Defining the structural basis of the optical properties of chromophoric dissolved organic matter
|Comparison of Suwanee River Fulvic Acid (SRFA) before and after irradiation with >305 nm. The expanded m/z region around m/z 413 is shown. (Kujawinski et al., 2004 Marine Chem.)|
Neil V. Blough (University of Maryland)
Rossana Del Vecchio (University of Maryland)
Chromophoric dissolved organic matter (CDOM) is a complex organic material found ubiquitously in aquatic systems where it plays a central role, not only through its impact on the aquatic light field and on the remote sensing of phytoplankton biomass by satellite ocean color sensors, but also through its photochemical reactions. Although the optical absorption and emission properties of CDOM have been known and studied extensively for over fifty years, no satisfactory explanation has yet been provided that can account for the long-wavelength absorption and emission behavior of these materials. Based on prior work, we propose that this behavior results from intramolecular charge transfer interactions between hydroxy-aromatic donors and quinoid acceptors formed by the partial oxidation of soluble lignin precursors. This hypothesis will be tested in the laboratory by combining wavelength-selective monochromatic (laser) and polychromatic photobleaching studies with ultrahigh-resolution mass spectrometry (9.4 T ESI Fourier-transform ion cyclotron resonance mass spectrometer at the NSF National ICR Users' Facility) to acquire structural information on the species that are specifically destroyed or produced by the wavelength-selective irradiations. Absorption and fluorescence spectroscopy, gel permeation chromatography and lignin phenol analyses will be employed to provide complementary structural information.
Standard humic substances and model lignin systems will be examined initially, followed by CDOM samples collected during a series of field studies in the Delaware Bay / Mid-Atlantic Bight region. These field studies will provide further tests of this hypothesis through an examination of the relationships among the optical properties and the lignin phenol content, molecular size and mass spectra of CDOM collected across a wide range of oceanic environments and seasons. Additional laboratory studies on these samples will test the importance of photochemistry as a source of the structural changes observed for this material in the environment.
Funding support: National Science Foundation, Grant# OCE-0443217
Personnel: Louisa Morrison (technician)
Publications: (1) Kujawinski, E. B., K. Longnecker, N. V. Blough, R. Del Vecchio, L. Finlay, J. B. Kitner and S. J. Giovannoni. 2009. Geochim. Cosmochim. Acta 73: 4384-4399; (2) Kujawinski, E. B., R. Del Vecchio, N. V. Blough, G. C. Klein and A. G. Marshall. 2004. Marine Chemistry 92 : 23-37.