Cryogen-free fully automated preconcentration unit to enable &#916;13CH3D and &#916;12CH2D2 analysis
Authors: Prokhorov Ivan , Mohn Joachim 
Published: March 2021

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&lt;p&gt;Clumped isotope analysis is a powerful tool to constrain methane's origin in various geological, biogeochemical, and environmental settings. The extremely low abundance of &lt;sup&gt;12&lt;/sup&gt;CH&lt;sub&gt;2&lt;/sub&gt;D&lt;sub&gt;2&lt;/sub&gt; and &lt;sup&gt;13&lt;/sup&gt;CH&lt;sub&gt;3&lt;/sub&gt;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 &amp;#956;mol of pure CH&lt;sub&gt;4&lt;/sub&gt;. To enable high-precision &amp;#916;&lt;sup&gt;12&lt;/sup&gt;CH&lt;sub&gt;2&lt;/sub&gt;D&lt;sub&gt;2&lt;/sub&gt; and &amp;#916;&lt;sup&gt;13&lt;/sup&gt;CH&lt;sub&gt;3&lt;/sub&gt;D analysis with dual-QCL (8.6 &amp;#956;m and 9.3 &amp;#956;m) absorption spectroscopy in a multipass cell (400 m), we have developed a new automated cryogen-free unit for methane &lt;em&gt;Clean&lt;/em&gt;ing and &lt;em&gt;Ex&lt;/em&gt;traction &amp;#8211;&lt;em&gt;&amp;#160;CleanEx&lt;/em&gt;.&lt;/p&gt;
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&lt;p&gt;The unit can process up to 18 liters of sample air at a high flow rate of 900 ml min&lt;sup&gt;&amp;#8722;1&lt;/sup&gt;. Methane (T&lt;sub&gt;BP&lt;/sub&gt; = &amp;#8722;161 &amp;#176;C) is separated from major air components on a high capacity trap filled with HayeSep D (Trap 1, &amp;#8722;176 &amp;#176;C). Sequential desorption and transfer to a second trap (Trap 2, &amp;#8722;181&amp;#176;C) ensure complete O&lt;sub&gt;2&lt;/sub&gt; and Ar removal. Substances with a higher boiling point, i.e., CO&lt;sub&gt;2&lt;/sub&gt;, N&lt;sub&gt;2&lt;/sub&gt;O, H&lt;sub&gt;2&lt;/sub&gt;O, C&lt;sub&gt;n&lt;/sub&gt;H&lt;sub&gt;m&lt;/sub&gt;, 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 &lt;em&gt;et al.,&lt;/em&gt; 2016), atmospheric O&lt;sub&gt;2&lt;/sub&gt; and Ar are effectively separated by cryo-focusing of CH&lt;sub&gt;4&lt;/sub&gt; on the second trap. Ongoing work is focused on the separation of atmospheric gases with boiling points close to methane, e.g., Kr (T&lt;sub&gt;BP&lt;/sub&gt; = &amp;#8722;153 &amp;#176;C).&lt;/p&gt;
&lt;p&gt;We present the instrument design, performance, and details of its operation. Fractionation effects, methane recovery efficiency, and implications for high-precision &amp;#948;&lt;sup&gt;13&lt;/sup&gt;C-CH&lt;sub&gt;4&lt;/sub&gt;, &amp;#948;D-CH&lt;sub&gt;4&lt;/sub&gt;, &amp;#916;&lt;sup&gt;13&lt;/sup&gt;CH&lt;sub&gt;3&lt;/sub&gt;D, and &amp;#916;&lt;sup&gt;12&lt;/sup&gt;CH&lt;sub&gt;2&lt;/sub&gt;D&lt;sub&gt;2&lt;/sub&gt; analyses are being discussed.&lt;/p&gt;
&lt;p&gt;&lt;em&gt;Acknowledgments.&lt;/em&gt; 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 &amp;#160;by &amp;#160;the &amp;#160;Participating &amp;#160;States &amp;#160;and &amp;#160;from &amp;#160;the &amp;#160;European &amp;#160;Union's &amp;#160;Horizon &amp;#160;2020 &amp;#160;research &amp;#160;and innovation programme.&lt;/p&gt;
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&lt;p&gt;&lt;strong&gt;References&lt;/strong&gt;&lt;/p&gt;
&lt;p&gt;Eyer, S., Tuzson, B., Popa, M. E., van der Veen, C., R&amp;#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&amp;#160;&amp;#948;&lt;sup&gt;13&lt;/sup&gt;C- and&amp;#160;&amp;#948;D-CH&lt;sub&gt;4&lt;/sub&gt;&amp;#160;in ambient air with laser spectroscopy: method development and first intercomparison results, Atmos. Meas. Tech., 9, 263&amp;#8211;280, https://doi.org/10.5194/amt-9-263-2016, 2016.&lt;/p&gt;
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