Chiral-coated stationary phases

In chiral chromatography, the two diastereomeric adducts ArEr and ArEs are formed during elution, rather than synthetically, prior to chromatography. The adducts differ with respect to their stability in the use of chiral stationary phases (CSPs) or chiral-coated stationary phases (CCSPs) and/or in their interphase distribution ratio with the addition of a chiral selector to the mobile phase (CMP). The difference between the interactions of the chiral environment with the two enantiomers is called enantioselectivity. 

CHIRAL STATIONARY PHASES AND CHIRAL-COATED STATIONARY PHASES... 

Chiral-coated stationary phases (CCSPs) consist of specific chiral selectors permanently adsorbed onto the surface of achiral thin layer chromatography (TLC) materials without covalently modifying their chemical characteristics. 

The chiral selector is derived from the well-known chiral chromatographic stationary-phase Chirasil-Val [22] and both enantiomers of the receptor have been applied as coatings in a sensor array. The chiral selector octyl-Chirasil-Val [N-(2-methylpropanoyl)-S/R-valinc tert-butylamide] contains chiral pep-... 

The use of a chiral liquid stationary phase. The packing of an LC column, e.g., silica, is coated with a film of a liquid, optically pure compound that must not be soluble in the mobile phase used. A possible phase pair is a tartrate derivative in combination with an aqueous buffer eluent. The handling of such liquid-liquid chromatographic systems is not simple, which is why they are not often used, and they are... 

New ILs and coating methods are being developed for high efficiency and high thermostability GC columns. Chiral GC stationary phases with high thermo-stability and broad enantiomeric selectivity are needed. One area that will continue to grow in imp>ortance is the use of ILs as absorbents in solid-phase extractions (SPE) and solid-phase micro-extractions (SPME). It is likely that ILs will fill the role of a polar absorbent for these techniques [Han Armstrong, 2007]. 

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 same is true for the chiral polysiloxanes described here. Their use as stationary phases in gas chromatography allows the calculation of the differences in enthalpy and entropy for the formation of the diaste-reomeric association complexes between chiral receptor and two enantiomers from relative retention time over a wide temperature range. Only the minute amounts of the polysiloxanes required for coating of a glas capillary are necessary for such determinations. From these numbers further conclusions are drawn on the stereochemical and environmental properties required for designing systems of high enantio-selectivity in condensed liquid systems. 

Extensive comparisons between GC and SFC have been reported in chiral separation [63-66]. Zoltan investigated the performance of SFC and GC using the same chiral capillary columns coated with cyclodextrin-based stationary phases. It was observed that chiral selectivity was higher in GC than in SFC using the same open tubular column at the identical temperature (e.g., >100°C). However, the selectivity in SFC was significantly increased at low temperatures, especially for polar compounds [67]. 

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. 

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). 

The various properties exhibited by ILs make them ideal stahonary phases in GLC. ILs exhibit a unique dual-nature selechvity that allows them to separate polar molecules like a polar stationary phase and nonpolar molecules like a nonpolar stationary phase. In addition, the combination of cations and anions can be tuned to add further selectivity for more complex separations. Viscosity, thermal stability, and surface tension are vital properties that dictate the quality and integrity of the stationary phase coating and are additional characteristics that can be controlled when custom designing and synthesizing ILs. Furthermore, thermal stability and the integrity of stationary phase film can be improved by immobilizing the IL by free radical polymerization to form stationary phases suitable for low- moderate-, and high-temperature separations. Chiral ILs have been shown to enantioresolve chiral analytes with reasonable efficiency. 

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]. 

Chiral chromatography can also be used in order to obtain resolution of stereoisomers from aspartame, its precursors, and its degradation products. Lin et al. (84), using a Chiracel OD column and a mobile phase of 2-propanol -hexane (1 1, v/v), achieved complete separation of aspartame precursors, dd-, dl-, LL-, and LD-[(Z)-AspOS-Bzl)-Phe-OCH3], Motellier and Wainer (85) separated four stereoisomers of aspartame, two of diketopiperazine, and three of aspartyl-phenylalanine using a stationary phase composed of a chiral crown either coated on a polymeric support—CrownPack CR( + )—a mobile phase of aqueous perchloric acid, pH 2.8, and modified... 

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