Interfaces et Capteurs Interfaces and Sensors

Raman

For several years prospective studies have been performed in the field of underwater measurements. The main objective is to open new ways for in situ analysis of parameters only previously accessible by sampling. In this context, Raman Spectroscopy offers the unique advantage of fast measurements a few tenth of seconds on any kind of sample (solid, liquid, gazeous) and identification, without any interaction, of a wide range of different coumpounds using the same equipment.

What is Raman Spectroscopy ?

Raman Spectroscopy consists in the a light beam excitation of molecules of a target compound to be identified. The photon energy is then absorbed providing a vibration that gives a secondary photon emission at a typical wavelength shift from the excitation representing the signature of the different atomic bonds. The spectra thus obtained gives a true image of the molécule composition which are present in the light beam.

Measurement set-up description

The excitation light source is generally provided by a sharp monochromatic laser beam, the energy of which is focussed on the target to be analysed. Raman light is then collected through a particular detection solid angle on a spectrograph coupled to a matricial CCD detector. The pixels frames recorded are processed in order to give a spectral signature containing all the peak vibrations which characterize the molecule.

Application to underwater measurements

Underwater oceanographic instrumentation requires a particular attention to the packaging of field equipment which must withstand harsh environmental conditions. Conventionnal technologies may not be always be suitable, and the challenge is to find original solutions. In close collaboration with the Jobin Yvon Company, the axial imaging spectrograph has been chosen as the core component of the instrumental set-up. The basic configuration relies on a co-linear optical axis alignment from the photonic input signal to the detection unit. As shown on the following picture, it becomes easy to integrate components in such way that the overall set-up can be made watertight and operated under hydrostatic pressure.

Size reduction of the new generation of laser sources is beneficial for their integration in limited spaces. In order to provide flexibility in operation, the measurements are achieved by of special probes linked to the core unit through optical fibre bundles as shown on the following drawing.
Since measurements have to be performed at a distance, all the components necessary for ensuring communications have been integrated in the system.

The final set-up is mounted on a frame to be placed on the lower part of the ROV. The optical probe is located on the front end of the carrier so that the pilot can visualise it by video and place it as close to the target to be analysed as possible using the controlled arm.

Few direct Raman spectra acquired by reflection with the system :

Among the in situ measuring features, one concerns the identification of rocks and more generally all solid matter. Though the best beam focalisation is usually required on the target compound, it is particularly interesting to get field informations on homogeneous structures before sampling. The following spectra have been obtained on solid samples collected during oceanographic cruises with the Raman spectrograph and the optode. The well-known signatures of carbonates or sulfates are quickly identified.

Direct Raman measurements on methane hydrate have been performed on the icy form without any preparation. On that form the Raman response was typically a 2902cm-1 signature followed by the presence of water, while the response of the gazeous form showed a normal peak at 2913cm-1.

SERS (Surface Enhance Raman Scattering)

Probes which are linked to the Core Unit have been designed so that two kinds of measurement are available. The first one, so called “direct Raman”, is devoted to the measurements of solid compound while the second is base on SERS technique which brings a higher sensitivity for the detection of dissolved analytes. Thanks to a close collaboration with “optische Institut” of TU Berlin funded by an EU project (MISPEC – EVK3-CT2000-00045), some in situ expériments have been carried out in the Baltic sea on the R/V Oceania from IOPAS- Sopot (Poland).