Superheated Water Ion-exchange Chromatography

Thermal effect on the separation selectivity of cation-exchange and anion-exchange processes was studied on a strongly acidic cation-exchange resin and a strongly basic anion-exchange resin using superheated water chromatography. The selectivity coefficient for a pair of identically charged inorganic ions, such as alkali metal ions and halide ions, approaches unity as the temperature increases, provided that the ions have the same effective size, such that the retention of an analyte ion decreases with an increase in temperature when the analyte ion has stronger affinity for the ion-exchanger than that of the eluent counter-ion, whereas it increases when it has weaker affinity. XAFS measurements on aqueous solutions containing the inorganic ions in the temperature range from ambient temperature to 175 degrees C at 3 MPa indicate that the number of coordinated first shell water molecules decreases with an increase in temperature. At elevated temperatures, electrostatic charge screening no longer effectively occurs, which may lead to smaller ion-exchange separation selectivity for different ions having an identical charge. Consequently, the electrostatic interaction or association of the ions with the fixed ion in the resin phase becomes predominant factors, resulting in a different separation selectivity from that obtained at ambient temperature.