The applicability of shifted excitation Raman difference spectroscopy (SERDS) for in-situ investigations in the deep sea is presented. For the laboratory experiments a distributed feedback (DFB) diode laser emitting at 785.0 nm and 785.4 nm and two low OH optical fibers with a length of 1000 m each were introduced to perform Raman measurements for selected minerals. The investigations for the polycyclic aromatic hydrocarbons (PAHs) pyrene, fluoranthene, and phenanthrene were carried out applying surface enhanced Raman scattering (SERS) which improves the Raman signal. It is shown that SERDS leads to a reduction of the background which improves the signal-to-background ratio by a factor of up to 100, i.e. even weak Raman bands can clearly be identified.
The requirement of using in-situ measurements for deep sea investigations arises from the necessity to obtain real-time information in the deep sea environment. Raman spectroscopy, which is a powerful tool to obtain in-situ "fingerprint" spectra from substances, is established as an attractive analytical method for deep ocean investigations due to the fact that water is a weak Raman scatterer. Furthermore, Raman spectroscopy is non-destructive and non-invasive as well as it needs no special sample preparation. Measurements in the deep ocean are of great economical interest since many important targets can be located and identified (e.g. minerals, gas hydrates). In order to perform in-situ Raman measurements, a sea going Raman instrument for deep sea investigation has been developed (Brewer, Malby, Pasteris, White, Peltzer, Wopenka, Freemann and Brown, 2004) and successfully tested to identify gas hydrates (Hester, Dunk, White, Brewer, Peltzer and Sloan, 2007) as well as at hydrothermal vents and cold seep systems (White, Dunk, Peltzer, Freeman and Brewer, 2006). Additionally, hydrothermal and cold seep minerals were investigated with a laboratory Raman set-up by White (2009).
