Capillary chiral selectors

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. 

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... 

Nishi et al. [110] used dextran and dextrin as chiral selectors in capillary-zone electrophoresis. Polysaccharides such as dextrins, which are mixtures of linear a-(l,4)-linked D-glucose polymers, and dextrans, which are polymers of D-glucose units linked predominantly by a-(l,6) bonds, have been employed as chiral selectors in the capillary electrophoretic separation of enantiomers. Because these polymers are electrically neutral, the method is applicable to ionic compounds. The enantiomers of basic or cationic drugs such as primaquine were successfully separated under acidic conditions. The effects of molecular mass and polysaccharide concentration on enantioselectivity were investigated. 

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. 

Main biodegradation products of LAS, sulfophenyl carboxylates (SPCs), were separated by CE using a-cyclodextrin as the chiral selector [6]. The best separation of enantiomers was achieved with 60 mM a-cyclodextrin in a 20 mM citrate buffer at pH 4.0 with an uncoated fused-silica capillary. The method was applied for the qualitative and quantitative analysis of SPC in primary sewage effluents with a detection limit of 1 p,g L-1. 

The postmodification strategy, in which apoly(glycydyl methacrylate-co-ethylene dimethacrylate) monolith was activated with hydrogen sulfide to a thiol-modified monolith and subsequently derivatized with an 0-9-(tert-butylcarbamoyl)quinine selector by radical addition reaction, yielded slightly less efficient capillary columns. However, this procedure has the advantage that only minute amount of chiral selector are needed to end-up with a useful enantioselective capillary column [84]. 

Aboul-Enein, H.Y. and Ali, I., Macrocyclic antibiotics as effective chiral selectors for enantiomeric resolntion by liquid chromatography and capillary electrophoresis, Chromatographia, 52, 679, 2000. 

Ward, T.J., Dann III, C., and Blaylock, A., Enantiomeric resolution using the macrocyclic antibiotics rifamycin B and rifamycin SV as chiral selectors for capillary electrophoresis, J. Chromatogr. A, 715, 337, 1995. 

Strege, M.A., Huff, B.E., and Risley, D.S., Evaluation of macrocyclic antibiotic A82846B as a chiral selector for capillary electrophoresis separations, LC-GC, 14, 144, 1996. 

Tesafova, E., Bosdkovd, Z., and Zuskova, L, Enantioseparation of selected iV-tert-butyloxycarbonyl amino acids in high-performance liquid chromatography and capillary electrophoresis with a teicoplanin chiral selector. J. Chromatogr. A, 879, 147, 2000. 

Perrin, C., Vander Heyden, Y, Maftouh, M., Massart, D.L. Rapid screening for chiral separations by short-end injection capillary electrophoresis nsing highly sulfated cyclodextrins as chiral selectors. Electrophoresis 2001, 22, 3203-3215. 

Chiral separation of drng molecules and of their precursors, in the case of synthesis of enantiomerically pure drugs, is one of the important application areas of HPLC in pharmaceutical analysis. Besides HPLC, capillary electrophoresis (CE) is another technique of choice for chiral separations. Chapter 18 provides an overview of the different modes (e.g., direct and indirect ones) of obtaining a chiral separation in HPLC and CE. The direct approaches, i.e., those where the compound of interest is not derivatized prior to separation, are discussed in more detail since they are cnrrently the most frequently used techniques. These approaches require the use of the so-called chiral selectors to enable enantioselective recognition and enantiomeric separation. Many different molecnles have been nsed as chiral selectors, both in HPLC and CE. They can be classified into three different groups, based on their... 

The aim of this chapter is to give an overview of chiral separations of pharmaceutical compounds by means of HPLC. Capillary electrophoresis, which is the most popular technique besides HPLC for performing chiral separations at the analytical level, will also be briefly discussed. A second reason to discuss chiral separation in CE in short is the large overlap in the chiral selectors applied in both techniques. 

Examined factors were the chiral selector concentration, the buffer or electrolyte concentration, the buffer or electrolyte pH, the capillary temperature, and the... 

