Analysis of copper complex lability using [formula omitted]-equilibration techniques and free-ion selective radiotracer extraction

Abstract

A novel analytical approach was applied to a conventional free-ion selective extraction technique in order to determine the lability of five copper complexes, viz. Cu–CDTA, Cu–EDTA, Cu–NTA, Cu–DTPA, and Cu–humic acid. The main purpose of this approach was to visualise dissociation kinetics of the copper complexes and to exclude reactions other than complex dissociation from the visualisation process. Therefore, three major modifications were applied to a conventional free-ion extraction with Chelex-100: (a) free-ligand concentrations were minimised, (b) the mobile phase as well as column material were provided with a surplus of free copper ions, and (c) the sample under study was equilibrated with a negligible mass of a high-specific activity 64 Cu radiotracer. The actual determination of copper complex lability was carried out in two different ways, either directly, by following dissociation reactions, or indirectly, by following association reactions. The two procedures were given different operationally defined detection windows, to maximise dissociation rate constant determinations. In a region where both detection windows overlapped, similar dissociation rate constants were found for the same copper species. Two species could be distinguished kinetically for Cu–CDTA, Cu–EDTA, as well as Cu–NTA. The dissociation rate constants of these two species differed about two orders of magnitude. The two main chemical forms of Cu–DTPA (viz. a 2:1 and 1:1 complex), predicted by speciation calculations, were found as well and equilibrium concentrations of both species were in accordance with theoretical values. Four species of the Cu–humic acid complex could be identified, with dissociation rate constants in a range of three orders of magnitude.