sample
was weighed into a PTFE vessel and 2 mL of concentrated
HNO3, 0.5 ml of concentrated HCl, 2 mL of 30% H2
O2 and
5.5 mL of H2
O were added. Samples were digested simultaneously with the optimized microwave digestion system. After
digestion, the solution was transferred into a volumetric flask
and made up to 25 mL with ultra-pure water. Concentrations
of mineral elements in Oggtt were determined with ICP–OES.