HR: 1340h
AN: OS43B-0557    [Abstracts]
TI: First Attempts at Direct Raman Detection of the Oceanic Carbonate System
AU: Brewer, P G
EM: brpe@mbari.org
AF: MBARI, 7700 Sandholdt Road, Moss Landing, CA 95039 United States
AU: * Dunk, R M
EM: dura@mbari.org
AF: MBARI, 7700 Sandholdt Road, Moss Landing, CA 95039 United States
AU: White, S N
EM: sheri@mbari.org
AF: MBARI, 7700 Sandholdt Road, Moss Landing, CA 95039 United States
AU: Peltzer, E T
EM: etp3@mbari.org
AF: MBARI, 7700 Sandholdt Road, Moss Landing, CA 95039 United States
AU: Bowie, B
EM: bowie@kosi.com
AF: Kaiser Optical Systems, Inc., 371 Parkland Plaza, Ann Arbor, MI 48103 United States
AU: Walz, P M
EM: wape@mbari.org
AF: MBARI, 7700 Sandholdt Road, Moss Landing, CA 95039 United States
AB: MBARI's Deep Ocean Raman {\it In-Situ} Spectrometer (DORISS) has been deployed on a number of scientific missions in the ocean, with the successful acquisition of spectra from a wide range of targets including minerals, gases, and gas hydrates. We are now exploring the future use of this instrument for the direct detection of aqueous species in natural waters, with a particular focus on the oceanic carbonate system. The ability to acquire {\it in-situ} data allowing a direct quantitative assessment of carbonate speciation would present a powerful new tool to the oceanographic community. Ocean scientists have long relied on indirect methods for determination of the individual carbonate system species in seawater through linking observations with apparent thermodynamic constants. However, direct Raman spectroscopic observation of HCO$_{3}$$^{-}$ and CO$_{3}$$^{2-}$ in solution is possible, although acquisition of spectra {\it in-situ} is difficult due to low concentrations in natural waters. We are attempting to address this problem using the DORISS instrument. The first objective is to determine the Raman scattering efficiency of HCO$_{3}$$^{-}$ and CO$_{3}$$^{2-}$, and thereby define the instrument requirements to enable direct determination of carbonate speciation in a $\sim$2mM TCO$_{2}$ solution. The second step is to increase the sensitivity of the system through improvements to the optical path and advanced post-processing techniques. To perform quantitative measurements using Raman spectroscopy a reference standard is required, where the relative scattering efficiency of the standard and target must be known (the Raman cross section ratio, $\sigma$$_{standard}$/ $\sigma$$_{target}$). We have chosen SO$_{4}$$^{2-}$ as the reference standard for seawater species, as the seawater SO$_{4}$$^{2-}$ signal is readily detected {\it in-situ} with short (sub 1 minute) spectral acquisition times. The scattering efficiencies of HCO$_{3}$$^{-}$ and CO$_{3}$$^{2-}$ with respect to SO$_{4}$$^{2-}$ were determined in the laboratory ($\sigma$$_{SO4}$/ $\sigma$$_{HCO3}$ = $\sim$4.2; $\sigma$$_{SO4}$/ $\sigma$$_{CO3}$ = $\sim$2.5). As both HCO$_{3}$$^{-}$ and CO$_{3}$$^{2-}$ are comparatively weak Raman scatterers, and the TCO$_{2}$ of seawater is ca. 10 times lower than the SO$_{4}$$^{2-}$ concentration, it is necessary to increase instrument sensitivity by a factor of 10-100 to enable rapid (sub 1 minute) {\it in-situ} detection of the seawater carbonate system. Current instrument sensitivity allows detection of $\sim$15mM HCO$_{3}$$^{-}$, or ca. 7 times the seawater concentration, using long ($\sim$30 minute) acquisition times and standard spectral processing software (GRAMS AI, ThermoGalactic). To increase sensitivity we have explored potential adaptations to DORISS and alternative methods of spectral processing. First, we tested a new sampling optic, the 532 nm PhAT system (Kaiser Optical Systems, Inc.). The current DORISS probe head utilizes a single optical fiber for signal collection, whereas the PhAT system utilizes a bundle of collection fibers, leading to a concomitant increase in instrument sensitivity to transparent and semi-transparent samples. Secondly, the application of partial least squares spectral processing (Eigenvector Research, Inc.) notably improves the Raman peak detection limit. The incorporation of the PhAT system, in addition to advanced spectral processing, could significantly increase the sensitivity of the DORISS instrument, enabling direct measurement of the seawater carbonate system.
DE: 4805 Biogeochemical cycles (1615)
DE: 4820 Gases
DE: 4894 Instruments and techniques
DE: 1094 Instruments and techniques
SC: Ocean Sciences [OS]
MN: 2004 AGU Fall Meeting