Conference Agenda

Isotope ratio analysis is used as a tool to distinguish between different sources of materials, and is applied in many scientific fields such as atmospheric sciences, food authenticity, and archeology. These small differences in isotope ratios require measurements of the highest precision. Unfortunately, several technical obstacles prevent us from reaching acceptable inter-laboratory measurement goals. One of the biggest impediments is the disagreement over the primary reference materials that set the measurement scales. Several recent studies, including a study performed at the NRC, have demonstrated that significantly different results for the same carbon dioxide sample can be obtained depending on which combination of calibration standards are used. Based on these studies, the scientific community has acknowledged that carbon isotope ratio measurements are indeed reported on two distinct measurement scales, depending on which calibration standards were used for the measurements.

Since the recommended discontinuation of the reference material LSVEC, reliable realization of the measurement scales has been questioned. The NRC undertook several measurement campaigns to provide additional data to address some of these questions, and to improve our knowledge about the isotopic composition of these widely used reference materials. First, the isotopic composition of both carbon and oxygen in the suite of IAEA reference materials IAEA-610-611-612 was measured to provide an independent isotope ratio data set for these materials. Second, we provide independent characterization of the NIST carbon dioxide reference gases SRMs 8562-8563-8564 to address the conflicting information about the traceability of the isotope delta values assigned to them. Finally, we will present preliminary results from a project aimed to evaluate the reliability of producing carbon dioxide reference gas from materials other than carbonates.

Progress in the development of pure CO₂ gas standards for δ¹³C, δ¹⁸O and Δ47 measurements as well as CO₂ in air gas standards (with mole fractions in the range 350 to 800 µmol/mol) for δ¹³C, δ¹⁸O measurements are described. Initial results indicate the potential to produce standards with internal consistencies at the 0.005 ‰ level for δ¹³C, and standard uncertainties of 0.015 ‰ in relation to the VPDB scale, with the magnitude of the latter principally limited by the homogeneity of primary carbonate reference materials.
Outputs of the project so far include:
• Establishment of a facility to produce stable pure CO₂ gas standards in 6L cylinders at 2 bar with δ¹³C values from −1 ‰ to −45 ‰ vs VPDB, with internal consistency approaching the 0.005 ‰ level, and an effective calibration option for dual inlet IRMS systems as demonstrated in the international comparison CCQM−P204 completed in 2021;
• Studies of Δ47 values of mixtures of different pure CO₂ gas, and the reproducibility and stability of these and their potential to act as reference standards for clumped isotope ratio measurements with IRMS systems;
• The development and validation of a cryogenic Air Trapping system to extract CO₂ from air for determination of δ¹³C and δ¹⁸O-CO₂ with IRMS, including a correction for the N₂O present in samples. The facility is currently being used for another international comparison (CCQM−P239) of 39 CO₂ in in air standards (gas cylinders) from 15 institutes containing CO₂ over the range of 380 μmol mol⁻¹ to 800 μmol mol⁻¹ and δ¹³C and δ¹⁸O-CO₂ values from +1 ‰ to −43 ‰ and −1 ‰ to −45 ‰, respectively. The method demonstrates excellent reproducibility, with standard deviations of 0.005% and 0.05% for δ¹³C and δ¹⁸O-CO₂, respectively, and will demonstrate the level of equivalence of new CO₂ in air isotope ratio standards currently being produced.

The precise measurement of the nitrous oxide (N₂O) isotope ratio in atmospheric air is required to understand global emission trends. There are no existing internationally accepted reference materials for atmospheric amount fraction N₂O with characterised isotope ratio including uncertainties. There is an urgent need for the development of reference materials to fill this traceability gap and meet the requirements to underpin global measurements. Reference materials are required with traceability to existing international stable isotope ratio scales: AIR-N₂ and VSMOW (Vienna Standard Mean Ocean Water) for δ¹⁵N and δ¹⁸O.

We will present progress towards the development of atmospheric amount fraction N₂O in synthetic air reference materials with characterised isotope ratios suitable for calibration of optical isotope ratio spectrometers (OIRS) operating in the field. The reference materials span a wide range of δ values. The pure N₂O used to prepare the N₂O in synthetic air reference materials was produced at Empa and has traceability to the primary AIR-N₂ and VSMOW scales. The N₂O in synthetic air reference materials were characterised for N₂O amount fraction and isotope ratio using OIRS. Nitrous oxide amount fractions in the range of 320-330 nmol mol^-1 were certified with combined (k=2) uncertainties of 0.5 %.

The reproducibility in the isotope ratio of dilutions of the pure N₂O reference materials to ambient amount fraction is shown to be less than 0.5 ‰ for δ¹⁵N, δ¹⁵N^α, δ¹⁵N^β and δ¹⁸O. A study on instrument calibration has demonstrated certification of reference materials with unknown isotope ratios with combined (k=2) uncertainties of 1.1 ‰ and repeatability within 0.3 ‰ for δ¹⁵N, δ¹⁵N^α, δ¹⁵N^β and δ¹⁸O. The sensitivities in the measured isotope ratios to commonly occurring synthetic air matrix impurities and to the N₂O amount fraction have been assessed to provide a full uncertainty budget for the N₂O in synthetic air reference materials.