Mercury speciation and nanoparticle analysis in foods from the Danish market

Mercury toxicity depends strongly on chemical form, yet dietary exposure assessments and regulatory monitoring are typically based on total mercury (THg). This study evaluated inorganic mercury (iHg), methylmercury (MeHg), THg, selenium (Se), and Hg- and Se-containing nanoparticles (NPs) in 44 seafood and 10 non-seafood products from the Danish market. A rapid reversed-phase HPLC-ICP-MS method was validated in-house and applied for Hg speciation, providing baseline separation of iHg and MeHg within 3.5 min. MeHg recoveries in certified reference materials were 94-101%, within-laboratory reproducibility was ≤12.3%, and the limit of quantification (LOQ) for fresh seafood samples was 0.29 µg kg^-1 wet weight. The sum of speciated Hg (∑Hg = iHg + MeHg) agreed with acid-digestion THg within 80-105% across seafood samples. In fish and crustaceans, MeHg was the predominant Hg species, accounting for approximately 90-94% of THg, supporting the use of THg as a practical proxy for MeHg exposure in these categories. In contrast, MeHg accounted for only 22.6-44.5% of THg in mussels and was below the LOQ in all non-seafood matrices except one pig liver sample, demonstrating that fixed species-proportion assumptions may overestimate MeHg exposure in some food categories. The highest THg concentration was observed in common dab (Limanda limanda) at 0.33 mg kg^-1 wet weight, slightly above the 0.30 mg kg^-1 EU maximum level applicable to this species group under Commission Regulation (EU) 2023/915. Hg-containing NPs represented approximately 1% of the iHg fraction within the detectable particle size range across analysed samples, indicating a minor contribution to the measured iHg pool.