Extensive grinding and pressurized extraction with water are key points for effective and species preserving extraction of arsenic from rice

Abstract

An adequate sample preparation is an essential prerequisite for an accurate assessment of exposure to arsenic (As) upon consumption of contaminated rice. Firstly, a well-defined amount of As must be released from the matrix following sample extraction. Secondly, given the toxicological importance of As species, the sample extraction procedure must preserve As speciation. We evaluated the effectiveness of closed and open microwave digestion procedures to extract As from a certified reference sample of rice and 3 commercial rice matrices. In addition, we investigated to what extent rice grain particle size after grinding, the ratio of rice over extraction liquid, hold time and temperature affect the release of different arsenic (As) species (As^III, As^V, and DMA^V) from rice samples. Particle size was found to have a major influence on arsenic extraction. Extraction efficiency of As was decreased to 75% when rice was treated as whole grain compared to powdered form. Extraction efficiency using microwave digestion in closed vessels was better than using microwave digestion in open vessels when the particle size was larger than 0.5 mm. For powdered samples, extraction efficiencies using both methods were similar. However, less time (30 min) was needed for complete extraction using microwave digestion in closed vessels compared to using microwave digestion in open vessels (180 min). The highest extraction efficiency for closed microwave digestion was obtained with powdered rice in 80 °C water at a liquid/solid ratio of 10 and a hold time of 30 min. The use of closed or open vessels during microwave digestion was indifferent to the speciation pattern. Extraction efficiencies of individual As species are affected by particle size to the same extent as that of total As, except for As^III. We concluded that closed microwave digestion of powdered sample under the proposed conditions is the most successful technique for species-preserving quantitative extraction of As species from rice.