Three different mixtures of reagents using HNO3+H2O2 were tested.Mixture I is 5 mL HNO3 (50:50 v/v),mixture II 5mL HNO3 (25:75 v/v), and mixture III (5 mL HNO3 (25:75 v/v) with 1 mL H2O2 1 mol/L). The resulting clear solutions of the digestion procedure were then brought to 10 mL with ultrapure water.