Qualitative and quantitative analysis of CO2 and CH4 dissolved in water and seawater using laser Raman spectroscopy.

Laboratory experiments have been performed using laser Raman spectroscopy to analyze carbon dioxide (CO2) and methane (CH4) dissolved in water and seawater. Dissolved CO2 is characterized by bands at ∼1275 and 1382 Δcm−1. Dissolved CH4 is characterized by a dominant band at ∼2911 Δcm−1. The laboratory instrumentation used for this work is equivalent to the sea-going Raman instrument, DORISS (Deep Ocean Raman In Situ Spectrometer). Limits of quantification and calibration curves were determined for each species. The limits of quantification are ∼10 mM for CO2 and ∼4 mM for CH4. A ratio technique is used to obtain quantitative information from Raman spectra: the gas bands are referenced to the O–H stretching band of water. The calibration curves relating band height ratios to gas concentration are linear and valid for a range of temperatures, pressures, and salinities. Current instrumentation is capable of measuring the highest dissolved gas concentration observed in end-member hydrothermal fluids. Further development work is needed to improve sensitivity and optimize operational configurations.