In situ measurement of trace metals in lakewater using the dialysis and DGT techniques

The technique of diffusion gradients in thin films (DCT) is designed to mea sure in situ labile trace metals; in a DGT device, the metal species are tr apped in a resin gel comprising a Chelex resin in its Na-form after their d iffusion across a diffusion layer comprising a polyacrylamide hydrogel. We deployed DCT and dialysis samplers (peepers) in the water column of the oli gotrophic and acidic Lake Tantare, Que. Canada. Contrary to expectation, co ncentrations of Cd and Ni measured by DCT in lake water were consistently h igher than those measured by in situ dialysis. We show that co-diffusion of other ions through the diffusion layer, not taken into account explicitly in the original DGT theory, explains our field observations. When the DGT d evices are deployed in a water of low major cation concentrations (Sigma[ca tions] less than or equal to 2 x 10(-4) M), a steep negative concentration gradient of Nai. across the diffusive layer as well as small positive conce ntration gradients of the other cations develop. The result is an increase in the effective diffusion coefficients of cations entering the DGT sampler and a decrease in the diffusion coefficient of Nat leaving the DGT device to fulfill electroneutrality. Non-steady-state conditions during the deploy ment time of the DGT devices make the prediction of these effects impractic able. We suggest that DGT devices, as presently designed, are not useful fo r measuring trace metal species in lakes of low cation concentrations. Such lakes are numerous in northern North America and in Scandinavia. Electrica l effects on the diffusion coefficients were not observed in other studies using DGT samplers deployed in surface waters with major cation concentrati ons higher than 2 x 10(-4) M. (C) 2000 Elsevier Science B.V. All rights res erved.