Geoscience Reference
In-Depth Information
of magnitude less than those obtained using other, more sensitive methods. Difficulties
encountered in the determination DOM-metal binding constants under environmentally
relevant conditions are reviewed by Town and Fillela (
2002
) and Gasper et al. (
2007
).
The interactions of metals with DOM can pose a problem for the measurement of fluo-
rescence data on natural samples. In particular, iron (Fe
3+
and Fe
2+
) is an issue because it is
often present at concentrations sufficiently large enough to influence DOM optical proper-
ties. Ferric iron (Fe
3+
) itself absorbs light at wavelengths important for both excitation and
emission in fluorescence analyses of water samples and is more problematic when mea-
suring absorbance than is Fe
2+
(Doane and Horwáth,
2010
). Weishaar et al. (
2003
) showed
the influence of Fe
3+
on the measurement of absorbance data at
λ
= 254 nm in the deter-
mination of specific UV absorbance of DOM. For fluorescence, the presence of Fe
3+
could
contribute to inner filter effects. Recently Doane and Horwáth (
2010
) described a method
whereby Fe
3+
is chemically reduced to Fe
2+
to correct for its influence in absorbance meas-
urements. However, both forms appear to influence the intensities and peak positions with
humic fluorophores. Cory (
2005
) demonstrated that addition of both Fe
2+
and Fe
3+
in the
range of 0.5-50 μM to a sample of aquatic fulvic acid reduced total fluorescence, resulted
in peak position shifts, and influenced the distribution of components based on PARAFAC
analyses. The presence of iron and its effects are often overlooked in reports describing the
optical properties of DOM in natural systems. These effects may be substantial and need to
be taken into account to improve data interpretation.
2.5.4 Charge Transfer Interactions
Charge transfer interactions are important in understanding both absorbance and fluores-
cence properties of organic molecules. Charge transfer interactions are weak electrostatic
interactions between two molecules wherein electron density is partially transferred from
one molecule (donor) to the other (acceptor), thereby forming a “charge transfer complex”
and altering the energy levels of both species. These interactions influence the optical prop-
erties of the molecules that are involved in the complex, resulting in absorption and fluo-
rescence spectra that cannot be expressed as the sum of the parts (March,
1968
; Barsberg
et al.,
2003
). For organic compounds, aromatic molecules with π-electrons can act as both
donors and acceptors. In the classic case, the π system of one molecule overlaps that of the
second molecule and yields an electron to the second molecule. This is the case for the for-
mation of the quinhydrone complex between hydroquinone (donor) and quinone (acceptor)
(Szent-Györgyi et al.,
1961
; D'Souza and Deviprasad,
2001
). Other types of charge trans-
fer interactions are possible involving both inorganic (e.g., some metals) and organic spe-
cies (March,
1968
). The optical properties of charge-transfer complexes will depend on
chemical properties, such as oxidation and reduction potentials, of both the acceptor and
donor compounds (Barsberg et al.,
2003
)
Charge transfer interactions have long been known to be important for a number of
compound classes of interest for DOM fluorescence. For instance, indoles, including tryp-
tophan, have been shown to be efficient electron donating molecules (Isenberg and Szent-
