Micellar electrokinetic chromatography resolution

The effects of pH on electrokinetic velocities in micellar electrokinetic chromatography was studied by using sodium dodecyl sulfate solutions [179]. Micellar electrokinetic capillary chromatography with a sodium dodecyl sulfate pseudostationary phase has been used to determine the partition constants for nitrophenols, thiazolylazo dyes, and metal chelate compounds [180]. A similar technique was used to separate hydroquinone and some of its ether derivatives. This analysis is suitable for the determination of hydroquinone in skin-toning creams [181]. The ingredients of antipyretic analgesic preparations have also been determined by this technique [182], The addition of sodium dodecyl sulfate improves the peak shapes and resolution in chiral separations by micellar electrokinetic chromatography [183]. 

CE was recently used for anthocyanin analysis because of its excellent resolution. This technique has different modes capillary zone electrophoresis (CZE), capillary gel electrophoresis (CGE), micellar electrokinetic chromatography (MEKC), capillary electrochromatography (CEC), capillary isoelectric focusing (CIEE), and capillary isotachophoresis (CITP)."° CZE is the most popular method for anthocyanin... 

Saitoh, K., Kiyohara, C., and Suzuki, N., Mobilities of metal [i-diketonato complexes in micellar electrokinetic chromatography, ]. High Resolut. Chromatogr., 14, 245, 1991. 

J. Vindevogel, P. Sandra, Resolution optimization in micellar electrokinetic chromatography use of Plackett-Burman statistical design for the analysis of testosterone esters,, 4na/. C/iem., 63 (1991) 1530-1536. 

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. 

Capillary electrophoresis (CE) coupled to MS has the advantage of high resolution and soft ionization for biomolecules, which may be used to differentiate post-translational modifications and variants of intact proteins and oligonucleotides. Different modes of CE (capillary zone electrophoresis, capillary isoelectric focusing, capillary electrochromatography, micellar electrokinetic chromatography, nonaqueous capillary electrophoresis) to MS as well as online preconcentration techniques (transient capillary isotachophoresis, solid-phase extraction, membrane preconcentration) are used to compensate for the restricted detection sensitivity of the CE methodology [77, 78]. 

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. 

Various papers related to the simultaneous determination of creatinine and uric acid can be found in the hterature. Several authors have developed capillary zone electrophoresis (CZE) methods for simultaneous analysis of these compounds in urine. The CE analysis of these renal markers offers some advantages when compared with chromatography, such as shortened separation time, reduced reagent consumption, and increased resolution. Capillar micellar electrokinetic chromatography has been applied to the simultaneous separation of creatinine and uric acid in human plasma and urine. However, chromatographic techniques are widely accepted for the determination of these compounds. Reversed-phase and ion... 

A promising alternative to HPLC and TLC is micellar electrokinetic chromatography (MEKC). Pietta et al. (1998a) used a 25 mM tetraborate buffer containing 30 mM sodium dodecyl sulfate (SDS) at pH 8.6 for the separation of CADs. The separation was completed within 20 min with good resolution. The advantage to this method is that the cost of operation is less than HPLC, because organic solvents are not needed. 

The separation number (or Trennzal) is a special case of the peak capacity. It is widely used as a column quality test in gas chromatography [217-219] and to a limited extent in micellar electrokinetic chromatography [220,221]. It is defined as the number of peaks that can be placed between the peaks of two consecutive homologous standards with z and z-i-1 carbon atoms and separated with a resolution of Rs = 1.177. It represents the... 

The general resolution equation for two neutral analytes with similar retention factors in micellar electrokinetic chromatography, is similar to the relationship for chromatography (section 1.6) with an additional term that arises from the limited migration time window [11,12,166,177,178]. 

Mikaeli, S., Thorsen, G, and Karlberg, B., Optimisation of resolution in micellar electrokinetic chromatography by multivariate evaluation of electrolytes, J. Chromatogr. A, 907, 267, 2001. 

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. 

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]. 

Table 1 A comparison of a GC-K/IS method and a CE method (in the micellar electrokinetic chromatography (MEKC) mode) for the separation of priority endocrine disrupters. Resolution and limit of detection (LOD) are used as parameters for comparison ...

Table 9.2 The chiral resolution of some environmental pollutants by capillary electrophoresis and by micellar electrokinetic chromatography...

Capillary electrophoresis (micellar electrokinetic chromatography MEKC) has also proven to be an effective method for the analysis of caffeic acid derivatives [44]. The method provides an excellent resolution and a very high sensitivity and enables the discrimination of the species (see Fig. 5). 

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