Comparison of Analytical Techniques for Dynamic Trace Metal Speciation in Natural Freshwaters

Several techniques for speciation analysis of Cu, Zn, Cd, Pb, and Ni are used in freshwater systems and compared with respect to their performance and to the metal species detected. The analytical techniques comprise the following: (i) diffusion gradients in thin-film gels (DGT); (ii) gel integrated microelectrodes combined to voltammetric in situ profiling system (GIME−VIP); (iii) stripping chronopotentiometry (SCP); (iv) flow-through and hollow fiber permeation liquid membranes (FTPLM and HFPLM); (v) Donnan membrane technique (DMT); (vi) competitive ligand-exchange/stripping voltammetry (CLE−SV). All methods could be used both under hardwater and under softwater conditions, although in some cases problems with detection limits were encountered at the low total concentrations. The detected Cu, Cd, and Pb concentrations decreased in the order DGT ≥ GIME−VIP ≥ FTPLM ≥ HFPLM ≈ DMT (>CLE−SV for Cd), detected Zn decreased as DGT ≥ GIME−VIP and Ni as DGT > DMT, in agreement with the known dynamic features of these techniques. Techniques involving in situ measurements (GIME−VIP) or in situ exposure (DGT, DMT, and HFPLM) appear to be appropriate in avoiding artifacts which may occur during sampling and sample handling.