Chiral stationary phases reverse elution

Xiao, T.L., Reversal of enantiomeric elution order on macrocyclic glycopeptide chiral stationary phases, J. Liq. Chrom. Rel. TechnoL, 24, 2673, 2001. 

Gaffney [89] reported the reverse order of elution of 2-phenoxypropanoic acid on Chiralcel OB CSP when different alcohols were used. In 2001 Aboul-Enein and Ali [63] observed the reverse order of elution of nebivolol enantiomers on a Chiralpak AD chiral stationary phase when ethanol and 2-propanol were used separately as the mobile phases. However, the best resolution was obtained when ethanol served as the mobile phase. The inversion of the elution may be due to the different conformation of the polysaccharide CSPs [63]. The pattern of conversion of order of elution using different ratios of ethanol and 2-propanol is shown in Figure 15. 

Persson and Andersson [65] reviewed the unusual effects in liquid chromatographic separations of enantiomers on chiral stationary phases with emphasis on polysaccharide phases. On protein phases and Pirkle phases, reversal of the elution order between enantiomers due to... 

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. 

Macaudiere, E., Lienne, M., Caude, M., Rosset, R., and Tambute, A. 1989. Resolution of Jt-acid racemates on Jt-acid chiral stationary phases in normal-phase liquid and subcriti-cal fluid chromatographic modes A unique reversal of elution order on changing the nature of the achiral modifier. Journal of Chromatography, 467 357-72. 

At the present state of knowledge, the mechanisms of GC enantiomer separation have not been elucidated. Unusual chromatographic behaviour and reversal of the elution order of enantiomers have been observed. Consequently, the usefulness of a given chiral stationary phase as well as the order of elution of separated enantiomers cannot be predicted. References of definite chirality are essential to identify the separated isomers, no matter whether directly stereoanalyzed with chiral stationary phases or via derivatized stereoisomers [12-19]. 

Okamoto, M., Nalcazawa, H., Reversal of elution order during direct enantiomeric separation of pyriproxyfen on a cellulose-based chiral stationary phase,/. Chromatogr., 1991, 588, 177-180. 

The second general approach to overcome poor reverse-phase retention is to employ NP-LC. By comparison to RP-LC, the historic use of NP-LC for bioanalysis is negligible because of several limitations. The most notable limitation is the inability to perform reproducible gradient elution. Nevertheless, NP-LC can be a viable option for analytes too polar for RP-LC and produces far less back pressure. Perhaps, the most visible application of NP-LC is for the bioanalysis of chiral drugs [96]. This critical niche is largely the result of the frequent use of NP mobile phases with chiral stationary phases. 

In order to broaden the capabilities of the Pirkle concept, both polar and polarizable groups were introduced into the molecule. The most popular of this type of chiral stationary phase are the (R,R) Whelk-01 and the (S,S)Whelk-01 phases, the structures of which are shown below. These phases are more versatile and have a wider field of application than the phases previously described. The phases are covalently bonded to the silica and so they can be used with almost any type of solvent. However, they have been found to operate most effectively in the normal phase mode. It should be noted that the polarizable character of the aromatic ring is essential for the stationary phase to function well. As the Pirkle phases are generally available in both the (R) and (S) configurations, the reversal of the elution order of a pair of enantiomers is possible. This stationary phase was originally designed for the separation of the Naproxen enantiomers but has found a wide application to the separation of epoxides, alcohols, diols, amides, imides and carbamates. 

An attractive method for the analysis of mixtures of enantiomers is chiral gas chromatography (GC). This sensitive method is unaffected by trace impurities, and is quick and simple to carry out. The premise upon which the method is based is that molecular association may lead to sufficient chiral recognition that enantiomer resolution results. The method uses a chiral stationary phase which contains an auxiliary resolving agent of high enantiomeric purity. The enantiomers to be analysed undergo rapid and reversible diastereomeric interactions with the stationary phase and hence may be eluted at different rates. There are certain limitations to the method, some of which are peculiar to the gas chromatographic method. The sample should be sufficiently volatile and thermally stable, and, of course, should be quantitatively resolved on the chiral GC phase. Occasionally this... 

The low vapor pressure and high thermal stability of CILs render them suitable for enantioseparations in gas chromatography (GC). Recently, CILs have been used as chiral stationary phases (CSPs) in GC [40]. Armstrong and coworkers carried out enantiomeric separation of chiral alcohols and diols, chiral sulfoxides, some chiral epoxides and acetamides using a CIL based on ephedrinium salt. Using an ephedrinium CIL (4) as the CSP, enantiomeric separation of alcohols and diols was achieved (Fig. 1). The presence of both enantiomeric forms of ephedrine makes it possible to produce CSPs of opposite stereochemistry, which could reverse the enantiomeric elution order of the analytes. This offers an additional advantage that may not be easily achieved with common and widely used chiral selectors in GC such as the cyclodextrins. However, there was a decrease in enantiomeric recognition ability of the CSP after a week which the authors attributed to dehydration-induced... 

Ma S, Shen S, Lee H, Eriksson M, Zeng X, Xu J, Eandrick K, Yee N, Senanayake C, Grinberg N. Mechanistic studies on the chiral recognition of polysaccharide-based chiral stationary phases using liquid chromatography and vibrational circular dichroism. Reversal of elution order of iV-substituted alpha-methyl phenylalanine esters. J. Chromatogr. A 2009 1216 3784-3793. 

Ghanem A. True and false reversal of the elution order of barbiturates on a bonded cellulose-based chiral stationary phase. J. Chromatogr. A 2006 1132 329-332. 

The reversal of the order of peak elution (peak inversion) for enantiomerically enriched samples by inverting the chirality of the stationary phase represents not only a diagnostic tool for verification of an enantiomer separation147, but also has important merits in enantiomeric analysis. Thus, this technique should be employed for the evaluation of trace amounts of... 

The overall observed retention of the enantiomers, and thus the elution order, is based on several kinetically and thermodynamically controlled parameters concerned with stereorecognition nonstereoselective interactions of all partners SO(R), SA(R S), and particularly of the [SO(RI-SA(KI] and [SO(K)-SA(Si] complexes with the achiral stationary phase, also play a role (Figure 21). Therefore the retention order may be reversed for a specific pair of enantiomers depending on whether a covalently bound CSP or a CMPA is applied, but using the same chiral molecule (part) as chiral selector. These general principles, shown schematically for a CLEC system, are further complicated by the complexity of the entire system, hence they are difficult to anticipate and each case must be studied individually. 

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