Effect of solution composition, flow and deployment time on the measurement of trace metals by the diffusive gradient in thin films technique

Diffusive gradients in thin films (DGT) is a recently developed technique c apable of in situ measurement of labile metal species in water. Tests of th e performance of DGT were extended in this work to cover a wide range of en vironmentally relevant conditions. DGT accurately measured Cd concentration s in 0.01 M NaNO3 solutions in the pH range 5-10 and Co, Mn and Zn concentr ations, between pH 3.5 and 10. Cu was measured accurately between pH 2 and 10. Above pH I I prolonged deployments were impractical due to swelling of the diffusive gel, but even at pH 12.9 significant fractions of the total C d and Cu concentrations were measured by DGT units deployed for a day or le ss. Measurements of trace metals by DGT were unaffected by the presence of Ca at concentrations found in hard water. Errors due to changes in flow vel ocity were less than 10% above a flow of 0.02 ms(-1), where the formation o f a diffusive boundary layer becomes negligible for diffusive gets of great er than or equal to0.7 mm thickness. Therefore, DGT devices prepared with t his diffusive gel thickness can confidently be used in most streams and riv ers. However, in quiescent solutions the DGT measurement may only be 50% of the true concentration, confirming the need in quiet waters for parallel d eployments of devices with different diffusive layer thickness. Quantitativ e DGT measurements of Cd over a 1 month period showed that neither the diff usive nor resin gel deteriorated with time. Collectively these results show that DGT behaves predictably over a wide range of solution conditions. For Cu it can be used in very acidic solutions. The upper pH limit for DGT mea surement is governed by the stability of the gel. For this gel composition the realistic limit is pH 11. (C) 2001 Elsevier Science B.V. All rights res erved.