Simultaneous dual-gas QEPAS detection based on a fundamental and overtone combined vibration of quartz tuning fork
A dual-gas quartz-enhanced photoacoustic spectroscopy (QEPAS) sensor system based on a frequency division multiplexing technique of a quartz tuning fork (QTF) was developed and experimentally demonstrated. Two beams from two independently modulated lasers are focused at two different positions between the QTF prongs to excite both the QTF fundamental and 1st overtone flexural modes simultaneously. The 2f-wavelength modulation technique is employed by applying two sinusoidal dithers, whose frequencies are equal to a half of the QTF fundamental and 1st overtone frequencies, respectively, to the currents of two excitation lasers. The resonance frequency difference between two flexural modes ensures that the correlated photoacoustic signals generated by different target gases do not interfere with each other. The proposed QEPAS methodology realizes a continuous real-time dual-gas monitoring with a simple setup and small sensor size compared with previous multi-gas QEPAS sensors.