Triple oxygen isotope analysis of nitrate using the denitrifier method and thermal decomposition of N2O

Interaction with ozone transfers its anomalous (non-mass-dependent) O-17 enrichment to atmospheric nitrogen oxides and nitrate. The O-17 anomaly (Delta O-17) in nitrate can be used to identify atmospheric nitrate inputs into terrestrial and aquatic environments as well as to study the role of ozone in the atmosphere's reactive nitrogen cycle. We report here on an online method for analysis of the O-17 anomaly, using a strain of denitrifiers to convert nitrate to N2O, which decomposes quantitatively to N-2 and O-2 in a gold furnace at 800 degrees C, followed by gas chromatographic separation and isotope analysis of O-2. This method requires similar to 50 nmol of nitrate, 2-3 orders of magnitude less than previous offline thermal decomposition methods to achieve a similar analytical precision of 0.5% for Delta O-17. There is no significant memory effect, but calibration via nitrate or N2O reference materials is required for scale normalization. The N2O decomposition method is shown to be well-suited for nitrate analysis in freshwater and seawater samples from various environments.