Jena Reference Air Set (JRAS): a multi-point scale anchor for isotope measurements of CO₂ in air

Abstract. The need for a unifying scale anchor for isotopes of CO₂ in air was brought to light at the 11th WMO/IAEA Meeting of Experts on Carbon Dioxide in Tokyo 2001. During discussions about persistent discrepancies in isotope measurements between the worlds leading laboratories, it was concluded that a unifying scale anchor for Vienna Pee Dee Belemnite (VPDB) of CO₂ in air was desperately needed. Ten years later, at the 2011 Meeting of Experts on Carbon Dioxide in Wellington, it was recommended that the Jena Reference Air Set (JRAS) become the official scale anchor for isotope measurements of CO₂ in air (Brailsford, 2012).

The source of CO₂ used for JRAS is two calcites. After releasing CO₂ by reaction with phosphoric acid, the gases are mixed into CO₂-free air. This procedure ensures both isotopic stability and longevity of the CO₂. That the reference CO₂ is generated from calcites and supplied as an air mixture is unique to JRAS. This is made to ensure that any measurement bias arising from the extraction procedure is eliminated. As every laboratory has its own procedure for extracting the CO₂, this is of paramount importance if the local scales are to be unified with a common anchor.

For a period of four years, JRAS has been evaluated through the IMECC¹ program, which made it possible to distribute sets of JRAS gases to 13 laboratories worldwide. A summary of data from the six laboratories that have reported the full set of results is given here along with a description of the production and maintenance of the JRAS scale anchors.

¹ IMECC refers to the EU project "Infrastructure for Measurements of the European Carbon Cycle" (http://imecc.ipsl.jussieu.fr/).