DEVELOPMENT OF A TUNABLE DIODE-LASER ABSORPTION SPECTROMETER FOR MEASUREMENTS OF THE C-13/C-12 RATIO IN METHANE

A tunable diode laser absorption spectrometer (TDLAS) for measuring the C-13/C-12 ratio in methane has been developed. Using a triple path arrangement the spectra of the CH4 sample, a (CH4)-C-13/(CH4)-C-12 isotope standard and pure (CH4)-C-13 are recorded simultaneously and compared to evaluate the (CH4)-C-13/(CH4)-C-12 ratio of the sample, using a (CH4)-C-13-(CH4)-C-12 absorption line pair near 3007 cm-1. Systematic effects due to variations in temperature, pressure, and optical density were measured for this rotational-vibrational transition pair. Optical interference effects are effectively suppressed by linearly polarizing the laser beam and using Brewster windows for gas cells and detectors. The overall deltaC-13 accuracy vs. the PDB scale is about +/- 1 parts per thousand for a CH4 concentration of 2.5% (sample size: 5 mumoles = 0.11 STP cm3 CH4) using 36 cm long absorption cells. The future application of a multipass cell should allow measurement concentrations of CH4 down to about 50 ppm. The main advantages of the new method are the short measurement time of 10-15 min for one sample and the direct measurement on the CH, molecule without the need to chemically convert it to CO2. With the present accuracy the new method should be useful for the measurement of CH4 sources, allowing a greater sample throughput compared to the conventional mass spectrometry technique.