Simultaneous measurements of cations and anions using diffusive gradients in thin films with a ZrO-Chelex mixed binding layer

A new method for the simultaneous measurements of eight cations [Fe(II), Mn(II), Co(II), Ni(II), Cu(II), Zn(II), Cd(II), and Pb(II)] and eight oxyanions [P(V), V(V), Cr(VI), As(III)/As(V), Se(VI), Mo(VI), Sb(V), and W(VI)] was established in this study using the diffusive gradients in thin films (DGT) technique with an improved ZrO-Chelex binding gel. The binding kinetics of each analyte in a mixed solution indicated a rapid uptake on the ZrO-Chelex gel. The anions exhibited slower absorption rates compared to the cations, which did not affect the DGT uptake. Stable elution efficiencies for these cations and anions were obtained based on a two-step extraction procedure using 1.0 M HNO₃ for elution of the cations followed by a mixed solution containing 0.2 M NaOH and 0.5 M H₂O₂ for elution of the anions. Simultaneous measurements of the cations and anions with the ZrO-Chelex DGT were validated in mixed solutions and found to be independent of solution pH (5–9) and ionic strength (2–3 mM to 750 mM). The capacities of the ZrO-Chelex DGT for single measurements of Fe(II), P(V), Cd(II), As(III), and As(V) were determined at 99.5, 127.6, 301.2, 33.3, and 87.4 μg cm⁻². Except for Se(VI) in seawater, accurate measurements of all of these cations and anions in natural freshwater and seawater were obtained using the ZrO-Chelex DGT. The performance of this technique for simultaneous measurements of 16 elements was also tested in sediments.