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.