Advances have been made in the field of high sensitivity and selectivity of trace gas detection by the Institute of Optics and Electronics of the Chinese Academy of Sciences: Using optical cavity ring down, combined with a pulse quantum cascade laser covering a wide spectral range in the mid-infrared band, Highly sensitive and highly selective determination of multiple broadband absorption gases. Ethanol, ether and acetone are commonly used volatile organic compounds in defense industry, medical and health, organic synthesis, food industry and industrial and agricultural production. Ethanol can be used in the manufacture of acetic acid, beverages, flavors, dyes and fuels. Medical commonly used volume fraction of 70% to 75% of ethanol as a disinfectant. Ether is an extractant and medical anesthetic used in the pharmaceutical industry for the manufacture of pharmaceuticals. Acetone is a representative compound of aliphatic ketones and is an important raw material for organic synthesis. However, they are irritating to the eyes, nose and throat. Long-term exposure to these gases can cause fatigue, nausea, headache, dizziness and irritability in the body. Severe vomiting, shortness of breath, spasms, and even coma. In order to reduce the harm of these volatile organic gases to the human body, its concentration needs to be monitored in real time. At present, there are many techniques that have been used to detect ethanol, ether and acetone. Among them, gas chromatography-mass spectrometry (GC-MS) is the most widely used technology. However, GC-MS is not suitable for real-time analysis. Open Path Fourier Transform Infrared Spectroscopy (OP / FT-IR) is another traditional method of detection that enables real-time monitoring. However, the high interference between the OP / FT-IR chemistries and the errors in the background spectra limit their detection limits. Electrochemical sensors, semiconductor sensors, and SAW quartz crystal sensors can also be used for gas detection, such as ether, acetone, but its sensitivity is limited and the selectivity is poor. Infrared laser spectroscopy, especially cavity ring down spectroscopy (CRDS), has very high sensitivity and selectivity in trace gas analysis. In practice, the high sensitivity to volatile organic compounds alone is not enough. In many applications, such as agricultural and industrial activities and national defense safety, multiple volatile organic compounds need to be tested at the same time. Although spectral detection techniques have been widely used for multicomponent gas detection, most of these reports focus on the detection of small-molecule gases with narrow-band absorption in the near-infrared region, whereas the predominant VOCs are often complex quantum structures Macromolecules, which have a broadband absorption spectrum in the mid-infrared band. In this case, a laser that is widely tunable in the desired spectral range is the key to successful detection. The rapid development of semiconductor laser technology, especially quantum cascade laser (QCL), provides an effective way for high sensitivity and high selectivity of trace gas detection. The main advantage of quantum cascade lasers is their wide spectral tuning range, which makes it possible to simultaneously detect mixed gas with a wide absorption band. Photoelectric team based on pulsed cavity ring down spectroscopy trace gas detection device shown in Figure. The light source is a tunable continuous quantum cascade laser with a center wavelength of 3.8um, a tuning range of 2610-2720 cm-1 and a maximum output power of 250mW. In pulse mode, the laser linewidth 0.7 cm-1, pulse period 0.5us, pulse repetition rate 50Khz. The ring-down chamber consists of two plano-concave high reflectors with a diameter of 25.4 mm, a concave radius of curvature of 100 cm and a reflectivity in the tuning range between 99.913% and 99.915%. The length of the ring is L = 50 cm. The output signal of the ring-down cavity is focused by the lens to the thermoelectric cooling infrared photodetector. The ring-down signal of the cavity detected by the detector is collected and recorded by the acquisition card, and the ring-down time is obtained by fitting the single exponential function to obtain the gas to be measured Absorb the coefficient and calculate the gas concentration. In order to avoid feedback light having an impact on the laser output, an optical isolator is inserted at the laser output so that the light fed back to the laser is below -30 dB. The ring down signal is triggered by a rising edge of the system signal that has a delay from the laser output pulse. To improve the signal-to-noise ratio, the signal is averaged 256 times for each acquisition. Schematic diagram of trace gas detection device based on pulsed cavity ring down technique Through this device, the research team detected the concentration of large molecular organic volatile gas (alcohol, ether, acetone) with wide-band absorption and prepared a multi-component analysis algorithm to realize the simultaneous measurement of volatile compounds with different concentrations. The detection sensitivity of the system to the measured organic gas is less than 1ppm. This research result is published in Applied Physics B, 122: 187 (2016).
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