Determination of methylmercury and inorganic mercury in shark fillets

Three samples of deep-frozen shark fillets were analysed according to the following procedure: dissolution in tetramethylammonium hydroxide, derivatization/ethylation with sodium tetraethylborate, extraction into iso-octane and measurement with gas chromatography hyphenated to inductively coupled plasma mass spectrometry (GC-ICPMS) for the identification and quantification of methylmercury (MeHg⁺) and inorganic mercury (Hg²⁺). For the correction of procedural errors, two internal standards were used. The sample pretreatment was corrected by spiking with dibutyldipentyltin (DBT-pe), and the GC-ICPMS measurements were controlled by the signal stability of xenon which was added to the GC carrier gas. Furthermore, for comparisons, the total amount of mercury was determined by an independent technique, i.e. atomic fluorescence spectroscopy. Standard reference materials, which are only available in the form of lyophilisates and not as fresh fish materials, were also analysed to ensure the procedural quality control. The concentration range of total mercury measured in the shark fillets was between 0.9 and 3.6 μg mercury per gram thawed-out shark fillet. Two samples contained higher concentrations than the European legislation. Speciation analysis leads to ≥94% mercury bound as MeHg⁺. Since MeHg⁺ is known as one of the most toxic compounds, this conclusion is of importance with respect to bioaccumulation of mercury species via fish into the human food chain.