Real-time monitoring of a chemical reaction through a dual-gas QEPAS sensor

Simultaneous detection of different gas species represents an indispensable asset for several applications, such as instantaneous quantification of isotope concentration ratios, self-calibrating sensors, and monitoring of the temporal evolution of a chemical reaction. In this research work, a dual-gas quartz-enhanced photoacoustic spectroscopy (QEPAS) sensor for a real-time analysis and in a continuous flow monitoring of one reactant and one product of a gas-phase chemical reaction involving nitrogen dioxide (NO₂) and water vapor (i.e., H₂O) – as reaction reactants – and nitrogen monoxide (NO) – as one of the reaction products – was realized. The QEPAS sensor implemented a spectrophone composed of a pair of metallic acoustic resonator tubes applied at both antinode points of a custom quartz tuning fork (QTF). In this configuration, two different quantum cascade lasers (QCLs) were used, having an emission wavelength centered at 5.26 μm – resonant with a nitrogen monoxide absorption feature located at 1,900.075 cm^-1 – and at 6.25 μm – resonant with a nitrogen dioxide absorption feature located at 1,601.77 cm^-1 –, respectively. The chemical reaction was studied by injecting in the gas line a certified concentration of 5,000 parts-per-million (ppm) of NO₂:N₂ and monitoring the QEPAS signals at four different total gas flow values, i.e., 10, 20, 30 and 50 standard cubic centimeters per minute (SCCM), respectively.