First time isotope ratio gas reference materials for d13CH4 and d2H-CH4

The aim of this work package is to develop first time gas reference materials of pure methane and at 1.85 µmol mol-1 in an air matrix on a large scale and with a repeatability in the preparation process of 0.02 ‰ for d13C-CH4 and of 1 ‰ for d2H-CH4 and with target uncertainties of 0.2 ‰ for d13C-CH4 and 5 ‰ for d2H-CH4. The stability of the isotopic composition reference materials should be more than two years. The d13C-CH4 and d2H-CH4 isotope ratio reference materials will be traceable to the VPDB and VSMOW/SLAP scales, respectively.
The requirement for methane in air reference materials is being driven by a significant increase in the use of commercial spectroscopic isotope analysers, which are dependent on frequent calibrations of amount fraction and isotope ratio with methane-air mixtures. There are currently no gas reference materials to meet the emergent high demand for high-volume d13CH4 and d 2H-CH4 calibration gases traceable to international isotope ratio scales, to underpin global measurements. In addition, a recent world-wide inter-laboratory comparison revealed discrepancies of up to 0.5 ‰ for d13C and 13 ‰ for d2H-CH4 isotopic measurements of ambient air samples which are 25 and 13 times their respective WMO-GAW network compatibility goals due to different calibration approaches and calibration propagations since the late 1970s (Umezawa et al., 2018). A new infrastructure based on gas reference materials of pure methane as well as methane in air mixtures will enable direct dissemination of unified standards and metrological traceability to meet the wide demand and improve international comparability.
Task 2.1 focuses on identifying suitable sources of methane and the development of pure gas reference materials with a range of isotopic compositions in high pressure cylinders, for underpinning measurements of d13C-CH4 and d2H-CH4. These will be made traceable to the existing VPDB and VSMOW/SLAP scales and act as parent mixtures in the development of diluted reference materials at ambient amount fractions (1.85 µmol mol-1) in task 2.2. As the realisation of the VPDB scale defines the traceability chain for both measurements of d13C-CH4 and d13C-CO2, task 2.3 will compare the reference materials for d13C-CH4 to the carbon dioxide reference materials developed in WP1 to provide an independent assessment of the accuracy and uncertainty claims.