Absolute 13C/12C isotope amount ratio for Vienna PeeDee Belemnite from infrared absorption spectroscopy

Measurements of isotope ratios are predominantly made with reference to standard specimens that have been characterized in the past. In the 1950s, the carbon isotope ratio was referenced to a belemnite sample collected by Heinz Lowenstam and Harold Urey1 in South Carolina’s PeeDee region. Due to exhaustion of the sample since then, reference materials that are traceable to the original artefact are used to define the Vienna PeeDee Belemnite scale for stable carbon isotope analysis2. However, these reference materials have also become exhausted or proven to exhibit unstable composition over time3, mirroring issues with the international prototype of the kilogram that led to a revised International System of Units4. A campaign to elucidate the stable carbon isotope ratio of Vienna PeeDee Belemnite is underway5, but independent measurement techniques are required to support it. Here we report an accurate value for the stable carbon isotope ratio inferred from infrared absorption spectroscopy, fulfilling the promise of this fundamentally accurate approach6. Our results agree with a value recently derived from mass spectrometry5 and therefore advance the prospects of International System of Units–traceable isotope analysis. Further, our calibration-free method could improve mass balance calculations and enhance isotopic tracer studies in carbon dioxide source apportionment. Isotope ratio measurements are complicated by the instabilities of composition in reference samples. Now a calibration-free method relying on infrared spectroscopy provides measurements that are traceable to International System of Units standards.