Rapid real-time monitoring of dissolved gases in seawater by cavity-enhanced Raman spectroscopy system with sub-ppm sensitivity.

Real-time monitoring of dissolved gases in seawater is of critical scientific significance for research on marine ecosystem, global carbon cycle and marine environmental change. We developed a portable cavity-enhanced gas Raman spectroscopy system to enhance the Raman scattering intensities for trace gases detection by extending the photon-gas interaction path based on retro-reflective near concentric cavity. By optimizing the non-resonant multi-reflection optical path, the optical path length between laser and target gas was extended, which effectively strengthened Raman scattering signals. Combined with a hollow fiber membrane as the gas-liquid separation unit, the system realized the in-situ degassing and quantitative detection of dissolved gases in water. Calibrated with standard gases, the limits of detection (LODs) for carbon dioxide (CO2), methane (CH4) and hydrogen (H2) were 0.86 ppm, 0.12 ppm and 0.92 ppm, respectively. For the detection of dissolved gases in actual water, the developed Raman spectroscopy system was applied to detect water samples collected from freshwater of campus lake and coastal seawater of Qingdao. The concentrations of trace extracted gases in lake water/seawater detected by this instrument were 761.2 ppm/636.9 ppm for CO2, 165.6 ppm/1.6 ppm for CH4 and 1.8 ppm/2.1 ppm for H2, respectively. These results indicated that the developed Raman system with sub-ppm sensitivity could satisfy the practical monitoring demands for the lake and seawater, exhibiting excellent rapid real-time monitoring performance in complex water environments. Its practical implementation provides a reliable technical support for marine environmental monitoring, aquatic ecological research and greenhouse gas traceability.