Geoscience Reference
In-Depth Information
PARAFAC Factor 7
Indensity
400
0.1
350
300
0.05
250
0
300
350
400
450
500
550
Emission Wavelength (nm)
Figure 3.5. EEM for factor 7 from a PARAFAC model for samples from the eastern Gulf of Mexico.
This component represents remaining fluorescence signal in highly photobleached samples.
challenging for humic-like components for several reasons. In PARAFAC modeling, EEMs
are resolved into a number of components to maximize the level of fit to the data while
minimizing the residual signal (Stedmon et al., 2003 ). The PARAFAC model can add
humic-like components with single excitation maxima, either in the UVC or UVA regions,
or add dual peak components that have maxima at wavelengths shorter and longer than
components C or M in order to obtain a best fit to the data. These adjustments to the model
account for the complexities of AOM fluorescence. In the first case, addition of single exci-
tation peaks accounts for the fact that humic-like C fluorescence does not show a constant
ratio between peak A C and C intensities (Coble, 1996 ). This could be caused by variability
in the A/C ratio for individual components, differential susceptibility to photobleaching,
or complete uncoupling of the two peaks such that there are independent sources of C and
A C fluorophores, in which case the PARAFAC model would be reflecting the underlying
chemistry of some samples. Prahl and Coble ( 1994 ) showed variability in the A/C ratio
during a tidal cycle in the Columbia River estuary and attributed it to new organic matter
produced on the tidal mud flats entering the main river flow on the ebb tide.
In the second case, the addition of dual maxima components with peaks at longer and
shorter wavelengths than observed in bulk EEMs accounts for the fact that the bulk sam-
ples most likely do have multiple fluorophores, more than just a component M or C. The
most common of these from the various models has an emission maximum at longer wave-
lengths than does peak C, and has been labeled humic-like C+ in Table 3.1 . This compo-
nent is often seen in soils and freshwaters close to terrestrial sources of CDOM. Although
it is tempting to conclude that the PARAFAC components represent actual fluorophores in
the environment, none have been demonstrated to exist in natural water samples. In lieu of
additional chemical analyses, the PARAFAC components can provide valuable informa-
tion regarding the relative contribution of components and how those contributions change
with changing environmental parameters.
The discussion in this section has covered only one method of multivariate analysis of
fluorescence data, that is, PARAFAC. Additional approaches are discussed in Chapter 10 .
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