HIGH-PRECISION DIRECT MEASUREMENTS OF (CH4)-C-13/(CH4)-C-12 AND (CH3D)-C-12/(CH4)-C-12 RATIOS IN ATMOSPHERIC METHANE SOURCES BY MEANS OF A LONG-PATH TUNABLE DIODE-LASER ABSORPTION SPECTROMETER

Measurements of (CH4)-C-13/(CH4)-C-12 and (CH3D)-C-12/(CH4)-C-12 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 H-2, we have developed a tunable diode laser absorption spectrometer (TDLAS), which permits rapid direct measurements of the (CH4)-C-13/(CH4)-C-12 and (CH3D)-C-12/(CH4)-C-12 ratios. An intercomparison between TDLAS and MS techniques for samples from natural wetlands, landfills, and natural gas sources resulted in a mean deviation of Delta delta(13)C = 0.44%0 and Delta delta D = 5.1%o. In the present system the minimum mixing ratios required are 50 parts in 10(6) by volume (ppmv) CH4 (sample size 2 mu mol CH4) for direct delta(13)C measurements and 2000 ppmv (sample size 80 mu mol CH4) for direct delta D measurements. These mixing-ratio limits are adequate for most CH4 source characterization studies without requiring sample preconcentration.