(1:1, v/v) was added to each tube and kept for 10 min at room temperature then close the tubes and placed in covered PTFE container. It was then heated following a one-stage digestion programmed at 80% of total power (900 W), 2–4 min were required for complete dissolution of all samples. The digestion tubes were cooled, and the resulting solution was diluted up to 10 mL with 0.2 M HCl. The prepared samples were stored in Teflon tubes as master sample solutions.