Rice grain usually contains ∼14% moisture; therefore, the harvested rice was lyophilized at −45 °C and then pulverized prior to digestion. The pulverized brown rice was pre-digested in nitric acid (HNO3) as described by the EPA method (US Environmenal Protection Agency, 2007.) Briefly, 500 mg of the pulverized rice sample was added to 7 mL of 70% HNO3 and was kept at room temperature for 6 h. Then, 1 mL of H2O2 was added prior to the digestion, which was carried out using a microwave oven. The digestion temperature was gradually increased from 30 °C to 180 °C in 50 min, and the digestion was completed when the digested sample was colorless. Finally, the digested sample volume was adjusted to 50 mL with ultrapure distilled water. Blank samples were prepared in the same way.