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

Authors

Edgar Flores, Bureau International Des Poids et Mesure
Joele Viallon, Bureau International Des Poids et Mesure
Philippe Moussay, Bureau International Des Poids et Mesure
David W. T Griffith, University of WollongongFollow
Robert Wielgosz, Bureau International Des Poids et Mesure

RIS ID

114198

Publication Details

Flores, E., Viallon, J., Moussay, P., Griffith, D. W. T. & Wielgosz, R. (2017). Calibration Strategies for FT-IR and Other Isotope Ratio Infrared Spectrometer Instruments for Accurate δ¹³C and δ¹⁸O Measurements of CO2 in Air. Analytical Chemistry, 89 (6), 3648-3655.

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).