news NEWS FROM THE EUROPEAN RESEARCH CONFERENCE ON NATURAL WATERS AND WATER TECHNOLOGY Raewyn M. Town reports from Castelvecchio Pascoli, Italy.
Bioavailability of metals The development of techniques for measuring the bioavailable metal fraction is of great interest for environmental regulation. Rational design of such monitoring systems requires a detailed understanding of both the mode of operation of the experimental technique and the biological response mechanisms. To obtain the full picture, equilibrium and dynamic aspects of trace metal speciation must be addressed from both chemical and biological perspectives. The chemists’ perspective. Techniques capable of responding to the bioavailable metal fraction should have a mode of operation that is analogous to the uptake of metals by biota. This implies a dynamic measurement system, involving diffusion of metal species to an interface, dissociation of complexes within a diffusive layer boundary, and detection of the resulting metal ion concentration. Furthermore, very low (environmentally relevant) concentrations must be detectable, which typically requires incorporation of a preconcentration step. Several approaches were presented that met these criteria. Jacques Buffle’s group at the University of Geneva (Switzerland) is working on anodic stripping voltammetry (ASV) measurements with gel-integrated microelectrodes. ASV has an inherent preconcentration step. The gel layer protects the electrode surface from adsorption of organic matter, and the microelectrode geometry allows measurements in fluorescence is unmodulated and the anasitu in low lyte signal is modulated, synchronous ionic strength demodulation with a lock-in amplifier freshwaters extracts the analyte signal from the backand on permeground. Detection limits on the order ation liquid of 10 µM were obtained for fluorescein membranes using this method. The researchers used (PLMs). In a high-voltage amplifier in the current PLMs, a flux setup but say they hope to reduce costs develops even further by replacing it with a DC through the power supply. membrane, Another problem with thermoplastics the permeabilis that, just like glass, they adsorb cations ity of which onto their surface. This is particularly can be varied troublesome for the separation of proto mimic bioteins. Wang and Morris have solved this logical interproblem, too. They report that an alkafaces. Hao line wash of < 0.1 mM polylysine conZhang and verts the carboxylic acids to carboxylates, Peter Teasdale resulting in a positively charged surface of Bill suitable for protein work.
Davison’s group at the University of Lancaster (U.K.) described the diffusive gradients in thin-film methodology. This is an adaptation of the traditional Chelex-labile measurement, in which a diffusive gradient is established within a gel layer placed over the ion-exchange resin. Gels of different pore sizes can be deployed, allowing size-based discrimination. Detailed understanding of the species that are detected by these experimental methods requires developing a theory to describe the processes occurring during measurements. The dynamic principles involved in voltammetric and PLM techniques were elucidated by Herman van Leeuwen of Wageningen University (The Netherlands). He highlighted how, analogous to the dynamic biouptake situation, kinetically controlled fluxes of free metal are generated during measurements with these techniques. Equilibrium speciation models were also presented by David Turner of Göteborg University (Sweden) and Ed Tipping of the Institute of Freshwater Ecology, Windermere (U.K.). This approach, however, remains largely empirical and is limited by lack of knowledge of both the appropriate species that should be included and their associated equilibrium formation constants. Although there are good models of systems, difficulties remain in the presence of significant amounts of organic matter. The biologists’ perspective. Progress in developing experimental techniques for monitoring bioavailable metal species also requires a detailed knowledge of biological uptake mechanisms. Traditionally, an equilibrium approach has been used, with the free metal ion concentration thought to be the best predictor of toxicity. However, this model does not hold in all situations. Joe Meyer of the University of Wyoming uses a model that offers better prediction because it extends the equilibrium concept to include terms for the competition of cations for binding to sites on the biological membrane. Any approach based solely on equilibrium is, however, likely to be too simplistic, and more detailed understanding of the processes occurring in biological systems requires the development of dynamic models that consider kinetic factors. Ronny Blust of the University of Antwerp (Belgium) detailed how functional characterization of biological membrane transport systems is key to understanding the mechanistic basis for the link between metal ion speciation and biotic metal uptake. By working with a dynamic model that accounts for the kinetics of membrane transport and internal complexation, the transport of metals through the biological membrane was found to be the rate-limiting step.
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