2.5. Sample preparation
Two samples of retail rice (sticky brown and Jasmine white rice) and two samples of rice products (wheat rice germ and brown rice milk) were purchased from a local supermarket and analyzed. The samples were digested by a HNO3eHClO4 digestion method. The solid components (sticky brown rice, Jasmine white rice and wheat rice germ) were ground and dehydrated in an oven at 105 _ 0.2 _C for 14 h. Then 0.5 g portions of above sample were digested with 1.5 cm3 of concentrated HNO3 and 1.5 cm3 of concentrated HClO4. The mixture was heated to boiling on a hot plate and evaporated to less than 2 cm3, whereupon 5 cm3 of 0.5 mol dm_3 HCl was added and heated until a clear solution was obtained. After that the solution was filtered through a 0.45 mm membrane filter and the filtrate volume was made up to 10 cm3 at 25 _C with 0.5 mol dm_3 HCl. For brown rice milk, an aqueous sample, 0.5 cm3was digested with HNO3eHClO4 following the same procedure as the solid samples above. Then, the standard addition method was used to determine the quantity of Pb2 and Cd2 in the digested samples. To validate the method, the results obtained by this method were compared with those obtained by ICP-OES.