Silica-based CSP

A CSP based on the adsorption of a chiral anthrylamine on porous graphitic carbon successfully resolved the enantiomers of tropic acid derivatives and anti-inflammatory agents in SFC [65]. The carbon-based CSP produced superior results when compared to an analogous silica-based CSP. Occasional washing of the column was necessary to remove highly retained substances. 

It is also worthwhile to outline at this place the immobilization procedure that was used for the preparation of type I CSPs A bifunctional linker with a terminal isocyanate on one side and a triethoxysilyl group on the other end (3-isocyanatopropyl triethoxysilane) was reacted with the native cinchona alkaloids quinine and quinidine and subsequently the resultant carbamate derivative in a second step with silica [30], Remaining silanols have been capped with silane reagents, yet, are less detrimental for acidic solutes because of the repulsive nature of such electrostatic interactions. CSPs prepared in such a way lack the hydrophobic basic layer of the thiol-silica-based CSPs mentioned earlier, which may be advantageous for the separation of certain analytes. 

Polysaccharide-based CSPs incorporate derivatives of cellulose and amylose adsorbed on silica gel. The selectivity of these CSPs depends upon the nature of the substituents introduced during the derivatization process. The secondary structure of the modified polysaccharide is believed to play a role in selectivity, but the chiral recognition mechanisms have not been fully elucidated [55]. 

The chiral recognition ability of the insoluble (+)-l was estimated by HPLC using a column packed with small particles of l.25 However, this column showed a poor efficiency because of a low theoretical plate number. This defect was overcome by coating soluble poly(TrMA) with a DP of 50 on macroporous silica gel.26 The 1-coated silica gel had higher resistance against compression and longer lifetime than the CSP of insoluble 1. Moreover, the two 1-based CSPs show quite different chiral recognition for several race-mates, which may be attributed to the different orientation of 1 in bulk and on the surface of the silica gel.27... 

The first silica-supported CSP with a cinchona alkaloid-derived chromatographic ligand was described by Rosini et al. [20]. The native cinchona alkaloids quinine and quinidine were immobilized via a spacer at the vinyl group of the quinuclidine ring. A number of distinct cinchona alkaloid-based CSPs were subsequently developed by various groups, including derivatives with free C9-hydroxyl group [17,21-27] or esterified C9-hydroxyl [28,29]. All of these CSPs suffered from low enantiose-lectivities, narrow application spectra, and partly limited stability (e.g., acetylated phases). 

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. 

Norbomene was functionalized with cyclodextrins and surface grafted onto silica-based supports using ROMP (44). The CSP are suitable for the enantioselective separation various amino acids, including jS-blockers and other compounds, such as chiral ferrocene derivates. 

In addition to the common use of vancomycin and teicoplanin, the use of other antibiotics as chiral selectors in HPLC were limited. Only a few reports are available in the literature dealing with these antibiotics as HPLC CSPs. Armstrong et al. [42] resolved about 230 racemates on the ristocetin A antibiotic covalently bonded to silica gel. The resolution was carried out using the three modes of mobile phases. The results were complimentary to those obtained on vancomycin and teicoplanin CSPs. In another study, the effect of selector coverage and mobile phase composition on enantiomeric resolution with ristocetin A CSP was carried out by Armstrong et al. [43]. Thiostrepton-based CSP was also used to resolve the enantiomers of thioridazine, 2,2,2-trifluoro-l-... 

The use of antibiotic-based CSPs has been reported in capillary electrochromatography (CEC) for chiral resolution [60]. Teicoplanin CSP covalently bonded to silica gel was used to resolve the enantiomers of tryptophan and dinitrobenzoyl leucine by CEC [61]. Good levels of enantioselectivity were obtained with optimized separations. Vancomycin covalently bonded to silica gel was also evaluated in CEC for the chiral resolution of thalidomide and jS-adrenergic blocking agents under all the three mobile phase modes. The... 

The obtained product was allowed to react with silica gel. The tertiary butyl protecting group was removed by the reaction of trifluoroacetic acid. The obtained product was washed thoroughly with 90% methanol followed by 100% methanol and dried at 60°C. The material was packed in appropriate stainless-steel columns. The ligand-exchange-based CSPs commercialized by different companies are given in Table 1. 

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