Dynamic flow-through approaches for metal fractionation in environmentally relevant solid samples

In recent 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. However, taking into account that naturally occurring processes always take place under dynamic conditions, recent trends have been focused on the development of alternative methods aimed at mimicking environmental events more correctly than their classical extraction counterparts. The present review details the state-of-the-art and the fundamental principles of automated, miniaturized strategies for on-line metal fractionation studies in solid substrates based on the concepts of stirring chambers, rotating coiled columns and packed microcartridges, mostly exploiting the attractive features of continuous-flow analysis, and of the first and second generations of flow-injection analysis. Special attention is also paid to a novel, robust, non-invasive approach for on-site continuous sampling of soil solutions, capitalizing on flow-through microdialysis, which presents itself as an appealing complementary approach to the conventional lysimeter experiments. In addition, a critical comparison between the time-resolved analytical information provided by these novel dynamic extraction procedures and that of the traditional schemes is given and thoroughly discussed in the bulk of the text.