spectroscopyNOW Blog-Mercury lights up swordfish: UV fluorophore

Organic vs inorganic

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A fluorescent polymer that lights up when it comes

into contact with mercury has been developed to test levels of the metal in edible fish, such as tuna and swordfish, according to researchers in Spain.

Researchers from the University of Burgos created the fluorescent polymer, JG25 to detect two forms of mercury in fish samples; details were published recently in the journal Chemical Communications. In the food chain, mercury can be present as the organic form methylmercury (MeHg+) or as inorganic salts, in the cationic Hg2+ form. Numerous studies have discussed the problems of exposure to methylmercury and one in particular recently linked high levels of mercury in swordfish and tuna to problems with foetal development and placental growth in women eating these fish during pregnancy.

A portable polymer probe that responds to ultraviolet radiation can be used as a simple tool for assessing mercury concentrations in fish samples. "The polymer remains in contact with samples extracted directly from the fish for around 20 minutes. Then, while is being irradiated with ultraviolet light, it emits a bluish light, which varies in intensity proportionally to the quantity of methylmercury and inorganic mercury present in the fish," explains lead author Tomás Torroba. The qualitative relationship between the mercury levels in fish and the increased fluorescence was validated using ICP mass spectrometry, the team reports.

Follow the leader

Fundamentally, the study showed that the larger the fish the higher the levels of mercury: between 1.0 and 2.0 parts per million (ppm) in swordfish, tuna and dogfish, and approximately 0.5 ppm in conger eel and 0.2 ppm in panga. The team found no mercury in their samples of farmed salmon, despite these creatures being high up the food chain. But, unlike their wild counterparts they are presumably not exposed to the range of oceanic environments where industrial or natural sources of mercury pollution exist.

The team explains that the toxicity of a given piece of fish also depends on the form in which any mercury is present. The European Food Safety Authority (EFSA) suggests that a tolerable weekly intake of methylmercury is no more than one serving containing amounts over 1.6 of micrograms per kilogram of fish or 4 micrograms per kilogram of inorganic mercury. This concentration is similar to that seen in Burgos team's work. That said, there is currently lobbying of the regulatory authorities to lower these safety thresholds as new evidence emerges regarding the risks to the unborn child associated with exposure to even lower levels of the metal. Viz, the US Food and Drug Administration, the FDA, now suggests that consumers should eat no more than one portion a week of any fish containing more than one microgram per kilogram of mercury; other countries are likely to follow this lead.

Contamination and cognition

"Contamination of above 0.5 ppm in a food is already thought to be a considerable level," Torroba explains. "Several of the fresh tuna and swordfish samples we analysed exceed and even double this amount. This is why experts recommend that pregnant women reduce their weekly intake of certain types of fish, such as swordfish, due to possible risks to the foetus."

Research from the Foundation for the Promotion of Health and Biomedical Research of the Community of Valencia (FISABIO) and the Spanish Consortium for Research on Epidemiology and Public Health (CIBERESP) has recently shown that there is an association between prenatal mercury exposure and reduced placenta size and foetal growth. This is one of the largest studies of its type carried out to date. The findings, published in the journal Environmental Research show a relatively high average mercury concentration in umbilical cord blood (8.2 micrograms per litre), with a 24% of samples exceeding the WHO's provisional tolerable weekly intake equivalent. Although the magnitude of the potential effects is small, they are nevertheless important risk factors for a range of developmental problems and issues in adulthood, such as lower IQ and high blood pressure.