Optimization of Ultrasound-Assisted Extraction Combined with AFS by Response Surface Methodology for Rapid Determination of Trace Mercury in Tea Leaves
XIONG Chun-hong, PENG Kang-nian, XIE Ming-yong*
State Key Laboratory of Food Science and Technology, Nanchang University, Nanchang 330047, China
Abstract：A rapid ultrasound-assisted extraction (UAE) procedure was developed for the determination of trace mercury (Hg) in tea leaves combined with atomic fluorescence spectrometry. Three variables including sonication time (St), ultrasonic bath temperature (T) and HNO3∶H2O2 (1∶1，φ)volume (A2) showed the significant effect on extraction rate of total Hg evaluated by a Plackett-Burman design, and they were further optimized by a central composite design and response surface methodology. The results showed that the optimum extraction conditions were as follows: presonication time 6 min, St 8.1 min, T 70.5 ℃, A2 4.4 mL, and sample mass 300 mg. GBW10016 (tea leaves) was used as certified reference material; for comparative purposes, a microwave-assisted digestion (MAD) was used. The result obtained by optimized UAE method showed good agreement with the certified values. Under optimal conditions, recovery was evaluated to be 94.2%～102.0% and the limit of detection 0.007 8 μg·L-1. The relative standard deviation (RSD) of replicate measurements was generally less than 10%. The proposed UAE method was successfully applied to the determination of Hg in 63 samples of fresh tea leaves and 10 different branded tea samples. No significant differences were established between the analytical results of UAE method and MAD method. The Hg concentrations of them were found in the range of 4.6～17.3 μg·kg-1 on a dried basis, which were within the permissible limit of the NY659-2003.
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