Mercury stock standard solution (1000 mg L1) was prepared by dissolving 0.0677 g of HgCl2 in the acid mixture HNO3–H2SO4–HClO3 (1:5:1) in a 50 mL digestion flask with heating on a hot plate at a temperature 200 ± 5 C until the solution became clear. The solution was then diluted to 50 mL with water