Chiral-coated stationary phases preparation

Chiral stationary phases prepared from silica-based coated poly(saccharide) derivatives... 

Lomsadze K, Jibuti G, Farkas T, Chankvetadze B. Comparative high-performance liquid chromatography enantioseparations using polysaccharide based chiral stationary phases prepared by coating of totally porous and core-shell silica particles. J Chromatogr A 2012 1234 50-55. 

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]. 

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. 

Mechanistic considerations (e.g., the extensive work published on brush-type phases) or the practitioner s experience might help to select a chiral stationary phase (CSP) for initial work. Scouting for the best CSP/mobile phase combination can be automated by using automated solvent and column switching. More than 100 different CSPs have been reported in the literature to date. Stationary phases for chiral pSFC have been prepared from the chiral pool by modifying small molecules, like amino acids or alkaloids, by the deriva-tization of polymers such as carbohydrates, or by bonding of macrocycles. Also, synthetic selectors such as the brush-type ( Pirkle ) phases, helical poly(meth) acrylates, polysiloxanes and polysiloxane copolymers, and chiral selectors physically coated onto graphite surfaces have been used as stationary phases. 

The plates coated with this stationary phase were recently prepared in laboratory and proposed by Lepri et al. [31] by using a CTB suspension for column chromatography. Until now, researchers have studied particularly applications about racemic alcohols containing one or more aromatic rings in such cases, CTB showed high resolution. Precoated plates are not still available to allow a larger use of such chiral adsorbent. 

Chiral stationary GC phases based on IL coating have been reported in which either a chiral IL was applied or a chiral molecule was dissolved within the IL film. Berthod et al. were the first to dissolve two chiral selector molecules, namely, permethylated-P-cyclodextrin (BPM) and dimethylated-p-cyclodextrin (BDM), in [BMIM]C1 and tested the as-prepared GC columns in the separation of 64 chiral compounds [41], Comparison with two commercial chiral columns, Chiraldex B-PM (20 m length, 250 pm inner diameter) and Chiraldex B-DM (20m length, 250 pm iruier diameter) was also made. The BDM-[BMIM]Cl-coated column produced efficiencies commonly 700% higher than the corresponding values obtained with the commercial BDM Chiraldex column. The difference was lower for the BPM-[BMIM]Cl-coated column, but stiU this column showed twice the efficiency compared to the BPM Chiraldex column. 

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