Capillary electrophoresis chiral additives

Capillary Electrophoresis. Capillary electrophoresis (ce) or capillary 2one electrophoresis (c2e), a relatively recent addition to the arsenal of analytical techniques (20,21), has also been demonstrated as a powerful chiral separation method. Its high resolution capabiUty and lower sample loading relative to hplc makes it ideal for the separation of minute amounts of components in complex biological mixtures (22,23). 

Catechin and epicatechin are two flavanols of the catechin family. They are enantiomers. The capillary zone electrophoresis (CE) methods with UV-detection were developed for quantitative determination of this flavanols in green tea extracts. For this purpose following conditions were varied mnning buffers, pH and concentration of chiral additive (P-cyclodextrin was chosen as a chiral selector). Borate buffers improve selectivity of separation because borate can make complexes with ortho-dihydroxy groups on the flavanoid nucleus. 

Library of Cyclic Oligopeptides as Additives to Background Electrolyte for Chiral Capillary Electrophoresis... 

Enantioresolution in capillary electrophoresis (CE) is typically achieved with the help of chiral additives dissolved in the background electrolyte. A number of low as well as high molecular weight compounds such as proteins, antibiotics, crown ethers, and cyclodextrins have already been tested and optimized. Since the mechanism of retention and resolution remains ambiguous, the selection of an additive best suited for the specific separation relies on the one-at-a-time testing of each individual compound, a tedious process at best. Obviously, the use of a mixed library of chiral additives combined with an efficient deconvolution strategy has the potential to accelerate this selection. 

Chiral additives have been shown to be very effective for chiral separations by capillary electrophoresis (CE) [4, 5]. Indeed, it may be argued that there has been considerably more research activity in chiral separations by CE than by EC methods since the introduction of the former technique. Chiral additives in CE have several advantages, some of which are highlighted in Table 11-2. 

Kang, J. and Ou, Q., Chiral separation of racemic mexilitine hydrochloride using cyclodextrins as chiral additive by capillary electrophoresis,. Chromatogr. A, 795, 394, 1998. 

Lin et al. [95] used capillary electrophoresis with dual cyclodextrin systems for the enantiomer separation of miconazole. A cyclodextrin-modified micellar capillary electrophoretic method was developed using mixture of /i-cyclodextrins and mono-3-0-phenylcarbamoyl-/j-cyclodextrin as chiral additives for the chiral separation of miconazole with the dual cyclodextrins systems. The enantiomers were resolved using a running buffer of 50 mmol/L borate pH 9.5 containing 15 mmol/L jS-cyclodextrin and 15 mmol/L mono-3-<9-phcnylcarbamoyl-/j-cyclodextrin containing 50 mmol/L sodium dodecyl sulfate and 1 mol/L urea. A study of the respective influence of the /i-cyclodcxtrin and the mono-3-(9-phenylcarbamoyl-/i-cyclodextrin concentration was performed to determine the optical conditions with respect to the resolution. Good repeatability of the method was obtained. 

Phinney et al. [Ill] investigated the application of citrus pectins, as chiral selectors, to enantiomer separations in capillary electrophoresis. Successful enantioreso-lution of primaquine and other antimalarials, was achieved by utilizing potassium polypectate as the chiral selector. Changes in pH, chiral additive concentration, and capillary type were studied in relation to chiral resolution. The effect of degree of esterification of pectin materials on chiral recognition was evaluated. 

Capillary electrochromatography-mass spectrometry (CE-MS), 4 641 Capillary electrodes, 14 27 Capillary electrophoresis (CE), 4 602-603, 631-633 6 385 9 751-752 antibody based columns with, 6 402 chiral additives, 6 77-79 applications, 4 641 basic principles, 4 606-609 detectors, 4 634-635 for DNA analysis, 4 636-637 flow profiles generated, 4 608 instrumentation, 4 633 as microfluidic assay technique,... 

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. 

With capillary electrophoresis (CE), another modern primarily analytically oriented separation methodology has recently found its way into routine and research laboratories of the pharmaceutical industries. As the most beneficial characteristics over HPLC separations the extremely high efficiency leading to enhanced peak capacities and often better detectability of minor impurities, complementary selectivity profiles to HPLC due to a different separation mechanism as well as the capability to perform separations faster than by HPLC are frequently encountered as the most prominent advantages. On the negative side, there have to be mentioned detection sensitivity limitations due to the short path length of on-capillary UV detection, less robust methods, and occasionally problems with run-to-run repeatability. Nevertheless, CE assays have now been adopted by industrial labs as well and this holds in particular for enantiomer separations of chiral pharmaceuticals. While native cyclodextrins and their derivatives, respectively, are commonly employed as chiral additives to the BGEs to create mobility differences for the distinct enantiomers in the electric field, it could be demonstrated that cinchona alkaloids [128-130] and in particular their derivatives are applicable selectors for CE enantiomer separation of chiral acids [19,66,119,131-136]. 

