Inter-laboratory calibration of natural gas round robins for δ²H and δ¹³C using off-line and on-line techniques

Compound-specific carbon and hydrogen isotopic compositions of three natural gas round robins were calibrated by ten laboratories carrying out more than 800 measurements including both on-line and off-line methods. Two-point calibrations were performed with international measurement standards for hydrogen isotope ratios (VSMOW and SLAP) and carbon isotope ratios (NBS 19 and L-SVEC CO₂). The consensus δ¹³C values and uncertainties were derived from the Maximum Likelihood Estimation (MLE) based on off-line measurements; the consensus δ²H values and uncertainties were derived from MLE of both off-line and on-line measurements, taking the bias of on-line measurements into account. The calibrated consensus values in ‰ relative to VSMOW and VPDB are:

NG1 (coal-related gas):

Methane: δ²H_VSMOW = − 185.1‰ ± 1.2‰, δ¹³C_VPDB = − 34.18‰ ± 0.10‰

Ethane: δ²H_VSMOW = − 156.3‰ ± 1.8‰, δ¹³C_VPDB = − 24.66‰ ± 0.11‰

Propane: δ²H_VSMOW = − 143.6‰ ± 3.3‰, δ¹³C_VPDB = − 22.21‰ ± 0.11‰

i-Butane: δ¹³C_VPDB = − 21.62‰ ± 0.12‰

n-Butane: δ¹³C_VPDB = − 21.74‰ ± 0.13‰

CO2: δ¹³C_VPDB = − 5.00‰ ± 0.12‰

NG2 (biogas):

Methane: δ²H_VSMOW = − 237.0‰ ± 1.2‰, δ¹³C_VPDB = − 68.89‰ ± 0.12‰

NG3 (oil-related gas):

Methane: δ²H_VSMOW = − 167.6‰ ± 1.0‰, δ¹³C_VPDB = − 43.61‰ ± 0.09‰

Ethane: δ²H_VSMOW = − 164.1‰ ± 2.4‰, δ¹³C_VPDB = − 40.24‰ ± 0.10‰

Propane: δ²H_VSMOW = − 138.4‰ ± 3.0‰, δ¹³C_VPDB = − 33.79‰ ± 0.09‰

All of the assigned values are traceable to the international carbon isotope standard of VPDB and hydrogen isotope standard of VSMOW.