Precise Measurements of ¹²CH₂D₂ by Tunable Infrared Laser Direct Absorption Spectroscopy

We present precise measurements of doubly deuterated methane (¹²CH₂D₂) in natural methane samples using tunable infrared laser direct absorption spectroscopy (TILDAS). Using a 413 m optical path length astigmatic Herriott cell and two quantum cascade lasers (QCLs) scanning the spectral regions of 1090.46 ± 0.1 and 1200.23 ± 0.1 cm^-1, the instrument simultaneously measures the five main isotopologues of methane. The ratios ¹³CH₃D/¹²CH₄ and ¹²CH₂D₂/¹²CH₄ are measured at 0.01‰ and 0.5‰ (1σ) instrumental precision, respectively. The instrumental accuracy was assessed by measuring a series of methane gases with a range of δ¹³C and δD values but with the abundances of all isotopologues driven to thermal equilibrium at 250 °C. The estimated accuracy of Δ¹²CH₂D₂ is 1‰ (1σ) on the basis of the results of the heated methane samples. This new TILDAS instrument provides a simple and rapid technique to explore the sources of methane in the environment.