Milk, banana, potato, carrot and soybean samples were purchased from local market. 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.