Charged chiral selector

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. 

The use of a charged chiral selector is probably the best solution to improve the classical PET when CE is hyphenated with MS. Better solubility, additional electrostatic interactions, and improvement of the stereoselective separation power afforded by the self-mobility of the chiral additives into the BGE are among the numerous advantages of these charged selectors. When electromigration of the chiral species and the analytes are opposite (PFT-countercurrent approach), the mobility difference between free and complexed analytes is increased, leading to a higher resolution than with a neutral chiral selector. In optimized countercurrent... 

Lammerhofer and Lindner [90] explained the chiral resolution of N-derivatized amino acids by CEC. The authors explained the formation of the transient diastereomeric ion-pairs between negatively charged analyte enantiomers and a positively charged chiral selector by multiple intermolecular interactions which might be differentially adsorbed to the ODS stationary phase. Furthermore, they claimed that the enantioseparation was achieved because of different observed mobilities of the analyte enantiomers originating from different ion-pair formation rates of the enantiomers and/or differential adsorption of the diastereoisomeric ion-pairs to the ODS stationary phase [90]. 

Cationic Analytes - Negatively Charged Chiral Selectors... 

Fig. 8.3 CE-ESI-MS enantioseparation of tramadol and its phase I metabolites (M1-M5) in the presence of a negatively charged chiral selector. Experimental conditions CE conditions running buffer, 20 mM ammonium formate at pH 2.5 in the presence of HS-y-CD (0.2%) fused silica capillary 50 cm 50 pm I.D. partial filling of the capil lary (52%) sample concentration, 1 pg mL pressure...

CD). The partial-filling technique (PFT) proved to be a suitable and efficient approach to avoid MS source contamination, as well as signal suppression due to nonvolatile additives. Therefore, fhe PFT technique is particularly adapted with chiral selectors added into fhe electrolyte solution. Because of fhe counter-current contribution, charged chiral selectors were found to be more suitable for the online MS detection of separated enantiomers. Capillary electrochromatography with chiral stationary phases has also been developed, but to a lesser extent. 

Separation of enantiomers in CE significantly differs from true electrophoretic separations that are based on the difference in charge-to-mass (size) ratio between the analyte molecules. These peculiarities need to be considered when looking at the differences between enantioseparations with uncharged and charged chiral selectors... 

In earlier papers, enantioseparations of charged analytes with neutral chiral selectors were attributed to CZE and the enantioseparations of neutral analytes with charged chiral selectors to EKC. However, from the mechanistic point of view, there is no principal difference whether an analyte or a chiral selector is charged. Actually, it is the subject of convention which counterpart of chiral recognition process will be named selectand and which one chiral selector. The reciprocal chiral recognition strategy for a design of effective chiral selectors proposed by Pirkle and co-workers in HPLC is based on this philosophy [9] and that principle certainly applies for enantioseparations in EKC also [10]. 

The enantioseparation of neutral chiral analytes with charged chiral selectors, which was considered impossible in the earlier studies on chiral CE, was suc-cessfiilly performed based on the understanding of the aforementioned concept [11,12],... 

Chankvetadze B (2009) Separation of enantiomers with charged chiral selectors in CE. 

The separation mechanism is based on stereoselective ion-pair formation of oppositely charged cationic selector and anionic solutes, which leads to a difference of net migration velocities of the both enantiomers in the electric field. Thus, the basic cinchona alkaloid derivative is added as chiral counterion to the BGE. Under the chosen acidic conditions of the BGE, the positively charged counterion associates with the acidic chiral analytes usually with 1 1 stoichiometry to form electrically neutral ion-pairs, which do not show self-electrophoretic mobility but... 

In general, charged CDs have shown superior discrimination abilities, especially the highly-sulfated (HS-CDs) ones. Furthermore, the separation mechanism is altered by the introduction of electrostatic interactions. Finally, the use of chiral selectors carrying a charge opposite to that of the analytes can greatly improve the mobility difference between the two enantiomers. The use of mixtures of CDs in chiral separation is also possible. ... 

The chiral recognition of enantiomers can be of three types (i) desionoselective, (ii) ionoselective, or (iii) duoselective, in which only the non-dissociated, the dissociated or both forms (charged and uncharged), respectively, of the enantiomers selectively interact with the chiral selector. In the case of ionoselective and duoselective interactions, a reversal of the migration order of the enantiomers is theoretically possible by the appropriate selection of CD concentration and the pH of the BGE. The addition of organic modifier to the BGE can also change selectivity by modifying the solubility of the chiral selector and/or of the solute, the complex equilibrium, the conductivity of the BGE and the electroendos-motic flow (EOE) level. Several other factors, such as the temperature, the type and the concentration of the BGE, and the level of the EOE can influence the separation. 

Schulte, G., Heitmeier, S., Ghankvetadze, B., and Blaschke, G. (1998). Ghiral capillary electrophoresis-electrospray mass spectrometry coupling with charged cyclodextrin derivatives as chiral selectors. /. Chromatogr. A 800, 77—82. 

B Chankvetadze, G Endresz, G Blaschke. Charged cyclodextrin derivatives as chiral selectors in capillary electrophoresis. Chem Soc Rev 25 141-146,... 

T Schmitt, H Engelhardt. Charged and uncharged cyclodextrins as chiral selectors in capillary electrophoresis. Chromatographia 37 475-481, 1993. 

CZE is the most widely used mode due to its simplicity of operation and its versatility. Selectivity can be most readily altered through changes in running buffer pH or by use of buffer additives such as surfactants or chiral selectors. The major drawback with CZE is that it deals with aqueous electrolytic systems, whereas components can only be separated if they are charged and soluble in water. CZE separation of various antibacterials including penicillins, tetracyclines, and macrolides has been reported (86). Determination of cefixime, an oral cephalosporin antibiotic, and its metabolites in human urine has been also successfully carried out with CZE (87). 

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