1 The Fluorescence Intensity Ratio (FIR): A New Way of Assessing the Efficiency of Oil Dispersion. J.B.C. Bugden 1 , P.E.Kepkay 2 and B.D. Johnson 2 1 Bedford Institute of Oceanography Fisheries and Oceans Canada P.O. Box 1006, Dartmouth, Nova Scotia Canada B2Y 4A2 2 Pro-Oceanus Systems Inc. 80 Pleasant Street Bridgewater, Nova Scotia Canada B4V 1N1 Abstract Ultraviolet fluorescence spectroscopy (UVFS) has been used to generate the excitation- emission matrices (EEM) of thirteen crude oils dispersed in seawater. The oils, with a wide range of dynamic viscosities (4 to 14,470 cP), were dispersed in seawater alone or by using the chemical dispersant Corexit 9500 at dispersant to oil ratios (DORs) of 1:10, 1:20 or 1:40. The matrices were simplified down to fluorescence intensity ratios (FIRs) obtained from two emission peaks at 340 and 445nm (with excitation fixed at 280nm), and the ratios compared to the solvent extracted concentrations of dispersed oil. When the dispersed oil concentrations were expressed as percent of oil dispersed (also known as the dispersion efficiency), FIRs of less than 4 were associated with efficiencies of greater than 40% while FIRs greater than 4 were characteristic of oils dispersed with efficiencies of 20% or less. Given that a FIR of 4 is a threshold between low and high dispersion efficiency, FIR fluorometry could be used as first-response tool to track marine oil dispersion.
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The Fluorescence Intensity Ratio (FIR): A NewWay of Assessing theEfficiency of Oil Dispersion.
J.B.C. Bugden1, P.E.Kepkay2 and B.D. Johnson2
1Bedford Institute of OceanographyFisheries and Oceans Canada
P.O. Box 1006, Dartmouth, Nova ScotiaCanada B2Y 4A2
2Pro-Oceanus Systems Inc.80 Pleasant Street
Bridgewater, Nova ScotiaCanada B4V 1N1
Abstract
Ultraviolet fluorescence spectroscopy (UVFS) has been used to generate the excitation-
emission matrices (EEM) of thirteen crude oils dispersed in seawater. The oils, with a
wide range of dynamic viscosities (4 to 14,470 cP), were dispersed in seawater alone or
by using the chemical dispersant Corexit 9500 at dispersant to oil ratios (DORs) of 1:10,
1:20 or 1:40. The matrices were simplified down to fluorescence intensity ratios (FIRs)
obtained from two emission peaks at 340 and 445nm (with excitation fixed at 280nm),
and the ratios compared to the solvent extracted concentrations of dispersed oil. When
the dispersed oil concentrations were expressed as percent of oil dispersed (also known as
the dispersion efficiency), FIRs of less than 4 were associated with efficiencies of greater
than 40% while FIRs greater than 4 were characteristic of oils dispersed with efficiencies
of 20% or less. Given that a FIR of 4 is a threshold between low and high dispersion
efficiency, FIR fluorometry could be used as first-response tool to track marine oil
dispersion.
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Introduction
Oil released into marine ecosystems can cause serious environmental damage if not
remediated (NRC, 1985). A number of mechanical remediation techniques such as
booming and vacuuming can be employed, but are often not practical due to the tendency
of spilled oil to spread as slicks over large swathes of the sea surface (NRC, 2005). Other
non-mechanical methods such as in situ burning and chemical dispersion have also been
tested, with dispersant application being the most practical technique in the majority of
marine environments (NRC, 2005, with references).
Oil will disperse naturally through the action of wave energy to create suspensions of
small droplets that can then be diluted by mixing and currents to concentrations below
toxic thresholds (Li and Garrett, 1998). When chemical dispersants are applied to an oil
slick, the formation of small droplets is accelerated, removing oil from the surface and
preventing it from stranding onshore (Fiocco and Lewis, 1999). In addition, the
generation of quasi-stable suspensions of small droplets by dispersants makes the oil
more accessible to natural populations of hydrocarbon-degrading bacteria in the water
column (Fiocco and Lewis, 1999; Lessard and Demarco, 2000; Venosa and Zhu, 2003,
with references).
It is important to know how well a slick has been dispersed after treatment with a
dispersant. Laboratory studies quantify this as dispersant effectiveness or dispersion
efficiency, which is defined as the amount of oil in the water column (measured after
extraction by a solvent) divided by the total amount of oil spilled (Sorial, et al., 2004).
