The dried samples were then powdered. Accurately, 1 g
of dried samples were weighted and put in polyethylene vials, and
digested with a mixture of concentrated H2SO4 and HNO3 (1:3, v/
v). A small amount of 30% H2O2 was added to eliminate nitrogen
dioxides. The digestion process was continued until the solution
became clear. The samples were transferred into 500 mL beaker
and diluted to 300 mL with distilled water