Development of a Laser Raman Spectrometer for In Situ Measurements in the Deep Ocean
Sheri N White1 (firstname.lastname@example.org), Peter G Brewer1, Edward T Peltzer III1, George E Malby1, Jill D Pasteris2
1MBARI, 7700 Sandholdt Rd., Moss Landing, CA 95039
2Dept. of Earth and Planetary Sciences, Washington University, St. Louis, MO 63130
We have developed an ROV-deployable laser Raman spectrometer (LRS) to make in situ measurements of solid, liquid and gaseous species in the ocean (up to 3600 m depth). The LRS can be used to determine chemical and structural composition by irradiating the target with a laser and measuring the inelastically scattered (Raman shifted) light. The frequency shift from the exciting wavelength is due to characteristic molecular vibrations of the molecule; thus, the Raman spectrum serves as a fingerprint of a substance based on molecular composition and crystal structure. Raman spectroscopy is rapid, and typically requires no sample preparation. However, the weak Raman effect (~ 1 in 108 photons), the need for precise laser positioning, and fluorescence, pose challenges.
We have acquired an LRS from Kaiser Optical Systems, Inc. and adapted it for use in the ocean by dividing the components into three pressure cases, building penetrating fiber optic cables, developing an Ethernet interface to control the system from shipboard, and redesigning and rebuilding non-robust components. Future improvements will include weight/size reduction, adding through-the-lens visualization, and using liquid core optical waveguides to increase sensitivity. An increase in sensitivity of x10 would permit direct observation of natural seawater HCO3 and CO3 peaks.
The LRS has been successfully deployed over 6 times on MBARI’s two remotely operated vehicles in 2002. Initial measurements of standards (e.g., isopropanol, calcite, and diamond) at depths as great as 3600 m have proven the effectiveness of the instrument in the deep ocean and have allowed us to advance methods for its use. Detailed spectra of seawater in situ and in the lab have also been obtained to better understand the ever-present seawater background (which includes water and SO4 peaks, and very little fluorescence). We have used the LRS in a number of deep-sea CO2 sequestration studies to acquire spectra of gaseous CO2 and CO2/N2 mixtures from the surface to 400 m depth, and of liquid CO2 and CO2 hydrate on the seafloor at 3600 m. Future plans include measurements of gas vents, hydrothermal vent fluids and minerals, natural gas hydrates, sediment pore waters, and bacterial mats.