Publication Date:
2022-05-25
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.
Keywords:
Fluorescence spectroscopy
;
Chemical oceanography
;
Optical oceanography
;
Columbus Iselin (Ship) Cruise
;
Knorr (Ship : 1970-) Cruise KN134-12
Repository Name:
Woods Hole Open Access Server
Type:
Thesis
Format:
application/pdf
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