Ligand-exchange CSP

Some ligand-exchange CSPs have been used at preparative level [31, 32]. In this case it must be taken into account that an extraction process, to remove the copper salts added to the mobile phase, must be performed following the chromatographic process [33]. Teicoplanin, in contrast, resolves all ordinary a and (3-amino acids with mobile phases consisting of alcohol/water mixtures. No buffer is needed in the... 

Different classifications for the chiral CSPs have been described. They are based on the chemical structure of the chiral selectors and on the chiral recognition mechanism involved. In this chapter we will use a classification based mainly on the chemical structure of the selectors. The selectors are classified in three groups (i) CSPs with low-molecular-weight selectors, such as Pirkle type CSPs, ionic and ligand exchange CSPs, (ii) CSPs with macrocyclic selectors, such as CDs, crown-ethers and macrocyclic antibiotics, and (iii) CSPs with macromolecular selectors, such as polysaccharides, synthetic polymers, molecular imprinted polymers and proteins. These different types of CSPs, frequently used for the analysis of chiral pharmaceuticals, are discussed in more detail later. 

FIGURE 2 Chemical structures of ligand-exchange CSPs. 

The coated ligand-exchange CSPs have been used with the mobile phases that did not cause bleeding effects. Otherwise, the CSP may lose its chiral ligand after a few runs [12]. It is very interesting to note that the values of separation factors were very high on the coated CSPs. The values of the separation factors for proline and valine amino acids were reported at 16.4 and 6.9, respectively... 

FIGURE 11 Graphical representation of the chiral recognition mechanism of (a) phenylalanine and (b) antifungal agents on the ligand-exchange CSPs. (From Refs. 18 and 70.)... 

There have also been examples of ligand-exchange CSPs. Schmid et al. [159] used a ligand-exchange monomer as a chiral selector. The chiral selector, monomer, cross-linker, and charged monomer were polymerized to produce monolithic capillaries capable of chiral recognition and generation of EOF. The separation is achieved due to the differences in the stability between the ternary mixed copper complexes formed by the enantiomers and the CSP. 

Scheme 7.11 A protocol for the development and optimization of mobile phases on ligand exchange CSPs. Note that this is only a brief outline of the procedure that should be followed in developing a resolution on ligand exchange CSPs.

Pyrethrin and pyrethroid Crops, food and other environment samples Ligand exchange CSPs 75... 

On other occasions, recognition requires adequate characteristics either in the CS or in the analytes that have to be provided by the mobile phase. Thus, when ligand-exchange CSPs are used, copper salts have to be added to the aqueous mobile phase. Also, when the separation of amines on CSPs containing crown-ethers as CSs is undertaken, an acid is added to the hydro-alcoholic mobile phase to ensure ionization of the analyte. Analogously, acidic hydro-alcoholic mobile phases are used with ion-exchange CSPs to foster protonation of the CS. 

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