Retention mechanisms enantioselective separation

In the elucidation of retention mechanisms, an advantage of using enantiomers as templates is that nonspecific binding, which affects both enantiomers equally, cancels out. Therefore the separation factor (a) uniquely reflects the contribution to binding from the enantioselectively imprinted sites. As an additional comparison the retention on the imprinted phase is compared with the retention on a nonimprinted reference phase. The efficiency of the separations is routinely characterized by estimating a number of theoretical plates (N), a resolution factor (R ) and a peak asymmetry factor (A ) [19]. These quantities are affected by the quality of the packing and mass transfer limitations, as well as of the amount and distribution of the binding sites. 

Booth, T.D. and Wainer, I.W. (1996a). Investigation of the Enantioselective Separations of alpha-alkylarylcarboxylic Acids on an Amylose tris(3,5-dimethylphenylcarbamate) Chiral Stationary Phase Using Quantitative Structure-Enantioselective Retention Relationships. Identification of a Conformationally Driven Chiral Recognition Mechanism. J.Chromat., 737A, 157-169. 

Figure 7 Enantiomer separation of methyl 2-chloropropanoate by gas chromatography on heptakis(3-0-trifluoroacetyl-2,6-di-0-/>pentyl)-)5-cyclodextrin (23) (undiluted) at three temperatures. Column 20 m x 0.25 mm (i.d.) nondeactivated fused-silica capillary. (From Berthod A, Li W, and Armstrong DW (1992) Multiple enantioselective retention mechanisms on derivatized cyclodextrin gas chromatography chiral stationary phases. Analytical Chemistry 64 873-879.)...

Enantioselective H P LC wrongly has the reputation of being a difficult to develop method that is very unreliable and mastered only by a few specialists. However, the multitude of commercially available stationary phases (CSPs) [2] with significantly different retention mechanisms makes an overview of the different factors contributing to a successful separation almost impossible. 

Wainer IW, Alembik MC (1986) Resolution of enantiomeric amides on cellulose based chiral stationary phases steric and electronic effects. J Chromatogr 358 85-93 Booth TD, Wainer IW (1996) Investigation of the enantioselective separations of a-alkyl arylctirboxylic acids on an amylose tris(3,5-dimethylphenylcatbamate) chiral stationary phase using quemtitative structure-enantioselective retention relationships (QSERR) identification of a conformationaUy driven chiral recognition mechanism. J Chromatogr A 737 157-169... 

CSPs, cyclodextrin-derived CSPs, " antibiotic-derived CSPs, " and protein-derived CSPs. As a general rule, starting conditions are applied to set retention of the analyte, and in the next step, enantioselectivity is tuned by stepwise modifying appropriate factors, considering the chemical nature of the analyte, compatibility with the chromatographic support, and recognition mechanism acting in the separation. 

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