Calibration Strategies for FT-IR and Other Isotope Ratio Infrared Spectrometer Instruments for Accurate δ¹³C and δ¹⁸O Measurements of CO₂ in Air

Abstract

This paper describes calibration strategies in laboratory conditions that can be applied to ensure accurate measurements of the isotopic composition of the CO₂ in 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 CO₂ in 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 CO₂ mole fraction of 400 μmol mol^-1 in air. In the case of the IRIS system, we demonstrate that the use of two standards of CO₂ in 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^-1 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 CO₂ obtained 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).