The separation is carried out in a capillary filled with a continuous background electrolyte (buffer). Micellar electrokinetic capillary chromatography (MEKC or MECC) is one other CE method based on differences in the interaction of the analytes with micelles present in the separation buffer, which can easily separate both charged and neutral solutes with either hydrophobic or hydrophilic properties. An alternative to MEKC is capillary... 

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. 

The apparent electrophoretic mobility of an analyte in micellar electrokinetic chromatography depends on three factors the electroosmotic mobility for the system the fraction of analyte in the electrolyte solution and its electrophoretic mobility and the fraction of analyte in the pseudostationary phase, and the electrophoretic mobility of the micelles (assuming that the mobility of the analyte-micelle complex is the same as the micelle). If we introduce the chromatographic retention factor, defined as the ratio of the number of analyte molecules in the pseudostationary phase to the number in the... 

Electrophoretic mobility of neutral solutes and anions in micellar electrokinetic chromatography... 

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-... 

The theories describing micellar electrokinetic chromatography (MEKC), capillary zone electrophoresis (CZE), and capillary gel electrophoresis (CGE) separations of small molecules and biopolymers are described in other chapters of this book and will not be discussed here. Here, we will briefly touch upon the theoretical aspects of the electrophoretic mobility of organelles, foregoing an in depth discussion of electrokinetic theory that can be found elsewhere in original publications and comprehensive reviews. The electrophoretic mobility (/ue) is defined as... 

While the migration principle, i.e., the driving forces moving the analytes through the separation capillary, is based on electrophoretic mechanisms the chiral separation is based on enantioselective interactions between the analyte enantiomers and a chiral selector and is, therefore, a chromatographic separation principle. The fact that the selector is in the same phase as the analytes in CE and not part of a stationary phase that is immiscible with the mobile phase as found in chromatography does not represent a conceptional difference between both techniques. The chiral selector in CE is also called pseudophase as it is not a physically different phase and may also possess an electrophoretic mobility. Enantioseparations in CE have also been termed capillary electrokinetic chromatography . 

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