The measurement setup is depicted in figure . The spectroscopic part is similar to that described in our previous article. In brief, it consists of a continuous-wave quantum cascade laser (QCL) and a photoacoustic cell containing an optically read cantilever microphone (PA201, Gasera). The laser is an InAs and AlSb based QCL with an emission wavelength of 14.8 um. The QCL used here was a custom build laser provided for this work by University of Montpellier, although now these lasers are commercially available (uniMir, mirSense). The operational temperature of the QCL is about 250 K, which is maintained with a water-cooled thermoelectric cooler. The output power is up to about 5mW. The laser beam passes through the CEPAS cell with anti-reflection coated ZnSe windows. The volume of the cell is about 8 mL and the cell temperature is stabilized to 50 °C. The optical power after the photoacoustic cell is measured with a thermopile detector and the power reading is used to normalize the photoacoustic signal. For the spectral measurements, the QCL wavelength is tuned by its current. Wavelength modulation absorption spectroscopy (WMAS) is used for the detection, and the wavelength modulation is also applied through the QCL current tuning. At the start of a measurement cycle, first several points are measured at the peak of a CO 2 line at 674.075 cm -1 , followed by a measurement of 23 spectral data points over the benzene adsorption feature. The CO 2 line is used to calibrate the wavelength axis, as well as to subtract the interference from the overlap of its tail with the benzene absorption band. The CO 2 corrections are described in more detail in our previous article. To determine the benzene concentration in a sample, linear least squares method is used to fit the measured benzene spectrum with a reference spectrum, which has been recorded from a sample with a known benzene concentration.