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fluoresce in the UV-visible region. More rigid aromatic molecules have fewer rotational
and vibrational degrees of freedom and are more likely to fluoresce than aliphatic and alicy-
clic molecules. Aromatic molecules that contain freely rotating substituents fluoresce less
intensely than those without these substituents because energy can be lost from the mol-
ecule through these functional groups. Finally, highly conjugated molecules have smaller
energy gaps between the excited and ground states and, therefore, fluoresce at longer, lower
energy wavelengths than less conjugated systems.
Excitation and emission maxima and fluorescence intensity are sensitive to structural
factors that can influence the energetics of the excited electronic state of the fluorophore.
Among these factors, substituent effects are very important in controlling radiative relaxa-
tion rates. Electron-donating groups associated with conjugated aromatic molecules (such
as -OH or -NH 2 ) can enhance fluorescence intensity by increasing rates of relaxation.
The presence of these substituents extends fluorescence maxima to longer wavelengths
compared to the parent molecule. Electron withdrawing groups (such as -COOH, -CHO,
and -NO 2 ) tend to diminish fluorescence quantum yields in aromatic molecules. It is
beyond the scope of this chapter to describe the reasons for these effects, and readers
are referred to other sources for more detailed discussion of the chemistry of substituent
effects (e.g., Schulman, 1985 ). It is important to realize, however, that, owing to aque-
ous solubility constraints, almost all DOM comprises polar molecules containing oxygen-,
nitrogen- or sulfur-containing substituents (Thurman, 1985 ). The largest percentages of
these molecules are organic acids with -COOH and -OH functional groups. Most of the
fluorophores in DOM, therefore, are heavily substituted conjugated molecules. Senesi et al.
( 1991 ), in a thorough overview of the fluorescence behavior of soil-derived humic sub-
stances, describe the effects of conjugation and substituent effects on fluorescence data for
50 samples obtained from soils and related materials.
Because of the greater degree of specificity, fluorescence is less universally applicable
in the study of DOM in comparison to UV-visible spectroscopy. UV-visible absorption is a
general property of analytes that contain chromophoric groups, whereas most compounds
are poor fluorophores. Therefore, a smaller set of compounds in DOM fluoresce relative
to those that absorb light. Because fluorescence and absorption are related phenomena,
however, it is possible to obtain more information on the chemistry controlling the optical
properties of DOM when they are employed together, as recently demonstrated by Boyle
et al. ( 2009 ).
2.3 DOM Fluorescence
Historically, organic matter in natural waters has been arbitrarily divided into dissolved
and particulate organic carbon based on filtration, generally through 0.2- to 1.2-µm filters.
No natural cut-off exists between these two fractions and the distinction is operational.
Dissolved organic matter itself is a complex, heterogeneous continuum of high to low
molecular weight species exhibiting different solubilities, reactivities, and optical proper-
ties depending on molecular structure. Overlapping the dissolved and particulate fractions
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