APPLICATION OF MACROCYCLIC LIGANDS TO ION CHROMATOGRAPHY

Macrocyclic ligands such as crown ethers and cryptands are highly selective in binding metal and other cations and have been employed in a variety of separation technologies, including solvent extraction, membranes and chromatography. In recent years, groups have succeeded in applying these ligands to high-performance ion chromatography. Typically, crown ethers have been adsorbed, covalently bonded or polymerized on particulate substrates. In some instances, excellent separations among alkali and other metal ions have been achieved. As many macrocycles are uncharged and are insensitive to pH, cations, together with their accompanying anions, may be eluted using pure solvents such as water or methanol as eluents. Further, anions may be separated through the formation of ion pairs with the bound cations. The latter effect has been refined to make possible a unique form of gradient anion chromatography. When metal hydroxide eluents are used for anion chromatography, the capacity of a macrocycle column is determined by the stability constant of the metal ion complexation reaction. If the eluent cation is changed during the course of a single chromatogram so that the column capacity drops, a wide range of anions can be separated with excellent resolution. Little baseline perturbation is experienced since the eluent (OH-) concentration remains constant throughout the gradient.