Geoscience Reference
In-Depth Information
substituted with polar functional groups (-COOH, -OH, -NH
2
). The fluorophores them-
selves can be categorized by the central conjugated system (e.g., phenols, indoles, etc.).
Understanding the positions of excitation/emission maxima for the common fluorophores
associated with DOM or soil humic substances can assist with data interpretation and
assignment of spectral regions in EEMs spectra. In considering the potential influence
of model compounds on DOM optical properties, it is important that spectral properties,
including absorptivity and quantum yields, are consistent with those exhibited by sam-
ples of interest. It is important to recognize two factors when considering the influences
of potential compounds or compound classes on DOM optical properties. First, the mere
similarity in fluorescence or absorbance behavior between model compounds or natural
products and DOM is not, in itself, strong evidence for the presence of these materials in
the pool of compounds comprising the DOM. In most studies, direct confirmation of the
presence of a compound or compound class is rare. Second, although all chromophores and
fluorophores in a sample contribute to its overall optical behavior, DOM absorbance and
fluorescence can rarely be explained as simply the sum of chromophores or fluorophores
(del Vecchio and Blough,
2004
).
Fluorescence of naturally occurring organic molecules has been studied for many years.
Wolfbeis (
1985
) published a thorough compendium of fluorescence data for organic nat-
ural products with more than 1400 references to original papers and experimental condi-
tions for the data he summarized. Many of the compounds described by Wolfbeis are found
throughout the microbial and plant world and can be considered as contributing to the
pool of organic compounds comprising DOM and soil humic substances. To learn more
about the chemistry, analyses, and biological functions of the compound classes described
in
Sections 2.4.1
-2.4.9, readers are referred to Robinson (
1991
). EEMs spectra for select
molecules described below are presented in
Figures 2.4
and
2.5
.
2.4.1 Amino Acids and Proteins
Among the common amino acids, phenylalanine, tyrosine, and tryptophan are fluores-
cent and have been heavily studied (Wolfbeis,
1985
). Phenylalanine is weakly fluores-
cent and of little importance with regard to the fluorescence of DOM. The fluorophore
associated with tyrosine is a simple phenol and tyrosine exhibits behavior similar to that
of phenols (Lackowicz, 2006), fluorescing strongly at ex 275 nm/em 303 nm). However,
tyrosine fluorescence is almost completely quenched when it occurs in proteins, being
only about 10-50% as intense in the protein form compared to free tyrosine (Wolfbeis,
1985
). Tryptophan, which contains an indole group, also fluoresces strongly (ex 287 nm/
em 348 nm) in a region common for indole-containing compounds (discussed further in
Section 2.4.3
).
Free amino acids are present in low concentrations in natural waters. Reynolds (
2003
)
reported low concentrations of free tryptophan (0.82-3.44 × 10
-8
mol L
-1
) in lake water
samples using a method employing high-performance liquid chromatography (HPLC)
analyses combined with synchronous fluorescence spectroscopy. In most cases, amino
