Quantum Cascade Laser absorption spectroscopy of clumped 12C18O2

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 ¹³C¹⁶O¹⁸O isotopologue [2]. Measurements of all isotopologues involved in isotope exchange reaction ¹²C¹⁶O₂ + ¹²C¹⁸O₂ ⇌ 2¹²C¹⁶O¹⁸O can provide an independent temperature estimate to conventional Δ₄₇ or Δ₆₃₈ 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].

In this work, we present an alternative approach based on laser spectroscopy for the simultaneous measurement of ¹²C¹⁸O₂, ¹²C¹⁶O₂, and ¹²C¹⁶O¹⁸O. The main experimental challenge to optically measure ¹²C¹⁸O₂ (natural abundance 3.95×10^-6) is the large spectral interference caused by the hot band transitions of the most abundant ¹²C¹⁶O₂ isotopologue. Our strategy to overcome this limitation is to analyze the sample CO₂ 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 10⁵.

The spectrometer deploys a thermoelectrically cooled, distributed feedback (DFB) QCL emitting at 2305 cm^-1. We operate the laser in intermittent continuous wave (iCW) mode [4] 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.

 Using pure CO₂ gas samples at 10 mbar pressure, we demonstrate a precision of 0.03 ‰ and 0.02 ‰ in ¹²C¹⁸O₂/¹²C¹⁶O₂ and ¹²C¹⁶O¹⁸O/¹²C¹⁶O₂ ratios in less than one minute averaging time. Repeated series of 30 consecutive measurements of Δ₄₈ has a standard deviation of 0.12 ‰ (SE = 0.022 ‰). The isotope ratio scale is investigated through the analysis of pure CO₂ samples, which range in δ¹⁸O from -25 ‰ to -14 ‰ vs VSMOW . The instrument reproduced the scale within 0.2 ‰, which corresponds to the uncertainty of the reported δ¹⁸O values.

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 CO₂ clumped isotopes, opening up new possibilities in geosciences.

 

[1] P. Sturm et al, Atmospheric Measurement Techniques, 6(7), 1659, 2013

[2] I. Prokhorov et al, Sci Rep, 9(1), p. 4765, 2019

[3] D. Bajnai et al., Nature Communications, 11(1), pg 4005, 2020

[4] M. Fischer et al  Opt. Express, 22(6), 7014, 2014

[5] M. Graf et al, Optics Letters, 43(11) 2434 2018