Introduction

Ligand exchange chromatography (LEC) was first introduced by Helfferich in 1961 as a general chromatographic technique to separate compounds which are able to form labile complexes with transition metal cations. The basic idea was to immobilize such ions as Cu(II) or Ni(II) on a stationary phase, in particular, a cation exchanger with sulfonic or carboxylic functional groups, and then let the ions form labile coordination compounds with analytes which possess electron-donating functional groups and can enter the coordination sphere of the metal ion, thus acting as ligands. Those analytes that form stronger complexes with the central ion, are retained longer on such a metal ion-incorporating stationary phase.

Starting with the idea that, in the densely packed coordination sphere, ligands enter multipoint interactions with each other and should therefore mutually recognize the spatial configuration of neighbours, Davankov and Rogozhin (1968-71) synthesized chiral complex-forming resins and further developed LEC into a powerful chiral chromato-graphic method. This technique, for the first time in liquid chromatography, resulted in a complete and reliable resolution of a racemate into constituent en-antiomers. Typical analytes for enantioseletive LEC are members of such important classes of chiral compounds as amino acids, hydroxy acids and amino alcohols. Having soon become one of the most extensively investigated methods for the direct resolution of enantiomers, LEC maintained for a long period of

Petterson C and Heldin E (1994) A practical approach to chiral separations by liquid chromatography. In: Subramanian G (ed.) Ion-Pair Chromatography in Enantiomer Separations, pp. 279-310. Weinheim: VCH.

Schill G, Wainer IW and Barkan SA (1986) Chiral separation of cationic drugs on an oxacid glycoprotein bonded stationary phase (Enantiopac®). Journal of Chromatography 365: 73-78.

time its leading positions in developing novel chiral chromatographic systems and evaluating mechanisms of chiral recognition and discrimination. This technique contributed much to the successful development of chiral silica-bonded stationary phases, both monomeric and polymeric, chiral coatings on column-packing materials and chiral mobile phase additives. In this last technique, it transpired that the central metal ion does not need to reside in the stationary phase, but, instead, can be a part of a chiral complex that is doped into the mobile phase to interact there with the analytes. This widens significantly the definition of LEC to a chromatographic process in which the formation and breaking of labile coordinate bonds to a central metal cation are responsible for the separation of complex forming analytes.

Ligand exchange has been realized in all known modes of enantioselective chromatography, including liquid chromatography, thin layer chromatography, gas chromatography, capillary electrophoresis and countercurrent liquid chromatography. In gas chromatography the principle of ligand exchange is more commonly known under the name 'complexa-tion chromatography'.

Solar Panel Basics

Solar Panel Basics

Global warming is a huge problem which will significantly affect every country in the world. Many people all over the world are trying to do whatever they can to help combat the effects of global warming. One of the ways that people can fight global warming is to reduce their dependence on non-renewable energy sources like oil and petroleum based products.

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