Geoscience Reference
In-Depth Information
Component M has now been observed in nonmarine environments (Stedmon and
Markager,
2005
), and thus may be more indicative of recent microbial activity rather than
arising exclusively from a marine source. In samples where there is mixing of old and new
humic material, the resulting EEM most commonly exhibits a UVA fluorescence max-
imum at a position intermediate between peak M and peak C, rather than two separate
peaks. Likewise, the UVC maximum appears as a mixture of A
C
and A
M
, with a much
broader range along both excitation and emission axes than is the case when component
M is absent. Coble (
1996
) reported the position of the wavelength-independent maximum
fluorescence (ex
max
/em
max
) at which the overall fluorescence maximum (F
max
) occurred and
showed that it was a function of environment, with coastal samples transitional between
river and marine samples. Thus, within any given sample, the position of the humic-like
component peak provides information regarding source, CDOM composition, and environ-
mental conditions.
3.1.3 Other EEM Components
In areas of active biological productivity, additional fluorescence peaks are observed that
more readily resemble pure compounds (
Figure 3.3
). These include the amino acid-like
peaks similar to tyrosine and tryptophan, a chlorophyll-like pigment peak, quinone-like
peaks (Cory and McKnight,
2005
), and several unidentified compounds, including fluoro-
phore N (Coble
1996
). These biological fluorophores can sometimes dominate the EEM
signature, but appear not be environmentally persistent.
Peak positions for the amino acid-like components are much less variable than for
humic-like components. Positions for tyrosine and tryptophan are
λ
ex/em
= 230,275/305 nm
and
λ
ex/em
= 230,275/340 nm, respectively (
Table 3.1
). Both components have dual exci-
tation maxima, although many studies do not present data for the 230 nm peaks (A
B
, A
T
).
Reported ranges from PARAFAC models for environmental samples have a broader range
of values for emission maximum, likely reflecting the fact that the signal in natural waters
arises from some combination of free and combined amino acids. The fluorescence emis-
sion maximum for proteins is also dependent on the hydrophobicity of the site surrounding
the amino acid moiety.
There is another fluorescence type that was not specified in early nomenclature schemes
that has now been widely observed and also is a component in some PARAFAC models
(Coble, unpublished). This type of fluorescence has been observed in very clear open ocean
surface waters and is most likely the result of extensive photobleaching.
Figure 3.5
shows
the EEM for a sample collected in the Gulf of Mexico along with the PARAFAC compo-
nent that is similar.
3.1.4 Reconciling PARAFAC Model EEM Components
Comparison of peak positions between bulk sample EEMs and PARAFAC components is
relatively straightforward for tyrosine-like and tryptophan-like components, but is more
