Geoscience Reference
In-Depth Information
9
Fluorescence Indices and Their Interpretation
Rachel S. Gabor, Andy Baker, Diane M. McKnight,
and Matthew P. Miller
9.1 Introduction
In aquatic ecosystems and soil interstitial waters, dissolved organic material (DOM) is
comprised of a heterogeneous mixture of organic compounds derived from plant and soil
decomposition and leachate, degradation of microbial biomass, and microbial exudates.
Organic material present in the solid phase of soils and sediments (SOM) is similarly com-
plex, with variations not only in chemical composition, but also in the extent to which
organic molecules are bound to minerals. The chemical characteristics and distribution of
classes of organic compounds within DOM can be dynamic. For example, DOM quality
can change seasonally with hydrologic drivers, such as snowmelt, or due to summertime
algal blooms in lakes and streams. In contrast, changes in SOM may occur more slowly as
soils and sediments age, a process historically referred to as humification.
Important fractions of DOM and SOM include chromophoric (light absorbing) and
fluorophoric (light absorbing and light emitting) compounds. Humic material is a major
chromophoric fraction of DOM and SOM, having a yellow to brown color associated
with aromatic carbon moieties derived from plant pigments, lignin, and other precursor
materials. In surface waters, for example, dissolved humics typically account for a sub-
stantial fraction of DOM and are the main light-absorbing constituents in the water col-
umn (McKnight et al., 2003 ). Humics derived from plant litter and soil are generally more
chromophoric than humics derived from microbial biomass, reflecting their greater con-
tent of aromatic carbon moieties. A fraction of both plant/soil-derived and microbially
derived humic molecules fluoresces with characteristic broad fluorophores occurring in the
useful analytical range of ex 240-370 nm, em 350-550 nm. Another important chromo-
phoric fraction is proteinaceous material associated with extracellular microbial products
and plant litter degradation products, which often include fluorescent amino acids, specifi-
cally tryptophan and tyrosine. These amino acid-like fluorophores occur at lower emission
wavelength ranges than typically observed for humic fluorophores. For these and other
fluorescent classes of organic compounds present in DOM and SOM, more chemical infor-
mation can be obtained by knowing the wavelength ranges at which the molecules both
absorb and emit light than by absorbance information alone. In addition, absorbance in a
natural water sample can be influenced by inorganic solutes, such as nitrate or iron. As a
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