| Raman spectroscopy is a molecular vibrational spectroscopic technique that is capable of real-time, continuous, non-contact and multicomponent detection, and it is gaining a growing popularity for in-situ detection. Marine sediment is transition layer from hydrosphere to lithosphere, and it contains the deposit from continent and sea water, which makes it is of great importance. Much of the geochemical knowledge sought is not from the sediments themselves but from the pore waters that contain the signature of the reactions at work. Geochemical analysis for sediment pore water have traditionally relied upon sample recovery, while the results show the big difference between the real concentration of the components in pore water because of the escape and decomposition during and after sampling. The sensors based on laser Raman spectroscopy have been used for in-situ detection of sediment pore water. While there is a long way to go for a wider range of application of sensors based on Raman spectroscopy, and for component analysis of pore water. With the hope of wider application of Raman spectroscopy underwater and more effective analysis of pore water, calibration to the DOCARS system is carried out, methods for Raman signal enhancement are investigated and spectral analysis of pore water are performed.In order to assess the capability of Raman spectroscopy for quantitative detection of acid radical ions, Raman spectra of samples prepared in laboratory are acquired using Deep Ocean Compact Autonomous Raman Spectrometer (DOCARS), and the calibration curves of DOCARS for SO42-、NO3-、HCO3- are obtained. Then the linear functions derived from the calibration curves are used for the analysis of spectra acquired in the sea trial. It is found that the normalized Raman signal intensity of SO42-, NO3- and HCO3-increased as a linear function of the concentration with correlation coefficient R2 of 0.99, 0.99 and 0.98 respectively, and the concentration of sulfate ion in seawater detected by DOCARS system is 30.5 mmol/L with 5% detection accuracy.The low detection sensitivity limits the underwater application of Raman spectroscopy. Simulation and experimental investigation for Raman signal enhancement technology based on LCOF is performed. With the aid of LCOF, Raman signal of SO42- is found to be enhanced over 10 times than that obtained with conventional Raman setup. For the detection of methane dissolved in water, a novel method based on enrich process is proposed. By CCl4 extraction, the methane with the concentration below 1.14 mmol/L has been indirectly detected with conventional Raman spectroscopy.For spectral analysis of pore water, two batches of pore water, which are obtained at the same station located at Zhangzidao waters but different time, are taken as samples to acquire Raman and fluorescence spectra. The results of Raman spectra analysis show that the concentration of SO42- in pore water samples decreases as the depth increases, which indicates the existence of sulfate reduction in pore water. The results of fluorescence spectra analysis shows that there are two kinds of fluorescent substances in pore water, and the fluorescence characteristics differ a lot.Quantitative analysis of methane dissolved in water using the proposed method is remained to be achieved, and spectral analysis of more pore water samples from different sea areas is remained to be performed, which will be accomplished in the further work. |
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