Rapid High-Sensitivity Analysis of Methane Clumped Isotopes (Δ13CH3D and Δ12CH2D2) Using Mid-Infrared Laser Spectroscopy

Mid-infrared laser absorption spectroscopy enables rapid and nondestructive analysis of methane clumped isotopes. However, current analytical methods require a sample size of 20 mL STP (0.82 mmol) of pure CH₄ gas, which significantly limits its application to natural samples. To enhance the performance of spectroscopic measurement of methane clumped isotopes, we established a laser spectroscopic platform with newly selected spectral windows for clumped isotope analysis: 1076.97 cm^–1 for ¹²CH₂D₂ and 1163.47 cm^–1 for ¹³CH₃D, and a custom-built gas inlet system. These spectral windows were identified through an extensive spectral survey on newly recorded high-resolution Fourier transform infrared (FTIR) spectra across the wavelength range of 870–3220 cm^–1, thereby addressing gaps for ¹²CH₂D₂ in existing spectral databases. In addition, we implemented several key technological advances, which result in superior control and performance of sample injection and analysis. We demonstrate that for small samples ranging from 3 to 10 mL (0.12–0.41 mmol) of CH₄ gas, a measurement precision comparable to high-resolution isotope ratio mass spectrometry for Δ¹²CH₂D₂ (∼1.5‰) can be achieved through 3 to 8 repetitive measurements using a recycle-refilling system within a few hours. Samples larger than 10 mL can be quantified in under 20 min. At the same time, for Δ¹³CH₃D analysis a repeatability of 0.05‰, superior to mass spectrometry, was realized. These advancements in reducing sample size and shortening analysis time significantly improve the practicality of the spectroscopic technique for determining the clumped isotope signatures of natural methane samples, particularly for applications involving low CH₄ concentrations or requiring consecutive analyses, which are feasible in conjunction with an automated preconcentration system.