Dynamic Flow-through Methods for Metal Fractionation in Environmental Solid Samples

Accummulation of metal ions in different compartments of the biosphere and their possible mobilization under changing environmental conditions induce a pertubation of the ecosystem and may cause adverse health effects. Nowadays, it is widely recognized that the information on total content of elements is quite insufficient to estimate their physicochemical mobility, potential bioavailability, and, consequently, toxicity.

Within the last two decades, batchwise equilibrium-based single or sequential extraction schemes have been consolidated as analytical tools for fractionation analyses to assess the ecotoxicological significance of metal ions in solid environmental samples. The background of end-over-end fractionation for releasing metal species bound to particular soil phases is initially discussed, its relevant features and limitations being thoroughly described.

However, taking into account that naturally occurring processes always take place under dynamic conditions, recent trends have been focused on the development of alternative flow-through dynamic methods aimed at mimicking environmental events more correctly than their classical extraction counterparts.

In this lecture particular emphasis is paid to the state-of-the-art and fundamental principles of automated, miniaturized strategies for on-line fractionation studies based on the concepts of stirring chambers, rotating coiled columns and packed microcartridges, mostly exploiting the attractive features of the first and second generations of flow injection analysis.

Selected examples are given to illustrate the unrivalled features and relevant information provided by these novel on-line approaches, that can be summarized as follows: (1) Monitoring of the kinetics of the ongoing leaching process for evaluation of the actual than potential trace metal availability; (2) Simplification of the overall procedure (from days to hours); (3) Automation of the extraction procedure and detection; (4) Minimization of risks of sample contamination; (5) Easy manipulation of reagents via Sequential Injection schemes; (6) Ability to implement a multitude of operational modes depending upon the kinetics of the targeted phases; and (7) Evaluation of the efficiency of extractants and the maximum pool of available fractions.