Inter-laboratory calibration of natural gas round robins for δ ~2H and δ ~(13)C using off-line and on-line techniques

摘要

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 CO _2). The consensus δ ~(13)C values and uncertainties were derived from the Maximum Likelihood Estimation (MLE) based on off-line measurements; the consensus δ ~2H 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: δ ~2H _(VSMOW)=-185.1‰±1.2‰, δ ~(13)C _(VPDB)=-34.18‰±0.10‰Ethane: δ ~2H _(VSMOW)=-156.3‰±1.8‰, δ ~(13)C _(VPDB)=-24.66‰±0.11‰Propane: δ ~2H _(VSMOW)=-143.6‰±3.3‰, δ ~(13)C _(VPDB)=-22.21‰±0.11‰i-Butane: δ ~(13)C _(VPDB)=-21.62‰±0.12‰n-Butane: δ ~(13)C _(VPDB)=-21.74‰±0.13‰CO ~2: δ ~(13)C _(VPDB)=-5.00‰±0.12‰NG2 (biogas):Methane: δ ~2H _(VSMOW)=-237.0‰±1.2‰, δ ~(13)C _(VPDB)=-68.89‰±0.12‰NG3 (oil-related gas):Methane: δ ~2H _(VSMOW)=-167.6‰±1.0‰, δ ~(13)C _(VPDB)=-43.61‰±0.09‰Ethane: δ ~2H _(VSMOW)=-164.1‰±2.4‰, δ ~(13)C _(VPDB)=-40.24‰±0.10‰Propane: δ ~2H ~(VSMOW)=-138.4‰±3.0‰, δ ~(13)C _(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.