Capillary electrophoresis micellar solutions

Currently, there are five major modes of operation of CE capillary zone electrophoresis (CZE), also referred to as free solution or free flow capillary electrophoresis micellar electrokinetic chromatography (MEKC) capillary gel electrophoresis (CGE) capillary isoelectric focusing (CIEF) and capillary isotachophoresis (CITP). Of these, the most commonly utilized capillary techniques are CZE and MEKC (Rabel and Stobaugh 1993 Issaq 1999 Smyth and McClean 1998). 

Anions and uncharged analytes tend to spend more time in the buffered solution and as a result their movement relates to this. While these are useful generalizations, various factors contribute to the migration order of the analytes. These include the anionic or cationic nature of the surfactant, the influence of electroendosmosis, the properties of the buffer, the contributions of electrostatic versus hydrophobic interactions and the electrophoretic mobility of the native analyte. In addition, organic modifiers, e.g. methanol, acetonitrile and tetrahydrofuran are used to enhance separations and these increase the affinity of the more hydrophobic analytes for the liquid rather than the micellar phase. The effect of chirality of the analyte on its interaction with the micelles is utilized to separate enantiomers that either are already present in a sample or have been chemically produced. Such pre-capillary derivatization has been used to produce chiral amino acids for capillary electrophoresis. An alternative approach to chiral separations is the incorporation of additives such as cyclodextrins in the buffer solution. 

A kit solutions for non-charged molecules based on micellar electrokinetic capillary electrophoresis... 

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]. 

The separation of phospholipids by micellar electrokinetic capillary electrophoresis (MEKC) has been described (17-19). In this technique, solutes are separated based on their distribution between a mobile (usually aqueous) and a pseudostationary (micellar) phase. Szucs et al. found that the major soybean phospholipids were fully resolved in only 7 minutes using deox ycholic acid for micelle formation in combination with 30% n-propanol at 50°C (18). However, quantification of the separated compounds remains troublesome. This is due first of all to the fact that only UV detection can be used, thus making the response highly dependent on the degree of unsaturation of the phospholipids. Besides, the comparison of peak areas in MEKC is more complicated than in HPLC, because all compounds are moving with different velocities. 

Hall et al. (127) compared free solution capillary electrophoresis (FSCE) and micellar elec-trokinetic capillary chromatography (MEKC) techniques with HPLC analysis. Four major food-grade antioxidants, propyl gallate (PG), BHA, BHT, and TBHQ, were separated. Resolution of the 4 antioxidants was not successful with FSCE, but was with MEKC. Separation was completed with excellent resolution and efficiency within 6 min and picomole amounts of the antioxidants were detectable using UV absorption. In contrast, reversed-phase HPLC separation was not as efficient and required larger sample amounts and longer separation time. 

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... 

Here free solution indicates capillary electrophoresis carried out in a solution without any additives SDS-PAGE is sodium dodecyl sulfate-polyacrylamide gel electrophoresis MECC is micellar electrokinetic capillary chromatography, at times, also called MEKC (micellar electrokinetic chromatography). 

Chromatographic and related electrophoretic methods for the separation of transition metal complexes or their ligands were reviewed . Micellar electrokinetic chromatography (MEKC) presents a new development in the field of capillary zone electrophoresis (CZE). The use of micellar solutions expands the application of CZE to electronically neutral solutes, as well as charged ones. Thus, electrically neutral / -diketonates Cr(dik)3, Co(dik)3, Rd(dik)3, Pt(dik)2 and Pd(dik)2 were separated by CZE in micellar solutions of sds. A linear log-log relationship was found between the distribution coefficient and the partition coefficient of the complex between dodecane and water, which was used for prediction of both the distribution coefficients and the migration times of different metal complexes . 

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. 

Figure 12.9. Amperometric CE detector with cathodic insulating assembly and ultramicroelectrode.7 [Reprinted, with permission, from R. A. Wallingford and A. G. Ewing, Anal. Chem. 60 (No. 3), 1988, 258-263. Amperometric Detection of Catechols in Capillary Zone Electrophoresis with Normal and Micellar Solutions . 1988 by American Chemical Society.]...

Cohen, A. S. High performance capillary electrophoresis of bases, nucleosides and ohgonucleotides retention manipulation via micellar solutions and metal additives. Ana/. Chem., 59, 1021,1987. 

Tadey, T. and W. C. Purdy, Capillary electrophoretic resolution of phosphorylated peptide isomers using micellar solutions and coated capillaries. Electrophoresis, 16, 574-579, 1995. 

MLC uses micellar mobile phases with classical RPLC columns. This chapter expands the field to include some mobile phases that can be considered close to micellar phases, such as normal and reverse microemulsions, bile salt solutions, and surfactant solutions in supercritical fluids. Also, this chapter rapidly surveys the use of micellar mobile phases with non-RPLC stationary phases such as size exclusion or gel permeation polymer phases. Allied techniques using micellar phases such as ion-exchange chromatography and capillary electrophoresis are also briefly presented. 

Artifactual-injection-related peak splitting can occur under certain conditions. When the sample diluent contains organic solvents and micellar electrokinetic capillary electrophoresis or cyclodextrin containing electrolytes are employed, splitting can occur due to the distribution of the solute between two phases moving at different speeds... 

This mouthful of words describes a form of capillary electrophoresis that separates neutral molecules as well as ions (Figure 23-21). The key modification in micellar electrokinetic capillary chromatography is the use of micelles in the capillary solution. Micelles are described in Box 23-2, which you should read now. 

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