Fig. 2.18 Structural formula of a linear heptadextrin "acyclodextrin", a new selector in enantiose-lective gas chromatography for a variety of amino acids. R = tert-butyldimethylsilyl; Ri = acetyl analytes such as specifically derivatized amino acids (Sicoli et al. 2005). Its molecular structure is depicted in Fig. 2.18.
As a consequence of the enantiomer-resolving properties of linear oligodextrins, one should be careful in referring the resolution of enantiomers exclusively to inclusion complexes in cyclodextrin host molecules.
The merits of gas chromatography in the resolution of enantiomers have successfully stimulated research in liquid chromatography. Today, many chiral selectors have been developed for use in the high-performance (HPLC) mode including molecularly imprinted polymers (Guibitz and Schmidt 2004) but also in supercritical fluid chromatography (SFC), and thin layer chromatography (TLC). Other separation techniques in use are the various electromigration techniques such as capillary electrophoresis (CE), and capillary electrochromatography (CEC), a hybrid of HPLC and CE, which has recently been used rather successfully.
In order to determine the quality of an obtained resolution of chiral analytes, the separation factor a and the resolution RS can be calculated. The chromatographic retention time t and the hold-up time tM (also called time of passage) allows the calculation of the required reduced retention time t' (see Eq. 2.25).
The ratio of the reduced retention times tR and t'S corresponding to the R-enantiomer and the S-enantiomer results in the separation factor a as written in Eq. 2.26.12 By convention, the separation factor a is equal or bigger than one
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