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This paper describes calibration strategies in laboratory conditions that can be applied to ensure accurate measurements of the isotopic composition of the CO 2 in ultradry air, expressed as δ 13 C and δ 18 O 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 2 in air with different mole fractions but identical isotopic composition was demonstrated to be highly accurate for measurements of δ 13 C and δ 18 O with standard uncertainties of 0.09‰ and 1.03‰, respectively, at a nominal CO 2 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 CO 2 in air of known but differing δ 13 C and δ 18 O isotopic composition allows standard uncertainties of 0.18‰ and 0.48‰ to be achieved for δ 13 C and δ 18 O 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 CO 2 obtained from different sources, namely, combustion; Northern Continental and Southern Oceanic Air and a gas well source, with δ 13 C 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).

The development and operation of a highly accurate primary gas facility for the dynamic production of mixtures of nitrogen dioxide (NO(2)) in nitrogen (N(2)) based on continuous weighing of a permeation tube and accurate impurity quantification and correction of the gas mixtures using Fourier transform infrared spectroscopy (FT-IR) is described. NO(2) gas mixtures in the range of 5 μmol mol(-1) to 15 μmol mol(-1) with a standard relative uncertainty of 0.4% can be produced with this facility. To achieve an uncertainty at this level, significant efforts were made to reduce, identify and quantify potential impurities present in the gas mixtures, such as nitric acid (HNO(3)). A complete uncertainty budget, based on the analysis of the performance of the facility, including the use of a FT-IR spectrometer and a nondispersive UV analyzer as analytical techniques, is presented in this work. The mixtures produced by this facility were validated and then selected to provide reference values for an international comparison of the Consultative Committee for Amount of Substance (CCQM), number CCQM-K74, (1) which was designed to evaluate the consistency of primary NO(2) gas standards from 17 National Metrology Institutes.