Simultaneous preparation of N2 and CO2 from water nitrates for δ15N and δ18O analysis on the example of the Zemborzycki Reservoir studies

Abstract

Nitrates are bioavailable compounds, which are necessary for every living organisms. However, their excess could be harmful, because it could lead to, i.e., methemoglobinemia, formation of carcinogenic nitrosamines and reservoir eutrophication. Isotopic analysis of δ¹⁵N and δ¹⁸O in nitrates allows to distinguish their different sources. To simplify the way of nitrate source monitoring, we developed a less complex and cheap "off-line" method for nitrate isotopic analysis, in which we obtain both gases required for isotopic analysis of NO₃^-, N₂ for δ¹⁵N and CO₂ for δ¹⁸O measurements, in one procedure. The developed method is based on simultaneous conversion of AgNO₃ (mixed with C and placed into PtIr boat) to N₂ and CO₂ in a glass vacuum apparatus. A small amount of sample (8 mg of AgNO₃) required for this procedure and a good reproducibility of the isotopic analysis (better than 0.3‰ for δ¹⁸O and 1‰ for δ¹⁵N) make this method a useful tool for environmental research. Using the developed method, we analyzed the isotopic composition of N and O in nitrates in the Zemborzycki Reservoir near Lublin (Poland), in 2015. The results indicate a large variation of δ¹⁵N (from -1.4 to +37.7‰) and δ¹⁸O (from +5.5 to +25.9‰) values in NO₃^- ions, while the concentration of NO₃^- varied from 0.2 to 15.3 mg/dm³. These preliminary results suggest that nitrate fertilizers probably are the main source of NO₃^- pollution. Our study points out the input of manure and/or sewage to the analyzed reservoir in the subsequent periods of sample collection.

Keywords: Eutrophication; Isotopic analysis; Nitrates; the Zemborzycki Reservoir; δ(15)N; δ(18)O.