Geoscience Reference
In-Depth Information
in situ
measurements can be made at 10- to 15-minute intervals with probes deployed for
weeks to months. For these reasons, fluorescence indices remain widely used to either (1)
simplify the large amounts of data present in an EEM, or (2) design an
in situ
probe to
obtain a quantified parameter that can be compared across sites or followed over time in a
field study.
This chapter reviews commonly used indices that have been developed in the analysis
of organic matter, along with their interpretation and potential problems. The chapter con-
cludes with an overview of the use of fluorescence indices, some recommendations and
ideas about future directions in their use, and some important spectroscopic considerations
for collecting fluorescence measurements. For further reading on the use of fluorescence
spectroscopy to understand DOM ecosystem dynamics, both
Chapter 8
(this volume) and
the review paper by Fellman et al. (
2010
) highlight additional ecological uses of fluores-
cence spectroscopy and indices. It is important to note that there are spectroscopic dif-
ficulties in the application of fluorescence methods as detailed in
Chapters 2
and
5
(this
volume). These are related to instrument-dependent characteristics of the fluorescence
spectra, inner-filter effects associated with the absorbance of emitted light, and fluores-
cence quenching by metal complexation and pH effects. All of these need to be addressed
to meaningfully compare results from different laboratories or in a single system over time
(Cory et al.,
2010
). Despite these potential difficulties, the use of indices, along with the
availability of standard reference materials, has allowed for comparisons across studies
done by different investigators in different study sites to build a broader understanding of
the dynamics of DOM and SOM extracts.
9.2 Overview of Common Fluorescence Indices
Table 9.1
summarizes the common fluorescence indices that have been employed in the
literature and are discussed in this chapter. Fluorescence indices were first used in the late
1990s, building on the improved interpretation of soil organic matter extracts using syn-
chronous scan techniques. Work such as that by Nicola Senesi and collaborators (e.g., Senesi
et al.,
1989
,
1991
; Miano and Senesi,
1992
) formed the foundations of our understanding
of how fluorescence spectroscopy could be used to classify and distinguish humic-like
substances from various soils, composts, and sludges. Using synchronous scan approaches
and humic and fulvic extracts analyzed as water solutions at relatively high concentrations
and fixed pH, several consistent peaks in fluorescence intensity were observed between 300
and 500 nm emission (Senesi et al.,
1991
). Comparison of fluorescence spectra with
13
C-
nuclear magnetic resonance (
13
C-NMR), ultraviolet (UV) absorbance, and Fourier trans-
form infrared (FTIR) spectroscopy demonstrated that fluorescence spectra were related to
the number of highly substituted aromatic nuclei and/or conjugated unsaturated systems
capable of a high degree of resonance (Senesi et al.,
1989
; Miano and Senesi,
1992
). They
found that a shift in maximum fluorescence intensity from shorter to longer wavelengths
due to the degree of polycondensation of the organic matter could be captured and quanti-
fied by use of an index. Later Kalbitz et al. (1999) developed a “humification index” using
