Geoscience Reference
In-Depth Information
to observed changes in fluorescence spectra is difficult (Spencer et al.,
2007b
), however,
the observations reported for different compound classes (e.g., Fink and Koehler,
1970
and
Wolfbeis et al.,
1986
) may be of relevance.
Natural water samples generally range in pH between 4 to 10 with most samples having
pH between 6 and 8. For most samples, therefore, it is sufficient to measure fluorescence
without pH adjustment.
2.5.3 Interactions with Metals
Metals bound to fluorescing ligands can influence the electronic state of the ligand in a
manner analogous to protonation reactions. Metals are Lewis acids, and, as such, the coor-
dination of the ligand with a metal ion is similar to protonation of the ligand (Sharma and
Schulman,
1999
). Fluorescence can be either quenched or enhanced by the coordination of
metals with fluorescing ligands, such as aromatic compounds with electron-rich functional
groups (-COOH, -OH, -NH
2
), depending on the ligand and the effect the metal has on
the nonradiative processes competing with fluorescence. Most interactions between main
group transition metals and the fluorescing ligands in DOM or humic substances result in
static quenching due to interactions of the π electrons of the ligand with the metal.
In some instances, the ability of an organic compound to form complexes with metals
results in enhanced fluorescence. This is the case for a number of colorimetric indicator com-
pounds used to indicate the presence of some metals and cations in water samples (Skoog
and West,
1982
). Flavones are one of the natural product classes that exhibit enhanced fluo-
rescence when complexed by metals, and this reaction has been used to detect the presence
of both flavanols and metals alike (Wolfbeis,
1985
). A common example is the compound
morin, which is nonfluorescent in the uncomplexed state but fluoresces in the presence of
Al
3+
and other metals (Brown et al.,
1990
). The reaction of morin with Al
3+
has been used to
study the speciation of Al
3
+ in natural waters as well as in the study of the effects of Al
3+
on
plant materials (Eticha et al.,
2005
). In a final example, the fluorescence of salicylic acid is
enhanced in the presence of As
3+
and sodium dodecyl sulfate (Karim et al.,
2006
), whereas
interactions with Fe
3+
result in fluorescence quenching (Cha and Park,
1998
).
Fluorescence quenching has been used to measure metal binding constants of a range
of metals with humic substances (Saar and Weber,
1980
) and plant extracts (e.g., Blaser
and Sposito,
1987
). Recent studies have used PARAFAC analyses to determine changes
in different regions of organic matter fluorescence spectra resulting from interactions
with metals (Ohno et al.,
2008
). However, while providing information about interactions
between naturally occurring fluorophores and metals, fluorescence quenching suffers from
limitations that restrict its application for the determination of environmentally relevant
DOM-metal binding constants (Cabaniss and Shuman,
1988
). The fluorescence quenching
approach lacks sensitivity, requires larger concentrations of metals than are environmen-
tally relevant, and provides information only about the fluorophores interacting with the
metals, not the nonfluorescent ligands that comprise most of the DOM. Binding constants
obtained using fluorescence quenching of soil extracts or DOM are usually many orders
