Preparation of Synthetic Isotope Mixtures for the calibration of carbon and oxygen isotope ratio measurements (in carbon dioxide) to the SI

Different sets of synthetic isotope mixtures for the calibration of carbon and oxygen ion current ratio measurements obtained by mass spectrometry have been prepared by mixing carbon dioxide isotopically enriched in 18O (natC18O2) and natural carbon dioxide (CO2)nat, and by mixing different natural CO2 gases with slightly different carbon and oxygen isotopic compositions. These mixtures act as Primary Standards to the SI for carbon and oxygen isotope amount ratio measurements in CO2. They will help to anchor existing carbon Isotope Reference Samples (i.e., NBS19, IAEA-CO-9) and therefore offer the basis for comparability of carbon (and oxygen) isotope measurement results, without any assumptive correction for the oxygen isotopes.Through such ‘absolute’ isotope amount ratio measurements of carbon and oxygen on CO2 produced from the Primary Standard for carbon to the VPDB-scale, NBS19CO2NBS19CO2, calibrated by means of synthetic isotope mixtures, ‘absolute’ isotope amount carbon and oxygen ratios for the zero point of the VPDB conventional scale were calculated to be R13/12 = (111,376 ± 16) × 10−7 and R18/16 = (208,824 ± 48) × 10−8, respectively. This approach makes these values traceable to the derived SI unit mol/mol.