Trace metal speciation in brackish water using diffusive gradients in thin films and ultrafiltration: comparison of techniques.

Diffusive gradients in thin films (DGT) and ultrafiltration were used to measure trace metal concentrations in the Baltic Sea. The results provide the first comparison of these two fundamentally different speciation methods for trace metals. Cd, Cu, Mn, Ni, and Zn were measured at two sites with different total trace metal concentrations. DGT units prepared with APA-gel as diffusive layer and Chelex 100 resin as binding agent were used throughout the study. The ultrafiltration was performed with Millipore Prep/Scale modules with cutoffs of 1 and 10 kDa. Concentration levels of Mn, Zn, and Cd measured by DGT agreed with the concentrations measured in 1 kDa ultrafiltered samples. For Cu and Ni the ultrafiltered concentrations exceeded the DGT-labile concentrations. The ability of DGT to preconcentrate metals was found to be an analytical advantage compared with ultrafiltration. DGT appears to be a good alternative to 1 kDa ultrafiltration for measurement of truly dissolved Mn, Cd, and Zn in the Baltic Sea.