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 ratio s in atmospheric methane (CH4) sources provide important information a bout the global CH4 budget as well as about CH4 production and consump tion 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 la ser absorption spectrometer (TDLAS), which permits rapid direct measur ements 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 n atural 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 direc t delta D measurements. These mixing-ratio limits are adequate for mos t CH4 source characterization studies without requiring sample preconc entration.