Optimization of a Near-Concentric Cavity Raman Spectroscopy System for Liquid Sample and Preliminary Results of CO32-/HCO3-

It is of great significance to study the carbon cycle in the ocean for environmental monitoring and resource detecting. In this field, one of the most important topics is to study carbonate. There is no direct detection method to monitor carbonate in seawater, and most traditional detection methods for carbonate are indirect. For example : with seawater sample acidified by phosphoric acid, the carbonate in the sample can be converted into CO2 and then be detected. Raman spectroscopy can be used in in-situ detection and has great potential to detect the carbonate directly. But it's sensitivity is still a limitation in the practical use of ocean detection. In the hope of developing an approach to directly detect the carbonate in the seawater, we build a near-concentric cavity Raman spectroscopy system and optimize the main parameters of the cavity (diameter= 25. 4 mm, reflectivity= 99. 66%@532 nm) including optical windows thickness of the liquid cell, the optical windows distance at two sides, and the focal length of the mirrors with simulation software. The results are listed as follows : (1)The number of the reflection is at a maximum when the focal length is 25 mm for the mirrors with diameter of 25. 4 mm; (2) For the optical windows of the liquid cell, with smaller thickness, the light would be denser in the center of the cell, and the totally luminous intensity in the center plane of the near-concentric cavity would be larger ; (3) with smaller distance between the optical windows, the light would be denser in the center of the cell, and the totally luminous intensity in the center plane of the near-concentric cavity would be larger; After optimization, the measurement of CO32- and HCO(3)(-)solutions on different concentration levels is carried out using the optimized near-concentric cavity Raman spectroscopy system. The spectral signal was pretreated using second order differential and Gaussian filter, and then calibration curves were established using the peak intensity of the corresponding concentrations. The results showed good linear relationship between concentration of solution and signal intensity of Raman spectrum, with R-2 of 0. 994 and 0. 998 for CO(3)(2-)and HCO3-, respectively. We calculated the LODs using the 3 times signal-to-noise ratio. The results showed that the LOD for CO32- and HCO3- is about 0. 06 and 0. 38 mmol . L-1 respectively. The LODs are lower than the typical concentrations of CO32- and HCO3- in seawater, which are about 0. 2 and 2 mmol . L-1 respectively. Compared to the current reported of the Raman spectroscopy system of in-suit ocean detection, the sensitivity of the system has increased by nearly ten times. So it is hoped to apply the system to the in-situ CO32- and HCO(3)(-)detection inseawater.