The detection of dissolved gases in seawater plays an important role in oceanic observations and exploration. As a potential technique for oceanic applications, Raman spectroscopy has been successfully applied in hydrothermal vents and cold seep fluids, but it has not yet been used in common seawater due to the technique's lower sensitivity. In this work, we present a highly sensitive underwater in situ Raman spectroscopy system for dissolved gas detection in common seawater. Considering the difficulty of underwater degassing and in situ detection, we designed a near-concentric cavity to improve the sensitivity, with a miniature gas sample chamber featuring an inner volume of 1 mL placed inside the cavity to reach equilibrium in a short period of time. According to the 3σ criteria, the detection limits of this system for CO2, O2, and H2 were calculated to be 72.8, 44.0, and 27.7 ppm, respectively. Using a hollow fiber membrane degasser with a large surface area, the CO2 signal was found to be clearly visible in 30 s at a flow rate of 550 mL/min. Moreover, we deployed the system in Qingdao's offshore seawater, and the field test showed that this system is capable of successfully detecting in situ the multiple gases dissolved in the seawater simultaneously.
Keywords: Raman spectroscopy; dissolved gas; multi-reflection; underwater in situ detection.