Quantum cascade laser absorption sensor for in-situ, real-time and sensitive measurement of high-temperature SO₂ and...

We report a mid-infrared quantum cascade laser absorption sensor capable of measuring SO₂ and SO₃ simultaneously and sensitively at elevated temperatures. In the sensor development, the intense transitions of SO₂ and SO₃ in the mid-infrared region of 1129 cm^-1 and 1398 cm^-1 were exploited by two quantum cascade lasers. A high-temperature multipass cell was adopted to increase the absorption path length to 10 m. The quantitative concentrations of SO_x were directly obtained from the calibration-free wavelength modulation spectroscopic method, which was validated at varied temperature and pressure conditions. From Allan deviation analysis, we achieved a minimum detection limit of 8 parts per billion (ppb) for SO₂ and 3 ppb for SO₃, with an average time of 100 s. Lastly, we successfully demonstrated the real-time and sensitive measurement of SO₂ and SO₃ during the oxidation reaction of SO₂ by O₃ at 460 K. Our laser sensor shows great potential for in-situ and real-time monitoring of SO_x from combustion emissions.