Nitrate (NO3) in Water

The Stable Isotope Facility (SIF) will be closing, effective July 26, 2026.

June 3, 2026
An Update on the Plant Sciences Stable Isotope Facility

Dear Faculty, Staff, Students, and Supporters,
After a lengthy review process and careful consideration, we have made the difficult decision to sunset the Stable Isotope Facility (SIF) in its current form, effective July 26, 2026.

The Department of Plant Sciences continues to face hard decisions surrounding funding allocation given the campus-wide request to reduce budgets. Over the past several years SIF has been operating with a significant and growing deficit and, despite extensive efforts over the last several months to find a solution that would allow the facility to continue to provide services to the research community, we have not found a model that is financially sustainable.

I want to thank SIF’s staff for their excellent work and dedication these past 25 years, and everyone who has played a role in supporting this facility.

We are committed to doing our best to support the researchers who rely on the facility during this transition, and will be in touch with individual clients about details of specific plans for handling existing orders over the next several weeks.

Sincerely,
Daniel Potter
Professor and Chair, Department of Plant Sciences
University of California, Davis

Original Letter

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operated in series such that the N2O is held in the first trap until the non-condensing portion of the sample gas has been replaced by helium carrier, then passed to the second, smaller trap.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