Seawater Nitrate Measurement Device Based On Deep Ultraviolet Wavelength Adjustable Light Source

Nitrate is one of the major forms of nitrogen in the ocean and one of the three major nutrients in seawater,which provides energy for marine ecosystems.In recent years,the eutrophication of coastal water is becoming more and more serious due to the discharge of human sewage and agricultural wastewater,which leads to the outbreak of red tide.The content of nitrate is closely related to the outbreak of red tide,and the study of the relationship between nitrate content and red tide outbreak can lay a foundation for the study of the outbreak mechanism of red tide.Therefore,the research on the technology which can realize long-term and in-situ monitoring of nitrate content in seawater is of great significance to the early warning of red tide and the study of marine ecosystem.At present,the accurate measurement of nitrate content mainly depends on the laboratory method,which is time-consuming,laborious and high cost.Therefore,the development of in-situ nitrate sensor with long-term stability is necessity to the early warning and accurate measurement of nitrate content in seawater.The commercially available in situ nitrate sensors mainly include the sensors based on wet chemical method and ultraviolet spectroscopy method.The sensors based on wet chemical method need to consume chemical reagents,which would pollute the seawater and cannot achieve long-term in situ measurement.Recently developed nitrate sensors based on ultraviolet absorption spectroscopy combined with the spectral analysis algorithm provide solution for the development of in-situ measurement.However,the use of a wide-band ultraviolet light source may excite unknown organic compounds in seawater and produce fluorescence,resulting in background noise and reduce measurement sensitivity.Meanwhile,the principle of Lambert Beer law requires monochromatic parallel light as source,the use of broadband light source may cause measurement errors.Therefore,to improve the current technology,the nitrate sensor based on deep ultraviolet wavelength tunable light source is proposed,designed,fabricated and tested in this thesis,which include the design of the deep ultraviolet wavelength tunable light source module and the establishment of the optimal prediction model.The results are compared and analyzed with those measured by the nitrate measurement system based on broadband light source,which provided verification to the new type of nitrate sensor proposed in this thesis.The innovation and contribution of this thesis are as follows:(1)By demonstrating the shortcomings of broad-spectrum measurement,a nitrate measurement technology based on a deep ultraviolet wavelength tunable light source is proposed,designed and fabricated,which includes the design of the deep ultraviolet narrowband tunable light source and the construction of the overall system.A wavelength tunable light source module was designed using ultraviolet narrowband filters,and the performance of the module was studied,which could avoid the background noise introduced by the fluorescence excitation of organic compounds and improve the system accuracy and resolution.(2)The algorithm model of nitrate measurement based on kernel partial least square method is proposed,and compared with the model of partial least square method.The optimal algorithm suitable for nitrate calculation is developed.In this study,the kernel function is used to map the data set,and the spectral inversion model is established based on kernel partial least square method.One hundred sea water samples were designed.The measurement data based on system made of broadband light source was modeled by the partial least square method and kernel partial least square method,respectively.KPLS regression fitting goodness of fit is close to 1,and the predicted goodness of fit is 0.88.By comparing the measurement results,KPLS is selected as the optimal measurement model.The data measured by the new type of narrow-band system was linearly fitted at the same wavelength,and compared with the data obtained from the wide-band system that provided the feasibility of the narrow-band nitrate measurement device proposed in this thesis.