Liquid chromatography with crown ether-containing mobile phases : I. Capacity factors of amino compounds on a hydrophobic stationary phase

Abstract

The retention behaviour of amino compounds (aromatic amines, amides, amino acids) in reversed-phase liquid chromatography with mobile phases containing crown ethers (18-crown-6 or dicyclohexyl-18-crown-6) have been investigated. The chromatographic process is viewed in terms of simultaneous equilibria involving protonation of the solute molecule, complexation of the cationic solute with the crown ether and associations of the complex and free species with the hydrophobic stationary phase. From these equilibria, an expression for the capacity factor as a function of the proton and crown ether concentrations in the mobile phase is derived. The equation predicts that the capacity factor should initially increase with increasing crown ether concentration at constant pH followed by a regular decrease at higher concentrations, and that the dependence of the capacity factor on the proton concentration should be rectangular hyperbolic at constant concentration of crown ether. The experimental results obtained at various crown ether concentrations at constant pH and at various pH values at constant concentration of crown ether are in accord with the predicted profiles. The analytical applicability of this method has been demonstrated in the reversed-phase separation of amino compounds.