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We report a mid-infrared quartz-enhanced photoacoustic sensor for highly sensitive nitrous oxide (N2O) detection using a 4.56 µm quantum cascade laser (QCL) with a butterfly package type. This new type of QCL features a compact size (30 mm × 12.7 mm × 13 mm), low threshold current (< 120 mA) at room temperature, and relatively high emission power (> 100 mW), which are suitable for portable trace gas sensor development. In this work, the QCL beam is directly coupled to the QEPAS detection module by using only one focusing lens. By measuring N2O mixtures with different concentrations (120 ppb to 3.16 ppm), the developed sensor shows a good linearity (R-square value of 0.999) and a minimum detection limit of below 1 ppb. The sensor has been successfully deployed for ambient air measurement, demonstrating the applicability and promising potential of using this QCL with butterfly package for various gas sensing applications. [ABSTRACT FROM AUTHOR]

[Display omitted] • Mid-infrared absorption spectra at 1129 cm−1 and 1398 cm−1 are exploited for SO 2 and SO 3 detection, respectively. • A high-temperature multipass cell is designed to enable calibration-free SO 2 and SO 3 detection with enhanced sensitivity. • The spectroscopic system achieves a minimum detection limit of 8 ppb for SO 2 and 3 ppb for SO 3 at the average time of 100 s. We report a mid-infrared quantum cascade laser absorption sensor capable of measuring SO 2 and SO 3 simultaneously and sensitively at elevated temperatures. In the sensor development, the intense transitions of SO 2 and SO 3 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 SO 2 and 3 ppb for SO 3 , with an average time of 100 s. Lastly, we successfully demonstrated the real-time and sensitive measurement of SO 2 and SO 3 during the oxidation reaction of SO 2 by O 3 at 460 K. Our laser sensor shows great potential for in-situ and real-time monitoring of SO x from combustion emissions. [ABSTRACT FROM AUTHOR]

• A multi-laser hybrid sensor is designed based on tunable diode laser absorption spectroscopy (TDLAS) for simultaneous measurement of NH 3 , NO, and flue gas temperature. • In calibration-free wavelength modulation spectroscopy (CF-WMS) measurements, we used a method to obtain the laser wavelength response by a simple fit to the S 2f/1f line pattern of a standard gas with a standard deviation of less than 5 × 10-3 cm-1. • Under static conditions, the relative accuracy of NH 3 and NO detected by the sensor is 4.2% and 2%, respectively, and the maximum relative standard deviation of temperature is 1.3%. • When tested under flowing conditions, the sensor can accurately capture NH 3 , NO, and temperature changes. The relative accuracy of NH 3 and NO is 4.7% and 2.2%, and the relative standard deviation of temperature is 0.9%. The real-time monitoring of NH 3 , NO, and flue gas temperatures is critical to controlling NO x emissions from coal-fired power plants, reducing ammonia slip, and realizing accurate ammonia injection. However, few previous studies have utilized laser absorption sensors to measure these three indicators simultaneously. In this paper, a multi-laser hybrid sensor is designed based on tunable diode laser absorption spectroscopy (TDLAS) for simultaneous measurement of NH 3 , NO, and flue gas temperature. The sensor uses a distributed feedback (DFB) laser to target the NH 3 and the H 2 O lines, and a quantum cascade laser (QCL) is used to target the NO line. In the measurements of NH 3 and NO using calibration-free wavelength-modulated spectroscopy (CF-WMS), we employ a method to obtain the laser wavelength response by fitting the demodulated signal of a standard gas. Additionally, temperature measurements are obtained using the time-division multiplexing (TDM) strategy and direct absorption spectroscopy (DAS) techniques with the help of the absorption spectrum of H 2 O. Under static conditions, the relative accuracy of NH 3 and NO detected by the sensor is 4.2% and 2%, respectively, and the maximum relative standard deviation of temperature is 1.3%. When tested under flowing conditions, the sensor can accurately capture NH 3 , NO, and temperature changes. The relative accuracy of NH 3 and NO is 4.7% and 2.2%, and the relative standard deviation of temperature is 0.9%. The sensor has great potential in optimizing denitrification processes in thermal power plants. [ABSTRACT FROM AUTHOR]