Comparison of infrared and mass-spectrometric measurements of carbon-13/carbon-12 ratios

High-precision isotopic ratio measurement system for methane (12CH3D/12CH4,13CH4/12CH4) by using near-infrared diode laser absorption spectroscopy

We demonstrate that the absorption spectroscopic method can be applied to a precise deltaD (an index of 12CH3D/12CH4) and delta13C (an index of 13CH4/12CH4) analysis for methane samples of natural isotopic abundance. We chose an appropriate absorption line pair whose absorption coefficients have nearly the same temperature dependences so as to minimize the temperature effect in absorbance ratio measurements. We measured 12CH3D/12CH4 ratio by using near-infrared external cavity diode lasers and a new type multi-pass cell. The deltaD value can be determined from the 12CH3D/12CH4 signal-intensity ratio with a fine correction by taking account of the interference of 13CH4 lines. Similarly, the delta13C value is determined from the 13CH4/12CH4 signal-intensity ratio, which is measured by using distributed-feedback laser and a modified Herriot-type cell and corrected for the abundance of 12CH3D. The precision was +/-0.7 and +/-0.027/1000 for deltaD and delta13C, respectively.

High-precision direct measurements of (13)CH(4)/(12)CH(4) and (12)CH(3)D/(12)CH(4) ratios in atmospheric methane sources by means of a long-path tunable diode laser absorption spectrometer

Measurements of (13)CH(4)/(12)CH(4) and (12)CH(3)D/(12)CH(4) ratios in atmospheric methane (CH(4)) sources provide important information about the global CH(4) budget as well as about CH(4) production and consumption processes occurring within the various sources. As an alternative to the conventional mass spectrometer (MS) technique, which requires conversion of CH(4) to CO(2) and H(2), we have developed a tunable diode laser absorption spectrometer (TDLAS), which permits rapid direct measurements of the (13)CH(4)/(12)CH(4) and (12)CH(3)D/(12)CH(4) ratios. An intercomparison between TDLAS and MS techniques for samples from natural wetlands, landfills, and natural gas sources resulted in a mean deviation of Δδ(13)C = 0.44‰ and ΔδD = 5.1‰. In the present system the minimum mixing ratios required are 50 parts in 10(6) by volume (ppmv) CH(4) (sample size 2 µmol CH(4)) for direct δ(13)C measurements and 2000 ppmv (sample size 80 µmol CH(4)) for direct δD measurements. These mixing-ratio limits are adequate for most CH(4) source characterization studies without requiring sample preconcentration.