Real-time detection of 13CH4 in ambient air by use of mid-infrared cavity leak-out spectroscopy.

We report on spectroscopic real-time detection of ${ }^{13}{\mathrm{CH}}_4$ in ambient air. Our measurements were carried out by means of cavity leak-out absorption spectroscopy employing a tunable cw laser in the mid-infrared spectral region near $\lambda =3 \mathit{\mu }\mathrm{m}$. A CO laser in combination with tunable microwave sideband generation was used as the light source. Using a 50-cm-long ringdown cell with $R=99.98\%$ mirrors, we achieved a detection limit of 290 parts in ${10}^{12}$ (ppt) ${ }^{13}{\mathrm{CH}}_4$ in ambient air (integration time, 100 s). The corresponding noise-equivalent absorption coefficient was $5\times {10}^{-9}/\mathrm{cm}$.