Methane concentration and isotopic composition measurements with a mid-infrared quantum-cascade laser

A quantum-cascade laser operating at a wavelength of $8.1 \mathit{\mu }\mathrm{m}$ was used for high-sensitivity absorption spectroscopy of methane $\left({\mathrm{CH}}_4\right)$. The laser frequency was continuously scanned with current over more than $3 {\mathrm{cm}}^{-1}$, and absorption spectra of the ${\mathrm{CH}}_4$ ${\nu }_4$ $P$ branch were recorded. The measured laser linewidth was 50 MHz. A ${\mathrm{CH}}_4$ concentration of 15.6 parts in ${10}^6 \left(\mathrm{ppm}\right)$ in 50 Torr of air was measured in a 43-cm path length with $\pm 0.5‐\mathrm{ppm}$ accuracy when the signal was averaged over 400 scans. The minimum detectable absorption in such direct absorption measurements is estimated to be $1.1\times {10}^{-4}$. The content of ${ }^{13}{\mathrm{CH}}_4$ and ${\mathrm{CH}}_3\mathrm{D}$ species in a ${\mathrm{CH}}_4$ sample was determined.