Speciation of trace mercury impurities in fish oil supplements

•Hg++/MeHg+ speciation by photochemical vapor generation at 311 nm vs. 254 nm.•Hg++ and MeHg+ averaged 0.67 ± 0.45 and 1.1 ± 1.3 ng mL−1 in fish oil.•Method validation by ICP-MS.•Low LODs: 0.50 ng mL−1 for Hg++ and 0.63 ng mL−1 for MeHg+.

Fish oil supplement is becoming increasingly popular worldwide because of beneficial long-chain omega-3 polyunsaturated fatty acids. However, mercury (Hg) impurity causes considerable concern because of its toxicity and bioaccumulation in the food chain. In this work, Hg impurities were extracted from fish oil by liquid-liquid partitioning. The sample solution was then mixed with a reductant (0.4% anthranilic acid-20% formic acid) and sequentially exposed to 311 and 254 nm UV radiation. The resulting Hg0 vapor was detected by atomic fluorescence spectrometry. Speciation was fulfilled by solving a set of two linear equations. Recovery of MeHg+ was 73%; total Hg was validated by ICP-MS. This method achieved 0.50 and 0.63 ng mL−1 limits of detection for Hg++ and MeHg+, respectively. Average Hg++ and MeHg+ contents in fish oil samples (n = 38), 0.67 ± 0.45 and 1.1 ± 1.3 ng mL−1, respectively, were 2-3 orders of magnitude lower than those in fish.

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