ANALYSIS OF OILS USING FLUORESCENCE.
3
In many cases, the only information available for the crude oil being investigated is
its location, source, or API gravity. API gravity is one of the simplest and most wideranging parameters used for describing crude oils. It is inversely related to the density by
the formula: API gravity = ((141.5/specific gravity at 15.6ºC) – 131.5). In general, the
higher the API gravity value, the lighter the oil. Specific gravities of oils tend to be in the
0.73 to 1.0 range, with paraffin type oils being lighter than asphalt-base oils.
3 Despite the
fact that it does not provide any significant chemical information, the parameter is easily
measured and widely used. Table 1 gives a brief survey of some recent fluorescence
studies of crude oils, clearly showing the dearth of chemical information usually
provided. Therefore, one must be aware that when discussing petroleum fluorescence,
qualitative descriptions dominate, and quantitative analysis may only apply to individual
studies.
3. PETROLEUM OIL FLUORESCENCE
The use of fluorescence for the analysis of crude oils has been in use for the past 60
years particularly for mud logging where UV light is used to detect the presence of oil in
drilling mud.
27
Fluorescence is also used in the analysis of core samples, again to
identify the presence of oils.
3 The fluorescence of crude petroleum oils derives from the
aromatic hydrocarbon fraction,
28 and this fluorescence emission is strongly influenced by
the chemical composition (e.g. fluorophore and quencher concentrations) and physical
characteristics (e.g. viscosity and optical density) of the oil. Unfortunately, crude
petroleum oils encompass a very wide range of physical and chemical characteristics,
making the fluorescence analysis of crude petroleum oils rather difficult.
Crude oils vary in appearance from black tars to clear liquids, indicating a complex
absorption profile. This is shown experimentally in Figure 1, where the spectra of 22 oils
of different compositions are displayed. It should also be noted that the shape of the
absorption edge is very similar in each case. Examination of the electronic absorption
spectra of crude oils reveals that the electronic absorption edge is similar to the “Urbach
tail,” in which the absorption coefficient depends exponentially on the photon energy.
17
The absorption edge moves to the red, as crude oils get heavier (lower API gravity),
and the distribution and number of chromophores increases. These absorption studies, in
conjunction with fluorescence emission studies reveal that when the Urbach tail
accurately describes the absorption, the dominant absorption process in the Urbach tail
region, corresponds to excitation of the lowest-energy electronic transitions of the
corresponding chromophores. In addition, Mullins and his co-authors suggest that the
absorption tail gives a direct measure of the chromophores population distribution, with
larger chromophores being present in exponentially decreasing quantities.
17
3
In many cases, the only information available for the crude oil being investigated is
its location, source, or API gravity. API gravity is one of the simplest and most wideranging parameters used for describing crude oils. It is inversely related to the density by
the formula: API gravity = ((141.5/specific gravity at 15.6ºC) – 131.5). In general, the
higher the API gravity value, the lighter the oil. Specific gravities of oils tend to be in the
0.73 to 1.0 range, with paraffin type oils being lighter than asphalt-base oils.
3 Despite the
fact that it does not provide any significant chemical information, the parameter is easily
measured and widely used. Table 1 gives a brief survey of some recent fluorescence
studies of crude oils, clearly showing the dearth of chemical information usually
provided. Therefore, one must be aware that when discussing petroleum fluorescence,
qualitative descriptions dominate, and quantitative analysis may only apply to individual
studies.
3. PETROLEUM OIL FLUORESCENCE
The use of fluorescence for the analysis of crude oils has been in use for the past 60
years particularly for mud logging where UV light is used to detect the presence of oil in
drilling mud.
27
Fluorescence is also used in the analysis of core samples, again to
identify the presence of oils.
3 The fluorescence of crude petroleum oils derives from the
aromatic hydrocarbon fraction,
28 and this fluorescence emission is strongly influenced by
the chemical composition (e.g. fluorophore and quencher concentrations) and physical
characteristics (e.g. viscosity and optical density) of the oil. Unfortunately, crude
petroleum oils encompass a very wide range of physical and chemical characteristics,
making the fluorescence analysis of crude petroleum oils rather difficult.
Crude oils vary in appearance from black tars to clear liquids, indicating a complex
absorption profile. This is shown experimentally in Figure 1, where the spectra of 22 oils
of different compositions are displayed. It should also be noted that the shape of the
absorption edge is very similar in each case. Examination of the electronic absorption
spectra of crude oils reveals that the electronic absorption edge is similar to the “Urbach
tail,” in which the absorption coefficient depends exponentially on the photon energy.
17
The absorption edge moves to the red, as crude oils get heavier (lower API gravity),
and the distribution and number of chromophores increases. These absorption studies, in
conjunction with fluorescence emission studies reveal that when the Urbach tail
accurately describes the absorption, the dominant absorption process in the Urbach tail
region, corresponds to excitation of the lowest-energy electronic transitions of the
corresponding chromophores. In addition, Mullins and his co-authors suggest that the
absorption tail gives a direct measure of the chromophores population distribution, with
larger chromophores being present in exponentially decreasing quantities.
17