Measurement of the oxygen isotopic composition of nitrate in seawater and freshwater using the denitrifier method

Analytical Chemistry
By: , and 

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Abstract

We report a novel method for measurement of the oxygen isotopic composition (18O/16O) of nitrate (NO3-) from both seawater and freshwater. The denitrifier method, based on the isotope ratio analysis of nitrous oxide generated from sample nitrate by cultured denitrifying bacteria, has been described elsewhere for its use in nitrogen isotope ratio (15N/14N) analysis of nitrate.1Here, we address the additional issues associated with 18O/16O analysis of nitrate by this approach, which include (1) the oxygen isotopic difference between the nitrate sample and the N2O analyte due to isotopic fractionation associated with the loss of oxygen atoms from nitrate and (2) the exchange of oxygen atoms with water during the conversion of nitrate to N2O. Experiments with 18O-labeled water indicate that water exchange contributes less than 10%, and frequently less than 3%, of the oxygen atoms in the N2O product for Pseudomonas aureofaciens. In addition, both oxygen isotope fractionation and oxygen atom exchange are consistent within a given batch of analyses. The analysis of appropriate isotopic reference materials can thus be used to correct the measured 18O/16O ratios of samples for both effects. This is the first method tested for 18O/16O analysis of nitrate in seawater. Benefits of this method, relative to published freshwater methods, include higher sensitivity (tested down to 10 nmol and 1 μM NO3-), lack of interference by other solutes, and ease of sample preparation.

Additional publication details

Publication type Article
Publication Subtype Journal Article
Title Measurement of the oxygen isotopic composition of nitrate in seawater and freshwater using the denitrifier method
Series title Analytical Chemistry
DOI 10.1021/ac020113w
Volume 74
Issue 19
Year Published 2002
Language English
Publisher American Chemical Society
Contributing office(s) Toxic Substances Hydrology Program
Description 8 p.
First page 4905
Last page 4912