To ensure the removal of organic impurities from the samples and thus prevent the interference in analysis, the samples were digested with concentrated Nitric acid.5 mL of conc. HNO3 was added to 100 mL of sampling water into the 250 mL conical flask then heated on a hot plate and evaporated till
20 mL was left. After cooling the flask again 5 mL of conc.HNO3 was added and heated the flask on the hot plate.