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In situ and simultaneous measurement of the three most abundantisotopologues of methane using mid-infrared laser absorption spectroscopy isdemonstrated. A field-deployable, autonomous platform is realized bycoupling a compact quantum cascade laser absorption spectrometer (QCLAS) toa preconcentration unit, called trace gas extractor (TREX). This unitenhances CH4 mole fractions by a factor of up to 500 above ambientlevels and quantitatively separates interfering trace gases such as N2Oand CO2. The analytical precision of the QCLAS isotope measurement onthe preconcentrated (750 ppm, parts-per-million, µmole mole−1) methane is0.1 and 0.5 ‰ for δ13C- and δD-CH4 at 10 min averaging time. Based on repeated measurements of compressed air during a 2-weekintercomparison campaign, the repeatability of the TREX–QCLAS was determinedto be 0.19 and 1.9 ‰ for δ13C and δD-CH4, respectively. In this intercomparisoncampaign the new in situ technique is compared to isotope-ratiomass spectrometry (IRMS) based on glass flask and bag sampling and real timeCH4 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 downspectrometer, was capable to deliver meaningful data for the isotopiccomposition. After correcting for scale offsets, the average differencebetween TREX–QCLAS data and bag/flask sampling–IRMS values are within theextended WMO compatibility goals of 0.2 and 5 ‰ for δ13C- and δD-CH4,respectively. This also displays the potential to improve theinterlaboratory compatibility based on the analysis of a reference airsample with accurately determined isotopic composition

atmospheric measurement techniques

Real-time analysis of &amp;lt;i&amp;gt;δ&amp;lt;/i&amp;gt;&amp;lt;sup&amp;gt;13&amp;lt;/sup&amp;gt;C- and &amp;lt;i&amp;gt;δ&amp;lt;/i&amp;gt;D-CH&amp;lt;sub&amp;gt;4&amp;lt;/sub&amp;gt; in ambient air with laser spectroscopy: method development and first intercomparison results