The rice sample was obtained from a Surin Provincial source,
Thailand. The samples were digested by an acid digestion method
[29]. A 0.5 g of rice sample was added in the mixing between
concentrated nitric acid and concentrated perchloric acid in the
ratio 1:1 (v/v) and heated to 150 C and stirred for 4 h. The solution
was evaporated to less than 2mL of volume. Sequentially, the
concentrated hydrogen peroxide was added dropwise under
heating until the solution was colorless after the solution was
evaporated. Finally, the sample solution was filtered through a
0.45mm membrane filter.