A sensitive sensor was built using frequency-division multiplexing quartz-enhanced photoacoustic spectroscopy (QEPAS) to simultaneously measure dissolved NH3 and H2S. Two distributed feedback (DFB) diode lasers with central wavelength located at 1512.2 nm and 1577.2 nm were employed as the light sources for the excitation of the photoacoustic signals which are detected by the quartz tuning fork (QTF). Two homemade analog lock-in amplifiers were utilized to retrieve the second harmonic (2f) components derived from the photoacoustic signals of NH3 and H2S, respectively. The results show that the interference between second harmonic signals of the two species can be effectively suppressed by using the varied modulation frequencies near the resonant frequency of the QTF according to the frequency-domain and time-domain analysis. An excellent linear response was observed within a wide range of concentration and the linear response coefficients were larger than 0.999. Allan deviation method was used to analyze the minimum detection limits of NH3 and H2S with 0.5 ppm and 4.6 ppm, corresponding to the normalized noise equivalent absorption coefficients (NNEA) of 1.2×10-8 cm-1W√Hz and 7.8×10-9 cm-1w√Hz, respectively. The detection system can satisfy the demand for precise measurements of NH3 and H2S.
A compact system based on mid-infrared quantum cascade laser (QCL) operated in room temperature was developed for the simultaneous monitoring of NO, NO2 and NH3 in the air. Laser beams of three QCLs with central wavelength located at 1900 cm-1, 1600 cm-1, 1103.4 cm-1 were coupled to pass through the 60m long gas cell together. With the technology of time division multiplexing, wavelength modulation spectroscopy (WMS) signals of three lasers can be detected at adjacent scan process. The real-time second harmonic analysis was implemented to achieve simultaneous detection of NO, NO2 and NH3. A minimum detection limit (MDL) of 0.2ppb for NO, 0.12ppb for NO2 and 0.1ppb for NH3 with an optimum integration time around 100 seconds can be achieved for this setup. An ambient monitoring of three gasses during 5 hours was performed to inspect the local air quality.
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