Dissolved Inorganic Carbon (DIC) 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

Analysis of Dissolved Inorganic Carbon (DIC) in Water by GasBench-IRMS

The SIF provides δ13C analysis of Dissolved Inorganic Carbon (DIC) in water samples using a GasBench II system interfaced to a Delta V Plus IRMS (Thermo Scientific, Bremen, Germany).

Analysis of 13C-DIC

Water samples (1-4 mL) are injected into helium-filled 12 mL septum capped vials (Exetainers, Labco, High Wycombe, UK) containing 1 mL 85% phosphoric acid, which forces the equilibrium between CO2 and H2CO3 to gaseous CO2. The evolved CO2 is purged from vials through a double-needle sampler into a helium carrier stream (20 mL/min). The gas is sampled using a six-port rotary valve (Valco, Houston TX) with either a 100 µL, 50 µL, or 10 µL loop programmed to switch at the maximum CO2 concentration in the helium carrier. The CO2 is passed to the IRMS through a Poraplot Q GC column (25m x 0.32mm ID, 45 °C, 2.5 mL/min). A reference CO2 peak is used to calculate provisional delta values of the sample CO2 peak. Final 13C delta values are obtained after adjusting the provisional delta values for changes in linearity and instrumental drift such that correct 13C delta values for laboratory reference materials are obtained. At least two laboratory reference materials are analyzed with every 10 samples. Laboratory reference materials are lithium carbonate dissolved in degassed deionized water and a deep seawater (both calibrated against NIST 8545). Final 13C delta values, delivered to the customer, are expressed relative to the international standard V-PDB (Vienna PeeDee Belemnite)

Limit of Quantitation and Long-term standard deviation for 13C-DIC by GasBench-IRMS

13C-DIC as CO2 : Limit of Quantitation: approx. 150 nanomoles

                                 Long-term standard deviation: 0.1 ‰