Nitrogen (N2) and/or Nitrous Oxide (N2O) in Gas

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

Returning SIF Samples (Facility closes July 26, 2026)

Dear Valued Client, 

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.

We regret to inform you that SIF staff will be unable to process your submitted samples before the facility's closure date. As a result, we would like to offer the option of returning your samples. If you would like your samples returned, please submit the request via return form on the SIF website.   If you do not wish to have the samples returned, please notify us, and we will arrange for their appropriate disposal in accordance with established protocols.

We sincerely appreciate your support of the Stable Isotope Facility and the opportunity to have served your research needs.

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

Analysis of N2 & N2O by GasBench-Precon-IRMS

The SIF provides isotope analysis (15N, 18O) of N2 and/or N2O in gas mixtures that may also contain O2 and CO2. This method is also used to analyze N2 and/or N2O purged from water samples with helium.

Analysis of 15N and 18O in Gas Mixtures

Stable isotope ratios of nitrogen (δ15N) and/or oxygen (δ18O) are measured using a ThermoScientific GasBench + Precon 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 (20 mL/min). Then, N2 and N2O are isolated and concentrated in preparation for isotopic analysis. First, N2 gas is sampled by a rotary 8-port valve fitted with a 5-100 µL sampling loop and timed to capture the peak N2 concentration in the carrier gas stream. This gas sample is passed to the IRMS through a molecular sieve 5A GC column (15 m x 0.53 mm ID,  25 °C, 3 mL/min). A reference N2 peak is used to calculate provisional isotope ratios of the sample N2 peak.

As N2 is analyzed, the rest of 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 a Poraplot Q GC column (25 m x 0.53 mm,  25 °C, 1.8 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 delta values are calculated by adjusting the provisional values for changes in linearity and instrumental drift such that correct 15N delta values for laboratory reference materials are obtained. Two laboratory reference materials are analyzed every 10 samples. The laboratory reference materials are mixtures of N2 and N2O (e.g., 3% N2 + 1 ppm N2O with the balance He or 1 ppm N2O with balance N2). The N2 is calibrated against an Oztech N2 standard (Oztech Trading Co., δ15N vs air = -0.61 ‰). The calibration of the N2O is problematic since there are no suitable international standards. Thus, we calibrated 15N and 18O by thermally decomposing N2O in a heated gold tube (800°C) to convert N2O to N2 + O2. The resulting N2 was calibrated against the Oztech N2 standard, and the O2 was calibrated against an Oztech O2 standard (δ18O vs VSMOW = 27.48 ‰).

N2O isotopomers (δ15Nα and δ15Nβ)

Analysis of N2O isotopomers (δ15Nα and δ15Nβ) are performed using the same multi-collector isotope-ratio mass spectrometer as above. Simultaneous measurement of N2O (m/z 44, 45, 46) and its NO fragment (m/z 30 and 31) allow determination of the central nitrogen atom (δ15Nα). Isotopic composition of the terminal nitrogen atom (δ15Nβ) can be calculated as the difference between the central position and the overall (δ15N) bulk measurement.

Limit of Quantitation and Long-term standard deviation for N2 & N2O Analysis by GasBench-Precon-IRMS

N2O : Limit of Quantitation: approx. 150 picomoles

           Long-term standard deviation: 15N, 0.1 ‰; 18O, 0.3 ‰

 

N2:    Limit of Quantitation: approx. 150 nanomoles

          Long-term standard deviation: 0.1 ‰


Maximum measurable gas concentrations are dependent upon both concentration and isotopic enrichment.  Please contact us if you intend to submit isotopically enriched samples at gas concentrations more than ten times that of ambient concentration.