Geoscience Reference
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of mechanisms. DOM fluorescence is a photophysical process under which excited species
expend absorbed light energy and return to their ground state; quantum yields are only
ca. 1%. Reactions between fluorophores and their environment that inhibit a molecule's
ability to release energy via photon emission at longer wavelength (i.e., at lower energy)
will reduce DOM fluorescence. In the environmental effects considered here, the dominant
effect will be the reduction in the intensity of fluorescence, termed here quenching . A num-
ber of processes can produce this effect at the molecular level.
Collisional quenching involves fluorophore (A) deactivation in the excited state (A*) on
contact with another molecule, termed the quencher [Q] (Lakowicz 2006 ). The fluorescent
energy that is quenched is released as heat and can be represented as:
k q
*+→++heat.
AQ
AQ
(7.1)
This is a diffusive process and so proximity of the fluorophore and the quencher are impor-
tant, but no chemical reaction results. Probably the most effective collisional quencher in
natural waters is molecular oxygen, which quenches nearly all fluorophores (Lakowicz,
2006 ). Molecular oxygen prevalence in surface waters means that DOM fluorescence will
be modulated by its presence, and it should be expected that anaerobic or anoxic systems
(e.g., hypolimnia of lakes and sediment pore waters) could exhibit higher fluorescence in
situ than if a sample were taken and allowed to equilibrate with air (see Chapter 4 in this
volume). Oxygen and paramagnetic metal ions (e.g. Cu 2+ , Pb 2+ , Mn 2+ ) quench DOM fluo-
rescence by enhancing intersystem crossing for the excited singlet state to a triplet state
and then deactivate the fluorophore to the ground state with loss of heat energy rather than
photon emission. The triplet state itself is readily quenched by oxygen and other species in
solution. In addition to intersystem crossing, several mechanisms, including charge trans-
fer and electron exchange, may also contribute to overall fluorescence quenching, and dis-
cerning individual processes can be difficult (Lakowicz, 2006 ).
Quenching may also be caused by inorganic and organic compounds. Inorganic species
in natural waters such as halides, with chloride and bromide being very effective collisional
quenchers, may exert some control on DOM fluorescence as ionic strength of naturals
waters increases. Like paramagnetic oxygen, halide (e.g., the heavy atom effect; Senesi,
1990 ) or amide quenching is most likely due to the enhancement of intersystem crossing
from the excited singlet to the excited triplet state, followed by rapid decay to the ground
state (Lakowicz, 2006 ). Organic compounds may also serve as collisional quenchers of
DOM fluorescence. Aromatic and aliphatic amines, chlorinated hydrocarbons, and some
olefins are known quenchers, and acrylamide and methyl viologen (a herbicide) have also
been shown quenching effects (e.g., Milne and Zika, 1989 ).
Like collisional quenching, resonant energy transfer is an excited-state interaction in
which the two participating molecules exist as a donor molecule that transfers the excita-
tion energy (via electronic coupling) to an acceptor molecule rather than emitting photons.
The result is an excited acceptor and a deactivated donor and altered absorption or fluores-
cence can detect the interaction, which is an important consideration for assessing DOM
optical properties in natural waters (Del Vecchio and Blough, 2004 ).
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