The development of
a dual-gas quartz-enhanced 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.
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