Photoacoustic spectroscopy, based on an external cavity diode laser operating at 1431 nm, was used for measuring CO<sub>2</sub> concentration as a minority component in a gas mixture. By using N<sub>2</sub> as a buffer gas, a molecular relaxation effect was observed, which influenced both the amplitude and the phase of the measured photoacoustic signal and consequently reduced the sensitivity of the PA system. This molecular relaxation effect could be suppressed by adding water vapor of a constant and relatively high (~4%) concentration to the gas sample. In parallel with this, the arising spectral interference between H<sub>2</sub>O and CO<sub>2</sub> necessitated the development of a simple yet efficient signal analysis method, which increased the sensitivity of the system by more than one order of magnitude and accordingly reduced the minimum detectable CO<sub>2</sub> concentration down to ~1000 ppm.

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