533
Fluorescent Dissolved Organic Matter in Natural Waters
3.7 Temperature
The fluorescence intensity of FDOM is inversely related to temperature because of
an increased collisional quenching of fluorescence at higher temperatures (Wehry
1973; Vodacek and Philpot 1987). Within the range 10–45 °C, the fluorescence
intensity can increase by approximately 1 % with a 1 °C decrease in temperature
in the case of tryptophan-like, humic-like and fulvic-like substances, depending on colloid size and fluorophore (Henderson et al. 2009; Baker 2005; Vodacek
and Philpot 1987; Elliott et al. 2006; Seredyńska-Sobecka et al. 2007). In contrast,
the fluorescence intensities of tryptophan standards are almost unaffected by a
temperature variation of ±8 °C (Reynolds 2003). The thermal quenching of fluorescence can be significant because of variation in water temperature between the
summer and winter seasons as well as the high variation between boreal, tropical
and Antarctic-Arctic regions. The effect of temperature on fluorescence quenching
is linear and reversible, and it can be prevented in the laboratory by measuring the
fluorescence of samples at a constant temperature (Vodacek and Philpot 1987). The
thermal quenching effects can also be overcome by applying simple correction factors, but such factors may be different for fluorophores of different size fractions
(Seredyńska-Sobecka et al. 2007). The mechanism of the temperature effect on fluorescence is that a rise in water temperature increases the likelihood that an excited
electron will return to its ground state by radiationless decay, leading to reduced fluorescence intensity (Henderson et al. 2009). It is suggested that a variation of water
temperature across a range of 20 °C or more between summer and winter would
lead to a corresponding decrease by 20 % of the fluorescence intensity during summer (Henderson et al. 2009). Fluorescence changes caused by temperature may have
no effect on the structure of the DOM. However, it has been shown that non-reversible changes may occur, possibly as a result of the application of a light-source that
may cause photodegradation or thermal decomposition (Vodacek and Philpot 1987).
4 Kinetics of Photodegradation of the Fluorescence
Intensity of Fulvic Acid and Tryptophan
Fulvic acid and tryptophan-like fluorescence intensity (FI) decreases monotonically with the number of absorbed UV photons or with integrated solar
intensity, as a result of solar effects on water (Fig. 10) (Mostofa et al. 2007a).
Photodegradation of fulvic acid often follows a two-step kinetics (Mostofa et al.
2007a; Ma and Green 2004), while tryptophan is photodegraded in a single step
(Mostofa et al. 2007a). A decrease of FI can be best fit to a first-order kinetics as
follows (Eq. 4.1):
where k 2 is the reaction rate constant for photodegradation of FI in waters, FI is
the fluorescence intensity obtained after illumination, FI o is the initial fluorescence
(4.1)
Ln (FI /FI o ) = −k 2 S
Fluorescent Dissolved Organic Matter in Natural Waters
3.7 Temperature
The fluorescence intensity of FDOM is inversely related to temperature because of
an increased collisional quenching of fluorescence at higher temperatures (Wehry
1973; Vodacek and Philpot 1987). Within the range 10–45 °C, the fluorescence
intensity can increase by approximately 1 % with a 1 °C decrease in temperature
in the case of tryptophan-like, humic-like and fulvic-like substances, depending on colloid size and fluorophore (Henderson et al. 2009; Baker 2005; Vodacek
and Philpot 1987; Elliott et al. 2006; Seredyńska-Sobecka et al. 2007). In contrast,
the fluorescence intensities of tryptophan standards are almost unaffected by a
temperature variation of ±8 °C (Reynolds 2003). The thermal quenching of fluorescence can be significant because of variation in water temperature between the
summer and winter seasons as well as the high variation between boreal, tropical
and Antarctic-Arctic regions. The effect of temperature on fluorescence quenching
is linear and reversible, and it can be prevented in the laboratory by measuring the
fluorescence of samples at a constant temperature (Vodacek and Philpot 1987). The
thermal quenching effects can also be overcome by applying simple correction factors, but such factors may be different for fluorophores of different size fractions
(Seredyńska-Sobecka et al. 2007). The mechanism of the temperature effect on fluorescence is that a rise in water temperature increases the likelihood that an excited
electron will return to its ground state by radiationless decay, leading to reduced fluorescence intensity (Henderson et al. 2009). It is suggested that a variation of water
temperature across a range of 20 °C or more between summer and winter would
lead to a corresponding decrease by 20 % of the fluorescence intensity during summer (Henderson et al. 2009). Fluorescence changes caused by temperature may have
no effect on the structure of the DOM. However, it has been shown that non-reversible changes may occur, possibly as a result of the application of a light-source that
may cause photodegradation or thermal decomposition (Vodacek and Philpot 1987).
4 Kinetics of Photodegradation of the Fluorescence
Intensity of Fulvic Acid and Tryptophan
Fulvic acid and tryptophan-like fluorescence intensity (FI) decreases monotonically with the number of absorbed UV photons or with integrated solar
intensity, as a result of solar effects on water (Fig. 10) (Mostofa et al. 2007a).
Photodegradation of fulvic acid often follows a two-step kinetics (Mostofa et al.
2007a; Ma and Green 2004), while tryptophan is photodegraded in a single step
(Mostofa et al. 2007a). A decrease of FI can be best fit to a first-order kinetics as
follows (Eq. 4.1):
where k 2 is the reaction rate constant for photodegradation of FI in waters, FI is
the fluorescence intensity obtained after illumination, FI o is the initial fluorescence
(4.1)
Ln (FI /FI o ) = −k 2 S