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Abstract
An ultra-high sensitivity multi-molecule sensor based on a photo-acoustic cell with two perpendicular acoustic resonators and a common microphone has been reported. In this work, a 4.5 µm distributed-feedback quantum cascade laser and a 1.5 µm external cavity diode laser (EC-DL) were used as optical excitation sources. Considering the spectral ranges of the lasers used, it is possible to analyze eight molecules (${\rm QCL}:\;{{\rm N}_2}{\rm O}$ and ${{\rm CO}_2}$, EC-DL: ${{\rm H}_2}{\rm O}$, ${{\rm H}_2}{\rm S}$, ${{\rm NH}_3}$, CO, ${{\rm CH}_4}$, and ${{\rm C}_2}{{\rm H}_2}$). The ${{\rm N}_2}{\rm O}$ molecule was used to evaluate the performance of the photo-acoustic spectroscopy (PAS)-based sensor. A sensitivity of 0.073 V/ppm and a linearity of 0.99 were found by analyzing the PAS signal as a function of ${{\rm N}_2}{\rm O}$ concentration at ${2237.656}\;{{\rm cm}^{- 1}}$. The long-term performance of the sensor was determined by performing an Allan deviation analysis. A minimum detection limit of 9.8 ppb for 90 s integration time was achieved. The simultaneous multi-trace gas detection capability was verified by measurement of ${{\rm N}_2}{\rm O}$, ${{\rm CO}_2}$, and ${{\rm H}_2}{\rm O}$. Depending on the coarse/fine-tuning ranges of the lasers used, the number of molecules analyzed can be further increased. Such a sensor could provide simultaneous diagnosis of many diseases through an analysis of breath air and simultaneous monitoring of the most important greenhouse gases.
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