Examined factors were voltage, buffer or electrolyte concentration, " buffer or electrolyte pH, chiral selector concentration, capillary temperature, detection wavelength and its bandwidth, reference wavelength and its bandwidth, peak width, threshold, data acquisition rate, filter and its peak width, and surfactant... 

For validation the following robustness factors should be considered different lots of the capillary, temperature ( 2°C), applied voltage/current ( 2% relative), buffer electrolyte concentration ( + 10% relative), pH ( 0.1), concentration of additives, e.g., organic modifiers or chiral additives ( 10% relative), injection time ( 0.5s), detection wavelength ( + 2nm), batch-to-batch variation of chiral selectors ( 2—3 different lots), CE instruments (two instruments of two different manufacturers preferentially). 

Because interfacing chiral CE with ESI/MS was severely troubled by the presence of CD, the PFT was investigated. The PFT, first introduced by Valtcheva et al. and further modified by Tanaka et al., involves the filling of a discrete portion of the CE capillary with BGE containing a chiral selector (i.e., partial filling). This method is particularly adapted in chiral CE—MS and presents major advantages over the traditional approach. Neutral CDs were initially used, but charged chiral selectors are currently preferentially employed. 

When using PFT with a neutral selector, it is quite difficult to avoid any entrance of the chiral selector into the ionization source, particularly at a high pH, where EOF is important. The use of BGE at low pH and/or coated capillary to minimize EOF is therefore mandatory. However, the coaxial sheath gas, which generally assists the ionization process, leads to an aspirating phenomenon of the chiral selector in the MS direction. Javerfalk et al. were the first to apply PFT with a neutral methyl-/i-CD for the separation of racemic bupivacaine and ropivacaine with a polyacrylamide-coated capillary and an acidic pH buffer (pH 3). Cherkaoui et al. employed another neutral CD (HP-/1-CD) with a PVA-coated capillary for the analysis of amphetamines and their derivatives. To prevent a detrimental aspiration effect, analyses were carried out without nebulization pressure. Numerous other studies presented excellent results such as the enantioselective separation of adrenoreceptor antagonist drugs using tandem mass spectrometry (MS/MS) the separation of clenbuterol enantiomers after solid-phase extraction (SPE) of plasma samples or the use of CD dual system for the simultaneous chiral determination of amphetamine, methamphetamine, dimethamphetamine, and p-hydroxymethamphetamine in urine. 

Brush-type, proteins, CDs, natural molecular imprint-based polymers (MIP), and macrocyclic antibiotics have been immobilized as chiral selectors on packed-CEC columns. Zheng and Shamsi demonstrated the possibility of using chiral CEC—ESI/MS with a commercially packed column for the determination of warfarin enantiomers in human plasma using coumachlor as an internal standard (IS). Robustness of this chiral CEC capillary was recently improved by a novel procedure and applied for the simultaneous enantiosepara-tion of height /1-blockers with multimodal CSP using different combinations of vancomycin and teicoplanin, as presented in Figure 5. ... 

Tanaka, Y., and Terabe, S. (1997). Separation of the enantiomers of basic drugs by affinity capillary electrophoresis using a partial filling technique and ai-acid glycoprotein as chiral selector. 

Amini, A., Pettersson, C., and Westerlund, D. (1997). Enantioresolution of disopyramide by capillary affinity electrokinetic chromatography with human alphal-acid glycoprotein (AGP) as chiral selector applying a partial filling technique. Electrophoresis 18, 950—957. 

Armstrong, D. W., and Nair, U. B. (1997). Capillary electrophoretic enantioseparations using macrocyclic antibiotics as chiral selectors. Electrophoresis 18, 2331—2342. 

Eanali, S., Desiderio, C., Schulte, G., Heitmeier, S., Strickmann, D., Chankvetadze, B., and Blaschke, G. (1998). Chiral capillary electrophoresis-electrospray mass spectrometry coupling using vancomycin as chiral selector. /. Chromatogr. A 800, 69—76. 

---