Evaluation of near-infrared laser Raman spectroscopy underwater research

The in-situ chemical detection in deep sea is very important for mineral resources exploitation. Laser Raman Spectroscopy as a powerful tool for chemical analysis has been used in deep sea detection recently. It was found that the fluorescence always exists as the background on Raman spectra detecting in ocean. It is apparent that the fluorescence background could be decrease effectively using near infrared wavelength as excitation source. While near infrared laser has few application underwater due to the absorption of water in this wavelength range. In this paper, a 785nm Raman spectrometer was used to evaluate the near infrared laser Raman spectroscopy for underwater application. It was found that the Raman shift of SO4 2- at 981cm-1 can be easily detected from Qingdao near shore sea water samples without sever fluorescence background. With mixed solution of SO4 2- and HCO3 -, the detection limit achieved for SO4 2- at 981cm-1 and HCO3 - at 1017cm-1 was obtained by 785nm compact Raman spectrometer to be 0.00625mol/l and 0.025mol/l with 10s integration time respectively. The obtained results proved the feasibility of near infrared laser Raman underwater detection.