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

Minet, Eddy and Goodhue, Robbie and Coxon, Catherine E. and Kalin, Robert M. and Meier-Augenstein, Wolfram (2011) Simplifying and improving the extraction of nitrate from freshwater for stable isotope analyses. Journal of Environmental Monitoring, 13 (7). pp. 2062-2066. ISSN 1464-0325 (https://doi.org/10.1039/c1em10289c)

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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 ‘ionexchange 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 mM or 25 mg L_1 NO3_) and low DOC (typically <417 mM 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.

ORCID iDs

Minet, Eddy, Goodhue, Robbie, Coxon, Catherine E., Kalin, Robert M. ORCID logoORCID: https://orcid.org/0000-0003-3768-3848 and Meier-Augenstein, Wolfram;
CORE (COnnecting REpositories)