Geoscience Reference
In-Depth Information
proven to be useful for studying the reactivity of both DOM and soil organic matter (SOM)
with other fluorophores, such as anthropogenic compounds (Chen et al.,
1994
; Backhus
et al.,
2003
). The effects of naturally occurring molecules comprising DOM or SOM on the
fluorescence properties of either of these materials are less well defined.
Static quenching results from interactions, such as chemical binding or the formation
of charge transfer complexes, of the ground state of a fluorescing molecule with another
chemical species. In this case, the interaction results in the formation of a non-fluorescing
“complex.” Quenching efficiency is determined by the strength of the interaction and the
concentration of the quencher. For studies of DOM and soil organic matter, interactions
with metals, such as iron, resulting in the formation of a complex and reduced fluorescence
of the DOM is an example of static quenching (e.g., Blaser and Sposito,
1987
). Other
examples involve the interactions of organic molecules with each other by weak electro-
static interactions resulting in decreased fluorescence of one or both molecules. For exam-
ple, the quenching of protein fluorescence resulting from interactions with compounds such
as cinnamic acids is an example of static quenching (Min et al.,
2004
). In environmental
chemistry, “partitioning” interactions of nonpolar organic pollutants with DOM have been
shown to result in static quenching of fluorescence probe molecules, such as fluorescent
polycyclic aromatic hydrocarbons (Gauthier et al.,
1986
; Backhus and Gschwend,
1990
;
Backhus et al.,
2003
). In these studies, fluorescence quenching of the polycyclic aromatic
hydrocarbon of interest was used to determine the equilibrium constant for the association
of the compound with DOM. In another example, the reduction of the fluorescence intensi-
ties of Suwannee River fulvic acid (FA) and humic acid (HA) bound to cationic nitroxides
was used to estimate surface potentials of the humic compounds (Green et al.,
1992
).
Collisional quenching, also known as “dynamic quenching” (Schulman,
1985
), occurs
when an excited fluorophore comes in contact (as via a collision) with the quenching spe-
cies. As part of the interaction of the excited fluorophore and the quenching molecule,
energy is transferred to the quenching molecule and the excited fluorophore is deactivated,
returning to the ground state via a nonradiative pathway. In this case, the fluorophore
returns to the ground state without a chemical reaction, and neither the quencher nor the
fluorophore is chemically altered. Molecular oxygen, halogens, amines, and electron-defi-
cient molecules are among the compounds that can act as quenchers (Lakowicz,
2006
).
For the case of DOM fluorescence, molecular oxygen could be an important collisional
quencher; however, the effects of molecular oxygen on DOM fluorescence have not been
well described, even in studies noting shifts in fluorophore intensities under different redox
conditions (see, e.g., Klapper et al.,
2002
)
A third type of quenching, the “inner filter effect,” does not involve direct interactions
between the excited molecule and another chemical species that result in loss of energy or a
change in the fluorophore energetics, but is a major problem associated with the collection
of fluorescence data (Tucker et al.,
1992
; Lakowicz,
2006
). It can result from molecules
that absorb excitation light, reducing the intensity of light available to excite a fluorophore
(primary effect), or by the absorption of light emitted by the fluorophore (secondary effect).
Any light-absorbing chemical species can result in inner filtration of light; when these are
