Towards simultaneous methane clumped-isotopologue and radiocarbon measurements by cavity-ring-down spectroscopy

Methane (CH₄) is a potent greenhouse gas, and proper source attribution is essential for effective climate change mitigation. Traditional measurement techniques, such as singly substituted isotope ratio measurements (δ¹³C and δD) by IRMS or optical spectroscopy, often struggle to distinguish between different methane sources because their isotopic ratios can overlap. Doubly substituted isotope analysis, commonly referred to as clumped isotope analysis, offers improved differentiation. However, these isotopologues are challenging to detect due to their low abundance. In this study, we demonstrate the detection of a doubly substituted methane isotopologue (¹³CH₃D) using cavity-ring-down spectroscopy (CRDS) at a wavelength of 4.5 µm, a region also used for the detection of ¹⁴CO₂. We further present a comprehensive data analysis routine for our CRDS measurements, including higher order mode filtering based on signal selection criteria, and relative and absolute frequency calibration. The approach is general and can be applied to other CRDS systems. Spectral lines recently assigned to ¹³CH₃D were addressed and characterized. The system achieves a precision of 19‰ at an averaging time of 50 minutes for ¹³CH₃D. Our results demonstrate that CRDS provides a sensitive and reliable approach for monitoring doubly substituted methane isotopologues on a shared platform with the capability for radiocarbon detection.