Mid-infrared sensors for marine monitoring

The emerging demands of marine monitoring have initiated increased efforts to develop sensor systems capable of screening organic pollutants in seawater. The construction of a sensor system based on a Fourier transform infrared (FT-IR) spectrometer coupled to a mid-infrared fiber-optic sensor head to be used in a tow-body for underwater applications is part of the European Union project "SOFIE - Spectroscopy using Optical Fibers in the Marine Environment", aiming at the introduction of an entirely optical approach for in-situ ocean monitoring. The investigated analytes of this initiative include heavy metals, chlorinated hydrocarbons and aromatic hydrocarbons, which will be addressed using fiber-optic sensing schemes based on absorption (mid-infrared fiber-optic evanescent wave spectroscopy (MIR-FEWS), fluorescence, surface enhanced Raman scattering (SERS) and refractivity (surface plasmon resonance (SPR)). Since the system is modular, adaptation of existing techniques or the incorporation of new modules ensures the detection of further analytes. As part of this research project, a compact FT-IR based sensor system has been developed, reconstructing a Bruker Vector 22 FT-IR spectrometer. For the first time, a spectrometer was redesigned to fit into a submersible tube, with an inner diameter of 270 mm and an overall length of 1100 mm. This approach proved capable of multi-component analysis in seawater, demonstrated for the example of various chlorinated hydrocarbons, as well as relatively low susceptibility to interfering parameters such as salinity and turbidity. First successful field test of the system integrated in a remotely operated vehicle corroborate the feasibility of this approach. With respect to the technological perspectives using miniaturized sensor components, a notable impact on this novel application area for MIR sensors can be expected.