Calibration Strategies for FT-IR and Other Isotope Ratio Infrared Spectrometer Instruments for Accurate δC and δO Measurements of CO in Air.

This paper describes calibration strategies in laboratory conditions that can be applied to ensure accurate measurements of the isotopic composition of the CO in ultradry air, expressed as δC and δO 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 CO in air with different mole fractions but identical isotopic composition was demonstrated to be highly accurate for measurements of δC and δO with standard uncertainties of 0.09‰ and 1.03‰, respectively, at a nominal CO mole fraction of 400 μmol mol in air. In the case of the IRIS system, we demonstrate that the use of two standards of CO in air of known but differing δC and δO isotopic composition allows standard uncertainties of 0.18‰ and 0.48‰ to be achieved for δC and δO 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 and 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 CO obtained from different sources, namely, combustion; Northern Continental and Southern Oceanic Air and a gas well source, with δC 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).