Quantum Cascade Laser absorption spectroscopy of clumped 12C18O2
Authors: Nataraj Akshay , Gianella Michele , Prokhorov Ivan , Tuzson Béla , Faist Jérôme , Emmenegger Lukas 
Published: March 2021

&lt;p&gt;Mid-infrared optical absorption spectroscopy techniques, in particular with quantum cascade lasers (QCLs), allow realization of highly sensitive and compact spectrometers for the detection of greenhouse gases such as carbon dioxide and its stable isotopes [1]. Clumped isotope analysis of carbon dioxide is evolving into an established method in environmental, geological, and biogeochemical research. Optical clumped isotope thermometry was recently demonstrated for the &lt;sup&gt;13&lt;/sup&gt;C&lt;sup&gt;16&lt;/sup&gt;O&lt;sup&gt;18&lt;/sup&gt;O isotopologue [2]. Measurements of all isotopologues involved in isotope exchange reaction&lt;sup&gt; 12&lt;/sup&gt;C&lt;sup&gt;16&lt;/sup&gt;O&lt;sub&gt;2&lt;/sub&gt; + &lt;sup&gt;12&lt;/sup&gt;C&lt;sup&gt;18&lt;/sup&gt;O&lt;sub&gt;2&lt;/sub&gt; &amp;#8652; 2&lt;sup&gt;12&lt;/sup&gt;C&lt;sup&gt;16&lt;/sup&gt;O&lt;sup&gt;18&lt;/sup&gt;O can provide an independent temperature estimate to conventional &amp;#916;&lt;sub&gt;47&lt;/sub&gt; or &amp;#916;&lt;sub&gt;638&lt;/sub&gt; thermometry. The added dimension can resolve kinetic effects and verify temperature measurements. However, applications of clumped isotopes using ultra-high resolution mass spectrometry are strongly limited by the sample amount, long measurement times, and isobaric interferences [3].&lt;/p&gt;&lt;p&gt;In this work, we present an alternative approach based on laser spectroscopy for the simultaneous measurement of &lt;sup&gt;12&lt;/sup&gt;C&lt;sup&gt;18&lt;/sup&gt;O&lt;sub&gt;2&lt;/sub&gt;, &lt;sup&gt;12&lt;/sup&gt;C&lt;sup&gt;16&lt;/sup&gt;O&lt;sub&gt;2&lt;/sub&gt;, and &lt;sup&gt;12&lt;/sup&gt;C&lt;sup&gt;16&lt;/sup&gt;O&lt;sup&gt;18&lt;/sup&gt;O. The main experimental challenge to optically measure &lt;sup&gt;12&lt;/sup&gt;C&lt;sup&gt;18&lt;/sup&gt;O&lt;sub&gt;2&lt;/sub&gt; (natural abundance 3.95&amp;#215;10&lt;sup&gt;-6&lt;/sup&gt;) is the large spectral interference caused by the hot band transitions of the most abundant &lt;sup&gt;12&lt;/sup&gt;C&lt;sup&gt;16&lt;/sup&gt;O&lt;sub&gt;2&lt;/sub&gt; isotopologue. Our strategy to overcome this limitation is to analyze the sample CO&lt;sub&gt;2&lt;/sub&gt; gas at low temperature, i.e. close to its sublimation point. This reduces the population of these hot band transitions, thereby reducing their linestrength by a factor of 10&lt;sup&gt;5&lt;/sup&gt;.&lt;/p&gt;&lt;p&gt;The spectrometer deploys a thermoelectrically cooled, distributed feedback (DFB) QCL emitting at 2305 cm&lt;sup&gt;-1&lt;/sup&gt;. We operate the laser in intermittent continuous wave (iCW) mode [4]&lt;strong&gt; &lt;/strong&gt;with a repetition rate of 6.5 kHz. The laser beam is coupled into a compact segmented circular multipass cell (SC-MPC) [5] with an optical path length of 6 m. The cell is enclosed in a high vacuum chamber and is stabilized at 150 K with a low-vibration Stirling- cooler.&lt;/p&gt;&lt;p&gt;&amp;#160;Using pure CO&lt;sub&gt;2&lt;/sub&gt; gas samples at 10 mbar pressure, we demonstrate a precision of 0.03 &amp;#8240; and 0.02 &amp;#8240; in &lt;sup&gt;12&lt;/sup&gt;C&lt;sup&gt;18&lt;/sup&gt;O&lt;sub&gt;2&lt;/sub&gt;/&lt;sup&gt;12&lt;/sup&gt;C&lt;sup&gt;16&lt;/sup&gt;O&lt;sub&gt;2 &lt;/sub&gt;and &lt;sup&gt;12&lt;/sup&gt;C&lt;sup&gt;16&lt;/sup&gt;O&lt;sup&gt;18&lt;/sup&gt;O/&lt;sup&gt;12&lt;/sup&gt;C&lt;sup&gt;16&lt;/sup&gt;O&lt;sub&gt;2&lt;/sub&gt; ratios in less than one minute averaging time. Repeated series of 30 consecutive measurements of &amp;#916;&lt;sub&gt;48 &lt;/sub&gt;has a standard deviation of 0.12 &amp;#8240; (SE = 0.022 &amp;#8240;). The isotope ratio scale is investigated through the analysis of pure CO&lt;sub&gt;2&lt;/sub&gt; samples, which range in &amp;#948;&lt;sup&gt;18&lt;/sup&gt;O from -25 &amp;#8240; to -14 &amp;#8240; vs VSMOW . The instrument reproduced the scale within 0.2 &amp;#8240;, which corresponds to the uncertainty of the reported &amp;#948;&lt;sup&gt;18&lt;/sup&gt;O values.&lt;/p&gt;&lt;p&gt;This unique approach of using cryogenically cooled MPC to reduce the interference of hot band transition from abundant isotopes, represents a promising method for high-precision quantification of the CO&lt;sub&gt;2&lt;/sub&gt; clumped isotopes, opening up new possibilities in geosciences.&lt;/p&gt;&lt;p&gt;&lt;strong&gt;&amp;#160;&lt;/strong&gt;&lt;/p&gt;&lt;p&gt;[1] P. Sturm et al, Atmospheric Measurement Techniques, 6(7), 1659, 2013&lt;/p&gt;&lt;p&gt;[2] I. Prokhorov et al, Sci Rep, 9(1), p. 4765, 2019&lt;/p&gt;&lt;p&gt;[3] D. Bajnai et al., Nature Communications, 11(1), pg 4005, 2020&lt;/p&gt;&lt;p&gt;[4] M. Fischer et al&amp;#160; Opt. Express, 22(6), 7014, 2014&lt;/p&gt;&lt;p&gt;[5] M. Graf et al, Optics Letters, 43(11) 2434 2018&lt;/p&gt;