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

A quantum-cascade laser operating at a wavelength of 8.1 μm was used for high-sensitivity absorption spectroscopy of methane (CH₄). The laser frequency was continuously scanned with current over more than 3 cm^-1, and absorption spectra of the CH₄ v₄ P branch were recorded. The measured laser linewidth was 50 MHz. A CH₄ concentration of 15.6 parts in 10⁶ (ppm) in 50 Torr of air was measured in a 43-cm path length with ±0.5-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 × 10^-4. The content of ¹³CH₄ and CH₃D species in a CH₄ sample was determined.