Geoscience Reference
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product compounds of interest in the study of DOM is presented, with an emphasis on the
influences of chemical reactions on their fluorescence behavior.
2.2 Theory
Fluorescence describes the phenomenon wherein a molecule that has been excited to a
higher electronic state by the absorption of UV-visible light returns to the ground state by
the direct emission of light. Molecular structure is the primary determinant for both absorp-
tion and fluorescence of organic molecules. Both methods are used to provide structural
information when studying individual molecules; however, the amount of structural infor-
mation that can be obtained on natural organic matter samples is limited by the chemical
complexities of these samples. Here a description of these phenomena is provided as a
basis for understanding the controls on DOM fluorescence and the limitations associated
with interpretation of fluorescence data. This topic is treated in a more thorough fashion
in a number of excellent references (e.g., Skoog and West, 1982 ; Schulmann, 1985 ; Del
Vecchio and Blough, 2004 ; Lakowicz, 2006 ).
2.2.1 Absorption
The absorption of light in the UV-visible range involves the excitation of electrons associ-
ated with chemical bonds from the ground electronic state (bonding orbital) to an excited
electronic state (antibonding orbital). This process is sensitive to chemical structure, and,
those structures that absorb light are referred to as chromophores. The wavelengths at
which light can be absorbed by an organic molecule are determined by the differences in
energy between bonding and antibonding orbitals. For many electronic structures, such
as those in alkanes and carbohydrates, absorption occurs only at wavelengths that are
shorter (higher energy) than those wavelengths available with most spectrophotometers.
The energies associated with π-bonds, such as those in alkenes, aromatic molecules, and
in some organic molecules containing heteroatoms, fall in the practical UV-visible range
(e.g., 190-780 nm). Therefore, UV-visible spectroscopy of organic molecules largely deals
with the absorption of light by conjugated systems (Silverstein et al., 1974 ). As molecules
become more conjugated, the energy differences between bonding and antibonding orbitals
decrease and molecules can absorb light at longer wavelengths (less energy), even into the
visible portion of the spectrum. The UV spectrum of a molecule, therefore, indicates the
presence of specific bonding arrangements within the molecule. In the case of absorption
in the near UV and visible portions of the spectrum, conjugated systems, such as those in
aromatic molecules, generally have the greatest absorptivities.
An advantage of the structural selectivity of UV-visible absorption is that characteristic
features or bonding arrangements may be recognized in molecules of varying complexity
(Silverstein et al., 1974 ). When measured in the near UV, many of the bonds present in
a complex molecule or complex mixtures of molecules are transparent to UV radiation.
Increased structural complexity, therefore, does not necessarily result in increased spectral
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