Development of Chemically Bonded Crown Ether Stationary Phases in Capillary Ion Chromatography

Abstract

Three types of crown ethers (i.e. 1-aza-15-crown-5-ether, 1-aza-18-crown-6-ether and 2-aminomethyl-18-crown-6-ether) were chemically bonded to silica gels via a simple 2-step reaction and the resulted stationary phases were evaluated for the separation of both anions and cations in capillary ion chromatography. It was found that the chemically bonded stationary phases were most stable and relatively sharper peaks were obtained when a mixture of toluene:acetonitrile = 7:3 was used as the reaction solvent in the second step reaction. The size of crown ether cavity as well as the presence of nitrogen atom within the cavity affected the retention behaviors of the ions. As for the separation of anions, it was found that the eluent cations were trapped on the stationary phases and acted as the anion-exchange site, viz. the anions were separated in the ion-exchange mode. Without the obstruction of nitrogen atom, 2-aminomethyl-18-crown-6-ether could easily form complexes with metal cations, and the retention as well as elution order were dependant on the complex formation constant between crown ether and the metal cations. Eluent with higher pH favored the retention and separation of the metal cations.