Advances in research on trace gas detection technology in the Institute of Optoelectronics, Chinese Academy of Sciences

China Instrument Network Instrument Research and Development Institute of Optoelectronic Technology, Chinese Academy of Sciences has made progress in the detection of highly sensitive and selective trace gases: using cavity ring-down technology, it can cover a very wide spectral range of pulsed quantum levels in the mid-infrared region. In conjunction with lasers, high-sensitivity and highly selective measurements of multiple broadband absorption gases are simultaneously achieved.
Ethanol, ethyl ether and acetone are volatile organic compounds commonly used in defense industry, medical and health, organic synthesis, food industry, and industrial and agricultural production. Ethanol can be used to produce acetic acid, beverages, flavors, dyes, and fuels. Medically, volumetric fractions of 70% to 75% ethanol are also used as disinfectants. Diethyl ether is used as an extractant and a medical anesthetic agent for pharmaceutical production in the pharmaceutical industry. 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 gas-filled environments can cause fatigue, nausea, headaches, dizziness, and irritability. Severe cases of vomiting, shortness of breath, paralysis, and even coma. In order to reduce the harmful effects of these volatile organic gases on the human body, we need to monitor the concentration in real time.

At present, there are many techniques used to achieve the detection of ethanol, ether and acetone. Among them, gas chromatography-mass spectrometry (GC-MS) is the most widely used technique. However, GC-MS is not suitable for real-time analysis. The open path Fourier Transform Infrared Spectroscopy (OP/FT-IR) is another traditional detection method that enables real-time monitoring. However, the high interference between OP/FT-IR chemicals and the background spectral errors limit their detection limits. Electrochemical sensors, semiconductor sensors, and surface acoustic wave quartz crystal sensors can also be used to detect gases such as ethers, acetone, etc., but their sensitivity and selectivity are limited. Infrared laser spectroscopy, and especially cavity ring down spectroscopy (CRDS), has a very high sensitivity and selectivity in trace gas analysis. In practical applications, the high sensitivity to separate detection of volatile organics is not enough. In many applications, such as agricultural and industrial activities and defense security, multiple volatile organic compounds need to be detected simultaneously. Although spectral detection techniques have been widely used for multi-component gas detection, most of these reports focus on the detection of small-molecule gases with narrow-spectrum absorption in the near-infrared region, and the major VOCs are often those with complex quantum structures. Macromolecules, which have broadband absorption spectra in the mid-infrared region. In this case, a wide-tunable laser in the desired spectral range is the key to successful detection. The rapid development of semiconductor laser technology, especially the quantum cascade laser (QCL), provides an effective method for the detection of trace gases with high sensitivity and selectivity. The main advantage of quantum cascade lasers is their wide spectral tuning range, which makes it possible to simultaneously detect mixed gases with broad absorption bands.

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 the pulse mode, the laser line width is 0.7 cm-1, the pulse period is 0.5us, and the pulse repetition rate is 50Khz. The ring-down cavity is composed of two flat concave-convex high-reflection mirrors with a diameter of 25.4mm, a concave radius of curvature of 100cm and a reflectance range of 99.913 and 99.915% in the tuning range. The cavity length is L=50cm. The output signal of the ring-down cavity is detected by an infrared photodetector focused by the lens to the thermoelectric refrigeration. The ring-down signal detected by the detector is collected and recorded by the acquisition card, and the ringdown time is obtained by fitting a single exponential function to obtain the gas to be measured. Absorption coefficient, and calculate the gas concentration. In order to prevent the feedback light from affecting the laser output, an optical isolator is inserted at the output of the laser 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 is delayed by the laser output pulse. In order to improve the signal-to-noise ratio, the signal is averaged 256 times per acquisition.

The research team conducted concentration detection of macromolecular organic vapors (alcohols, ethers, acetones) with broad-spectrum absorption through the device, and prepared a multi-component analysis algorithm to achieve simultaneous measurement of various concentrations of volatile agents. The detection sensitivity of the system to the measured organic gas is less than 1ppm.

(Original title: A multi-component gas simultaneous measurement technique based on mid-infrared cavity ring-down spectroscopy proposed by Optoelectronics)

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