Mid-Infrared Spectroscopic Sensors for In-Situ Monitoring of Methane Dissolved in Sea Water

摘要

A mid-infrared spectroscopic measurement technique isrnpresented for determination of the concentration of methanernand other short-chained aliphatic hydrocarbons dissolved inrnsea water. This approach will be used for studying gas hydraternreservoirs providing analytical information on the amount ofrnmethane, ethane, and propane in the vicinity of hydrate fields.rnOn account of their methane content gas hydrates arernconsidered a potential energy resource for the future.rnFurthermore, environmental concerns are related to methanernas greenhouse gas. If large amounts of methane are releasedrnfrom hydrate sites through the water column into thernatmosphere major impact on the climate is expected.rnFurthermore, gas hydrates are stabilizing the sea floor andrndisturbances e.g. by oil exploration drillings can result inrnrelease of large amounts of gas posing serious risks for oilrnrigs.rnThe proposed target analytes are detected at lowrnconcentrations due to their molecule specific absorptionrnspectra in the mid-infrared spectral range (3-20 μm). As thernwater matrix itself is a strong IR absorber, conventionalrntransmission spectroscopy can not be applied. The presentedrnapproach is based on attenuated total reflection (ATR)rnspectroscopy utilizing a polymer coated waveguide as sensorrnhead. This measurement principle is based on IR radiationrnevanescently guided as exponentially decayingrnelectromagnetic field at the waveguide-polymer interface.rnHence, resonant absorptions of IR-active molecules present atrnthe waveguide-polymer interface result in spectra similar tornconventional transmission absorption experiments.rnHydrophobic polymer coatings at the waveguide surfacernprevent water from being in contact with the evanescent fieldrnand simultaneously enrich analytes at the waveguide-polymerrninterface for sensitivity enhancement.rnThe deep sea sensor will be based on a compact Fourierrntransform infrared (FT-IR) spectrometer and a high-pressurernsensor head for ATR spectroscopy encapsulated into arnspherical glass pressure vessel.rnConcentrations of multiple analytes dissolved in sea waterrnwith overlapping spectral features are evaluated usingrnchemometric1 data evaluation algorithms, which are modifiedrnto handle spectroscopic difficulties encountered in deep searnapplications.