Chemical Conversion of Nitrate and Nitrite to Nitrous Oxide for Nitrogen and Oxygen Isotopic Analysis in Freshwater and Seawater

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Abstract
We present a novel method for nitrogen and oxygen natural isotopic abundance analysis of nitrate and nitrite of seawater and freshwater at environmental concentrations. The method involves the reduction of nitrate to nitrite using spongy cadmium with further reduction to nitrous oxide using sodium azide in an acetic acid buffer. For separate nitrite analysis, the cadmium reduction step is simply bypassed. Nitrous oxide is purged from the water sample and trapped cryogenically using an automated system with subsequent release into a gas chromatography column. The isolated nitrous oxide is then analyzed on a continuous flow isotope ratio mass spectrometer via an open split. This paper describes the basic protocol and reaction conditions required to obtain reproducible natural abundance level nitrogen and oxygen isotopic ratios from nitrate, nitrite, or both, and the results obtained to support these conclusions. A standard deviation less than 0.2‰ for nitrogen and 0.5‰ for oxygen was found for nitrate samples ranging in concentration from 40 to 0.5 μM (better for nitrite), with a blank of 2 nmol for 50-mL samples. Nitrogen and oxygen isotopic fractionation and oxygen atom exchange were consistent within each batch of analysis. There was no interference from any seawater matrixes. Only one other method published to date can measure the nitrate oxygen isotopic abundance in seawater and none that do so for nitrite alone in the presence of nitrate. This method may prove to be simpler, faster, and obtain isotopic information for lower concentrations of nitrate and nitrite than other methods.