Fluorescence Indices and Their Interpretation - Aquatic Organic Matter Fluorescence - page 308

Table 9.2. Common labels for identified regions of observed fluorescence peaks in an
excitation-emission spectrum of aquatic DOM

Component ex (nm) em (nm) Coble ( 1996 ) Parlanti et al. ( 2000 )

Humic-like 250-260 380-480 A α ′

Tyrosine-like 270-280 300-320 B γ

Humic-like 330-350 420-480 C α

Marine humic-like 310-320 380-420 M β

Tryptophan-like 270-280 320-350 T δ

Note : Note that peak M has subsequently been observed in nonmarine environments.

synchronous fluorescence spectra. At the same time, Zsolnay et al. ( 1999 ) developed a
“humification index” using line scan spectroscopy.
Aquatic scientists also explored the application of fluorescence spectroscopy to study
the chemical quality and source of DOM. For example, Stewart and Wetzel ( 1980 , 1981 )
showed that in a freshwater lake the larger molecular weight aquatic humic fractions had
a greater absorbance but lower fluorescence than smaller molecular weight fractions and
that the higher molecular weight humic fractions were removed in calcium-rich waters.
In marine systems, the fluorescence characteristics of DOM were shown to provide a use-
ful means for its characterization despite low DOC concentrations (Coble, 1996 ). Coble
identified common fluorophores present in marine and coastal waters. Five component
peaks were identified as either protein-like or humic-like and their regions and labels can
be found in Figure 9.1a and Table 9.2 . Based on this work, fluorescence indices were
developed by aquatic scientists as a means to understand the variations in DOM quality in
natural waters. Parlanti et al. ( 2000 ) developed the “freshness index” (the β / α index, later
modified to the “BIX” index) to identify microbial influence on marine DOM. McKnight
et al. ( 2001 ) put forward a “fluorescence index” (FI) to examine differences in precursor
organic materials for aquatic humics. Later, Miller et al. ( 2006 ) proposed a “redox index”
(RI) as an indicator of the oxidation state of quinone-like moieties in DOM. Indices were
also developed to quantify information in more specialized studies. Proctor et al. ( 2000 )
applied an index to fluorescent organic matter preserved in a cave stalagmite. Perrette et al.
( 2005 ) also developed a similar index for use in measuring fluorescent organic matter pre-
served in stalagmites, using a laser excitation source for high spatial resolution analyses.
Figure 9.1b shows the locations of the wavelength pairs measured in the fluorescence
indices tabulated in Table 9.1 , presented together with the regions of interest typically
associated with identified peaks indicated in Table 9.2 . Most of the indices focus on vari-
ations in the fluorescence intensity associated with what has historically been referred to
as the “humic-like” peak in natural organic matter, sometimes also comparing it with what
has been called the “protein-like” peak, although it is now clear that fluorescence in these
regions can be attributed to more than one class of fluorescent organic matter. In all cases,
the interpretation of the index is likely to vary with the organic matter source, matrix, and