Geoscience Reference
In-Depth Information
result, fluorescence is a more sensitive indicator of organic material source and chemical
quality than absorbance and there is less potential for interference from other light absorb-
ing compounds. For these reasons, and the observation of ubiquitous fluorescent organic
materials in natural waters, soils, and sediments, fluorescence spectroscopy has proven to
be a robust technique for studying the source and chemical composition of organic mat-
ter (e.g., Fellman et al.,
2010
). As explained in
Chapter 2
, 2-D fluorescence scans can be
either a line scan (typically a fixed excitation wavelength and scan of emitted fluorescence
over a range of wavelengths) or synchronous scan (scanning both excitation and emission
wavelengths with a constant wavelength offset between the two). Three-dimensional fluo-
rescence scans, referred to as excitation-emission matrices (EEMs), are acquired for a
range of emission wavelengths when excited at multiple wavelengths.
Spectroscopic (absorbance and fluorescence) indices have been used to characterize
natural organic material for many decades. For example, the determination of platinum
color units is a chromophoric index that was employed in the 1950s (American Public
Health Association,
1965
) by limnologists and was based on comparison of a water sample
to a standard series of platinum solutions. This absorbance index was used as a proxy for
dissolved organic carbon (DOC) concentration. Although color units are no longer rou-
tinely measured, another chromophoric index, specific ultraviolet absorbance (SUVA), is
commonly used. SUVA
254
, reported in units of L mg
-1
m
-1
, is the absorbance of a water
sample at 254 nm divided by the concentration of DOC and is associated with bulk aro-
maticity (Weishaar et al.,
2003
). Soil scientists employ an absorbance index called the E4/
E6 ratio (the ratio of absorbance of a solution of extracted humic materials at 400 nm and
600 nm), which is used as an index of humification (Chen et al.,
1977
). However, in humic-
poor aquatic samples, absorption at 600 nm is minimal, and this ratio is not often useful.
Fluorescence indices were developed subsequently and can be generally defined as the
ratio of fluorescence intensity measured at two different points or regions in optical space.
These indices can be thought of as a sub-sample of the information contained in EEMs
(
Figure 9.1
) of organic matter. Beginning several decades ago, indices were developed in
association with the commonly available line scan or synchronous scan methodologies.
The various indices were targeted to address several specific questions about the nature
of organic material in a broad range of systems, such as soils, groundwater (Kalbitz et al.,
1999
), lakes, and streams (McKnight et al.,
2001
). These indices focus on different por-
tions of the EEM and the utility of an index must be considered in the context of the ques-
tions to be addressed. Indices have been applied to understand a wide range of ecosystem
processes including SOM associated with land cover and the changes in DOM that may
occur on decadal time scales under a changing climate.
Currently, fluorescence indices are frequently developed in the context of data contained
in full EEMs, which can now be obtained with high resolution in a reasonable time period
(minutes) in a laboratory setting. It is also now possible to measure DOM fluorescence
in
situ
continuously at one excitation wavelength with a laser source, and one, or possibly
several, emission wavelengths;
in situ
methods are considered further in
Chapter 6
. Indices
have proven to be a valuable tool for remote sensing applications owing to the current
