Rapid and quantitative microwave-assisted recovery of methylmercury from standard reference sediments
Résumé
A simple and rapid method has been developed to determine methylmercury in sediments. The procedure is based on the quantitative microwave-assisted leaching of methylmercury from sediments with an acidic extradant. Sample preparation is achieved in an open focused microwave field by heating about 1 g of dry sediment suspended into 10 ml of 2 mol l-1 HNO3 during 3-4 min at a power of 40-60 W. The extracted mercury compounds are ethylated, cryogenically trapped in a chromatographic phase, successively eluted and detected in an electrothermally heated quartz furnace by CV AAS. The method has been validated by the analysis of the certified IAEA-356 and CRM 580 reference materials and one sediment sample, BCR S19, from an intercomparison study within the framework of a certification exercise sponsored by the Community Bureau of Reference (BCR) of the European Commission. The detection limit of the procedure is 0.5 ng of methylmercury per gram of dry sediment. After analysis of the sediments mentioned above (methylmercury ranging from 5 to 80 ng g-1), a mean methylmercury recovery of 97% and an RSD of 7% were obtained. Under optimum conditions, sample throughput is restricted by the instrumental analysis time (about 20 min per sample), rather than by the sample preparation step. In addition, the methylmercury extraction efficiency of four different acids, nitric, hydrochloric, sulfuric and acetic acid, has been investigated for comparison. Results on the behaviour and stability of methylmercury in a microwave field are also provided.