Diffusional characteristics of hydrogels used in DGT and DET techniques

Hydrogels are increasingly being used to control mass transport in the analytical determination of trace elements. Quantitative use of the techniques of diffusive gradients in thin-films (DGT) and voltammetry at gel covered microelectrodes relies on precise knowledge of the diffusion coefficients in the gels. Two simple procedures, one relying on establishing pseudo steady state (Fick's first law) and the other on a dynamic response (Fick's second law), have been used to measure the diffusion coefficients of trace metals and fulvic and humic substances in five different hydrogels. To obtain consistent results it was essential to treat gels with an electrolyte prior to measurement. The different procedures agreed to within 5% with each other and with measurements made by DGT. For polyacrylamide gels cross-linked with an agarose derivative (APA) and with Bis acrylamide at 5% monomer concentration (BPA2) and for pure agarose gel (AGE), there was no significant difference between the diffusion coefficients of trace metals measured in the gels and those reported for water. Bis cross-linked gels containing 15% monomer (BPA1) and the constrained gel (CGa) impeded the diffusion of metal ions compared to water. There was little evidence for the AGE gel impeding the diffusion of fulvic substances, but it appeared to increasingly restrict the diffusion of humic substances as molecular weight increased. There was clear restriction of fulvic and humic substances by the APA gel and much greater restriction by the constrained gel (CGa). The results are consistent with the known open structures of AGE and APA gels and the dependence of the structure of BPA gels on monomer concentration. The much greater retardation of humic and fulvic substances in the constrained gels compared to trace metals opens up the possibility of using gel techniques to discriminate organically and inorganically complexed metals in natural waters. (C) 1999 Elsevier Science B.V. All rights reserved.