Zeng-Feng Du

The levels of dissolved sulfate and methane are crucial indicators in the geochemical analysis of pore water. Compositional analysis of pore water samples obtained from sea trials was conducted using Raman spectroscopy. It was found that the concentration of SO₄²⁻ in pore water samples decreases as the depth increases, while the expected Raman signal of methane has not been observed. A possible reason for this is that the methane escaped after sampling and the remaining concentration of methane is too low to be detected. To find more effective ways to analyze the composition of pore water, two novel approaches are proposed. One is based on Liquid Core Optical Fiber (LCOF) for detection of SO₄²⁻. The other one is an enrichment process for the detection of CH₄. With the aid of LCOF, the Raman signal of SO₄²⁻ is found to be enhanced over 10 times compared to that obtained by a conventional Raman setup. The enrichment process is also found to be effective in the investigation to the prepared sample of methane dissolved in water. By CCl₄ extraction, methane at a concentration below 1.14 mmol/L has been detected by conventional Raman spectroscopy. All the obtained results suggest that the approach proposed in this paper has great potential to be developed as a sensor for SO₄²⁻ and CH₄ detection in pore water.

While under laboratory conditions, the concentration of methane dissolved in water is too low to be detected because of the low solubility of methane using Raman spectroscopy. In the present paper, a novel approach based on CCl4 extraction was introduced, and used in the measurement of methane dissolved in water using Raman spectroscopy under laboratory conditions. Saturated aqueous solution of CH4, CCl4 solution after extraction of CH4 from the saturated aqueous solution and the saturated CCl4 solution of CH4 were prepared, and the Raman spectra of three samples were obtained. The obtained results show that the CH4 dissolved in saturated aqueous solution(the concentration of CH4 is about 1.14 mmol x L(-1)) can't been detected using Raman spectroscopy under laboratory conditions, but the CH4 Raman peak can be clearly seen in the spectra obtained for CCl4 solution after extraction. All the results demonstrate that the proposed approach can improve the Raman spectroscopy sensitivity of methane detection dissolved in water, and this approach has significant potential to be developed as an effective method for detection of methane dissolved in water.