Chiral separation mechanisms interactions

The degree of substitution in derivatized CDs may affect the chiral separation mechanism [21], The changes in enantioselectivity of a derivatized CD were attributed to the loss of hydrogen bonding groups in the or-position to the chiral centers of the CD molecule, or to a gain of additional interaction sites around the base of the derivatized CD molecule [27]. 

Booth and Wainer [50] have demonstrated the role of multiple hydrogenbonding interactions in the stability of the diastereomeric complex occurring throughout conformational changes of both ATPC chiral selector and solute. Authors pointed out in this study that chiral recognition does not fit a three-point interaction mechanism. Other aspects of chiral separation mechanisms on polysaccharide triphenylcarbamate derivatives have been recently reviewed by Yamamoto andOkamoto [51],... 

Chiral separations present special problems for vaUdation. Typically, in the absence of spectroscopic confirmation (eg, mass spectral or infrared data), conventional separations are vaUdated by analysing "pure" samples under identical chromatographic conditions. Often, two or more chromatographic stationary phases, which are known to interact with the analyte through different retention mechanisms, are used. If the pure sample and the unknown have identical retention times under each set of conditions, the identity of the unknown is assumed to be the same as the pure sample. However, often the chiral separation that is obtained with one type of column may not be achievable with any other type of chiral stationary phase. In addition, "pure" enantiomers are generally not available. 

In general, charged CDs have shown superior discrimination abilities, especially the highly-sulfated (HS-CDs) ones. Furthermore, the separation mechanism is altered by the introduction of electrostatic interactions. Finally, the use of chiral selectors carrying a charge opposite to that of the analytes can greatly improve the mobility difference between the two enantiomers. The use of mixtures of CDs in chiral separation is also possible. ... 

When a CSP is applied, the separation mechanism is based on the differences in the interaction between the chiral selector in the stationary phase and the enantiomers of the solute. Depending on the nature of the selector and the type of the solute, the stereoselective interaction can be based on interactions of one or more different types such as inclusion complexation, Tr-jr-interaction, dipole stacking, hydrogen bonding, electrostatic interaction, hydrophobic interaction, and steric interaction [35]. In order to obtain chiral discrimination between the enantiomers, a three-point interaction is required between at least one of the enantiomers and the CSP [36]. The interactions can be of attractive as well as repulsive nature (e.g., steric and electrostatic interactions). 

Since one or more of the interactions in these systems might originate from the stationary phase, only a two- or a one-point interaction between the solute and the selector is necessary for mechanisms (2) and (3) to occur [50]. However, some of the CMPAs used in HPLC [37,40,51,52] have also been used as chiral selectors in CE [53-56], which indicates that at least one of the separation mechanisms between the selector and enantiomers is selective complex formation in the mobile phase in these cases, since there is no stationary phase present in CE. A recent example by Yuan et al. [57] is presented in Eigure 17.1. The authors introduced the use of (R)-A,A,A-trimethyl-2-aminobutanol-bis(trifluoromethane-sulfon)imidate as the chiral selector for enantioseparation in HPLC, CE, and GC. This chiral liquid serves simultaneously as a chiral selector and a co-solvent. 

Solutes undergo retention when weak interaction occurs with the stationary phase. The analyst must select a stationary phase according to the nature of the solute (apolar, polar, ionizable). For convenience we shall consider the following different mechanisms adsorption, partition, chromatography of ionizable substances, chiral separations, exclusion. Table 1 is. a quick selection guide. 

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