Compound-Specific Stable Isotope Analysis (CSIA)

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 2H and 13C in Polycyclic Aromatic Hydrocarbons (PAHs)

Polycyclic aromatic hydrocarbons, or PAHs, are readily measured following extraction from char, soil, sediment, or petroleum sources [1-3]. CSIA primarily targets the 7 PAHs as listed on the US EPA Priority Chemical list and closely related compounds (Table 1), though additional compounds may be measureable upon request. The PAHs are dissolved in pentane, injected at 300 ºC (splitless, 1 min), and separated on an Agilent DB-5ms column (30 m x 0.25 mm ID x 1 µm film thickness) at constant flow rate of 1.2 mL/min under the following temperature program: 50 °C (hold 1 min); 120 °C (10 °C/min); 310 °C (5 °C/min; hold 40 min.). GC-C-P-IRMS is performed on a Thermo Trace GC 1310 gas chromatograph coupled to a Thermo Finnigan MAT 253 isotope-ratio mass spectrometer via a GC IsoLink II combustion interface. For δ13C analysis, individual PAHs are converted to CO2 within a combustion reactor composed of a NiO tube containing CuO/NiO wires maintained at 1000 °C. Water is subsequently removed through a nafion dryer and the analyte gases transferred to the IRMS. For δ2H analysis, individual PAHs are converted to H2 within a high-temperature thermal conversion reactor of graphitized Al2O3 tube maintained at 1425 °C.

One of every five samples are analyzed in duplicate. Replicates of the quality control and assessment materials are measured every 5 samples.

Table 1. UCD SIF PAHs Compound List (* indicates listed as EPA Priority Chemical).
1    Acenaphthene*
2    Acenaphthylene*
3    Anthracene*
4    Benzo(a)anthracene
5    Benzo(b)fluoranthene
6    Benzo(g,h,i)perylene*
7    Benzo(a)pyrene
8    Benzo(k)fluoranthene
9    Chrysene
10    Dibenzo(a,h,i)perylene
11    Fluoranthene
12    Fluorene*
13    Indeno(1,2,3-cd)pyrene
14    Naphthalene
15    Phenanthrene*
16    Pyrene*

Calibration and Reporting of Stable Isotope Ratios

Quality control and assessment mixtures are composed of pure n-alkanes that have been calibrated separately by EA- and TC/EA-IRMS using certified reference materials (e.g. NBS-22, IAEA-CH-7) distributed by NIST (Gaithersburg, MD U.S.A.) and the USGS (Reston, VA U.S.A.) or are certified mixtures distributed by Indiana University (Bloomington, IN U.S.A.). All are directly traceable to the primary isotopic reference material for each element (δ13C: V-PDB; δ2H: V-SMOW). All calibration procedures for CSIA of PAHs are applied identically across reference and sample materials. First, the provisional isotopic value for each PAH is obtained by normalization to an isotopically-calibrated internal reference compound (e.g. c11:0 FAEE, c13:0 FAME). Isotopic values of the individual PAHs are then scale-normalized to the primary reference material (δ13C: V-PDB; δ2H: V-SMOW) using an external mixture composed of n-alkanes with a broad range of calibrated δ13C and δ2H values, IU A7 (or equivalent). Through each calibration step, n-alkanes mixtures and secondary QA materials are monitored for accuracy and precision. Final quality assessment and acceptance of analysis is based on the accuracy and precision of the unbiased quality assessment materials.

Acceptance or rejection of calibrated data is based on the accuracy and precision of an unbiased quality assurance materials, a second δ13C- and δ2H-calibrated n-alkanes mixture, IU B6 (or equivalent). Acceptance of sample measurements require that the final calibrated isotopic values and mean standard deviation of the quality assurance replicates fall within expected measurement error (δ13C <±0.4‰; δ2H <±6‰). Precision estimates from the co-measured calibrated n-alkanes mixtures and quality assurance materials are provided with data reports.

Measurement Uncertainty

Sample materials are inherently variable in PAH composition. Some PAHs may not be measureable and measurement error among PAHs varies between different sample types due to differences in composition and chromatographic resolution. Accuracy and precision of the co-measured calibrated PAHs mixtures and quality assurance materials are provided with data reports. Limit of quantification, based on total peak area, is generally 1 V-s for δ13C and > 10 V-s for δ2H.   

References

[1] W. Wilcke, M. Krauss, W. Amelung. 2002. Stable carbon isotopic ratio measurement of polycyclic aromatic hydrocarbons as a tool for source identification and apportionment - a review of analytical methodologies. Environ. Sci. Technol. 36: 3530-3535. doi: 10.1021/es020032h

[2] B. Aichner, B. Glaser, W. Zech 2007. Polycyclic aromatic hydrocarbons and polychlorinated biphenyls in urban soils from Kathmandu, Nepal. Organic Geochemistry 38: 700-715. doi: 10.1016/j.orggeochem.2006.11.002

[3] A.J. Buczynska, B.Geypens, R.Van Grieken, K.De Wael 2012. Stable carbon isotopic ratio measurement of polycyclic aromatic hydrocarbons as a tool for source identification and apportionment - a review of analytical methodologies. Talanta 105: 435-450 doi: 10.1016/j.talanta.2012.10.075