Abstract

Laboratory experiments have been performed using laser Raman spectroscopy to analyze carbon dioxide (CO<sub>2</sub>) and methane (CH<sub>4</sub>) dissolved in water and seawater. Dissolved CO<sub>2</sub> is characterized by bands at ∼1275 and 1382 Δcm<sup>−1</sup>. Dissolved CH<sub>4</sub> is characterized by a dominant band at ∼2911 Δcm<sup>−1</sup>. 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 CO<sub>2</sub> and ∼4 mM for CH<sub>4</sub>. 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.

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