Chiral selectors concentration

The concentration of the chiral selector, for instance, has considerable influence on the mobility and separation of the enantiomers. Optical resolution varies with the chiral selector concentration and reaches a maximum value at a given optimum concentration. Wren and Rowe proposed a model that describes the influence of the selector concentration on selectivity, and which was extended by Vigh s group ° by including the pH as a separation parameter for weak acidic enantiomers. The latter model shows that the chiral selectivity is determined by the complex s relative mobility, the CD concentration, the degree of dissociation... 

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 organic solvent concentration, buffer or electrolyte concentration, - - buffer or electrolyte pH,7 7 - T83,85,86 voltage,chiral selector concentration " and supplier, active pharmaceutical ingredient (API) concentration, internal standard concentration, injection time - and pressure, ... 

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... 

Capillary electrophoresis has been applied for the enantioselective determination of the binding constants of chiral drugs with cyclodextrins for basically the following two reasons (1) optimization of chiral selector concentration and (2) understanding the fine mechanisms of enantioseparations in CE. The first group of studies have been published mainly on the early stage of chiral CE development, whereas the second goal is followed in the most recent studies, mainly by Rizzi and Kremser (10,13) and Scriba et al. 

The discussion clearly shows the limitations of Eq. (18) for calculating the optimal chiral selector concentration. As already mentioned, Eq. (18) was derived based on the assumption of equal mobility of both diastereomeric complexes, which indeed is not always the case. This means that Eq. 

Two critical points should be mentioned when applying the aforementioned model for optimization purposes in chiral CE (1) the maximum mobility difference between the enantiomers does not a priori mean the maximum resolution and (2) the model does not cover several important parameters affecting chiral CE separations. However, this model without any doubt markedly contributed to the development of chiral CE and good correlations between the values of the optimal chiral selector concentrations calculated based on this model and observed experimentally have been reported [182-185]. 

In this experiment the enantiomers of cyclobarbital and thiopental, and phenobarbital are separated using MEKC with cyclodextran as a chiral selector. By adjusting the pH of the buffer solution and the concentration and type of cyclodextran, students are able to find conditions in which the enantiomers of cyclobarbital and thiopental are resolved. 

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. 

Another possibility of constructing a chiral membrane system is to prepare a solution of the chiral selector which is retained between two porous membranes, acting as an enantioselective liquid carrier for the transport of one of the enantiomers from the feed solution of the racemate to the receiving side (Fig. 1-5). This system is often referred to as membrane-assisted separation. The selector should not be soluble in the solvent used for the elution of the enantiomers, whose transport is driven by a gradient in concentration or pH between the feed and receiving phases. As a drawback common to all these systems, it should be mentioned that the transport of one enantiomer usually decreases when the enantiomer ratio in the permeate diminishes. Nevertheless, this can be overcome by designing a system where two opposite selectors are used to transport the two enantiomers of a racemic solution simultaneously, as it was already applied in W-tube experiments [171]. 

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. 

Only few studies addressed the effect of the silica surface concentration of glycopeptides antibiotics on the chiral performances. The first was performed by Armstrong on a TE CSP toward five racemic test solutes [30]. The study evidenced a modest increase of a and Rs values with increasing the initial selector concentration in the grafting reaction mixture, but no data were reported regarding the effective... 

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. 

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