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 mu m), while the QCL emission wavelength is centered at 7.71 mu m 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.