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

We present precise measurements of doubly deuterated methane ((CH2D2)-C-12) 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 (CH3D)-C-13/(CH4)-C-12 and (CH2D2)-C-12/(CH4)-C-12 are measured at 0.01 parts per thousand and 0.5 parts per thousand (1 sigma) instrumental precision, respectively. The instrumental accuracy was assessed by measuring a series of methane gases with a range of delta C-13 and delta D values but with the abundances of all isotopologues driven to thermal equilibrium at 250 degrees C. The estimated accuracy of Delta(CH2D2)-C-12 is 1 parts per thousand (1 sigma) 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.