Configuration for cw-CRDS: Optical cavity composed of two highly reflective mirrors (reflectivity 99.98%, radius of curvature 100 cm), separated by a distance of 54.5 cm; cavity length continuously modulated by 4 μm; quantum cascade laser with a tuning range of 1224 - 1234 cm−1; absorption feature probed within the spectral range of 1231.57 - 1232.00 cm−1