A total of forty rice grain samples (10 samples each mine) were taken at the paddy field using stainless steel scissors. The samples were thoroughly washed in de-ionized water prepared by filtration of tap water through a Millipore-Q system (Millipore, France), dried in a clean room at 20 °C for 7 days, and then milled to a fine powder in a stainless steel electric grinder. The prepared grain samples were added to concentrated fuming nitric acid for the removal of organic compounds and then decomposed by aqua regia (3:1 mixing ratio of HCl and HNO3).