Geoscience Reference
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(Kraus et al.,
2003
), and as major constituents in the more complex molecules that comprise
both soil and aquatic humic substances (Aiken et al.,
1985
). In molecules of this group,
phenolic structures are the fluorophores of interest, and many have similar excitation and
emission spectra (
Figure 2.4c
, d). As a point of interest, especially with regard to DOM
studies, phenols are the subject of much interest in the wine industry, and fluorescence is an
important analytical tool for detecting their presence (Bonerz et al.,
2008
).
Most of the aromatic carboxylic acids produced by plants also have phenolic groups
and share properties with phenols (Robinson,
1991
), including fluorescence. The pres-
ence of hydroxyl groups is important in controlling the fluorescent behavior of this
class of compounds. Unsubstituted aromatic carboxylic acids, such as benzoic acid,
have been shown to be nonfluorescent in aqueous solutions except under acidic condi-
tions wherein the undissociated form of the acid is predominant (Martin and Clarke,
1978
; Wolfbeis,
1985
). Addition of a phenolic hydroxyl group results in stronger fluo-
rescence. For example, salicylic acid (2-hydroxybenzoic acid), a compound commonly
associated with humic substances and present in wastewaters (Flaherty et al.,
2002
),
fluoresces under a range of pH conditions observed in natural waters (ex 296 nm/em
408 nm; Wolfbeis,
1985
). Other simple hydroxylated benzoic acids of significance in
the study of DOM include gentistic acid (2,5-dihydroxybenzoic acid; ex 318 nm/em
442 nm; Wolfbeis,
1985
) and gallic acid (3,4,5-dihydroxybenzoic acid; ex 260 nm;/em
346 nm; Maie et al.,
2007
).
Tannins are the fourth most abundant class of molecules in terrestrial biomass (Hernes
and Hedges,
2000
) and have been identified as components of terrestrially derived DOM
(Sleighter et al.,
2010
). Polyphenolic compounds similar to tannins are also produced by
brown algae (Geiselman and McConnell,
1981
). Tannins, which are classified as either
condensed or hydrolyzable, are large molecules containing numerous phenolic moieties
(Kraus et al.,
2003
). Condensed tannins are polymers of three-ring flavanols, whereas
hydrolyzable tannins contain various phenolic acids linked to sugar moieties, the most
common of which is gallic acid (3,4,5-dihydroxybenzoic acid; Robinson,
1991
). Some tan-
nins are a mixture of both types (Kraus et al.,
2003
). The fluorescence spectral properties
for condensed and mixed tannins (
Figure 2.4d
) are similar to those of gallic acid and other
lower molecular weight phenolic compounds (e.g., cresol;
Figure 2.4c
). Maie et al. (
2007
)
presented compelling evidence that the usefulness of the T-peak intensity as a proxy for
dissolved organic nitrogen (DON) was limited in coastal waters of Florida because of the
overlap of this peak with gallic acid and tannin from red mangrove (
Rhizophora mangle
)
leaves.
One of the most important phenolic compounds potentially contributing to the fluores-
cence of DOM is tyrosine. Although it is categorized as an amino acid, tyrosine could be
considered a simple phenol from the perspective of fluorescence because the phenol group
is the fluorescing portion of the molecule. Overlap of the fluorescence properties of tyro-
sine with other phenols complicates the interpretation of fluorescence data with regard to
peak assignments. For example, Goldberg and Weiner (
1993
) employed specific compound
analyses to demonstrate that cresol, a common wastewater component (e.g., Tertuliani
