Research On Determination Of Nitrate In Seawater Based On Ultraviolet Spectra Combined With PLS Method

Traditional wet chemical method for measuring nitrate has many shortcomings,such as operational complexity, venomous chemic reagents and long detection time.The method based on ultraviolet spectra is quicker, simpler and easier to achieve insitu continuous measurement but it is disturbed by the materials in the seawater. Theapproach which combined partial least squares method with ultraviolet spectraanalysis can separate the overlapping spectra effectively and calculate theconcentration of nitrate accurately in seawater.The main research contents and innovation points in this thesis are summarizedas follows:1. Research on the influence of seawater on the measurement of nitrate usingultraviolet spectra. Based on the composition of seawater, the mainly interferingsubstances were Cl-、Br-、NO3-、NO2-and humic acid. Meanwhile, the linearabsorption range of nitrate was researched. The results showed that the linearabsorption range of nitrate was widened with the increase of the wavelength from190nm-240nm, the maximum linear range was5mg/L in190nm and10mg/L in220nm.2. Determine the optimal measuring wavelength band using interval PLS (iPLS)method. The ultraviolet spectrum was divided into21subintervals where the nitratemeasurement model was developed using PLS method. The subintervals whoseprediction accuracy was higher were combined to determine the optimal measuringwavelength band. The study showed that the optimal measuring wavelength banddetermined by iPLS not only reduce the number of independent variable from211to31, but also improved the prediction accuracy.3. Choose the optimal spectra preprocessing methods. Seven spectrapreprocessing methods were used to develop model and compared the modelparameters with the model without spectra preprocessing. The results showed that theprediction accuracy of the model without preprocessing was highest.4. Study on the relationship between the number of dependent variable and themodel prediction accuracy. The parameters of the model developed by five dependentvariables and single dependent variable were compared. Although the model whichwas developed by five dependent variables increased calculation costs and the numberof principal components, it improved the model prediction accuracy. 5. Established the method for measuring the concentration of nitrate usingultraviolet spectra. Different concentration and combinations of five compounds wereadded into deionized water to build the modeling samples. The concentration of fivecompounds were dependent variables, the model was built using the original spectrumin the optimal measurement band210nm-240nm. Finally, the method for measuringthe concentration of nitrate using ultraviolet spectra was established.6. Study on the applicability of the method for seawater. The recoveryexperiments were conducted using seawater collected from different areas. The resultsshowed that the recovery of82of84samples was between90%~110%, whichsatisfied the demands of 《The specification for marine monitoring》. It means that themethod can be applied to measure the concentration of nitrate in seawater.