In situ Raman quantitative detection of methane concentrations in deep-sea high-temperature hydrothermal vent fluids

Raman spectroscopy is an ideal approach for measuring methane concentrations in deep-sea high-temperature hydrothermal vent fluids due to its advantages of being nondestructive and noninvasive and not requiring sample pretreatment. However, no application of Raman spectroscopy in the measurement of hydrothermal methane has yet been reported because of the lack of Raman quantitative calibration models for CH₄ suitable for hydrothermal fluid detection and available for deep-sea in situ Raman experiments. In this study, a new Raman quantitative calibration model suitable for hydrothermal fluid detection was established with the linear equation (Formula presented.) = (2.61E-3 ± 8.52E-6) × (Formula presented.), where (Formula presented.) is the peak area ratio of CH₄ and H₂O and (Formula presented.) is the concentration of dissolved CH₄ in mmol/kg. In situ Raman spectra of deep-sea hydrothermal fluids were acquired using an adapted deep-sea in situ Raman spectrometer, Raman insertion probe (RiP) system, and then the methane concentrations were determined based on the quantitative calibration model for CH₄. The concentrations of methane measured by RiP are approximately 1.5–4.0 times higher than those derived from the gas-tight samples collected simultaneously at the same vents, which indicates that the amount of methane released from the hydrothermal system has probably been underestimated.