A group of researchers from Italy have proposed a very simple, highly selective method for the speciation analysis of methylmercury in seafood.
Mercury and its species pose a major risk to all organisms within our biosphere. Environmental behaviour, bioavailability, bioaccumulation and toxicity are species specific, with methylmercury showing higher toxicity than inorganic mercury. The general population is exposed to methylmercury through food consumption, mainly via seafood and rice. In order to address the human health risk through food consumption, methods for speciation analysis distinguishing between inorganic and organic forms have been accordingly developed since decades. Food samples are extracted usually by the aid of HCL or digested enzymatically under conditions preventing species interconversion. Species specific detection is achieved by chromatographic or non-chromatographic separation methods followed by highly sensitive atomic fluorescence spectrometry (AFS) or inductively coupled plasma mass spectrometry (ICP-MS). The use of such hyphenated systems is relatively costly, both related to purchase and running costs. In case that only two species have to be differentiated (binary speciation), the high separation power of chromatography may exceed the need, as is the case for inorganic mercury and methylmercury. Methylmercury is the only organic mercury species found in food samples. The new method:
A team of researchers from Italy have now applied the technique of frontal chromatography for the selective separation of methylmercury from seafood extracts. The method is based on the insertion of a low pressure column packed with a strong anionic exchanger in front of the ICP nebulizer, without the need of any chromatographic pump or injection valve (see figure 1).
Figure 1: FC-ICP-MS instrumental configuration for the determination of methylmercury in extracts of seafood samples
Methylmercury is selectively determined in 100 s by in-line blocking inorganic mercury on the column with a detection limit of 7.8 ng/kg in solution or 1.6 µg/Kg in biological tissue.
Eleven samples representing different trophic positions within the marine food web of the Gulf of Tadjoura were analysed using the proposed method: zooplankton, squid, oil-sardinella and different shark species.
Methylmercury extraction was performed by a fast procedure using 5 M HCl for 15 min assisted by ultrasound irradiation for sample masses between 50 and 250 mg. The resulting suspension was centrifuged at 4000 rpm for 10 min. and the supernatant was collected and diluted with ultrapure water. Sample solutions were spiked with lutetium solution as internal standard to correct for matrix effects. ICP-MS measurements were performed using a He-collision cell in kinetic energy discrimination (KED) mode. The high capacity of the column for retaining inorganic mercury was regenerated after 100 determinations by passing 2% nitric acid for 30 min. In order to reduce memory effects, thiourea was added online to the eluent from the column. Anyhow, samples containing more than 10 µg/kg MeHg should be diluted for avoiding extensive memory effects. The whole procedure was validated by using different certified reference materials from the marine food web. All analytical results were within the certified range.
The authors concluded that their method compared favourable with other existing methods when considering its simplicity and the high sample throughput.
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