Precise measurement of the 13CH4/12CH4 ratio of diluted methane using a near-infrared laser absorption spectrometer

We measure the 13CH4/12CH4 ratio of methane diluted with air or nitrogen to investigate the applicability of our laser spectrometer to real environmental samples. We developed a 1.66 μm diode laser absorption spectrometer and demonstrated high precision on pure methane, but for practical observations it is appropriate to measure the samples directly without purification. We examine effects of different pressure, concentration and the kind of gases for diluting between samples and a working-standard on the measurement. The effects are found to be small and the high repetitive precision (reproducibility) of ∼0.1‰ (1σ) is obtained at 10 and 1% methane concentrations without any correction or automatic control of the gas conditions. At 0.1% concentration, however, the precision stays at ∼1‰. It is expected that the high precision of ∼0.1‰ at the 0.1% concentration cannot be achieved, due to saturation of the signal intensity at high pressure of the sample in a multi-pass cell, caused by pressure broadening of methane absorption lines. The relationship between the isotopic ratio measured by the laser and that by the isotopic ratio mass spectrometer (IRMS) is linear with small fluctuations, but the slope is slightly different from unity.