Design and Application of a Nitrate Measurement System Based on a Narrowband Tunable Ultraviolet Light Source

Abstract

The nitrate concentration in seawater is an important parameter for marine environment monitoring, which is of great significance for the analysis of seawater ecosystems. In this paper, a novel nitrate measurement system based on a narrowband tunable ultraviolet light source and SVM-based algorithm is proposed and demonstrated. The system is composed of a tunable light source module, an optical fiber splitter, a temperature control module, a spectrometer and a data processing module. By controlling the motorized rotation stage and motorized filter wheel, a continuous and automatic narrowband tunable deep ultraviolet (DUV) light source module is developed. Different seawater samples from Aoshan Bay (Qingdao, China), the Western Pacific, Sanggou Bay (Weihai, China) and the South China Sea with nitrate concentrations from 0 to 102 μmol/L are measured by this system based on a narrowband tunable DUV light source and the results are compared with a broadband DUV light source system. The calculation model is established based on the support vector machine (SVM) algorithm with/without temperature and salinity correction (TSC), and the prediction results are compared with those of the in situ ultraviolet spectrophotometry (ISUS) algorithm. The results show that the method based on a narrowband tunable light source system and the TSC-SVM algorithm has the best performance in predicting the nitrate concentration, with the root mean squared error (RMSE) reduced by 29.16% (from 1.20 to 0.85 μmol/L) compared with the results of the broadband light source system, and the error range is from -3.01 to 2.99 μmol/L. The method proposed in this paper has the potential to provide more accurate measurements in marine environment monitoring.