Chiral selectors mobile phase additives

Q Sun, S V Olesik. Chiral separation by simultaneous use of vancomycin as stationary phase chiral selector mobile phase additive. J Chromatogr B 745 159-166, 2000. 

An alternative model has been proposed in which the chiral mobile-phase additive is thought to modify the conventional, achiral stationary phase in situ thus, dynamically generating a chiral stationary phase. In this case, the enantioseparation is governed by the differences in the association between the enantiomers and the chiral selector in the stationary phase. 

A82846B [24] and LY307599 [25] were developed as chiral selectors for CE LY333328 [26] and A35512B [27] were applied as chiral mobile phase additives in narrow-bore HPLC. 

Sharp, V.S. and Risley, D.S., Evaluation of the macrocyclic antibiotic LY333328 as a chiral selector when used as a mobile phase additive in narrow bore HPLC, Chirality, 11,75, 1999. 

CSPs and chiral mobile phase additives have also been used in the separation of amino acid enantiomers. Another technique that should be mentioned is an analysis system employing column-switching. D-and L- amino acids are first isolated as the racemic mixture by reverse-phase HPLC. The isolated fractions are introduced to a second column (a CSP or a mobile phase containing a chiral selector) for separation of enantiomers. Long et al. (2001) applied this technique to the determination of D- and L-Asp in cell culture medium, within cells and in rat blood. 

In recent years, for analytical purposes the direct approach has become the most popular. Therefore, only this approach will be discussed in the next sections. With the direct approach, the enantiomers are placed in a chiral environment, since only chiral molecules can distinguish between enantiomers. The separation of the enantiomers is based on the complex formation of labile diastereoisomers between the enantiomers and a chiral auxiliary, the so-called chiral selector. The separation can only be accomplished if the complexes possess different stability constants. The chiral selectors can be either chiral molecules that are bound to the chromatographic sorbent and thus form a CSP, or chiral molecules that are added to the mobile phase, called chiral mobile phase additives (CMPA). The combination of several chiral selectors in the mobile phase, and of chiral mobile and stationary phases is also feasible. 

The development of a plethora of HPLC CSPs in the 1980s and 1990s has, to a large extent, made the use of chiral mobile-phase additives (CMPAs) redundant in most modem pharmaceutical analytical laboratories [23]. Before this period, chiral selectors were used routinely as additives in HPLC, but are now only used for a small number of specific applications [23]. CMPAs are used to form... 

For obvious reasons CDs (and other dextrins) are potentially good chiral selectors for chromatography on the one hand they can be used as mobile phase additives (CMPA) in TLC45, HPLC46 and CE47 49 and on the other they can be covalently bonded onto solid supports50,51 and silica gei 52- 54 xhis approach can be extended to the preparative resolution of enantiomers41,55,56. 

In contrast to the various CSPs mentioned so far, but still based on covalently or at least very strongly adsorbed chiral selectors (from macromolecules to small molecules) to, usually, a silica surface, the principle of dynamically coating an achiral premodified silica to CSPs via chiral mobile phase additives (CMPA) has successfully been adapted for enantioseparation. The so-called reverse phase LC systems have predominantly been used, however, ion-pairing methods using nonaqueous mobile phases are also possible. 

On the other hand, the direct chromatographic approach involves the use of the chiral selector either in the mobile phase, a so-called chiral mobile phase additive (CMPA), or in the stationary phase [i.e., the chiral stationary phase (CSP)]. In the latter case, the chiral selector is chemically bonded or coated or allowed to absorb onto a suitable solid support. Of course chiral selectors still can be used as CMPAs, but the approach is a very expensive one owing to the high amount of chiral selector required for the preparation of the mobile phase, and the large amount of costly chiral selector that is wasted (since there is very little chance of recovering this compound). Moreover, this approach is not successftd in the preparative separation of the enantiomers. 

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... 

In CE a successful method for enantiomer separation is the addition of a chiral selector to the mobile phase. This practice can be transferred to p-CEC by using a packing bed which consists of bare silica or ODS (octadecylsilica) and a mobile phase containing a chiral additive. At the first time, Lelievre et al. added hydroxypropyl-[5-cyclodextrin to the mobile phase using an ODS packed capillary. The enantiomer separation of chlorthalidone with a resolution Rs of 1.4 was feasible [41]. Deng et al. [59] used an ODS-packed column and (3-cyclodextrin as a mobile phase additive. A theoretical model for the enantiomer separation of salsolinol was developed and compared with the experimental data. For pressure supported CEC, very high pressure (about 100 bar) was applied to the inlet vial so that the mobile phase was mainly driven by the applied pressure. 

The numerator in equation (22-26) represents the processes occurring in the mobile phase, while the denominator represents the processes occurring in the stationary phase. Such a situation can be realized by combining a chiral stationary phase in a push-pull mode with a chiral mobile phase of opposite con-hguration, where two enantiomers of the chiral selector are involved, one for the chiral stationary phase and the other for the chiral mobile phase. The most selective chiral chromatographic system should be encountered when one enantiomer binds to the immobilized chiral selector in the stationary phase, whereas the other enantiomer predominantly associates with the chiral mobile-phase additive [158]. The above treatment is applicable to all applications regarding the use of chiral mobile phases. 

For a number of reasons it is usually preferable to use a chiral stationary phase (CSP) rather than a chiral derivatising agent. A CSP will normally be preferred to a chiral mobile phase additive, partly because the use of a chiral selector as a mobile phase additive will lead to much... 

It is well known that a chiral environment is essential for the enantiomeric resolution of racemates. In CE, this situation is provided by the chiral compounds used in the BGE and is known as the chiral selector or chiral BGE additive. Basically, the chiral recognition mechanisms in CE are similar to those in chromatography using a chiral mobile-phase additive mode, except that the resolution occurred through different migration velocities of the diastereoisomeric complexes in CE. The chiral resolution occurred through diastereomeric complex formation between the enantiomers of the pollutants and the chiral selector. The formation of diastereomeric complexes depends on the type and nature of the chiral selectors used and the nature of the pollutants. 

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