Geoscience Reference
In-Depth Information
Györgyi,
1959
; Green and Malrieu,
1965
), as well as purines, pyrimidines, and flavins
(Pullman and Pullman,
1958
). Charge-transfer complexation is also thought to be impor-
tant in color intensification reactions (copigmentation) involving anthocyanins, the major
red, blue, and purple pigments in plants (Quina et al.,
2009
). da Silva et al. (
2005
) showed
that flavylium ions associated with anthocyanins are excellent electron acceptors forming
charge-transfer complexes with a number of naturally occurring polyphenols (ferulic acid,
gallic acid, caffeic acid, protocatechuic acid, and quercitin) wherein the polyphenols were
electron donors. As a final example, charge-transfer interactions are thought to be important
in the absorption behavior of lignin, especially with regard to visible absorbance (Furman
and Lonsky,
1988
). The potential importance of charge-transfer interactions within the
pool of molecules that comprise DOM is large. In an effort to elucidate the factors con-
trolling the fluorescence behavior of Suwannee River humic substances, Del Vecchio and
Blough (
2004
) performed experiments wherein the optical properties of the humic sub-
stances were altered by the selective photolytic destruction of chromophores using nar-
rowly focused laser light. The observed changes were inconsistent with a model for humic
substance fluorescence resulting from the simple, conservative addition of chromophores
and flourophores. Rather, the results suggested the presence of charge-transfer interactions.
Del Vecchio and Blough (
2004
) proposed that these interactions involve hydroxyl-aromatic
donors and quinoid acceptors resulting from the partial oxidation of lignin precursors. In
a subsequent paper, Boyle et al. (
2009
) present a strong argument for the importance of
the partial oxidation products of lignin in this process, with aromatic ketones possibly
playing important roles in absorption and emission properties of DOM (Ma et al,
2010
).
Other possibilities exist for donor (e.g., indoles, tryptophan, simple phenols, polyphenols)
and acceptor compounds (e.g., flavonoids, anthocyanins,). It is, at this time, difficult to
pinpoint the significance of these species owing to the complexity of DOM samples and
analytical constraints. However, given the reactive natures of aromatic compounds likely
to be precursors for DOM, it is possible that charge-transfer interactions associated with
a myriad of compound types contribute to observed optical behavior of DOM and aquatic
humic substances.
2.6 Conclusions
Fluorescence spectroscopy has been used as a method for studying organic matter in soils
and aquatic systems for many years. In the last 20 years, however, technological advances
in instrumentation and data management have resulted in greatly improved efficiency of
fluorescence data collection. Associated with these advances have been developments of
new approaches for analyzing the large amounts of data that can be generated for each
sample. Together, these developments have resulted in a resurgence of fluorescence as an
important tool for studying organic matter in a myriad of environments and there has been
a large increase of fluorescence data reported throughout the water sciences. Ease of data
collection and manipulation, however, do not, in themselves, result in improved data inter-
pretation. The goals of this chapter have been twofold: (1) to survey the literature related to
