Donnan effects in metal speciation analysis by DET/DGT.

Cross-linked polyacrylamide gels have found wide application in analytical techniques, such as diffusional equilibration in thin film (DET) and diffusive gradient in thin film (DGT). In these applications, the assumption is made that the gel matrix is effectively uncharged and chemically inert with respect to the species of interest. Recent data has shown significant nonideal behavior at low ionic strengths, which strongly suggests a finite structural charge within the gel matrix. The present investigation explores the possible ramifications of fixed charged sites within the gel on metal speciation analysis by DET and DGT. The results indicate that structural charge within the gel layer will give rise to different DET equilibrium concentrations than in the sample because of Donnan partitioning. The steady-state diffusion of ions through a diffusive gel (DGT) is also influenced by the gel charge, but the net effect will depend on the details of the speciation. The results indicate that for submillimolar ionic strength solutions the quantitative interpretation of DGT data benefits from combination with DET.