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Abstract

In 1985, the primary recommendation of a Consultants' Group Meeting of the International Atomic Energy Agency (IAEA)1 was that a new (Vienna Peedee Belemnite) VPDB δ¹³C scale be established with NBS 19 carbonate assigned to be +1.95‰ as its single anchor. This recommendation improved δ¹³C measurement uncertainties,2 especially those of materials with δ¹³C values near 0‰. A fundamental problem remained that materials with δ¹³C values far from 0‰, such as NBS 22 oil, had much poorer uncertainties.2, 3 Recognizing that two-point calibrations of the δ²H and δ¹⁸O scales substantially improved the agreement among laboratories,4 the IAEA convened a panel in 2004 to review stable carbon isotopic reference materials and to recommend a second reference material for two-point normalization of the δ¹³C scale. Four laboratories (Centrum voor Isotopen Onderzoek, Groningen, The Netherlands; Max-Planck-Institute for Biogeochemistry, Jena, Germany; UFZ Leipzig-Halle, Leipzig, Germany; US Geological Survey, Reston, VA, USA) performed analytical measurements. Participants at the US National Institute of Science and Technology (NIST) headed the task to estimate consensus means and uncertainties using multivariate Bayesian techniques. Collectively, the laboratories performed 1055 state-of-the-art continuous-flow elemental-analyzer mass spectrometry measurements using the general method of Qi et al.5 on selected organic and inorganic carbon isotopic reference materials. Aims of the work included determining consensus δ¹³C values of stable carbon isotopic reference materials and confirming that isotopic materials measured in this study are isotopically homogeneous in amounts used in continuous-flow methods [m(C) of approximately ∼40 µg]. Based on high precision mass spectrometric measurements,6, 7 a consensus value of −46.6‰ was assigned to L-SVEC lithium carbonate.8 The results (Table 1) were provided to the International Union of Pure and Applied Chemistry (IUPAC).