Determination of methylmercury in rice using microwave-assisted extraction coupled with thermal decomposition amalgamation atomic absorption spectrometry (MAE-TDA-AAS)

Abstract

This work developed a method for rapid determination of methylmercury (MeHg) in rice, a staple cereal widely consumed in Asia, based on microwave-assisted extraction coupled with thermal decomposition amalgamation atomic absorption spectrometry (MAE-TDA-AAS). Orthogonal design was applied to optimize the parameters of MAE, and the optimal conditions were found to be as follows: 1.5 g of rice powder extracted using 12 mL toluene and 6 mL 30% HCl solution at 110 °C for 5 min under microwave irradiation. MeHg extracted into the organic phase (4.0 mL) was then back-extracted into 2.0 mL 1.5% L-cysteine solution, followed by quantitation by TDA-AAS on a direct mercury analyzer. The MeHg recovery rate of this method determined from a spiked rice sample was 80.8% (77.5–84.1%, 95% confidence limits), while those for MeHg in certified reference materials, TORT-2 and SRM-2976, were 97.6% (91.0–104.1%, 95% confidence limits) and 93.8% (90.4–97.2%, 95% confidence limits), respectively. The limit of detection and limit of quantification for MeHg in rice were 0.13 and 0.43 ng g⁻¹, respectively, and the relative standard deviation was below 5%. The extracts of 15 rice samples collected from two provinces in China were analyzed by both TDA-AAS and the well-developed high performance liquid chromatography coupled with inductively coupled plasma-mass spectrometry (HPLC-ICP-MS) method, and comparable results were obtained. Overall, these findings demonstrate that the MAE-TDA-AAS method developed in this study is a convenient and reliable method for quick determination of MeHg in rice and probably other food grains, and it is an order of magnitude more sensitive than the previous methods established for analysis of mercury species in fish and seafood.