Clumped isotope analysis in nitrous oxide by mid-IR laser spectroscopy: analytical developments and validation
Authors: Kantnerová Kristýna , Yu Longfei , Zindel Daniel , Zahniser Mark S. , Nelson David D. , Tuzson Béla , Emmenegger Lukas , Nakagawa Mayuko , Toyoda Sakae , Yoshida Naohiro , Bernasconi Stefano M. , Mohn Joachim 
Published: March 2020

&lt;p&gt;Nitrous oxide (N&lt;sub&gt;2&lt;/sub&gt;O) has been for long a major focus of all greenhouse gas accounting agreements. Understanding the mechanisms of its formation and clarifying its sources and sinks are highly important for mitigating N&lt;sub&gt;2&lt;/sub&gt;O emissions. In this context, measuring the doubly substituted isotopocules of N&lt;sub&gt;2&lt;/sub&gt;O can add new and unique opportunities to fingerprint and constrain the biogeochemical N&lt;sub&gt;2&lt;/sub&gt;O cycle, similar to other atmospheric species such as CO&lt;sub&gt;2&lt;/sub&gt;, CH&lt;sub&gt;4&lt;/sub&gt;, and O&lt;sub&gt;2&lt;/sub&gt;.&lt;/p&gt;&lt;p&gt;We address this challenging research field by developing and validating a laser spectroscopic technique for selective analysis of the eight most abundant N&lt;sub&gt;2&lt;/sub&gt;O isotopic species including the doubly substituted isotopocules &lt;sup&gt;14&lt;/sup&gt;N&lt;sup&gt;15&lt;/sup&gt;N&lt;sup&gt;18&lt;/sup&gt;O, &lt;sup&gt;15&lt;/sup&gt;N&lt;sup&gt;14&lt;/sup&gt;N&lt;sup&gt;18&lt;/sup&gt;O, and &lt;sup&gt;15&lt;/sup&gt;N&lt;sup&gt;15&lt;/sup&gt;N&lt;sup&gt;16&lt;/sup&gt;O. This method is based on quantum cascade laser absorption spectroscopy (QCLAS) and reaches a precision of 0.01 &amp;#8211; 0.20 &amp;#8240; with 1 &amp;#8211; 2 min spectral averaging on samples of 4&amp;#160;&amp;#956;mol of N&lt;sub&gt;2&lt;/sub&gt;O in N&lt;sub&gt;2&lt;/sub&gt; at 4&amp;#160;hPa.&lt;/p&gt;&lt;p&gt;Furthermore, we have established a new reference frame combining two independent approaches: (1) clumped N&lt;sub&gt;2&lt;/sub&gt;O isotopocule abundances were linked to stochastic distribution by equilibrating the N&amp;#8211;O bond in the N&lt;sub&gt;2&lt;/sub&gt;O molecule over activated Al&lt;sub&gt;2&lt;/sub&gt;O&lt;sub&gt;3&lt;/sub&gt; at 100 and 200 &amp;#176;C, and (2) individual isotopocule concentrations were calibrated using a set of high-accuracy gravimetric N&lt;sub&gt;2&lt;/sub&gt;O-in-N&lt;sub&gt;2&lt;/sub&gt; gas mixtures. The latter approach, applied for the first time to clumped isotope measurements, has a particular potential in realizing regular multi-point calibration for species like &lt;sup&gt;15&lt;/sup&gt;N&lt;sup&gt;15&lt;/sup&gt;N&lt;sup&gt;16&lt;/sup&gt;O, because no procedure for equilibration of the N&amp;#8211;N bond has been successful yet.&lt;/p&gt;&lt;p&gt;Results of the validation measurements, using the QCLAS method and calibration approach, are presented for a large range of &amp;#948; values (approx. 100 &amp;#8240; for d&lt;sup&gt;15&lt;/sup&gt;N and d&lt;sup&gt;18&lt;/sup&gt;O). Inter-comparison measurements with high-resolution mass spectrometry show compatible results for bulk isotopic composition (d&lt;sup&gt;15&lt;/sup&gt;N, d(458+548)), but superior performance of QCLAS for determining site-selectivity (SP, SP&lt;sup&gt;18&lt;/sup&gt;). In summary, this work provides new methodological basis for the measurements of clumped N&lt;sub&gt;2&lt;/sub&gt;O isotopes and has a high potential to stimulate future research in the N&lt;sub&gt;2&lt;/sub&gt;O community by establishing a new class of reservoir-insensitive tracers and molecular-scale insights.&lt;/p&gt;