Research On HCN Gas Detection Based On QEPAS Technique

Trace gas detection technology emerges as the times require for the need of environmental monitoring and human production activities,and its development is of great significance.Quartz-enhanced photoacoustic spectroscopy(QEPAS)technique is a new type of gas detection based on photoacoustic spectroscopy,and use the micro piezoelectric quartz tuning fork as a sensing element to detect the sound waves generated by the target analyte gas as a result of light absorption.It is an innovation in traditional photoacoustic spectroscopy(PAS)technique,and possess advantages of large dynamic range,good reliability,easy to operate,without wavelength selectivity,small volume,and high sensitivity.Its technical principle involves the photoacoustic effect,the infrared absorption spectrum,absorption law,the quartz tuning fork and the piezoelectric effect,wavelength modulation technique with second harmonic detection technology,among them,the wavelength modulation technique and the second harmonic detection technology is the key to the QEPAS detection.In recent years,many domestic and foreign agencies have used this technology to detect various trace gases.Among them,HCN gas has a wide range of sources and of great harm,and the industrial production activity of human makes its content increase.It is of great significance to study the HCN spectrum and to detect HCN gas.In this paper,QEPAS technique is used to detect HCN gas.Before the experiment,simulation and calculation are carried out by using the relevant theoretical model,Comsol software was used to build the numerical model of the photoacoustic spectrum detection system,and the model of photoacoustic detector is simulated to solve the coupling problem of finite element multiphysical field,it provides guidance for the selection of the structure and placement position of some photoacoustic sensors in the experiment,so the optimal optical acoustic geometric parameters of two models(bare tuning fork model and resonant QEPAS detector model)are obtained.In the preparation of the experiment,the best HCN absorption spectrum was selected,the performance test of the laser was carried out,the modulation depth required for the experiment was optimized,and the two kinds of tuning fork were compared.The experiment used HCN as the target gas to detect and dilute the 50 ppm HCN:N2 mixture with a mass flow controller with an uncertainty of 3% and control the flow rate of 120 m L/min to the system gas chamber.Using wavelength modulation technology and two harmonic(2f)detection technology,the high-frequency sine wave signal with frequency of f is superimposed on the driving current of the saw tooth wave.The resonance frequency of the quartz tuning fork is about 30.72 k Hz,and taken the continuous wave distributed feedback single longitudinal mode diode laser with output wavelength of 1.53 um as the laser source,the traditional QEPAS system and improved EDFA-QEPAS system has been carried on the experiment separately.Among them,the detection limit of 1.7 ppm was obtained by using the traditional QEPAS system.In the improved EDFA-QEPAS miniaturized system,the erbiumdoped fiber amplifier(EDFA)and the miniaturized 3D-printed photoacoustic detection channel(PADC)were applied and have achieved the ultra-high sensitive and stable detection of hydrogen cyanide based on a quartz-enhanced photoacoustic spectroscopy sensor for the first time.A HCN molecule absorption line located at 6536.46 cm-1 was selected which was in the range of the EDFA emission spectrum.The detection sensitivity of the reported EDFA-QEPAS sensor was enhanced significantly due to the high available EDFA excitation laser power.A 3D printing technique was used to develop the compact PADC,resulting in a size of 29 × 15 × 8 mm3 and a mass of ~5 g in order to improve the sensor stability and implement sensor applications requiring a compact size and light weight.At atmospheric pressure,room temperature and a 1 s acquisition time,a minimum detection limit(MDL)of 29 parts per billion(ppb)was achieved,corresponding to a normalized noise equivalent absorption(NNEA)coefficient of 1.08 × 10-8 cm-1W/Hz-1/2.In addition,since no saturable absorption effects were observed with the excitation laser power applied in the experiment,the sensor's capability can be further improved when an EDFA with a higher amplified output power and an optimized humidity condition are implemented.