Potato and carrot were washed with tap water several times and ultra pure water in turn. Potato, banana, carrot and soybean were dried at 110 C and ground into fine power, respectively. Thereafter, 10 mL of milk or 1 g of potato, banana, carrot or soybean samples were placed into 250 mL beaker, and 5 mL of 0.5 mol L1 nitric acid were added to moisten the samples thoroughly. It was followed by addition of 10 mL concentrated nitric acid and heating on a hotplate at about 130 C for 3 h in fume hood. After cooling to room temperature, 5 mL concentrated perchloric acid was added dropwise. The mixture was heated gently until completion of sample decomposition. The above procedures were repeated until resulting in a clear solution and pH of final solution was maintained at about 3. The solution was left to cool and filtered, then transferred into a 100 mL volumetric flask and made up to the mark with ultra pure water.