Effect of Preservation Techniques on the Determination of Nitrite in Freshwater Samples

摘要

In the view of accepted ideas about the preservation of nitrite in freshwater, this short note presents experiments questioning standard laboratories procedures and tries to be a caveat to experimenters looking for sole total nitrite values more than for total nitrate plus nitrite concentrations. To validate an adequate preservation technique for nitrite, we realized a series of three experiments investigating the effect of filtration, freezing and ageing on initial nitrite concentrations found in freshwater samples collected in an agricultural watershed. Experiment results demonstrate no adsorption or dilution of NO_2~-concentrations during filtration. Nonetheless, we recommend using filtration only when sample turbid- ity is visible at the eye to avoid any contamination with laboratory materials during manipulations. Furthermore, results also indicate that freezing samples induces a significant decrease of NO_2 concentrations whereas the long term storage at 4℃ of samples induces a smaller variability on nitrite concentrations. In consequence, we forbid the use of freezing as a preservation technique for nitrite analysis from freshwater samples and rather recommend storage of samples at 4℃ for a period of 48 h.