NO₃ Analysis by Bacteria Denitrification

The SIF provides the preparation and isotope analysis (¹⁵N, ¹⁸O) of NO₃^- in water by bacteria denitrification assay.

Analysis of ¹⁵N and ¹⁸O of N₂O from NO₃^- by bacterial denitrification assay

Isotope ratios of ¹⁵N and ¹⁸O are measured using a ThermoFinnigan GasBench + PreCon trace gas concentration system interfaced to a ThermoScientific Delta V Plus isotope-ratio mass spectrometer (Bremen, Germany). Gas samples are purged from vials through a double-needle sampler into a helium carrier stream (25 mL/min).  The gas sample passes through a CO₂ scrubber (Ascarite) and N₂O is trapped and concentrated in two liquid nitrogen cryo-traps operated in series such that the N₂O is held in the first trap until the non-condensing portion of the sample gas has been replaced by helium carrier, then passed to the second, smaller trap. Finally, the second trap is warmed to ambient, and the N₂O is carried by helium to the IRMS via an Agilent GS-Q capillary column (30 m x 0.32 mm, 40 °C, 1.0 mL/min). This column separates N₂O from residual CO₂. A reference N₂O peak is used to calculate provisional isotope ratios of the sample N₂O peak.

Final ¹⁵N and ¹⁸O delta values are calculated by adjusting the provisional values such that correct ¹⁵N and ¹⁸O delta values for laboratory reference materials are obtained. The calibration standards are the nitrates USGS 32, USGS 34, and USGS 35, supplied by NIST (National Institute of Standards and Technology, Gaithersburg, MD).  Additional laboratory reference materials are included in each batch to monitor and correct for instrumental drift and linearity.

Limit of Quantitation and Long-term standard deviation for ¹⁵N and ¹⁸O of N₂O from NO₃^- by bacterial denitrification

Limit of Quantitation: 4 - 7000 μM NO₃^- in water

Accepted Precision: 0.4 ‰ for ¹⁵N

                                     0.5 ‰ for ¹⁸O