A new isolation procedure of nitrate from freshwater for nitrogen and oxygen isotope analysis


B. Huber, Swiss Federal Research Institute WSL, Zürcherstrasse 111, 8903 Birmensdorf, Switzerland. E-mail: benjamin.huber@wsl.ch


The nitrogen (δ15N) and oxygen isotope (δ18O) analysis of nitrate (NO3) from aqueous samples can be used to determine nitrate sources and to study N transformation processes. For these purposes, several methods have been developed; however, none of them allows an accurate, fast and inexpensive analysis. Here, we present a new simple method for the isolation of nitrate, which is based on the different solubilities of inorganic salts in an acetone/hexane/water mixture. In this solvent, all major nitrate salts are soluble, whereas all other oxygen-bearing compounds such as most inorganic carbonates, sulfates, and phosphates are not. Nitrate is first concentrated by freeze-drying, dissolved in the ternary solvent and separated from insoluble compounds by centrifugation. Anhydrous barium nitrate is then precipitated in the supernatant solution by adding barium iodide. For δ18O analysis, dried Ba(NO3)2 samples are directly reduced in a high-temperature conversion system to CO and measured on-line using isotope ratio mass spectrometry (IRMS). For δ15N analysis, samples are combusted in an elemental analyzer (EA) coupled to an IRMS system. The method has been tested down to 20 µmol NO3 with a reproducibility (1SD) of 0.1‰ for nitrogen and 0.2–0.4‰ for oxygen isotopes. For nitrogen we observed a small consistent 15N enrichment of +0.2‰, probably due to an incomplete precipitation process and, for oxygen, a correction for the incorporation of water in the precipitated Ba(NO3)2 has to be applied. Apart from being robust, this method is highly efficient and low in cost. Copyright © 2011 John Wiley & Sons, Ltd.