Abstract
12ch4 and 13ch4 were measured by wavelength modulation spectroscopy and a Herriott-type multi-pass cell with an optical path length of 29.9 m using a distributed feedback laser diode in the 2.4-μm wavelength region. 10 select the appropriate absorption lines for isotope ratio measurement, we comparatively analyzed the absorption spectra of 12CH4 and 13CH4 measured using their pure samples in the wavelength range of 4147 - 4154 cm-1 because the available molecular spectroscopy database does not explicitly list the data for 13ch4 in this region.Resultantly we found two well-isolated lines of 12CH4 at 4151.845 cm-1 and 13ch4 at 4151.966 cm-1. The wavelength modulation spectrum of 1020 ppm ch4 in synthetic air obtained at 10 kPa indicates that the selected lines are capable of determining the stable carbon ratio in CH4.
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