The influence of sample matrix on the accuracy of nitrite N and O isotope ratio analyses with the azide method.

Rationale: The isotope ratios of nitrogen ( N/ N) and oxygen ( O/ O) in nitrite (NO ) can be measured by conversion of the nitrite into nitrous oxide (N O) with azide, followed by mass spectrometric analysis of N O by gas chromatography isotope ratio mass spectrometry (GC/IRMS). While applying this method to brackish samples, we noticed that the N and O isotope ratio measurements of NO are highly sensitive to sample salinity and to the pH at which samples are preserved.

Methods: We investigated the influence of sample salinity and sample preservation pH on the N and O isotope ratios of the N O produced from the reaction of NO with azide.

Constraining the Oxygen Isotopic Composition of Nitrate Produced by Nitrification.

Measurements of the stable isotope ratios of nitrogen (N/N) and oxygen (O/O) in nitrate (NO) enable identification of sources, dispersal, and fate of natural and contaminant NO in aquatic environments. The O/O of NO produced by nitrification is often assumed to reflect the proportional contribution of oxygen atom sources, water, and molecular oxygen, in a 2:1 ratio. Culture and seawater incubations, however, indicate oxygen isotopic equilibration between nitrite (NO) and water, and kinetic isotope effects for oxygen atom incorporation, which modulate the NO O/O produced during nitrification.