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 cm-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‰ (1) instrumental precision, respectively. The instrumental accuracy was assessed by measuring a series of methane gases with a range of 13C and D values but with the abundances of all isotopologues driven to thermal equilibrium at 250 C. The estimated accuracy of 12CH2D2 is 1‰ (1) based on 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.