High-precision continuous-flow measurement of delta C-13 and delta D of atmospheric CH4

We describe our development of a CH4 preconcentration system for use with c ontinuous-flow gas chromatograph combustion isotope ratio mass spectrometry (GC/C/ IRMS). Precision of measurement of delta C-13-CH4 is 0.05 parts per thousand (1 sigma) on multiple 60-mL aliquots of the same ambient air samp le. The same front-end on-line CH4 preconcentration system allows us to mea sure deltaD of CH4 by gas chromatography IRMS when the combustion furnace i s replaced with a pyrolysis oven (GC/P/IRMS). Precision of measurement for deltaD-CH4 is 1.5 parts per thousand (1 sigma) using 120 mL of ambient air based on multiple aliquots of the same air sample. These are the first repo rted measurements of atmospheric CH4 using GC/P/IRMS methodology. Each isot ope analysis can be made much more rapidly (30-40 min) than they could usin g off-line combustion of an air sample (1-6 h) followed by conventional dua l-inlet RMS measurements (12-20 min), while requiring much less total volum e and retaining a comparable level of precision and accuracy. To illustrate the capabilities of our preconcentration GC/C/IRMS system, we compare the results of measurement of 24 background air samples made using both GC/C/IR MS and conventional vacuum line/dual-inlet HMS methodology. The air samples were collected on a shipboard air sampling transect made across the Pacifi c Ocean in July 2000 and are part of an ongoing atmospheric CH4 research pr ogram. The average difference between the two methods of IRMS analyses on t hese 24 samples is 0.01 +/- 0.03 parts per thousand (95% confidence interva l) for delta C-13-CH4. These are the first measurements to be reported of a ir samples directly intercompared for delta C-13-CH4 using both GC/C/IRMS a nd dual-inlet IRMS measurement methodology. Measurement of deltaD-CH4 of th ese air samples is also presented as an illustration of the ability of this system to resolve small isotopic differences in remote air. High-precision measurement of delta C-13 and deltaD of atmospheric CH4 made using our cou pled preconcentration GC/IRMS system will greatly improve our ability to ut ilize isotopic data in understanding spatial and temporal changes in atmosp heric CH4 and the biogeochemistry of its sources and sinks.