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 CO 2 isotopologues, including 12 C 16 O 2 (44), 13 C 16 O 2 (45), 12 C 16 O 18 O (46), and 13 C 16 O 18 O (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 δ 45 / 44, 0.28 ‰ for δ 47 / 46, 0.47 ‰ for δ 46 / 44, 0.35 ‰ for δ 47 / 45, and 0.3 ‰ for δ 47 / 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 δ 13 C, δ 18 O, and δ 47 / 44 isotope ratios were successfully achieved.