PERFORMANCE-CHARACTERISTICS OF DIFFUSION GRADIENTS IN THIN-FILMS FOR THE IN-SITU MEASUREMENT OF TRACE-METALS IN AQUEOUS-SOLUTION

The technique of diffusive gradients in thin films (DGT) provides an i n situ means of quantitatively measuring labile species in aqueous sys tems. By ensuring that transport of metal ions to an exchange resin is solely by free diffusion through a membrane, of known thickness, Delt a g, the concentration in the bulk solution, C-b, can be calculated fr om the measured mass in the resin, M, after time, t, by C-b = M Delta g/DAt, where D is the molecular diffusion coefficient and A is the exp osure surface area of the membrane. If a sufficiently thick (similar t o 1 mm) diffusion layer is selected, the flux of metal to the resin is independent of the hydrodynamics in solution above a threshold level of convection. Deployment for 1 day results in a concentration factor of similar to 300, allowing metals to be measured at extremely low lev els (4 pmol L(-1)). Only labile metal species are measured, the effect ive time window of typically 2 min being determined by the thickness o f the diffusion layer. Because metals are quantified by their kinetics of uptake rather than the attainment of equilibrium, any deployment t ime can be selected from 1 h to typically 3 months when the resin beco mes saturated. The measurement is independent of ionic strength (10 nM -1 M). For Chelex-100 as the resin, the measurement is independent of pH in the range of 5-8.3, but a subtheoretical response is obtained at pH <5 where binding to Chelex is diminished. The effect of temperatur e can be predicted from the known temperature dependence of the diffus ion coefficient and viscosity. The application of DGT to the in situ m easurement of Cd, Fe, Bin, and Cu in coastal and open seawater is demo nstrated, and its more general applicability as a pollution monitoring tool and for measuring an in situ flux, as a surrogate for bioavailab ility, is discussed.