Consistency of δ13C measurements can be improved 39-47% by anchoring the δ13C scale with two isotopic reference materials differing substantially in 13C/12C. It is recommended that δ13C values of both organic and inorganic materials be measured and expressed relative to VPDB (Vienna Peedee belemnite) on a scale normalized by assigning consensus values of -46.6‰ to L-SVEC lithium carbonate and +1.95‰ to NBS 19 calcium carbonate. Uncertainties of other reference material values on this scale are improved by factors up to two or more, and the values of some have been notably shifted: the δ13C of NBS 22 oil is -30.03‰.
