| Nitric oxide(NO)is an important diatomic molecule,playing an significant role in atmospheric pollution and biochemical processes.For example,NO participates in the conversion and formation of secondary fine particles in the atmosphere,and the breath NO concentration is an important diagnostic indicator for cardiovascular diseases such as asthma.Therefore,an analyzer with strong portability,fast time response and wide detection range is highly demanded.Miniaturization is one of hot fields in developing sensitive NO analyzers based on laser spectroscopy,such as mid-infrared QCL laser combined with multiple reflection cells,quartz tuning based photoacoustic spectrometry and cavity ring-down spectroscopy.These new laser technologies can achieve very high sensitivity and selectivity,but the cost of tunable laser is high,which limits their widely application.The reaction of NO and O3 could produce strong visible chemiluminescence spectroscopy,which allows sensitive measurement of NO concentration,and has become one of the standard methods for measuring NO concentration.However,in the existing commercial NO chemiluminescence analyzer,a photomultiplier tube(PMT)has been used as a weak light detecting device,which results in the analyzer bulky,and it is difficult to adapt in a mobile platform.Based on chemiluminescence of NO and ozone,the development of a low-cost,portable,highly sensitive NO analyzer has a wide range of applications.In this paper,a photodiode was proposed as a weak light detector for chemiluminescence from the reaction of NO and 03.The spectral response characteristics of different photosensitive materials,such as Si and InGaAs,were first compared,as well as their sensitivity and noise levels of different size of photodiodes.Finally,UV coating Si-based photodiode with photosensitive area of 2.5×2.5 mm was selected as the weak chemiluminescence detector.A high gain transconductance circuit was first designed to achieve high sensitivity and low noise detection of weak light signals,and a third-order active filtering was used to reduce the dark noise of the detector.In order to reduce the base line drift induced by the temperature fluctuation from the detection system,a temperature control device with temperature control accuracy of ±0.01 0C was designed by multi-stage heat insulation.The photoelectric detection noise of the system had been controlled below 0.1 mV,which allowed the measurement of low NO concentration.In order to reduce the volume of the chemiluminescence cavity and shorten the conversion time of chemical reaction,the non-sequence light field analysis module in Zemax software were used to simulate and optimize the collection efficiency of different cavity shapes,such as cylindrical,cone and circular cavity.Then,the fluid mechanics module in Solidworks software was used to optimize the position of gas introducing into the optical cavity and the mixing state of the airflow.A circular cavity with a diameter of 6 mm,a diameter of 10 mm and a height of 8 mm was selected,and the optical cavity was designed for approximate hexagonal for easier processing and assembly in the final.The laser 3D printing technology was used to build the micro-optical cavity.The aluminized reflective lens was attached to the cavity wall to further enhance the effective collection of weak light.In order to reduce the volume and weight of the miniaturized analyzer,miniaturized components,such as the corona-discharge micro-ozone generator,the sampling and gas flow controller,the highly integrated circuit,the high-precision data acquisition,the interaction and display interface have been developed.The NO analyzer developed had a weight of only 1.04 kg(including battery)and a volume of 16 cm × 9 cm × 7.5 cm,which could be deployed on different portable and mobile platforms.The analyzer had two operation modes,fast response and high sensitivity.The fast response mode for rapid measurement had a linear range of 0.2-200 ppm,the linear correlation coefficient R2 was 0.99,and the response time was less than 1.0 s.In the high sensitivity mode,the detection limit was better than 5 ppb,a linear response up to 20 ppm with R2 of 0.999,and the response time was 4 s.Finally,the paper introduces the analytical ability of the miniatured NO analyzer in measurements of NO produced from different emission sources.The concentration of NO in the exhaust gas of the vehicle under idle speed and dynamic speed was studied,and the NO analyzer was able to capture the fast change NO concentration in the exhaust gas with working states of the vehicle in real time.The concentration of NO gas after burning cigarettes in the automatic smoking machine was also monitored,and the difference of NO gas concentration between the flue-cured tobacco type and the mixed cigarette was compared.The concentration of NO in the exhaled breath of the volunteers after smoking was analyzed by the sampling air bag. |
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