ANALYSIS OF OILS USING FLUORESCENCE.
13
3.3. Multidimensional Techniques
Multidimensional techniques involve the collection of multiple fluorescence
parameters and offer a more detailed method for studying petroleum photophysics or
providing a spectroscopic fingerprint for identification purposes. Excitation-Emission
Matrix (EEM) methods are a convenient way of mapping the fluorescence space that
complex oils occupy.
33, 34 They can be used in the laboratory to determine the optimum
excitation-emission parameters for simpler measurements/instrumentation, or as a
method for the discrimination of oil types. The main drawbacks of EEM include timeconsuming data collection and the requirement for complex instrumentation.
Synchronous Fluorescence Spectroscopy (SFS) has been applied to the study of
petroleum-based materials since the mid-1970’s.
35, 36 Constant wavelength mode SFS has
been used to study crude oils,
37 motor oils,
38, 39 and asphaltenes.
40 SFS has also been
applied to the study of Shale oils,
41
fluid inclusions,
42
and reservoir
compartmentalization.
43 Constant energy SFS was used for the analysis of both crude oil
and gasoline.
44
3-D fluorescence spectra have been used to discriminate oils into
condensate, light oil, and heavy oils.
45, 46
Smith and Sinski
47 studied the red-shift
cascade,* which has also been used to investigate the degradation of aged petroleum by
mycobacteria.
48
More recently, Total Synchronous Fluorescence Scan Spectroscopy
(TSFS) has been used to discriminate different refined and crude petroleum liquids.
49, 50
The compositional diversity of crude petroleum oils is evident in Figure 9 where the
TSFS plots of nine different oils are displayed. For comparison purposes, and to account
for instrument instability / sampling effects, each TSFS plot was normalised to the point
of maximum fluorescence intensity. This allows for a general comparison between the
different oils based on the identity of the emitting species. All the TSFS plots show a
general diagonal contour trend from short ex /large , to long ex /short which
represents a maximum fluorescence emission in the 350 – 500 nm range for these
excitation wavelengths. This diagonal trend represents the extensive impact energy
transfer processes have on crude petroleum oil fluorescence. The top row of Figure 9
shows the TSFS plots of 3 light oils (API > 40º) with low polar concentrations (<4%).
There are considerable differences in the plot topology and this is due to changes in the
aromatic concentration (as measured). The measured aromatic concentration increases
across the top row from 1.8% to 6.6%, and then to 18.2% for (C). Oil (A) is classed as a
late maturity oil and as such, most of the larger polyaromatic species will have been
broken down to alkanes and small aromatic species. This results in a very tight TSFS
contour plot centred at ex = 390 nm, = 40 nm, indicating a somewhat homogenous,
and restricted mixture of fluorophores, with an emission maximum around 430 nm. The
more diverse and wider ranging contour plot of oil (B) cannot be explained just on the
basis of a ~4% increase in aromatic concentration, but also by a change in the type of
aromatic species present. Since most crude oil fluorophores are aromatic, it follows that
the increase in aromatic concentration causes TSFS contours to spread out over a larger
parameter space. The primary process driving this is the increased rates of collisional
energy transfer from small to large aromatic species.
* The red shift cascade is the greater degree of energy transfer, which occurs at high concentrations of crude
oils. The excitation energy will continue to cascade to larger fluorophores, producing greater red shifts in the
emission spectra.
25
13
3.3. Multidimensional Techniques
Multidimensional techniques involve the collection of multiple fluorescence
parameters and offer a more detailed method for studying petroleum photophysics or
providing a spectroscopic fingerprint for identification purposes. Excitation-Emission
Matrix (EEM) methods are a convenient way of mapping the fluorescence space that
complex oils occupy.
33, 34 They can be used in the laboratory to determine the optimum
excitation-emission parameters for simpler measurements/instrumentation, or as a
method for the discrimination of oil types. The main drawbacks of EEM include timeconsuming data collection and the requirement for complex instrumentation.
Synchronous Fluorescence Spectroscopy (SFS) has been applied to the study of
petroleum-based materials since the mid-1970’s.
35, 36 Constant wavelength mode SFS has
been used to study crude oils,
37 motor oils,
38, 39 and asphaltenes.
40 SFS has also been
applied to the study of Shale oils,
41
fluid inclusions,
42
and reservoir
compartmentalization.
43 Constant energy SFS was used for the analysis of both crude oil
and gasoline.
44
3-D fluorescence spectra have been used to discriminate oils into
condensate, light oil, and heavy oils.
45, 46
Smith and Sinski
47 studied the red-shift
cascade,* which has also been used to investigate the degradation of aged petroleum by
mycobacteria.
48
More recently, Total Synchronous Fluorescence Scan Spectroscopy
(TSFS) has been used to discriminate different refined and crude petroleum liquids.
49, 50
The compositional diversity of crude petroleum oils is evident in Figure 9 where the
TSFS plots of nine different oils are displayed. For comparison purposes, and to account
for instrument instability / sampling effects, each TSFS plot was normalised to the point
of maximum fluorescence intensity. This allows for a general comparison between the
different oils based on the identity of the emitting species. All the TSFS plots show a
general diagonal contour trend from short ex /large , to long ex /short which
represents a maximum fluorescence emission in the 350 – 500 nm range for these
excitation wavelengths. This diagonal trend represents the extensive impact energy
transfer processes have on crude petroleum oil fluorescence. The top row of Figure 9
shows the TSFS plots of 3 light oils (API > 40º) with low polar concentrations (<4%).
There are considerable differences in the plot topology and this is due to changes in the
aromatic concentration (as measured). The measured aromatic concentration increases
across the top row from 1.8% to 6.6%, and then to 18.2% for (C). Oil (A) is classed as a
late maturity oil and as such, most of the larger polyaromatic species will have been
broken down to alkanes and small aromatic species. This results in a very tight TSFS
contour plot centred at ex = 390 nm, = 40 nm, indicating a somewhat homogenous,
and restricted mixture of fluorophores, with an emission maximum around 430 nm. The
more diverse and wider ranging contour plot of oil (B) cannot be explained just on the
basis of a ~4% increase in aromatic concentration, but also by a change in the type of
aromatic species present. Since most crude oil fluorophores are aromatic, it follows that
the increase in aromatic concentration causes TSFS contours to spread out over a larger
parameter space. The primary process driving this is the increased rates of collisional
energy transfer from small to large aromatic species.
* The red shift cascade is the greater degree of energy transfer, which occurs at high concentrations of crude
oils. The excitation energy will continue to cascade to larger fluorophores, producing greater red shifts in the
emission spectra.
25