Detecting mercury contamination in fish

Researchers from the University of Burgos (Spain) have developed a fluorescent polymer that lights up in contact with mercury that may be present in fish.

Due to mercury’s toxicity, consumers — and especially pregnant women — are very keen to minimise their exposure to the metal and are well aware that consumption of fish is by far the most significant source of ingestion-related mercury exposure in humans.

Some mercury in the environment comes from natural sources; however, in the last decades industrial waste has caused an increase in concentrations of the metal in the aquatic environments. In the food chain, mercury can be diluted either in organic form as methylmercury, (CH₃Hg)⁺ or as an inorganic salt, the cation Hg²⁺.

The concentration of mercury in any given fish depends on the species of fish, the age and size of the fish and the type of water body in which it is found. The concentration is usually greater in larger, older fish, such as tuna, walleye, largemouth bass and northern pike, and in fish-eating fish like shark, swordfish and marlin.

Mercury content is not reduced by processing techniques such as canning, freezing or cooking.

Now, researchers from the University of Burgos have created a fluorescent polymer, JG25, which can detect the presence of both forms of mercury 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,” explained Tomás Torroba, lead author of the paper, which has been published in the journal Chemical Communications.

A portable polymer probe, which can be used in situ, was used to apply the technique to 2 g samples from a range of fish species. The qualitative relationship between the mercury levels in fish and the increased fluorescence was verified using chemical analysis using ICP-mass spectroscopy.

The research showed that the larger the fish, the higher the levels of mercury: between 1–2 ppm for swordfish, tuna and dogfish, around 0.5 ppm in conger eels and 0.2 ppm in panga. No mercury was found in farmed salmon. These are large fish at the top of the food chain, but the metal is not present in captivity due to the lack of an industrial or natural source.