Evaluation of paper-based diffusive gradients in thin film samplers for trace metal sampling

Abstract

Diffusive gradients in thin films (DGT) samplers rely on diffusion of metals through a permeable diffusion layer of known thickness for in situ measurement of trace metals in natural waters, with the principal diffusion layer used to date being based on polyacrylamide hydrogels. Alternative diffusion layers based on commercially available chromatography paper have been investigated and applied as a diffusion layer in DGT samplers, with Cd diffusion coefficients being within 20% of the value in polyacrylamide hydrogels. Cd diffusion coefficients were, however, found to increase with decreasing ionic strength at ionic strengths ≤5 × 10 −4 mol L −1 NaNO 3, but unlike the polyacrylamide diffusion layer the chromatography paper showed reproducible behaviour at these low ionic strengths. Samples of the chromatography paper equilibrated with various ionic strength solutions containing Cd showed that the Cd was weakly bound by the paper, providing a potential explanation for the enhanced diffusion. The chromatography paper diffusion layer was also tested with a commercially available phosphate ion exchange binding phase where diffusion coefficients were determined for Cd, Cu, Pb, Zn, Co, Ni and Mn. The paper-based DGT samplers were found to be easier to prepare and handle than the polyacrylamide hydrogel based samplers, making them ideal for application as a routine monitoring technique.