A digestion solution was prepared with 6 ml of 65% nitric acid (HNO3) (Carlo Erba) and 2 ml of 30% peroxide hydrogen (H2O2-Carlo Erba) over a 50 min operation cycle at 200 _C. After mineralization, the vessels were opened if a temperature < 25 _C was reached, Then the content was decanted in falcon tubes and ultra-pure water (Merck) was added to the samples up to 30 ml; for quantification of metals an ICP-MS Elan-DRC-e (Perkin– Elmer, USA) was used.