Rapid High-Sensitivity Analysis of Methane Clumped Isotopes (ΔCHD and ΔCHD) 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 CH 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 for CHD and 1163.47 cm for CHD, 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, thereby addressing gaps for CHD 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 CH gas, a measurement precision comparable to high-resolution isotope ratio mass spectrometry for ΔCHD (∼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 ΔCHD 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 CH concentrations or requiring consecutive analyses, which are feasible in conjunction with an automated preconcentration system.