Surfactants, capillary electrophoresis

Keywords Capillary electrophoresis Surfactant analysis Dodecylpyridinium bromide Triton X-100 Critical micelle concentration... 

Capillary Electrophoresis. Capillary electrophoresis (ce) is an analytical technique that can achieve rapid high resolution separation of water-soluble components present in small sample volumes. The separations are generally based on the principle of electrically driven ions in solution. Selectivity can be varied by the alteration of pH, ionic strength, electrolyte composition, or by incorporation of additives. Typical examples of additives include organic solvents, surfactants (qv), and complexation agents (see Chelating agents). 

Capillary electrophoresis employing chiral selectors has been shown to be a useful analytical method to separate enantiomers. Conventionally, instrumental chiral separations have been achieved by gas chromatography and by high performance liquid chromatography.127 In recent years, there has been considerable activity in the separation and characterization of racemic pharmaceuticals by high performance capillary electrophoresis, with particular interest paid to using this technique in modem pharmaceutical analytical laboratories.128 130 The most frequently used chiral selectors in CE are cyclodextrins, crown ethers, chiral surfactants, bile acids, and protein-filled... 

Heinig, K., Vogt, C., Werner, G. (1998). Separation of nonionic surfactants by capillary electrophoresis and high-performance liquid chromatography. Anal. Chem. 70(9), 1885-1892. 

Petrovic, M., Barcelo, D. (2003). Capillary electrophoresis in surfactant analysis. Comprehensive Anal. Chem. 40, 77-87. 

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. 

Yeung, K. K. C., Kiceniuk, A. G., and Li, L. (2001). Capillary electrophoresis using a surfactant-treated capillary coupled with offline matrix-assisted laser desorption ionization mass spectrometry for high efficiency and sensitivity detection of proteins. /. Chromatogr. A 931, 153 — 162. 

Lu, W, Poon, G. K., Carmichael, P. L., and Cole, R. B. (1996). Analysis of tamoxifen and its metabolites by online capillary electrophoresis-electrospray ionization mass spectrometry employing nonaqueous media containing surfactants. Anal. Chem. 68, 668—674. 

Nonaqueous capillary electrophoresis is a new technique that matured over the last couple of years to complement the electrophoretic techniques performed under aqueous conditions. Consequently, it extends the applicability of CE to those analytes that are insoluble or only sparsely soluble in water, including several classes of pharmaceutical compounds and long-chain fatty acids, vitamins, and surfactants, to name only a few. 

Schwarz et al. used ESI-MS detection to characterize the composition of binary (bile salt/phosphatidylcholine) and ternary (bile salt/phophatidy 1-choline/fatty acid) mixed micelles that were used in micellar affinity capillary electrophoresis (43,44). The detrimental effects of the surfactants turned out to be tolerable for short-time qualitative determinations. 

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]. 

N. E. Baryla, J. E. Melanson, M. T. McDermott, and C. A. Lucy, Characterization of Surfactant Coatings in Capillary Electrophoresis by Atomic Force Microscopy, Anal. Chem. 2001, 73, 4558 M. M. Yassine and C. A. Lucy, Enhanced Stability Self-Assembled Coatings for Protein Separations by Capillary Zone Electrophoresis Through the Use of Long-Chained Surfactants, Anal. Chem. 2005, 77, 62. 

The optimum pH for separating cations is pK + 0.30 K. K.-C. Yeung and C. A. Lucy, Isotopic Separation of [14N]- and fI5N] Aniline by Capillary Electrophoresis Using Surfactant-Controlled Reversed Electroosmotic Flow, Anal. Chem 1998, 70. 3286. 

Capillary electrophoresis (CE) provided an orthogonal separation technique. The retention mechanism can be manipulated with buffers or addition of surfactants to form micelles for the analysis where size and charge differences... 

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... 

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