Geoscience Reference
In-Depth Information
Although not a new technique (e.g., Hartley,
1893
) for studying naturally occurring
organic compounds, fluorescence spectroscopy is increasingly being used to study DOM
in a range of aquatic systems including freshwater, marine waters, and wastewaters. It
has been applied both to quantify DOM concentrations (Saraceno et al.,
2009
) and as a
tool to “fingerprint” DOM composition in almost all aquatic environments (Green and
Blough,
1994
; Cory and McKnight,
2005
; Spencer et al.,
2007a
; Larsen et al.,
2010
). It
is an approach that is rapidly developing with respect to
in situ
probes, allowing for the
collection of environmental data in real time (Downing et al.,
2009
; see
Chapter 6
by
Conmy et al., this volume). In addition, DOM fluorescence has been found to be useful
in forensic applications, such as tracing sources of ship ballast water (Hall and Kenny,
2007
; Murphy et al.,
2006
), is critical for remote sensing applications (Vodacek,
1989
;
Vodacek et al.,
1995
; Siegel et al.,
2005
), and has been applied to the determination of
physical properties such as the electrostatic properties (Green et al.,
1992
) and diffu-
sion coefficients (Lead et al.,
2000
) of humic substances. Excellent reviews (Blough and
Del Vecchio,
2002
; Coble,
2007
; Hudson et al.,
2007
; Henderson et al.,
2009
; Fellman
et al.,
2010
) provide thorough descriptions of the application of fluorescence analy-
ses to the quantification and characterization of DOM across the range of systems and
disciplines.
On the surface, fluorescence is an attractive method for studying DOM because data
collection is easy and straightforward; it provides information about DOM composition;
and, when properly calibrated with DOM measurements, can be used as a proxy for DOM
concentration (e.g., Pellerin et al.,
2011
). For many practitioners in the water sciences, an
underlying assumption in applying fluorescence spectroscopy to the characterization of
DOM is that the compounds comprising DOM behave similarly to pure components in
solution. In this context, changes in parameters such as intensity, peak width, fluorescence
efficiency, and wavelengths of maximum intensity are often interpreted as variability in
the provenance of DOM chemical components, as well as the production and removal of
components via biological activity of bacteria and plankton. However, the ease of data col-
lection and the potentially powerful applications of fluorescence spectroscopy to monitor
compositional changes in DOM belie the inherent complexity of the method.
The measurement and comparability of fluorescence signals is nontrivial owing to many
complicating factors. Some of these are nonchemical, such as instrumental inefficiencies
and variability. Others are chemical in nature, such as inner filter effects and the depen-
dency of measured data on environmental conditions (e.g., pH, temperature, redox sta-
tus). In addition, fluorophores are very sensitive to chemical interactions. Indeed, the vast
majority of papers in the biochemical and chemical literature using fluorescence make use
of this sensitivity to chemical conditions to study structure and reactivity of fluorophores
of interest (Lakowicz,
2006
). Common approaches for interpreting the environmental and
ecological significance of DOM fluorescence, however, often fail to adequately address the
nonlinear behavior of fluorophores in natural samples. A goal of this chapter is to examine
the potential influences of these chemical factors on the fluorescence properties of DOM.
In addition, a cursory review of the large body of literature on the fluorescence of natural
