Use of complexing reagents as additives to the eluent for optimization of separation selectivity in high-performance chelation ion chromatography

This paper details a study of the selectivity characteristics of high-performance chelation ion chromatography when separating a range of metal ions with a number of complexing eluents. It shows that exploitation of competitive metal complexation between ligands in the eluent and surface bonded chelating groups allows a wide range of control over the retention order and selectivity coefficients of groups of metal ions for specific applications. An indication of the metal separation characteristics found for simple non-complexing eluents on iminodiacetic acid (IDA) silica bonded substrates is given first, followed by an illustration of the selectivity changes that can be achieved by using complexing eluents. Using a novel approach, plots of logβ1 of the metal complexes of a chosen eluent ligand against the surface bonded IDA metal complexes were found to be useful indicators of which metals may show unusual selectivity changes during separation. Example chromatograms of metal separations are given for three selected complexing eluent reagents, namely, oxalic acid, picolinic acid, and chloride, either singly or in admixture. For special mention it was found that very specific retention control could be achieved for Cu(II) with picolinic acid, Fe(III) and Fe(II) speciation with oxalic acid, Pb with dipicolinic acid and Cd with chloride.