AF₄-MALS-SP ICP-ToF-MS analysis gives insight into nature of HgSe nanoparticles formed by cetaceans

Cetaceans are known to accumulate very high levels of mercury (Hg) and unlike fish species, most of the mercury is not present as methylmercury (MeHg) but instead as inorganic Hg. It has been suggested this is a result of a detoxification process which involves the sequestering of Hg by Se-containing proteins, concluding with the formation of HgSe nanoparticles. However, their elemental compositions besides Hg and Se, as well as their genesis are unknown. We applied asymmetric flow-field flow fractionation-multiangle laser light scattering (AF4-MALS) coupled to single particle inductively coupled plasma-time of flight-mass spectrometry (SP ICP-ToF-MS) for the size-dependant separation, counting and further elemental characterisation of Hg/Se-containing nanoparticles in the liver and cerebellum of a sperm whale. Using a proteinase K digestion method, we isolated and identified particles which had size-independent masses of Hg and as well as a continuous molar ratio of Hg to Se. We also identified Cd and Sn on particles alongside Hg and Se, which indicated the detoxification process resulting in Hg/Se nanoparticles may not be specific to MeHg. This data indicates nanoclusters of toxic elements, bound to selenium, make up a nanoparticle core which is surrounded by a larger non-metal(loid) corona.