Precise Measurements of CHD by Tunable Infrared Laser Direct Absorption Spectroscopy.

We present precise measurements of doubly deuterated methane (CHD) 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, the instrument simultaneously measures the five main isotopologues of methane. The ratios CHD/CH and CHD/CH 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 δC and δD values but with the abundances of all isotopologues driven to thermal equilibrium at 250 °C. The estimated accuracy of ΔCHD 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.