Certification of methylmercury in sediment: from controversial facts to scientific evidence

Techniques used for the determination of methylmercury (MeHg) in sediment generally involve a succession of analytical steps (extraction, derivatisation, separation, detection) which may all be prone to systematic errors. A programme to evaluate and possibly improve the state-of-the-art of MeHg determination in sediment has been launched by the Standards, Measurements and Testing Programme (formerly BCR) of the European Commission, which was continued by a successful certification of this compound in a sediment reference material (CRM 580). However, controversial discussions occurred with respect to the certified McHg contents in sediment reference materials owing to the suspicion of artefact McHg formation in some analytical procedures (in particular distillation-based methods). This paper gives an account of the methods used in the certification of CRM 580 and responds to the doubts expressed. A workshop organised with world wide experts to discuss this problem (this issue) concluded that available CRMs are fit-for-purpose for ensuring comparability of McHg data with respect to the present state-of-the-art.