Comparison of carbon dioxide sources for use as spectroscopic radiocarbon (Δ14CO2) standards

Advances in spectroscopic techniques for measuring radiocarbon (¹⁴C) in carbon dioxide (CO₂) allow near-real-time analyses of atmospheric CO₂ and the characterization of the fossil fuel fraction of CO₂ emissions on sub-hourly timescales. Calibration and drift correction of these measurements require high-purity CO₂ with near-modern (atmospheric) radiocarbon and stable isotopic (δ¹³C) signatures. Most commercially available compressed CO₂ gases are fossil fuel-derived (¹⁴C-dead), while biogenic CO₂ remains a niche product with limited purity specifications. Both total gas purity and the presence of ppbv-level impurities of nitrous oxide (N₂O) can adversely impact the spectroscopic Δ¹⁴C-CO₂ measurements.

We present results on purity and isotopic characterization (δ¹³C, Δ¹⁴C) of different CO₂ gas sources as part of ongoing work to develop CO₂ standard gases with modern Δ¹⁴C and δ¹³C signatures. We tested CO₂ from distinct biogenic sources, including brewery, ethanol production, and biogas production. We also characterize several high-purity fossil CO₂ sources. Gases were tested for Δ¹⁴C-CO₂ and N₂O impurities by saturated-absorption cavity ring-down spectroscopy (SCAR) using a commercial instrument (ppqSense). These measurements were calibrated against spectra from CO₂ released from a NIST oxalic acid standard (SRM 4990C). We also characterized Δ¹⁴C-CO₂ by accelerator mass spectrometry, δ¹³C-CO₂ using isotope ratio mass spectrometry, and the presence of trace impurities using different techniques. Preliminary results show the N₂O impurities from three tested biogenic gas sources vary by a factor of 10³, the ¹⁴C content varies by almost 30%, and the δ¹³C values vary by over 25 ‰. These results will be presented in the larger context of supporting semi-automated SCAR Δ¹⁴C-CO₂ measurements in Dübendorf, Switzerland.