Geoscience Reference
In-Depth Information
the study of CDOM composition. In this technique, multiple emission scans are collected
at different excitation wavelengths and combined to form a three-dimensional matrix of
data. The ability of spectrophotometers to collect data from
λ
ex
= 220-455 nm and
λ
em
=
230-700 nm has also revealed the presence of additional fluorescent materials in many
samples, both natural and anthropogenic in origin. Reported fluorescent components of
AOM include proteins, tyrosine (Tyr), tryptophan (Trp), pigments, lignin phenols, humic
substances, and hydrocarbons. This chapter provides a broad review of the current under-
standing of AOM fluorescent components, starting with a general description of peaks
identified in samples from all environments. This is followed by brief summaries of the
distribution and characteristics of fluorescence in discrete environments, including fresh-
water, seawater, groundwater, wastewater, and drinking water.
3.1.1 Peak Nomenclature
Traditional spectroscopic techniques use parameters such as position of excitation and emis-
sion wavelength maximum and quantum efficiency to characterize fluorescence properties
of a compound. The fluorescence of AOM is complicated because it results from a mixture
of compounds, some of which have overlapping excitation and emission spectra. The posi-
tion of maximum fluorescence is not constant across environments, but rather shifts along
both excitation and emission axes not only in response to variation in the relative amounts
of fluorophores and thus CDOM chemistry, but also due to matrix effects, water content
of the matrix, and changes due to alteration of the tertiary physicochemical structure of
fluorophores during sampling and sample handling (Zsolnay,
2003
). Fluorescence maxima
also can shift with variation in solvent properties (e.g., ionic strength, pH; see Osburn et al.,
Chapter 7
, this volume). The following discussion of peak nomenclature is applicable to all
aquatic environments, as well as to soil-derived waters, but was developed from analysis of
freshwater and marine samples.
The earliest peak nomenclature, and the one still most widely used is that of Coble et al.
(
1990
), which denotes two peaks for humic-like fluorescence, peaks A and C, and one for
tyrosine-like fluorescence, peak B. Coble (
1996
) introduced peaks T (tryptophan-like) and
M (marine humic-like). A similar naming scheme was proposed by Parlanti et al. (
2000
).
Since the introduction and expanding use of the multicomponent analysis technique par-
allel factor analysis (PARAFAC; Bro,
1997
; Stedmon et al.,
2003
), peak nomenclature has
evolved into a numbering scheme based on the output of the model. PARAFAC models
have now been developed for diverse environments, both freshwater and marine, and the
outputs have resulted in an ever-increasing number of peak designators.
In the following section, we discuss the fluorescence properties of AOM in terms of
peaks observable in the spectrally corrected EEM data according to the scheme of Coble
(
1996
), while at the same time attempting to reconcile the myriad of peak tables published
in the past 20 years, with some speculation as to commonality among results. For the
purpose of this chapter, we use the term “peak” in the context of spectroscopy practice
as anything that exceeds the signal to noise of the background of the spectrum. The term
