Multifunctional ion-exchange stationary phases for high-performance liguid chromatography

Abstract

The preparation and properties of multifunctional ion-exchangers, including one cation and two zwitterion types, for high-performance liquid chromatography (HPLC) are described. These ion-exchange stationary phases (IXSPs) were synthesized through two major steps; first by bonding the corresponding organic moieties, either undecenoic acid, aniline or ethyl p-fluorobenzoate, onto silica surfaces with appropriate silane coupling agents. The bonded silica was subsequently derivatized to the corresponding ion exchanger by free-radical partial addition, sulfonation or hydrolysis, respectively, and worked up by end-capping. The ion exchangers thus prepared were characterized by elemental analysis and FTIR spectroscopy. These IXSPs produce effective separations of aromatic amines, aromatic acids, amino acids, aminobenzoic acids and nitroanilines by HPLC. Effects of pH, concentration of the electrolyte and the polarity of the mobile phase on the capacity (retention) factor were investigated. The chromatographic results for these IXSPs as separators of the positional isomers of aminobenzoic acids and nitroanilines were compared with those for commercial phases. On the basis of the chromatographic behaviour observed in this study, it is concluded that the effective selectivity of these IXSPs is due to the sulfo-(SO 3H) group or the carboxylic group for the cation-exchange ability, the amino group for the anion-exchange ability, the long-chain alkenyl moiety for the hydrophobic character, and the double bond of the long-chain alkenyl moiety as well as the aromatic rings for the π-π charge-transfer interactions. These multifunctional interaction mechanisms render these readily preparable phases more efficient in the separation of organic weak bases and organic weak acids than a simple ion exchanger.