Development of a Midinfrared Cavity Ring-Down Spectrometer for High-Precision Analysis of Carbon Dioxide Clumped Isotopic Compositions

A midinfrared cavity ring-down (CRD) spectrometer was developed employing a 4.44 μm quantum cascade laser (QCL) for the sensitive detection of CO₂ isotopologues, including ¹²C¹⁶O₂ (44), ¹³C¹⁶O₂ (45), ¹²C¹⁶O¹⁸O (46), and ¹³C¹⁶O¹⁸O (47). Instead of using an acousto-optic modulator (AOM), rapid QCL current switching was employed to initiate ring-down events, effectively reducing optical losses typically introduced by the AOM and simplifying the system design. The spectrometer incorporated a high-finesse (∼50,000) optical cavity, achieving a minimum detectable absorption coefficient (MDAC) of 2.2 × 10⁻¹¹ cm⁻¹ with 14.5 s average time. A continuous frequency sweep within a 0.2 cm⁻¹ spectral window enabled simultaneous analysis of the four target isotopologues. Allan deviation analysis over a 6 h continuous measurement revealed optimal detection precisions of 0.45 ‰ for δ⁴⁵/⁴⁴, 0.28 ‰ for δ⁴⁷/⁴⁶, 0.47 ‰ for δ⁴⁶/⁴⁴, 0.35 ‰ for δ⁴⁷/⁴⁵, and 0.3 ‰ for δ⁴⁷/⁴⁴ at 65 min averaging time, demonstrating excellent long-term stability and high detection precisions. Coupled with a dedicated gas handling system, the instrument enabled analysis of gas volumes as small as ∼10 mL. The CRDS system further demonstrated robust performance in real-sample analyses, exhibiting strong linear agreement with isotope ratio mass spectrometry (IRMS). High-precision determinations of the δ¹³C, δ¹⁸O, and δ ⁴⁷/⁴⁴ isotope ratios were successfully achieved.