Real-time analysis of δ13C- and δD-CH4 in ambient air with laser spectroscopy: method development and first intercomparison results.

In situ and simultaneous measurement of the three most abundant isotopologues of methane using mid-infrared laser absorption spectroscopy is demonstrated. A fielddeployable, autonomous platform is realized by coupling a compact quantum cascade laser absorption spectrometer (QCLAS) to a preconcentration unit, called TRace gas EXtractor (TREX). This unit enhances CH₄ mole fractions by a factor of up to 500 above ambient levels and quantitatively separates interfering trace gases such as N₂O and CO₂. The analytical precision of the QCLAS isotope measurement on the preconcentrated (750 ppm, parts-per-million, μmole/mole) methane is 0.1 and 0.5‰for δ¹³C and δD-CH₄ at 10min averaging time. Based on replicate measurements of compressed air during a two-week intercomparison campaign, the repeatability of the TREX-QCLAS was determined to be 0.19 and 1.9‰ for δ¹³C and δD-CH₄, respectively. In this intercomparison campaign the new in situ technique is compared to isotope-ratio mass-spectrometry (IRMS) based on glass flask and bag sampling and real time CH₄ isotope analysis by two commercially available laser spectrometers. Both laser-based analyzers were limited to methane mole fraction and δ¹³C-CH₄ analysis, and only one of them, a cavity ring down spectrometer, was capable to deliver meaningful data for the isotopic composition. After correcting for scale offsets, the average difference between TREX-QCLAS data and bag/flask sampling-IRMS values are within the extended WMO compatibility goals of 0.2 and 5‰ for δ¹³C- and δD-CH₄, respectively. Thus, the intercomparison also reveals the need for reference air samples with accurately determined isotopic composition of CH₄ to further improve the interlaboratory compatibility. [ABSTRACT FROM AUTHOR]