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 Research^3,4), 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