Fillet, M., Hubert, P., Crommen, J. Method development strategies for the enantiosep-aration of drugs by capillary electrophoresis using cyclodextrins as chiral additives. Electrophoresis 1998, 19, 2834—2840. 

Riekkola, M. L., and Wiedmer, S. K. (1997). Potential of capillary electrophoresis with micelles or chiral additives as a purity control method in pharmaceutical industry. Process Control Qual. 10, 169-180. 

S Cladrowa-Runge, A Rizzi. Enantioseparations of 6-aminoquinolyl-lV-hydrox-ysuccinimidyl carbamate derivatized amino acids by capillary zone electrophoresis using native and substituted /3-cyclodextnns as chiral additives II. Evaluation of complexation constants. J Chromatogr A 759 167-175, 1997. 

P Sun, N Wu, GE Barker, RA Hartwick. Chiral separations using dextran and bovine serum albumin as run buffer additives in affinity capillary electrophoresis. J Chromatogr 648 475-480, 1993. 

J Yang, DS Hage. Chiral separations in capillary electrophoresis using human serum albumin as a buffer additive. Anal Chem 66 2719-2725, 1994. 

M Hedeland, R Isaksson, C Pettersson. Cellobiohydrolase I as a chiral additive in capillary electrophoresis and liquid chromatography. J Chromatogr A 807 297-305, 1998. 

Beads = pure polymeric particles with similar chiral information to the corresponding sorbent (CSp) coated on silica gel CE = capillary electrophoresis CSP = chiral stationary phase CMPA — chiral mobile phase additive MEKC = micellar electrokinetie capillary chromatography. 

Chiral crown ethers can be generally utilized as chiral selectors. They have been used as additives to mobile phases or running buffer in MECK and capillary electrophoresis (CE) systems124,125 (see Section 3.1.6.4.). 

Another method for creating a chiral environment is lo add an optically pure chiral selector to a bulk liquid phase. Chiral additives have several advanlages over chiral stationary phases and continue lo be the predominant mode for chiral separations by tic and capillary electrophoresis (cc). First of all, the chiral selector added to a bulk liquid phase can be readily changed. The use of chiral additives allows chiral separations lo be done using less expensive, conventional stationary phases. A wider variety of chiral selectors are available [ be used as chiral additives than are available as chiral stationary phases, thus, providing the analyst with considerable flexibility. Finally, the use of chiral additives may provide valuable insight into (he chromatographic conditions and/or likelihood ol success with a potential chiral stationary-phase chiral selector. This is particularly important for the development of new chiral stationary phases because of the difficulty and cosl involved. 

Alternatively, chiral separation can be performed by capillary electrophoresis (CE) with, for example, quaternary ammonium-/ -cyclodextrin as chiral buffer additive [184, 185]. 

Contrary to conventional HPLC, almost 98% of chiral resolution in CE is carried out using the chiral selector as a mobile phase additive. Again all the common chiral selectors used in NLC can also be used in NCE. But, unfortunately, few chiral molecules have been tested in NCE for enantiomeric resolution of some racemates. To the best of our knowledge only cyclodextrins and protein-based chiral mobile phase additives have been used for this purpose. Manz and coworkers discussed chiral separations by NCE in their reviews in 2004 [21] and 2006 [22], Later on, Pumera [16] reviewed the use of microfluidic devices for enantiomeric resolutions in capillary electrophoresis. Not much work has been carried out on chiral resolution in NCE but the papers that are available are discussed here. 

The chiral recognition mechanisms in NLC and NCE devices are similar to conventional liquid chromatography and capillary electrophoresis with chiral mobile phase additives. It is important to note here that, to date, no chiral stationary phase has been developed in microfluidic devices. As discussed above polysaccharides, cyclodextrins, macrocyclic glycopeptide antibiotics, proteins, crown ethers, ligand exchangers, and Pirkle s type molecules are the most commonly used chiral selectors. These compounds... 

Capillary electrophoresis (CE) provides a valid alternative to HPLC methods for chiral separations. The direct resolution of racemates requires only an enantiomerically pure additive (chiral selector) to be dissolved in the running buffer. The experimental conditions affecting the separations and an overview of practical applications have been compiled <1999ELP2605>. 

Fran9ois, Y. et al. Evaluation of chiral ionic liquids as additives to cyclodextrins for enantiomeric separations by capillary electrophoresis. J. Chromatogr. A. 2007, 1155, 134-141. 

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