18-crown analytical chromatography

Although the majority of reports of macrocycles in analytical chromatography have involved ligand association with the stationary phase, their use as mobile phase constituents has also been investigated. Lamb and Drake [11] showed that addition of water-soluble crown ethers to the mobile phase altered the retention of alkali metal cations on an underivatized reversed phase column. Nakagawa et al. [63-66] also used crown ether-containing mobile phases in the separation of protonated amines, amino acids and peptides, and [1-lactam antibiotics. 

Analytically, the inclusion phenomenon has been used in chromatography both for the separation of ions and molecules, in Hquid and gas phase (1,79,170,171). Peralkylated cyclodextrins enjoy high popularity as the active component of hplc and gc stationary phases efficient in the optical separation of chiral compounds (57,172). Chromatographic isotope separations have also been shown to occur with the help of Werner clathrates and crown complexes (79,173). 

Capillary electrophoresis employing chiral selectors has been shown to be a useful analytical method to separate enantiomers. Conventionally, instrumental chiral separations have been achieved by gas chromatography and by high performance liquid chromatography.127 In recent years, there has been considerable activity in the separation and characterization of racemic pharmaceuticals by high performance capillary electrophoresis, with particular interest paid to using this technique in modem pharmaceutical analytical laboratories.128 130 The most frequently used chiral selectors in CE are cyclodextrins, crown ethers, chiral surfactants, bile acids, and protein-filled... 

In contrast, CSPs have achieved great repute in the chiral separation of enantiomers by chromatography and, today, are the tools of the choice of almost all analytical, biochemical, pharmaceutical, and pharmacological institutions and industries. The most important and useful CSPs are available in the form of open and tubular columns. However, some chiral capillaries and thin layer plates are also available for use in capillary electrophoresis and thin-layer chromatography. The chiral columns and capillaries are packed with several chiral selectors such as polysaccharides, cyclodextrins, antibiotics, Pirkle type, ligand exchangers, and crown ethers. 

Kimura, K., Nakajima, M., and Shono, T. (1980) Poly(crown ether)-Modified Silica for Stationary Phase of Liquid Chromatography, Analyt. Lett. 13, 741-750. 

Crownpak CR(+). Chiral separation of gemifloxacin was performed in analytical counter-current chromatography using (- -)-(18-crown-6)-tetra-carboxylic acid as CSP. A successful separation of gemifloxacin enantiomers could be achieved using a two-phase solvent system composed of 1-butanol-ethyl-acetate-bis(2-hydroxyethyl)amino tris (hydroxymethyl)methane acetate buffer with a small amount of CgH4 [19]. 

Toda procedure for obtaining enantiomeri-cally pure compounds will find broad application very soon. This development could make preparative HPLC with chiral columns obsolete and be applied to distillable amino acid derivatives as well. After all, analytical resolution of amino acids was quite successful by host/guest complexation chromatography with reversed-phase packings loaded with Cram s chiral 1,1 -binaphthyl crown ethers (similar to 1). [20]... 

A steadily growing application of crown compounds is observed in analytical chemistry and related fields. Fundamentals of their use in this field have already been discussed in a review of Weber [249]. Since then, a great many improvements and further applications have been demonstrated, including ion-selective solvent extraction [250,251], ion chromatography [252], ion-selective electrodes [253-256], chromoionophores and fluoroionophores [257-2591, biology [260-262], and medicine [263]. Of the numerous paper dealing with these subjects, only a few important topics have been singled out here in order to make new concepts and trends evidents. 

Fig. 5. Schematic diagram of a thermai conductivity detector, TCD. Reproduced from i.A. Fowtis, Gas Chromatography Analytical Chemistry by Open Learning, 2nd edn, 1995, with permission from Her Majesty s Stationery Office. Crown Copyright.

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