| Nitrate is an important nutrient, and also one of the projects which marine survey is stipulated to measure. Traditionally, chemical analysis method is usually adopted in water nitrate measurement. Owe to chemical reagent being involved in the measurement process, chemical analysis method always has high maintain frequency and secondary pollution. UV absorption spectroscopy measurement on seawater nitrate has the advantage of high measuring speed, no chemical reagents and secondary pollution. UV absorption spectroscopy seawater nitrate measurement system is constructed in the paper, and the measurement model and confounders were studied to achieve rapid measurement of seawater nitrate, laying the foundation for the nitrate optical position sensor.The main contents are summarized as follows:(1) UV absorption spectrum stability data acquisition method was studyed, and seawater nitrate measurement system was designed. Through the analysis of the feature and function of key component, the model of ultraviolet light source, micro spectrometer, optical fiber, sample measuring chamber, development board were determined; By the secondary development of micro spectrometer, PC and embedded nitrate measurement software were developed using of the spectrometer function interface, realizing spectral preprocessing, absorbance data acquisition and storage, and nitrate concentration measurement, UV absorption spectroscopy seawater nitrate measurement system was built, realizing seawater nitrate fast measurement.(2) The relation model between ultraviolet absorption spectrum and the nitrate concentration was studyed. According to the chemical composition of seawater and nitrate ultraviolet interfering substances, nitrate mixed sample solutions were prepared, absorption spectra data was acquisited using seawater nitrate measurement system, UV absorption spectrum seawater nitrate inversion model was established based on the partial least squares; taking prediction standard deviation and correlation coefficient as measure index, the effect of different spectra pretreatment methods on the performance of the model was studyed, the results showed that, model prediction standard deviation after SG smoothing convolution was 74.12, better than the original spectra model prediction standard deviation 158.44, model measuring performance was improved.(3) The nitrate seawater ultraviolet absorption spectrum curve characteristic was studyed using of ultraviolet absorption spectroscopy measurement system. Nitrate has a wide absorption peak, absorbance gradually increases from 200 nm to 210 nm, reaching the maximum at 210 nm, then decreases close to zero at 240 nm.(4) The relation model between absorbance and turbidity in 210nm-240 nm and model verification were conducted. The results showed that, contrast with the true nitrate concentration value, measurement results with UV absorption spectroscopy on solutions containings of turbidity had large errors. After model calibration, maximum relative error of predicted and measured values of absorbance was 6.7%, maximum relative error of predicted and measured nitrate concentrations values was 17.37%, the model measuring performance was improved.(5) The experiment was carried out to determine the measuring precision, percent recovery, and detection limit of ultraviolet absorption spectroscopy seawater nitrate measurement system. System measurement precision is better than 15.9%, detection limit of 11 Indian Ocean sea water samples is 15.98 μg/L.Percent recoverys of 15 samples were measured, and all of the samples meet the "marine monitoring" requirements. The system is reliable for on-site measurements of seawater spiked samples. |
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