Cryogen-free fully automated preconcentration unit to enable Δ13CH3D and Δ12CH2D2 analysis

<div> <p>Clumped isotope analysis is a powerful tool to constrain methane's origin in various geological, biogeochemical, and environmental settings. The extremely low abundance of <sup>12</sup>CH<sub>2</sub>D<sub>2</sub> and <sup>13</sup>CH<sub>3</sub>D isotopologues poses a challenge for the existing analytical systems. Mole fraction enhancement and cleaning of the environmental samples are necessary to fully exploit the potential of modern analyzers, requiring 50 to 1000 &#956;mol of pure CH<sub>4</sub>. To enable high-precision &#916;<sup>12</sup>CH<sub>2</sub>D<sub>2</sub> and &#916;<sup>13</sup>CH<sub>3</sub>D analysis with dual-QCL (8.6 &#956;m and 9.3 &#956;m) absorption spectroscopy in a multipass cell (400 m), we have developed a new automated cryogen-free unit for methane <em>Clean</em>ing and <em>Ex</em>traction &#8211;<em>&#160;CleanEx</em>. </p> </div><div> <p>The unit can process up to 18 liters of sample air at a high flow rate of 900 ml min<sup>&#8722;1</sup>. Methane (T<sub>BP</sub> = &#8722;161 &#176;C) is separated from major air components on a high capacity trap filled with HayeSep D (Trap 1, &#8722;176 &#176;C). Sequential desorption and transfer to a second trap (Trap 2, &#8722;181&#176;C) ensure complete O<sub>2</sub> and Ar removal. Substances with a higher boiling point, i.e., CO<sub>2</sub>, N<sub>2</sub>O, H<sub>2</sub>O, C<sub>n</sub>H<sub>m</sub>, remain on Trap 1 to be removed at a later conditioning phase. CleanEx demonstrates equal performance for gas mixtures with initial methane mole fraction ranging from 2 ppm up to 2%. In contrast to the previously developed single trap TREX system (Eyer <em>et al.,</em> 2016), atmospheric O<sub>2</sub> and Ar are effectively separated by cryo-focusing of CH<sub>4</sub> on the second trap. Ongoing work is focused on the separation of atmospheric gases with boiling points close to methane, e.g., Kr (T<sub>BP</sub> = &#8722;153 &#176;C). </p> <p>We present the instrument design, performance, and details of its operation. Fractionation effects, methane recovery efficiency, and implications for high-precision &#948;<sup>13</sup>C-CH<sub>4</sub>, &#948;D-CH<sub>4</sub>, &#916;<sup>13</sup>CH<sub>3</sub>D, and &#916;<sup>12</sup>CH<sub>2</sub>D<sub>2</sub> analyses are being discussed. </p> <p><em>Acknowledgments. </em> This work was supported by the Swiss National Science Foundation (SNSF) under R'Equip project QCL4CLUMPS (no. 206021_183294) and the project STELLAR (grant no. 19ENV05; Stable isotope metrology to enable climate action and regulation) which has received funding from the EMPIR programme co-financed &#160;by &#160;the &#160;Participating &#160;States &#160;and &#160;from &#160;the &#160;European &#160;Union's &#160;Horizon &#160;2020 &#160;research &#160;and innovation programme. </p> <p>&#160;</p> <p><strong>References</strong></p> <p>Eyer, S. , Tuzson, B. , Popa, M. E. , van der Veen, C. , R&#246;ckmann, T. , Rothe, M. , Brand, W. A. , Fisher, R. , Lowry, D. , Nisbet, E. G. , Brennwald, M. S. , Harris, E. , Zellweger, C. , Emmenegger, L. , Fischer, H. , and Mohn, J.: Real-time analysis of&#160;&#948;<sup>13</sup>C- and&#160;&#948;D-CH<sub>4</sub>&#160;in ambient air with laser spectroscopy: method development and first intercomparison results, Atmos. Meas. Tech., 9, 263&#8211;280, https://doi.org/10.5194/amt-9-263-2016, 2016. </p> <p>&#160;</p> <p>&#160;</p> </div>