Development of a compact deep-sea Raman spectroscopy system and direct bicarbonate detection in sea trials

In recent years, Raman spectroscopy techniques have been successfully applied to the area of deep-sea exploration. However, there are still some problems impeding the further application of Raman systems. For example, the large size of an underwater Raman system makes it difficult to deploy on the underwater vehicle. Meanwhile, the sensitivity is often a disadvantage, requiring improvement for detecting more trace components. To solve these problems, a new compact deep-sea in situ Raman spectroscopy system is presented in this paper. The whole system weighs 60 kg and is housed in an L800  mm×ϕ258  mm pressure vessel with an optical window on the front end cap. The main components include a 532 nm Nd:YAG laser, an optics module, a high-throughput spectrograph with 0∼4900  cm^-1 spectral range and 8  cm^-1 spectral resolution, a TEC-cooled 2000  pixel×256  pixel CCD detector, a PC104 embedded computer, and an electronics module. To evaluate the performance of the newly developed Raman system, systematic experiments have been carried out with solutions in laboratory, and the results have shown that the system limit of detection of SO42- is 0.4 mmol/L. The Raman system has been successfully deployed on a remote-operated vehicle on the Kexue research vessel in June 2015. The typical in situ detection results are presented in this paper, and it is shown that the Raman system is capable of detecting the Raman signal of SO42- and fluorescence of chlorophyll a (chl-a) and chromophoric dissolved organic matter (CDOM) in seawater. With 500 spectra accumulations and some data processing, the Raman signal of HCO3- is obtained. This is the first report of direct measurement of HCO3- by Raman system in in situ experiments. After further optimization, it is hoped to apply the Raman system in seafloor observation networks for long-time carbon cycling research.