Simplifying and improving the extraction of nitrate from freshwater for stable isotope analyses

Abstract

Determining the isotopic composition of nitrate (NO₃⁻) in water can prove useful to identify NO₃⁻ sources and to understand its dynamics in aquatic systems. Among the procedures available, the ‘ion-exchange resin method’ involves extracting NO₃⁻ from freshwater and converting it into solid silver nitrate (AgNO₃), which is then analysed for ¹⁵N/¹⁴N and ¹⁸O/¹⁶O ratios. This study describes a simplified methodology where water was not pre-treated to remove dissolved organic carbon (DOC) or barium cations (added to precipitate O-bearing contaminants), which suited samples with high NO₃⁻ (≥400 μM or 25 mg L⁻¹NO₃⁻) and low DOC (typically <417 μM of C or 5 mg L⁻¹ C) levels. % N analysis revealed that a few AgNO₃ samples were of low purity (compared with expected % N of 8.2), highlighting the necessity to introduce quality control/quality assurance procedures for silver nitrate prepared from field water samples. Recommendations are then made to monitor % N together with % O (expected at 28.6, i.e. 3.5 fold % N) in AgNO₃ in order to better assess the type and gravity of the contamination as well as to identify potentially unreliable data.