Retention Characteristics of Lanthanide Ions on a Mixed Phosphonic Acid-Carboxylic Acid Cation Exchanger

The Chromatographic behaviour of lanthanide ions on a dual phosphonate- and carboxylate-functionalized ion-exchange resin (Dionex IonPac CS12A and CG12A) was studied using various mobile phase compositions. By suppressing ion-exchange interactions through increasing the ionic strength of the eluent, the column selectivity changed to that of a chelation mechanism via the phosphonate functional groups. Retention was based on the varying conditional stability constants of the lanthanide ions with the phosphonate groups. Eluent pH was additionally found to exert a strong influence on analyte retention, but no further changes to the column selectivity occurred. The inclusion of small concentrations of organic solvents could also be used to manipulate retention to a more minor extent, as could altering the column temperature. Examples of separations are given, especially the separation of gadolinium and europium, which are often co-eluted when using nitrogen-oxygen-chelating groups such as iminodiacetic acid.