Stable isotopes: ¹²C/¹³C analysis of CH₄ and CO₂

Stable carbon isotope analyses of CH₄ and CO₂ are being made on a Delta XP mass spectrometer by gas chromatography combustion isotope ratio mass spectrometry (GC-C-IRMS).

Gas samples are stored in glass crimp vials sealed with butyl septa, containing a 10% KCl solution adjusted to a pH~1. The gases are extracted into a gas tight syringe fitted with a gas tight valve by displacement with an equivalent volume of KCl solution.

The concentration of CH4 and CO2 in samples is used to determine the volume of gas injected into the GC-C-IRMS for stable isotope analyses.

Methane is converted to CO2 for delta 13C/12C determination by combustion at 1050 C in a reactor containing Cu, Ni and Pt wires. The optimum quantity of CH4 injected which allows both quantatitive conversion to CO2 and a sufficient mass 45/44 signal is between 2 to 3 nmols.

Analyses are checked against an inhouse CH4 standard that has been cross-calibrated at external laboratories. The laboratory reference CO2 gas has been calibrated against International Atomic Energy Agency (IAEA) CO2 gas standards. Final stable isotope results are expressed in permil relative to Vienna Pee Dee Belemnite (VPDB).

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