Process gas analyser for analysing a process gas mixture guided in a plant component, - having two different measuring heads (5, 6) which can be assembled on diametrically opposed sides of the plant component, which comprise windows (10, 11) and contain the ambient air, - with a wavelength-tuneable laser diode and a detector, which are arranged in the measuring heads (5, 6) separated from the interior of the plant component by means of the windows (10, 11), so that after passing through the plant component the light of the laser diode strikes the detector, - with a control device (12, 13, 15, 17), which is embodied to activate the laser diode with a periodic current-time function (i(t)) so that upon detection of the light a time-resolved absorption spectrum of the process gas mixture is obtained, and - with an evaluation device, which is embodied to determine the concentration of this gas component in the process gas mixture on the basis of a selected absorption line of a gas component of interest in the detected absorption spectrum (18, 18 2 f) of the process gas mixture, characterised in that - a temperature sensor (23, 24) is arranged in at least one of the two measuring heads (5, 6), which is embodied to capture the respective current temperature when the absorption spectrum is measured, - ambient reference spectra (27, 27 2 f) of the ambient air contained in the two measuring heads (5, 6) which are recorded at different temperatures measured with the temperature sensor (23, 24) and without a process gas mixture are stored in a storage device of the evaluation device, and - the evaluation device is also embodied to select one of the stored ambient reference spectra (27, 27 2 f) as a function of the current temperature captured with the temperature sensor (23, 24) or to calculate it from two stored ambient reference spectra (27, 27 2 f), to subtract it from the currently detected absorption spectrum (18, 18 2 f) of the process gas mixture and finally to determine the oxygen concentration in the process gas mixture on the basis of a selected oxygen absorption line. - having two different measuring heads (5, 6) which can be assembled on diametrically opposed sides of the plant component, which comprise windows (10, 11) and contain the ambient air, - with a wavelength-tuneable laser diode and a detector, which are arranged in the measuring heads (5, 6) separated from the interior of the plant component by means of the windows (10, 11), so that after passing through the plant component the light of the laser diode strikes the detector, - with a control device (12, 13, 15, 17), which is embodied to activate the laser diode with a periodic current-time function (i(t)) so that upon detection of the light a time-resolved absorption spectrum of the process gas mixture is obtained, and - with an evaluation device, which is embodied to determine the concentration of this gas component in the process gas mixture on the basis of a selected absorption line of a gas component of interest in the detected absorption spectrum (18, 18 2 f) of the process gas mixture, characterised in that - a temperature sensor (23, 24) is arranged in at least one of the two measuring heads (5, 6), which is embodied to capture the respective current temperature when the absorption spectrum is measured, - ambient reference spectra (27, 27 2 f) of the ambient air contained in the two measuring heads (5, 6) which are recorded at different temperatures measured with the temperature sensor (23, 24) and without a process gas mixture are stored in a storage device of the evaluation device, and - the evaluation device is also embodied to select one of the stored ambient reference spectra (27, 27 2 f) as a function of the current temperature captured with the temperature sensor (23, 24) or to calculate it from two stored ambient reference spectra (27, 27 2 f), to subtract it from the currently detected absorption spectrum (18, 18 2 f) of the process gas mixture and finally to determine the oxygen concentration in the process gas mixture on the basis of a selected oxygen absorption line.