Preparation, Characterization, and Value Assignment of Carbon Dioxide Isotopic Reference

Three carbon dioxide isotopic reference materials (RM 8562, RM 8563, and RM 8564) have been prepared and characterized for composition, heterogeneity, and stability using dual-inlet isotope ratio mass spectrometry. RMs 8562-8564 are not certified, but their use as proxies for carbonate standards allows users to realize precisely the Vienna PDB scale and to provide comparability of stable carbon and oxygen isotope ratio data obtained by inves-tigators in different laboratories. Samples (400 ímol each) are sealed in borosilicate glass tubes (9-mm outer diameter) for distribution by the National Institute of Standards and Technology (NIST) and the International Atomic Energy Agency (IAEA). ä13C and ä18O value assignments and uncertainties for the CO2 RMs were determined by two complementary methods: (1) throug