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.

同位素比率的测量主要参照过去已表征的标准样本进行。在 1950 年代，碳同位素比率参考了 Heinz Lowenstam 和 Harold Urey ¹在南卡罗来纳州皮迪地区收集的箭石样本。由于此后样本耗尽，可追溯到原始人工制品的参考材料被用于定义用于稳定碳同位素分析的 Vienna PeeDee Belemnite 标度²。然而，随着时间的推移，这些参考材料也已耗尽或被证明表现出不稳定的成分³，反映出国际千克原型的问题导致修订后的国际单位制⁴. 一项阐明 Vienna PeeDee Belemnite 稳定碳同位素比率的活动正在进行中⁵，但需要独立的测量技术来支持它。^{在这里，我们报告了从红外吸收光谱推断出的稳定碳同位素比率的准确值，实现了这种从根本上准确的方法6}的承诺。我们的结果与最近从质谱法⁵中得出的值一致，因此推进了国际单位制 - 可溯源同位素分析的前景。此外，我们的免校准方法可以改进质量平衡计算并加强二氧化碳来源解析中的同位素示踪剂研究。