title: Calibration Strategies for FT-IR and Other Isotope Ratio Infrared Spectrometer Instruments for Accurate δ13C and δ18O Measurements of CO2in Air.
abstract: This paper describes calibration strategies in laboratory conditions that can be applied to ensure accurate measurements of the isotopic composition of the CO2in ultradry air, expressed as δ13C and δ18O on the VPDB scale, with either FT-IR (in this case a Vertex 70 V (Bruker)) or an isotope ratio infrared spectrometer (IRIS) (in this case a Delta Ray (Thermo Fisher Scientific)). In the case of FT-IR a novel methodology using only two standards of CO2in air with different mole fractions but identical isotopic composition was demonstrated to be highly accurate for measurements of δ13C and δ18O with standard uncertainties of 0.09‰ and 1.03‰, respectively, at a nominal CO2mole fraction of 400 μmol mol-1in air. In the case of the IRIS system, we demonstrate that the use of two standards of CO2in air of known but differing δ13C and δ18O isotopic composition allows standard uncertainties of 0.18‰ and 0.48‰ to be achieved for δ13C and δ18O measurements, respectively. The calibration strategies were validated using a set of five traceable primary reference gas mixtures. These standards, produced with whole air or synthetic air covered the mole fraction range of (378-420) μmol mol-1and were prepared and/or value assigned either by the National Institute of Standards and Technology (NIST) or the National Physical Laboratory (NPL). The standards were prepared using pure CO2obtained from different sources, namely, combustion; Northern Continental and Southern Oceanic Air and a gas well source, with δ13C values ranging between -35‰ and -1‰. The isotopic composition of all standards was value assigned at the Max Planck Institute for Biogeochemistry Jena (MPI-Jena).
authors.affiliation: Bureau International des Poids et Mesures (BIPM) , Pavillon de Breteuil, F-92312 Sèvres Cedex, France.
authors.forename: Edgar
authors.identifier: 0000-0003-1910-7265
authors.initials: E
authors.lastname: Flores
authors.affiliation: Bureau International des Poids et Mesures (BIPM) , Pavillon de Breteuil, F-92312 Sèvres Cedex, France.
authors.forename: Joële
authors.initials: J
authors.lastname: Viallon
authors.affiliation: Bureau International des Poids et Mesures (BIPM) , Pavillon de Breteuil, F-92312 Sèvres Cedex, France.
authors.forename: Philippe
authors.initials: P
authors.lastname: Moussay
authors.affiliation: University of Wollongong , Wollongong, New South Wales 2500, Australia.
authors.forename: David W T
authors.initials: DW
authors.lastname: Griffith
authors.affiliation: Bureau International des Poids et Mesures (BIPM) , Pavillon de Breteuil, F-92312 Sèvres Cedex, France.
authors.forename: Robert Ian
authors.initials: RI
authors.lastname: Wielgosz