Quick and robust method for trace determination of MeHg in rice and rice products without derivatisation†
Recent studies of methylmercury (MeHg) in rice have shown that rice grown on mercury contaminated soil contributes to the human MeHg intake similar to a fish diet. Trace levels of MeHg in biological samples are often determined via a complex multi-stage process following EPA method 1630. We developed a simple and cost effective method suited for food quality monitoring based on a simple sample preparation procedure and the subsequent analysis of the sample with online preconcentration via solid phase extraction high performance liquid chromatography cold vapor atomic fluorescence spectrometry (SPE-HPLC-CV-AFS). The reliability of this method for MeHg in rice and rice products in the low ppb range was investigated for 4 different rice product samples. At present, no CRM for MeHg in rice or rice products is available. Therefore we cross-validated our method against standard addition and species-specific isotope dilution gas chromatography inductively coupled plasma mass spectrometry (SSID-GC-ICPMS), which showed no significant difference versus the external calibration with SPE-HPLC-CV-AFS. Potential species interconversion during sample preparation and measurement was ruled out by using a spike of isotopically enriched inorganic mercury. The SPE-HPLC-CV-AFS developed in our work has proven to be a robust, fast, cost efficient, competitive and reliable method for MeHg speciation in rice and rice products with a limit of detection of 0.12 µg kg−1 and a reproducibility comparable to the SS-ID-GC-ICPMS method which is sufficient for the determination of MeHg in the four market rice samples. The concentrations of MeHg ranged from 1.6 to 2.7 µg kg−1.