Calibration strategies for FTIR and other IRIS instruments for accurate δ13C and δ18O measurements of CO2 in air

This talk will describe calibration strategies in laboratory conditions that can be applied to ensure accurate measurements of the isotopic composition of the CO2 in air, expressed as δ13C and δ18O on the VPDB scale, with either FTIR (in this case a Vertex 70V (Bruker)) or an Isotope Ratio Infrared Spectrometer (IRIS) (in this case a Delta Ray (Thermo Fisher Scientific)). In the case of FTIR, two standards with known CO2 mole fraction, and isotopic composition, in air are sufficient to make accurate measurements with standard uncertainties of 0.05 ‰ and 0.77 ‰ for δ13C and δ18O respectively at a nominal CO2 mole fraction of 400 μmol/mol in air. In the case of the IRIS system, two pure CO2 gas isotope standards, diluent air and two standard of CO2 certified for mole fraction and isotopic composition (δ13C and δ18O) are sufficient to make accurate measurements of δ13C and δ18O with standard uncertainties of 0.29 ‰ and 0.60 ‰ respectively. The calibration strategy was validated using a set of five traceable Primary Reference Standards. The standards, produced with whole air or synthetic air as the matrix over the mole fraction range of 378- 420 μmol mol-1, were prepared and/or certified either by the National Institute of Standards and Technology (NIST) and the National Physical Laboratory (NPL). The standards were prepared in three subsets of different δ13C values between -35 ‰ and -1 ‰ using pure CO2 obtained from specific sources, namely: combustion; Northern Continental and Southern Oceanic Air and a gas well source. The isotopic composition of all standards was value assigned at the Max Planck Institute for Biogeochemistry Jena (MPI-Jena).