ALBERT

Description: Submitted in partial fulfillment of the requirements for the degree of Doctor of Philosophy at the Massachusetts Institute of Technology and the Woods Hole Oceanographic Institution February 1992

Description: The work presented in this thesis consists of three parts. The first is a photophysical study of the mechanism of fluorescence quenching by stable nitroxyl radicals, which are becoming an important analytical tool for the study of reactive transients in surface waters (1, 2). In part two, quenching of dissolved organic matter (DOM) fluorescence by nitroxides is employed to investigate the electrostatic propertie~ of DOM in aqueous solution, with the goal of elucidating the apparent ionic strength and pH dependence of metal-fulvic acid binding constants. In part three, the intrinsic optical properties (absorbance, fluorescence, and fluorescence efficiency) of DOM are examined in a coastal region to understand how these properties vary with source. age and sunlight exposure time. Nitroxide-fluorophore adducts were employed to investigate the mechanism by which nitroxyl radicals quench fluorescence (3). Fluorescence quantum yields and lifetimes were measured for a series of adducts, and quenching rates were shown to be quite high (kq≈108-1010 s-1), even at distances of ≈12 Å. Forster or Dexter energy transfer mechanisms are unable to account for the observed rates and lack of solvent dependence in quenching. An excellent correlation is observed between kq and the non-radiative relaxation rate. These results confirm that nitroxyl radicals are very non-selective in their quenching abilities, and suggest that the best analytical probe adducts will include a fluorophore with an appreciable non-radiative relaxation rate. Diffusional quenching by charged and neutral nitroxides was employed to explore the electrostatic properties of fulvic (FA) and humic (HA) acids. Cationic nitroxides were found to be up to 16 times more effective than neutral analogues in quenching the fluorescence of humic materials. This result is attributed to the enhanced coulombic attraction of cations to the anionic FA or HA surface, and is interpreted as an estimate of surface electrostatic potential. Reduction of molecular charge at low pH and shielding of charge at high ionic strength (I) produced diminished enhancements. consistent with this interpretation. The potential was found to be particularly sensitive to ionic strength. suggesting that this electrostatic effect should be of particular importance in transition zones. such as estuaries, where I increases from 〈5 mM to 0.7 M as river water and seawater mix. High molecular weight fractions of HA have a higher apparent surface potential than lower molecular weight fractions. indicating that larger humic molecules may have an enhanced ability to bind metal ions. Optical properties of colored DOM may vary with source and age of the matertal. Absorption spectra can be characterized by their log-linearized slopes (S) as well as by their absolute intensities. The slope, S, is found to be much greater (steeper decrease in absorbance with increasing wavelength) for blue-water samples than for rtverine and coastal samples, indicating that the visible-light absorbing fraction of DOM may be preferentially removed from surface waters. Fluorescence quantum yields were very similar for a wide variety of samples, but do show some minor differences; changes in quantum yield with excitation wavelength within a single sample are an indication of the heterogeneity of the chromophore mixture present in these materials. To better compare fluorescence data, complete excitation/ emission matrix spectra were collected. When normalized to their respective absorbance spectra, these provide a full 'map' of fluorescence quantum efficiency over the entire uv-visible range. This technique is showing promise as a way to identify important spectral regions in these complex chromophore mixtures. DOM isolated on C-18 columns had somewhat different optical charateristics than whole water samples, suggesting selective isolation of absorbing material.