First Synthetic Isotopic Methane Gas Reference Materials for Optical Isotope Ratio Spectroscopy

Isotope ratio is a powerful tool for discriminating between sources and sinks of greenhouse gases, enabling their source apportionment. We have produced the first high volume and high-pressure methane (CH₄) gas reference materials with a certified δ¹³C-CH₄ and δ²H-CH₄ in a synthetic air matrix. This matrix contained ambient amount fractions of the major air components and key greenhouse gases CO₂ and N₂O.

The SI traceable methane in air reference materials were gravimetrically produced in 10 L cylinders at a pressure of 100 bar to fulfil the high-volume calibration requirements of optical isotope ratio spectroscopy (OIRS) techniques. The methane sources were chosen to span the observed atmospheric δ¹³C-CH₄ and will be used to monitor the isotope ratio of atmospheric methane at sites across Europe.

The δ¹³C-CH₄ and δ²H-CH₄ of pure methane sources and diluted methane references were measured via isotope ratio mass spectrometry (IRMS). The diluted methane reference materials were used to characterise an OIRS (Aerodyne Research Inc.) analyser fitted with an in-house built pre-concentrator. Further methane sources were characterised for δ¹³C-CH₄ and δ²H-CH₄ after dilution on the calibrated Aerodyne to create a suite of reference materials with δ¹³C-CH₄ values spanning -51.94 to -39.19 ‰ and -202.56 to – 182.04 ‰ for δ²H-CH₄.

Uncertainty budgets for the diluted reference materials will be presented with gravimetric amount fraction uncertainties aiming towards the World Metrological Organization Global Atmosphere Watch (WMO-GAW) compatibility goal of 2 nmol mol^-1 across the amount fraction range of 1.75-2.10 µmol mol^-1. Reference materials were produced at a range of amount fractions (1.7 to 569 µmol mol^-1) required to calibrate different OIRS techniques. Commonly interfering matrix components Ar, O₂, CO₂ and N₂O were added at ambient amount fractions (420 µmol mol^-1 and 335 nmol mol^-1 for CO­₂ and N₂O respectively). Approximately 30 reference materials, each containing around 1000 litres of gas were distributed to seven partners in the EMPIR Metrology for European emissions verification on methane isotopes (isoMET) project. Measurements using these reference materials will enable comparability of isotopic methane data collected as part of national and international monitoring networks.