Measurements of 13CH4/12CH4 and 12CH3D/12CH4 ratios in atmospheric methane (CH4) sources provide important information about the global CH4 budget as well as about CH4 production and consumption processes occurring within the various sources. As an alternative to the conventional mass spectrometer (MS) technique, which requires conversion of CH4 to CO2 and H2, we have developed a tunable diode laser absorption spectrometer (TDLAS), which permits rapid direct measurements of the 13CH4/12CH4 and 12CH3D/12CH4 ratios. An intercomparison between TDLAS and MS techniques for samples from natural wetlands, landfills, and natural gas sources resulted in a mean deviation of Δδ13C = 0.44‰ and ΔδD = 5.1‰. In the present system the minimum mixing ratios required are 50 parts in 106 by volume (ppmv) CH4 (sample size 2 μmol CH4) for direct δ13C measurements and 2000 ppmv (sample size 80 μmol CH4) for direct δD measurements. These mixing-ratio limits are adequate for most CH4 source characterization studies without requiring sample preconcentration.
© 1994 Optical Society of AmericaPDF Article