Chirality/Chiral coated phases

In chiral chromatography, the two diastereomeric adducts ArEr and ArEr are formed during elution, rather than synthetically, prior to chromatography. The adducts differ in their stability using chiral stationary phases (CSP) or chiral coated phases (CCP) and/or in their interphase distribution ratio adding a chiral selec-... 

Chiral Stationary Phases and Chiral Coated Phases... 

Chiral stationary phases in tic have been primarily limited to phases based on normal or microcrystalline cellulose (44,45), triacetylceUulose sorbents or siHca-based sorbents that have been chemically modified (46) or physically coated to incorporate chiral selectors such as amino acids (47,48) or macrocyclic antibiotics (49) into the stationary phase. 

Diamide Chiral Separations. The first chiral stationary phase for gas chromatography was reported by GH-Av and co-workers in 1966 (113) and was based on A/-trifluoroacetyl (A/-TFA) L-isoleucine lauryl ester coated on an inert packing material. It was used to resolve the tritiuoroacetylated derivatives of amino acids. Related chiral selectors used by other workers included -dodecanoyl-L-valine-/-butylamide and... 

Gyclodextrins. As indicated previously, the native cyclodextrins, which are thermally stable, have been used extensively in Hquid chromatographic chiral separations, but their utihty in gc appHcations was hampered because their highly crystallinity and insolubiUty in most organic solvents made them difficult to formulate into a gc stationary phase. However, some functionali2ed cyclodextrins form viscous oils suitable for gc stationary-phase coatings and have been used either neat or diluted in a polysiloxane polymer as chiral stationary phases for gc (119). Some of the derivati2ed cyclodextrins which have been adapted to gc phases are 3-0-acetyl-2,6-di-0-pentyl, 3-0-butyryl-2,6-di-0-pentyl,... 

The gas chromatographic separation of some sulphoxide enantiomers was observed on quartz fused silica capillaries coated with the chiral silicon phase chirasil-val280. 

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. 

Figure 11. Reaction scheme for synthesis of intrinsically chiral polymer phase (CSP) and coated onto silica.

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. 

Case II dynamically coated CSP via a chiral mobile phase additive = (S)-CMPA ... 

The ability to design chiral ILs in which the cation and anion is of fixed chirality represents additional tuning features of ILs. Two approaches have incorporated ILs as new stationary phases for chiral GC. One method involves the use of chiral ILs as stationary phases in WCOT GC [37]. In the second approach, chiral selectors (e.g., cyclodextrins) were dissolved in an achiral IL and the mixture coated onto the wall of the capillary colunm [38]. Both approaches can separate a variety of different analytes, but the observed enantioselectivities and efficiencies do not rival those observed with commercially available chiral stationary phases (CSPs). 

Capillary gas chromatography (GC) using modified cyclodextrins as chiral stationary phases is the preferred method for the separation of volatile enantiomers. Fused-silica capillary columns coated with several alkyl or aryl a-cyclo-dextrin, -cyclodextrin and y-cyclodextrin derivatives are suitable to separate most of the volatile chiral compounds. Multidimensional GC (MDGC)-mass spectrometry (MS) allows the separation of essential oil components on an achiral normal phase column and through heart-cutting techniques, the separated components are led to a chiral column for enantiomeric separation. The mass detector ensures the correct identification of the separated components [73]. Preparative chiral GC is suitable for the isolation of enantiomers [5, 73]. 

Using a chiral column, coated with a definite modified cyclodextrin as the chiral stationary phase, the elution orders of furanoid and pyranoid linalool oxides are not comparable [11, 12]. Consistently, the chromatographic behaviour of diastereomers and/or enantiomers on modified cyclodextrins is not predictable (Fig. 17.1, Table 17.1). Even by changing the non-chiral polysiloxane part of the chiral stationary phase used, the order of elution may significantly be changed [13]. The reliable assignment of the elution order in enantio-cGC implies the coinjection of structurally well defined references [11-13]. 

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. 

Cass et al. [66] used a polysaccharide-based column on multimodal elution for the separation of the enantiomers of omeprazole in human plasma. Amylose tris (3,5-dimethylphenylcarbamate) coated onto APS-Hypersil (5 /im particle size and 120 A pore size) was used under normal, reversed-phase, and polar-organic conditions for the enantioseparation of six racemates of different classes. The chiral stationary phase was not altered when going from one mobile phase to another. All compounds were enantioresolved within the elution modes with excellent selectivity factor. The separation of the enantiomers of omeprazole in human plasma in the polar-organic mode of elution is described. 

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