Geoscience Reference
In-Depth Information
This group of molecules is common in the plant world, is not chemically well defined, and
compound classification is often based on the nature of the nitrogen-containing ring struc-
ture (Robinson,
1991
). Alkaloids have long been of interest to pharmacological chemists
because they exhibit a wide range of physiological reactivities. The fluorescence proper-
ties of alkaloids are often determined by the central heterocyclic ring structure. Wolfbeis
(
1985
) presents a thorough review of alkaloid fluorescence through 1985. Unlike most
molecules comprising DOM, alkaloids are often basic in nature and can become more
basic in the excited state (Wolfbeis,
1985
).
Of major importance for DOM fluorescence are the indoles and quinolines. The indoles
represent one of the largest groups of alkaloids. As is the case for tryptophan (see discus-
sion in
Section 2.4.3
), simple indole derivatives exhibit fluorescence behavior similarly to
that of indole itself (see
Figure 2.4b
). These compounds are efficiently excited in the wave-
length range of 270-290 nm, with emission in the range of 330-350 nm (Wolfbeis,
1985
).
As noted previously, a number of indole-containing compounds are present in sewage-
related wastewaters. Another wastewater compound of interest is caffeine, a compound
that has found utility as a marker for the presence of sewage wastewater in surface water
samples (Buerge et al.,
2003
). Caffeine, with a purine-type fluorophore, fluoresces in a
region (ex 270 nm/em 320 nm) similar to that of “B” type fluorophores (
Table 2.1
) and
simple phenols.
One of the best known and most studied of the fluorescent alkaloids is the isoquino-
line alkaloid, quinine, which is responsible for the bluish hue of tonic water. Much of the
early research defining the fluorescence phenomenon was carried out on solutions con-
taining quinine (Lakowicz,
2006
). Quinine sulfate (ex 331 nm/em 382 nm) is often used
as a “quantum counter” to correct EEMs spectra (Lakowicz,
2006
). Its use as a standard
to quantify fluorescence response in DOM studies has been proposed to properly correct
DOM spectra and improve comparability of results between different instruments and ana-
lysts (Hoge et al.,
1993
, Murphy et al.,
2010
).
2.5 Factors Influencing DOM Fluorescence
2.5.1 Quenching
Fluorescence quenching describes processes that reduce fluorescence intensity of a fluoro-
phore. These processes generally involve interactions that influence one or more chemi-
cal aspects of the fluorophore, such as rates of decay, intermolecular energy transfer, or
the population of molecules in the excited state that decrease the fluorescence intensity
of the molecule (Lakowicz,
2006
). The pathways leading to fluorescence quenching are
described according to whether the quenching results from interactions of the quenching
species with the ground state of the fluorescing molecule (static quenching), interactions
with the excited state of the fluorophore (collisional quenching), or if the quenching results
from nonmolecular mechanisms (Lakowicz,
2006
). Each type of quenching can influence
the fluorescence of DOM, and experiments designed to determine quenching effects have
