Abstract
A detailed review on the development of quartz-enhanced photoacoustic sensors (QEPAS) for the sensitive and selective quantification of molecular trace gas species with resolved spectroscopic features is reported. The basis of the QEPAS technique, the technology available to support this field in terms of key components, such as light sources and quartz-tuning forks and the recent developments in detection methods and performance limitations will be discussed. Furthermore, different experimental QEPAS methods such as: on-beam and off-beam QEPAS, quartz-enhanced evanescent wave photoacoustic detection, modulation-cancellation approach and mid-IR single mode fiber-coupled sensor systems will be reviewed and analysed. A QEPAS sensor operating in the THz range, employing a custom-made quartz-tuning fork and a THz quantum cascade laser will be also described. Finally, we evaluated data reported during the past decade and draw relevant and useful conclusions from this analysis.
Highlights
The detection and measurement of trace gas concentrations is important for both the understanding and monitoring of a wide variety of applications, such as environmental monitoring, industrial process control analysis, combustion processes, detection of toxic and flammable gases, as well as explosives.For example, trace gas sensors capable of high sensitivity and selectivity are required in atmospheric science for the monitoring of different trace gas species including greenhouse gases and ozone, and in breath diagnostics, nitric oxide, ethane, ammonia and numerous other biomarkers
The dependence of quartz-enhanced photoacoustic sensors (QEPAS) detection sensitivity upon the trace gas concentration in a specific absorbing gas sample is a function of the sample pressure. This dependence is influenced by different parameters: (i) the Q factor of a quartz tuning fork (QTF) decreases rapidly with increasing pressure due to energy losses by the mechanical viscosity and friction losses via the trace gas; (ii) at low gas pressures the collisional line broadening of the absorption peak is less than Doppler broadening the merging of closely spaced absorption lines should be taken into account at higher pressures; (iii) vibrational to translation (V-T) relaxation rates are faster at higher pressures which is in competition with the opposite trend of the Q factor; and (iv) the speed of sound depends on the gas pressure
If we assume that the absorption coefficient has a pure Lorentzian lineshape, S1ω has a pure first derivative line-shape with a constant background; S2ω consists of two terms: the first term, arising from a residual amplitude modulation is proportional to the first derivative, whereas the second is the second-derivative expression arising from the laser wavelength modulation
Summary
The detection and measurement of trace gas concentrations is important for both the understanding and monitoring of a wide variety of applications, such as environmental monitoring, industrial process control analysis, combustion processes, detection of toxic and flammable gases, as well as explosives. Electrochemical gas sensors can be relatively specific to individual gas, have usable resolutions of less than one part per million (ppm) of gas concentration, and operate with very small amounts of current, making them well suited for portable, battery powered instruments [1]. They experience hysteresis and are influenced by water humidity. The measurement procedure is based on the comparison between the signal amplitude both at the input and the output of the cavity [8] Both techniques require precise information about mirror reflectivity, a sensitive photodetector with a fast-response, perfect optical alignment and the use of long optical pathlengths. One of the most robust and sensitive trace-gas optical detection techniques is photo-acoustic spectroscopy (PAS), which is capable of extremely high detection sensitivities with a compact and relatively low-cost absorption detection module (ADM) [9]
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