Role of ligand purity in separations of alkaline earth metals as arsenazo I complexes by capillary zone electrophoresis

In the separation of metal ions by capillary electrophoresis in the form of kinetically labile complexes formed through the incorporation of an auxiliary complexing ligand into the background electrolyte (BGE), it has been shown that the purity of this auxiliary complexing ligand can play a crucial role in the selectivity and efficiency of the resultant separations. Using the separation of alkaline earth metals as complexes with the metallochromic ligand arsenazo I as a model system, the effects of the addition of low concentrations of simulated impurities, in the form of various metal ions and competing ligands, were studied. Addition of Fe(III) at low micromolar levels to a BGE containing 1 mM arsenazo I resulted in severe peak tailing. The addition of the competing ligands diethylenetriaminepentaacetic acid (DTPA) or arsenazo III, at a molar ratio as low as 1:1000 to arsenazo I, also caused substantial peak broadening and altered the separation selectivity. The practical implications of the above results for the separation of metals as labile complexes, using capillary zone electrophoresis (CZE) systems similar to the above, are discussed.