Pros and cons of methane clumped isotope analysis by high-resolution isotope-ratio mass spectrometry and laser absorption spectroscopy

Bulk isotope analytical methods of CH4 quantify carbon and hydrogen isotope ratios (δ13C and δD) to provide information on the sources and sinks of CH4 in natural environments. A more extensive tracing of CH4 pathways, especially when multiple processes and sources are involved, has been realized by novel measurements techniques capable of methane clumped isotope analysis (termed as Δ13CH3D and Δ12CH2D2) during the past decade. These paired datasets can either be used as proxy for exploring CH4 formation temperatures under thermodynamic equilibrium, or studying contributions of kinetically controlled processes during CH4 formation and consumption1. Currently, methane clumped isotope analysis is performed by two different techniques: isotope-ratio mass spectrometry (e.g. 253 Ultra from Thermo Fisher Scientific2 or Panorama from Nu Instruments1) or laser absorption spectroscopy (e.g. QCLAS from Aerodyne Research3,4), both of which have demonstrated a precision better than 0.5‰ for Δ13CH3D and 1.5‰ for Δ12CH2D2, which is sufficient for most applications. This work will provide insights about the main instrumental features, measurement protocols and performance of the 253 Ultra HR-IRMS at Tokyo Institute of Technology (Japan)2,5, and the QCL absorption spectrometer at Empa (Switzerland)4. Furthermore, advantages and limitations of both techniques during current applications in natural methane samples are discussed. Finally, perspectives for future applications at low CH4 concentrations, such as atmospheric monitoring, are provided.   References: Young et al., 2017, Geochimica et Cosmochimica Acta; 2. Dong et al., 2020, Thermo Scientific white paper; 3. Gonzalez et al., 2019, Analytical Chemistry; 4. Prokhorov and Mohn, 2022, Analytical Chemistry; 5. Zhang et al., 2021, Geochimica et Cosmochimica Acta