Chiral anion exchanger, polymeric

Pioneering attempts at using cinchona alkaloids as a platform for chiral stationary phase preparation have been reported as early as in the mid-1950s by Grubhofer and Schleith [52]. The chiral anion exchange polymeric materials were prepared by immobilization of quinine (and other cinchona alkaloids) via the 9-hydroxyl group or quinuclidine nitrogen to a polymer support. However, this resulted in very low selectivities of these phases toward racemic mandelic acid as a test analyte. Results of the early studies have been reviewed in detail by Davankov [53]. 

Thirteen different enantiomers were tentatively used to obtain a significant number of A-V chiral solute descriptors. They were selected because they were well resolved on the four Chirobiotic columns. The set contained eight ionic or zwit-terionic amino acids or amino acid derivatives and five molecular solutes. The 13 enantiomers were separated on 5 well-characterized achiral columns, namely a classical polymeric C18, a diphenyl bonded column, a weak anion exchanger (Primesep D, SiELC), a divinyl benzene bonded column, and a strong anion exchanger (SAX, Alltech). Five different ethanol-water mobile phases were used making more than 300 chromatograms to develop and to analyze. 

The CSPs based on chiral crown ethers were prepared by immobilizing them on some suitable solid supports. Blasius et al. [33-35] synthesized a variety of achiral crown ethers based on ion exchangers by condensation, substitution, and polymerization reactions and were used in achiral liquid chromatography. Later, crown ethers were adsorbed on silica gel and were used to separate cations and anions [36-39]. Shinbo et al. [40] adsorbed hydrophobic CCE on silica gel and the developed CSP was used for the chiral resolution of amino acids. Kimura et al. [41-43] immobilized poly- and bis-CCEs on silica gel. Later, Iwachido et al. [44] allowed benzo-15-crown-5, benzo-18-crown-6 and benzo-21-crown-7 CCEs to react on silica gel. Of course, these types of CCE-based phases were used in liquid chromatography, but the column efficiency was very poor due to the limited choice of mobile phases. Therefore, an improvement in immobilization was realized and new methods of immobilization were developed. In this direction, CCEs were immobilized to silica gel by covalent bonds. 

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