海洋中碳循环研究对环境监测和资源探测有着重要意义, 其中研究海水中的碳酸盐又是研究碳循环的重要环节, 目前对海水中碳酸盐的测量没有直接的现场测量手段, 传统海水中碳酸盐的探测主要采用间接探测方法, 例如: 向海水中加入磷酸, 将海水中的碳酸盐转化为二氧化碳, 然后再对二氧化碳进行探测。 拉曼光谱作为一种可用于海水现场测量的技术, 具有对海水中碳酸盐直接检测的潜力, 但要在海洋探测中实际应用主要受限于灵敏度。 针对海水中碳酸盐的检测需求, 搭建了一套近共心腔液体拉曼光谱系统, 利用软件分别对反射率为99.66%(@532 nm)、 直径为25.4 mm的近共心腔的主要参数（腔镜的焦距、 液体样品池两端窗片的厚度及间距）进行了模拟和优化, 模拟结果显示: ①对直径为25.4 mm的腔镜, 焦距为25 mm时, 反射次数最多; ②对液体样品池光学窗片而言, 厚度越小, 样品池中心处的光斑越密集, 总光通量越大; ③液体样品池光学窗片距离越短, 样品池中心处的光斑越密集, 总光通量越大。 基于模拟结果对近共心腔液体拉曼光谱系统优化后, 在实验室配置了一系列浓度的碳酸氢根和碳酸根溶液进行测量, 并对光谱进行二次微分和高斯滤波预处理, 然后提取各浓度下碳酸根和碳酸氢根的峰强信息, 建立定标曲线。 结果显示: 碳酸根、 碳酸氢根的拉曼信号强度与其浓度之间线性关系良好, R2分别为0.994和0.998。 按照3倍信噪比计算系统对碳酸根和碳酸氢根检测限, 结果分别为0.06和0.38 mmol·L-1, 检测限低于海水中碳酸根和碳酸氢根浓度（海水中碳酸根浓度约为0.2 mmol·L-1, 碳酸氢根浓度约为2.0 mmol·L-1）。 该系统灵敏度比目前报道的海洋现场探测拉曼光谱系统提高了10倍以上, 为下一步海水中碳酸根和碳酸氢根的现场探测提供了可能。

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 CO2-3 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 R2 of 0.994 and 0.998 for CO2-3 and HCO-3, respectively. We calculated the LODs using the 3 times signal-to-noise ratio. The results showed that the LOD for CO2-3 and HCO-3 is about 0.06 and 0.38 mmol·L-1 respectively. The LODs are lower than the typical concentrations of CO2-3 and HCO-3 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 CO2-3 and HCO-3 detection inseawater.