Applications

Two principal advantages of chiral LEC should be mentioned with respect to practical application of the method. Firstly, the detection of compounds which do not absorb in the near UV region is greatly facilitated by the fact that they partially elute from the LEC column in the form of metal complexes. Even having no strong chromophore in the molecule, they strongly absorb at 254 nm in the complexed form. When com-plexed to Fe(III), hydroxy acids can be selectively detected at 420 nm. The second great advantage of ligand exchange is that enantioselectivity of the separation is largely unaffected by the temperature of the column and the presence of buffer salts and organic modifier in the eluent. This makes the method easily compatible with numerous other separation techniques, including multidimensional and multicolumn chromatography.

In general, the efficiency and easiness of the direct resolution of amino acids and hydroxy acids, without any prior derivatization of the analyte, make LEC a technique of choice for serial determinations of enantiomeric compositions. This is the case, for example in fossil dating where, depending on the rate of racemization of the amino acid under investigation, the dating can be performed in the time range from several hundred years to one million years. Another application area of this kind is the determination of d-amino acids in food products caused by partial racemization through heat or microwave treatment, and to fermentation processes. Occurrence of d-amino acids in biological fluids (e.g. serum, cerebrospinal fluid, urine), bone and tissues is also of great importance from a diagnostic point of view. Rapid enantiomeric analysis techniques are needed when asymmetric synthesis or chemical or enzymic racemate resolution processes are being developed. Examination of the enan-tiomeric composition of hydroxy acids, in particular malic acid, helps to investigate adulteration of apple juice and other soft drinks. Rapid preparation of enantiomers of compounds labeled with highly radioactive atoms, where only dilute solutions can be handled, is another excellent application area for chiral LEC.

Three decades of the existence and intensive development of enantioselective LEC have shown this technique to be an extremely versatile and productive general approach to resolving racemates of compounds that form complexes with metal ions. Other types of molecular interactions have been later found to provide alternative successful ways of organizing the formation of labile diastereomeric adducts with appropriate chiral selectors, e.g. formation of inclusion complexes of analytes with cyclodextrins and cyclic antibiotics or formation of charge-transfer complexes between electron-donating and electron-deficient aromatic groups in Pirkle-type chiral stationary phases. Designing novel chiral selectors which simultaneously offer to the analyte several different modes of interactions could probably result in developing chiral chromatographic systems with wider application areas or higher enantioselectivity with respect to particular important racemates. Thus first attempts of providing cyclodextrins with a copper complexed residue moiety have recently been reported in the literature. Another practically unexplored field for further development could be ligand exchange in nonaqueous and even nonpolar media. With the solvent molecules not competing for the coordination positions of the metal ion, much weaker electron donors than carboxylic or amino functional group of the analyte, should suffice for the complex formation. Ether, sulfide and carbon-carbon double bonds should thus be suitable for complexation. Achievements of complexing gas chromatography and 'argentation' thin-layer chromatography corroborate the feasibility of this development of ligand exchanging enantioselective column liquid chromatography.

See also: N/Chromatography: Liquid: Chiral Separations in Liquid Chromatography: Mechanisms. N/Chromatography: Thin-Layer (Planar): Ion Pair Thin-Layer (Planar) Chromatography. III/Amino Acids and Derivatives: Chiral Separations. Chiral Separations: Capillary Electrophoresis; Cellulose and Cellulose Derived Phases; Chiral Derivatization; Cyclodextrins and Other Inclusion Complexation Approaches; Ion-Pair Chromatography; Liquid Chromatography; Molecular Imprints as Stationary Phases; Protein Stationary Phases; Synthetic Multiple Interaction ('Pirkle') Stationary Phases; Thin-Layer (Planar) Chromatography.

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