Determinazione del metilmercurio in sedimenti e biota d'acqua dolce mediante Analizzatore Automatico di mercurio e GC-MS

In freshwater ecosystems mercury is mainly adsorbed on sediments and it can be partially transformed into methylmercury (MeHg), which is more toxic than inorganic species of mercury and shows higher bioaccumulation capability. Thus, MeHg analysis can provide useful information for environmental risk assessment in mercury contaminated sites. The determination of MeHg in environmental matrices is often difficult due to low concentrations and to the presence of interfering substances. The aim of this work is to develop and compare two analytical protocols for the analysis of MeHg in freshwater organisms and sediments, by using an Automated Mercury Analyzer (AMA 254) and by GC-MS. The evaluation of the methods was carried out by analysis of certified reference materials and samples of riverine benthic invertebrates, as well as lacustrine fish and sediments. AMA 254 protocol involves a double liquid-liquid extraction with toluene and L-cysteine. GC-MS protocol is carried out by static headspace, using the internal standard isotopic dilution quantification method. Both methods showed a good performance, with recoveries of 86-102 % compared to certified values and with variation coefficients (CV) of 2.5-13.2 %. For natural samples, repeatability was good, with CVs below 20 %. The quantification limit calculated for AMA 254 protocol (21 µg MeHg kg-1) may be not adequate for the analysis of natural sediments, while GC-MS protocol proved to be more sensitive (LOQ = 1.4 µg MeHg kg-1). A preliminary comparison between methods showed potential agreement, even if GC-MS protocol generally showed higher recoveries than AMA 254 protocol.