Quantum cascade laser absorption spectrometer with a low temperature multipass cell for precision clumped 12C18O2 measurement

High precision measurement of multiply substituted ("clumped") isotopologues of CO₂ is a topic of significant interest in the fields of isotope geochemistry and paleoclimate research [1, 2]. The temperature-dependent behavior of ¹³C and ¹⁸O isotopes in gaseous carbon dioxide is widely used as a temperature proxy for paleoclimate reconstruction. The basis for it lies in the temperature dependence of the equilibrium constant, K(T), of the isotope exchange reactions ¹²C¹⁸O₂ + ¹²C¹⁶O₂ ↔ 2٠¹²C¹⁶O¹⁸O and ¹³C¹⁶O¹⁸O + ¹²C¹⁶O₂ ↔ ¹³C¹⁶O₂+¹²C¹⁶O¹⁸O [3, 4] as these reactions have a slight tendency to move towards the right at higher temperatures. Currently, the established method to perform clumped isotope thermometry is Isotope Ratio Mass Spectrometry (IRMS) [5]. However, IRMS measurements, in particular for rare isotopologues, typically require several hours of analysis time and extensive sample preparation to properly separate isobaric interferences. In contrast to IRMS, optical absorption spectroscopic techniques allow the realisation of isotopologue specific, non-destructive, and compact spectrometers with short analysis time and high-precision capabilities. Recently, Wang et al. [6], Prokhorov et al. [3], and Nataraj et al. [4] have demonstrated the great promise of laser absorption spectroscopy for measurements of clumped isotopes of carbon dioxide.

The major challenge for clumped isotope thermometry using ¹²C¹⁸O₂ resides in its very low natural relative abundance (4.1 ppm) and the spectral interference from the major (¹²C¹⁶O₂) and singly substituted isotopologues. These factors seriously limit the achievable analytical performance of spectroscopic measurements and thus the applicability of this technique. However, the interference caused by the hot-band transitions of the abundant species can be suppressed by reducing the gas temperature. Moreover, working at low pressure (5 mbar) narrows the absorption lines and reduces the overlap between neighbouring transitions.

Here, we present a novel quantum cascade laser absorption spectrometer (QCLAS) employing a low-volume segmented circular multipass cell (SC-MPC) [7] operated at cryogenic temperatures (153 K) and low pressure (5 mbar). For the first time, we optically measure the abundances of all three isotopologues involved in the reaction ¹²C¹⁸O₂ + ¹²C¹⁶O₂ ↔ 2٠¹²C¹⁶O¹⁸O simultaneously. We report a precision of 0.05 ‰ in the isotope ratios [¹²C¹⁸O₂/¹²C¹⁶O₂] and [¹²C¹⁶O¹⁸O/¹²C¹⁶O₂] with 25 s integration time. In addition, we determine and resolve the tiny variation in the equilibrium constant, K(T), of the above exchange reaction for carbon-dioxide samples equilibrated at 300 K and 1273 K, respectively. This versatile system can be extended to other chemical species where spectroscopic measurements are impacted by the hot-band transitions of abundant isotopologues — (e.g., methane and its deuterated isotopologues, CH₃D and CH₂D₂, or propane and the two isotopomers, ¹²CH₃¹³CH₂¹²CH₃ and ¹³CH₃¹²CH₂¹²CH₃) — thereby opening up new perspectives in environmental sciences and fundamental research.

[1] J. M. Eiler, Quaternary Science Reviews,doi: 10.1016/j.quascirev.2011.09.001.

[2] S. M. Bernasconi et al., Applied Geochemistry doi: 10.1016/j.apgeochem.2011.03.080.

[3] I. Prokhorov et a.l, Sci Rep, doi: 10.1038/s41598-019-40750-z.

[4] A. Nataraj et al., Optics Express accepted, doi:10.1364/OE.447172

[5] Fiebig et al, Chemical Geology doi: 10.1016/j.chemgeo.2019.05.019

[6] Z. Wang et al., Anal. Chem., doi: 10.1021/acs.analchem.9b04466.

[7] M. Graf et al.,Optics Letters doi: 10.1364/OL.43.002434.