Dual-Gas Quartz-Enhanced Photoacoustic Sensor for Simultaneous Detection of Methane/Nitrous Oxide and Water Vapor

The development of a dual-gas quartzenhanced photoacoustic (QEPAS) sensor capable of simultaneous detection of water vapor and alternatively methane or nitrous oxide is reported. A diode laser and a quantum cascade laser (QCL) excited independently and simultaneously both the fundamental and the first overtone flexural mode of the quartz tuning fork (QTF), respectively. The diode laser targeted a water absorption line located at 7181.16 cm−1 (1.392 μm), while the QCL emission wavelength is centered at 7.71 μm and was tuned to target two strong absorption lines of methane and nitrous oxide, located at 1297.47 and 1297.05 cm−1, respectively. Two sets of microresonator tubes were positioned, respectively, at the antinode points of the fundamental and the first overtone flexural modes of the QTF to enhance the QEPAS signal-to-noise ratio. Detection limits of 18 ppb for methane, 5 ppb for nitrous oxide and 20 ppm for water vapor have been achieved at a lock-in integration time of 100 ms.