VOCs Detection Based On Photoionization Technology

Volatile Organic Compounds are mostly toxic and explosive.A certain concentration of VOCs can seriously pollutes the environment and affects people's health.At present,photoionization technology has developed rapidly in the detection of volatile organic compounds.A variety of photoionization detection systems have been established in domestic and foreign laboratories,and have been continuously improved and optimized,including improvements in ionization chamber structure,design of weak signal amplifying circuits,and rich functions of instruments.Based on the principle of photoionization technology,a portable photoionization detector was designed and its performance was tested.Based on the principle of photoionization technology,the parameters of the ionization chamber in the system were determined by theory,simulation and practice.The distance between the electrode plates is the same as the diameter of the UV lamp,which is about 6 mm.The plates are made of stainless steel.The depth of the ionization chamber is about 10 mm,and the processing material of the ionization chamber is polytetrafluoroethylene.The ionization source is vacuum ultraviolet lamp with ionization energy of 10.6 eV made by Heraeus Company of Germany.The structure of the ionization chamber was determined as the axial flow type.A logarithmic micro-current detection circuit was designed to detect the microcurrent generated by ionizing VOCs.The wide range acquisition and display of micro-current signal was realized by the combination of logarithmic amplifier and STM32F103ZET6.PWM wave generated by STM32F103ZET6 was used to control the flow rate of gas in the micro suction pump.The power management module of the whole detection system was designed,and 12 V/1 A was determined as the only power input,which reduced the volume of the whole detection circuit module.The interference of the signal was reduced by combining the amplitude limiting filter with the recursive average filter.With the control core of microprocessor STM32F103ZET6,the electronic clock function module,temperature and humidity monitoring module,acoustooptic alarm module and SD card storage module were designed for the system,which greatly enriched the function of the whole detection system.The structure diagram of the ionization chamber was drawn by Solidworks2016,and then the ionization chamber was processed based on this structure diagram.The Altium designer16 was used to draw the PCB structure diagram of various functional modules such as micro current detection,power management,and detection system and perform proofing.The experimental platform was set up to optimize the flow rate of the VOCs gas and the plate voltage.The gas flow rate was determined to be 200 mL/min,and the plate voltage was 200 V.The performance parameters of the designed photoionization detection system were tested.The experimental response time of the system was about 4~6 s,and the lower detection limit was about 1.910 ppm.The system measures the same concentration of isobutene gas with good stability,and the repeatability of short-term test and long-term test was ±1.37% and ±2.28%,respectively.The system was calibrated with isobutylene gas,and the calibration curve of the system was obtained in many experiments in the range of 0~2000 ppm.The calibration curve was linearly fitted in three sections(0~178.5 ppm,178.5~657.7 ppm,657.7~2000 ppm),and showed a better linear relationship,the correlation coefficient were 0.982,0.985 and 0.969 respectively.In the range of 0~178.5 ppm,the sensitivity of the detection was 13.39 pA/ppm.When the system was applied to the detection of acetone and ethanol,there was obvious response.The system can be applied to the detection of other volatile organic compounds.