Re-determination of R(¹³C/¹²C) for Vienna Peedee belemnite (VPDB)

Rationale: The isotope ratio for the internationally agreed but virtual zero-point of the carbon isotope-delta scale, Vienna Peedee belemnite (VPDB), plays a critical role in linking carbon isotope delta values to the SI. It is also a quantity used for various data processing procedures including ‘¹⁷O correction’, clumped isotope analysis and conversion of carbon isotope delta values into other expressions of isotopic composition. A value for R_VPDB(¹³C/¹²C) with small uncertainty is therefore desirable to facilitate these procedures. Methods: The value of R_VPDB(¹³C/¹²C) was determined by errors-in-variables regression of isotope delta values traceable to VPDB measured by isotope ratio mass spectrometry against isotope ratios traceable to the SI by use of gravimetric mixtures of ¹²C- and ¹³C-enriched d-glucose measured by multicollector inductively coupled plasma mass spectrometry. Results: A value of R_VPDB(¹³C/¹²C) = 0.0111105 ± 0.0000042 (expanded uncertainty, k = 2) was obtained. Conclusions: The new value for R_VPDB(¹³C/¹²C) agrees very well with the consensus values calculated from previous measurement results proposed by Kaiser and by ourselves, as well as recent determinations independent of mass spectrometry. The expanded uncertainty of 0.4‰ when expressed as an isotope delta value is a tenfold improvement over the previous best measurement of the isotopic composition of carbon.