Disequilibrium effects in metal speciation by capillary electrophoresisinductively coupled plasma mass spectrometry (CE-ICP-MS); theory, simulations and experiments

Abstract

A theoretical–experimental approach to evaluate disequilibrium effects in capillary electrophoresis inductively coupled plasma mass spectrometry (CE-ICP-MS) is presented. Electrophoresis requires metal ligand (ML) complexes to be stable on the time scale of separation and detection. By expressing ML complex stability in terms of half-life during a CE separation, an evaluation of separation artifacts can be made. Kinetically slow metals like Cr, Al or Fe form complexes that are stable on the time scale of electrophoretic separations. Kinetically fast metals, like Pb, Hg, Cu, Cd and REE, however tend to form labile complexes which unless complexed by strong chelators will dissociate during CE separations. A reactive transport simulation model of CE separations involving ML complexes allows a more detailed prediction of disequilibrium bias and identifies kinetically limited from mobility-limited types of dissociation. Complementary experimental results are given for kinetic and equilibrium binding experiments of Sm with humic acid. The equilibrium logK for Sm–Leonardite humic acid (HA) binding at pH 7 and 0.01 mol L⁻¹ ionic strength was determined to be 13.04. Kinetic rates of formation and dissociation for SmHA were 5.9 10⁸ and 5.3 10⁻⁵ mol s⁻¹.