Cyclodextrin-based CSPs obtained

Macaudiere et al. first reported the enantiomeric separation of racemic phosphine oxides and amides on native cyclodextrin-based CSPs under subcritical conditions [53]. The separations obtained were indicative of inclusion complexation. When the CO,-methanol eluent used in SFC was replaced with hexane-ethanol in LC, reduced selectivity was observed. The authors proposed that the smaller size of the CO, molecule made it less likely than hexane to compete with the analyte for the cyclodextrin cavity. 

TABLE 5 Examples of Separations Obtained on Cyclodextrin-based CSP... 

A combination of acidic and basic additives is used for the separation of primary amines in the polar organic mode. Table 1 lists the chromatographic data obtained using different combinations of acidic and basic additives. The optimized additive composition is 0.3/0.2 (% v/v) acetic acid/triethylamine in pure methanol, which happens to be the recommended buffer for the polar organic mobile phase separation on cyclodextrin-based CSPs [40,41]. 

A chiral selector can also be dissolved in the IL solvent and be subsequently coated on the capillary wall [38]. In this approach, the achiral [C4CiIm]Cl was used to dissolve permethylated p-cyclodextrin (p-PM) and dimethylated P-cyclodextrin (p-DM). The chromatographic separations obtained from these two columns were compared to two commercially available CSPs based on p-PM and p-DM dissolved in polydimethylsiloxane. From a set of 64 chiral molecules separafed by fhe commercial p-PM column, only 21 of the molecules were enantioresolved by the IL-based p-PM column. Likewise, from a collecfion of 80 analytes separated by the p-DM column, only 16 analytes could be separated on the IL-based p-DM column. The authors also noted a considerable enhancement in the separation efficiency of fhe IL-based CSPs. This resulf, coupled to fhe loss of enantioselecfivify for mosf separations, suggests that the imidazolium cation may occupy the cavity of the cyclodextrin preventing the analyte-cyclodextrin inclusion complex-ation that is crucial for chiral recognition. The ability for ILs to form inclusion complexes wifh cyclodextrin molecules has been recently studied by Tran and coworkers using near-infrared spectromefry [39]. 

The most popular and commonly used chiral stationary phases (CSPs) are polysaccharides, cyclodextrins, macrocyclic glycopeptide antibiotics, Pirkle types, proteins, ligand exchangers, and crown ether based. The art of the chiral resolution on these CSPs has been discussed in detail in Chapters 2-8, respectively. Apart from these CSPs, the chiral resolutions of some racemic compounds have also been reported on other CSPs containing different chiral molecules and polymers. These other types of CSP are based on the use of chiral molecules such as alkaloids, amides, amines, acids, and synthetic polymers. These CSPs have proved to be very useful for the chiral resolutions due to some specific requirements. Moreover, the chiral resolution can be predicted on the CSPs obtained by the molecular imprinted techniques. The chiral resolution on these miscellaneous CSPs using liquid chromatography is discussed in this chapter. 

Because plasma and urine are both aqueous matrixes, reverse-phase or polar organic mode enantiomeric separations are usually preferred as these approaches usually requires less elaborate sample preparation. Protein-, cyclodextrin-, and macrocyclic glycopeptide-based chiral stationary phases are the most commonly employed CSPs in the reverse phase mode. Also reverse phase and polar organic mode are more compatible mobile phases for mass spectrometers using electrospray ionization. Normal phase enantiomeric separations require more sample preparation (usually with at least one evaporation-to-dryness step). Therefore, normal phase CSPs are only used when a satisfactory enantiomeric separation cannot be obtained in reverse phase or polar organic mode. 

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