Robust ultrasound assisted extraction approach using dilute TMAH solutions for the speciation of mercury in fish and plant materials by cold vapour atomic absorption spectrometry (CVAAS)

Abstract

A simple and rapid ultrasound assisted extraction (UAE) protocol with dilute solutions of tetramethylammonium hydroxide (TMAH) for the speciation of mercury in fish and plant tissues was developed as an alternative to conventional methods which require intensive treatments. The main operational parameters, such as extractant concentration (TMAH), sonication time and amount of sample, were optimized using BCR ERM-CE 464 (tuna fish) and mercury loaded coriander powder, an in-house reference material, taken as representatives of fish and plant tissues, respectively. Quantitative extractions of the inorganic mercury (iHg) and methyl mercury (MeHg) species were obtained using 8 mL of 2% TMAH with a sonication time of 5 min for a sample weight of <0.5 g. After sonication, the supernatant obtained upon centrifugation was directly used for the determination of iHg by cold vapour atomic absorption spectrometry (CVAAS). Inorganic mercury was determined using SnCl₂ as a reducing agent, while total mercury was determined after the oxidation of methyl mercury (MeHg) with KMnO₄ solution. Amount of organic mercury, basically MeHg, was obtained by using the difference. The analytical results were in good agreement with the certified reference values of iHg, MeHg and total mercury at a 95% confidence level. The method was further validated through the analysis of additional certified reference materials: BCR CE-463 (tuna fish), IAEA-350 (fish homogenate), BCR-60 (aquatic plant Lagarosiphon major) and BCR-482 (lichen). The detection limit of the overall procedure was found to be 0.014 μg g⁻¹ for both inorganic and methyl mercury species.