Dissolved Organic Carbon (DOC) 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 Organic Carbon (DOC) in Water by TOC-IRMS

THE SIF IS NOT CURRENTLY ACCEPTING DOC PROJECTS IN 2025.
The analytical method listed is for historical record.

The SIF previously provided  13C isotope analysis of dissolved organic carbon using a TOC analyzer interfaced to an IRMS.

Dissolved inorganic carbon (DIC) is no longer analyzed using this system; please refer to DIC in Water.

Analysis

DOC is analyzed for 13C using an O.I. Analytical Model 1030 TOC Analyzer (Xylem Analytics, College Station, TX) interfaced to a PDZ Europa 20-20 isotope ratio mass spectrometer (Sercon Ltd., Cheshire, UK) utilizing a GD-100 Gas Trap Interface (Graden Instruments). Samples are acidified and purged with helium off-line to remove all dissolved inorganic carbon (DIC). Depending on sample concentration, a 1- to 9-mL aliquot of sample is transferred into a heated digestion vessel and reacted with sodium persulfate to convert DOC into a pulse of CO2. The CO2 is carried in a helium flow to the isotope ratio mass spectrometer where the 13C/12C ratios are measured.

During analysis, samples are interspersed with several replicates of at least three different laboratory reference materials. These reference materials have been previously calibrated against international reference materials, including: IAEA-600, USGS-40, USGS-41, and Elemental Microanalysis (EM) reference materials. A sample’s provisional isotope ratio is measured relative to a reference gas peak analyzed with each sample. These provisional values are finalized by correcting the values for the entire batch based on the known values of the included laboratory reference materials.

The final 13C delta values, delivered to the customer, are expressed relative to international standard VPDB (Vienna Pee Dee Belemnite). For information on delta notation and the international standards, please refer to a stable isotope reference such as Sharp, Z. (2005) Principles of Stable Isotope Geochemistry (Prentice Hall).

Limit of Quantification and Analytical Precision

0.4‰ for 13C of DOC at DOC concentrations greater than 0.5ppm