Environmental Engineering Reference
In-Depth Information
state returns to the ground state. When non-radiative processes are deactivating
the excited state, the measured lifetime (
τ
) can be expressed as (Eq.
2.4
) (Senesi
1990a
):
τ = Φ
0
τ
0
(2.4)
where
0
is the fluorescence quantum efficiency. Fluorescence intensity (
F
)
is proportional to the number of excited states which, in turn, depends on the con-
centration of the absorbing molecules in solution. According to a basic equation,
F
can be expressed as follows (Senesi
1990a
):
Ф
<
Ф
F
= Φ
I
0
1
−
exp
(−ε
bc
)
(2.5)
where
is the quantum efficiency,
I
0
is the intensity of the incident radiation that
is directly proportional to the fluorescence intensity (
F
),
ε
is the (neperian) molar
absorptivity of the molecule at the excitation wavelength (higher
ε
values produce
higher fluorescence intensities),
b
is the path length of the cell and
c
is the molar
concentration of the molecule.
For very diluted solutions, where
ε
bc
is sufficiently small, Eq.
2.5
can be
expressed as follows (Senesi
1990a
):
Ф
F
= Φ
I
0
ε
bc
(2.6)
Equation
2.6
predicts a linear relationship between fluorescence intensity and
concentration of the molecule when
ε
bc
is small. If the concentration of the mole-
cule and, as a consequence,
ε
bc
increases, a non-linear relationship is followed by
Eq. (
2.5
). Small
ε
bc
values indicate that the fluorescence intensity of the molecule
is essentially homogeneous throughout the sample. On the other hand, larger
ε
bc
values result in a fluorescence intensity of the molecule that is no longer homo-
geneous within the cell, but is increasingly localized at its front surface (Senesi
1990a
).
2.1 Fluorophores in the Fluorescent Molecule
and their Controlling Factors
A fluorophore is defined as a part of an organic molecule, with or without elec-
tron-donating heteroatoms such as N, O, and S, or as a functional group of a
highly unsaturated aliphatic molecule with a structure that can hold up an excited
electron, or having extensive
π
-electron systems, which exhibits fluoresce with
significant efficiency (Mostofa et al.
2009a
). The major fluorophores in various
fluorescent organic molecules in natural waters are composed of Schiff-base deriv-
atives (-N
=
C-C
=
C-N-), -COOH, -COOCH
3
, -OH, -OCH
3
, -CH
=
O, -C
=
O,
-NH
2
, -NH-, -CH
=
CH-COOH, -OCH
3
, -CH
2
-(NH
2
)CH-COOH, S-, O- or
N-containing aromatic compounds (Mostofa et al.
2009a
; Senesi
1990a
; Leenheer
and Croué
2003
; Malcolm
1985
; Corin et al.
1996
; Peña-Méndez et al.
2005
;
Seitzinger et al.
2005
; Zhang et al.
2005
).