The assessment of dispersion efficiency currently requires the collection, extraction and
analysis of a sample. What is needed is a less involved method to rapidly assess
efficiency.
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A consortium of government agencies has developed the SMART (Special Monitoring of
Applied Response Technologies) program (www.response.restoration.noaa.gov/smart) to
better assess dispersion efficiency during and after a spill. Given the ever changing
conditions encountered during dispersant operations and the need to monitor a number of
key processes, the SMART program includes three levels (or tiers) of monitoring: Tier I
is the simplest and is based on visual observations at sea and in the air. Tier II combines
visual monitoring with single-depth monitoring in the water column and sample
collection for later analysis. Tier III expands on Tier II, with sampling at multiple depths
and the possible redeployment of resources to address specific problems.
A key component of Tier II and III monitoring is the deployment of fluorometers to
determine if oil has been dispersed down into the water column. The utilization of ultra-
violet fluorescence spectrometry (UVFS) to identify and characterize hydrocarbons is
well documented (Bugden, et al., 2008, with references). The strength of this method is
that it does not require the extraction and concentration procedures required of many
other spectroscopic techniques (Patra and Mishra, 2002). Bugden, et al. (2008) have
already described how UVFS can be used to distinguish between oil and chemically
dispersed oil in seawater. They have also shown how complex excitation-emission
matrix spectra (EEM spectra) can be simplified down to a fluorescence intensity ratio
(FIR) that can be used as an index of how well oil is dispersed. Even though FIRs have
been used to characterize oil (Higashi and Hagiwara, 1980; Thruston and Knight, 1971),
crude petroleum (Sotelo et al., 2008) and oil quality (Markova, et al., 2007), as well as
estimate the API gravity of oils (Horvitz, 1986; Ryder, 2002), Bugden et al. (2008) have
pointed out that very little attention has been paid to the application of FIR fluorometry to
oil dispersion. They have also highlighted the main advantage of the FIR concept: Ratios
and freshwater wetlands. Spill Science and Technology Bull. 8, 163-178.
Venosa, A.D., D.W. King, and G.A. Sorial. 2002. The baffled flask test for dispersant
effectiveness: A round robin evaluation of reproducibility and repeatability. Spill Sci.
& Technol. Bull. 7(5&6), 299-308.
Von der Dick, H. & W. Kalkreuth. 1985. Synchronous excitation and three-dimensional
fluorescence spectroscopy applied to organic geochemistry. Adv. Org. Geochem. 10,
633-639.
Wakeman, S. 1977. Synchronous fluorescence spectroscopy and its application to
indigenous and petroleum-derived hydrocarbons in lacustrine sediments. Environ. Sci
& Technology. 11(3), 272-276.
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Table 1. Dynamic viscosity (DV), fluorescence intensity ratio (FIR), and the reduction inFIR of thirteen crude oils when dispersed in seawater (SW) in the presence of Corexit9500. The change in FIR was calculated by dividing the FIR of chemically dispersedoil (DOR 1:10, 1:20, and 1:40) by the FIR of oil dispersed in seawater alone (DOR 0)to obtain the fraction or percent reduction.
Oil DV SW (DOR 0) SW + Corexit (DOR 1:10) SW + Corexit (DOR 1:20) SW + Corexit (DOR 1:40)(cP) FIR FIR % Reduction FIR % Reduction FIR % Reduction
Table 2. Dispersion efficiency (% dispersed) of thirteen oils, calculated by dividingdispersed oil concentration (measured as total petroleum hydrocarbon - TPH) dividedby the amount of oil added during each treatment and expressed as a percentage.
Gulfaks 10 88.5 5.3 Terra Nova 10 90.1 6.7Gulfaks 20 88.9 0.3 Terra Nova 20 87.0 6.5Gulfaks 40 88.4 8.7 Terra Nova 40 86.3 2.4Gulfaks 0 16.6 4.8 Terra Nova 0 6.3 2.3
Vasconia 10 76.8 10.9 Lago 10 82.7 9.5Vasconia 20 40.8 0.7 Lago 20 66.0 2.9Vasconia 40 41.9 0.9 Lago 40 74.0 15.8Vasconia 0 2.2 0.6 Lago 0 10.0 3.8
Maya 10 68.8 10.1 Santa Clara 10 85.1 2.2Maya 20 58.7 16.6 Santa Clara 20 85.2 2.8Maya 40 55.8 10.1 Santa Clara 40 75.5 3.5Maya 0 2.1 0.3 Santa Clara 0 12.3 3.8
Figure 1. Blank-corrected contour plot of IFO 180 crude oil fluorescence at fourdispersant to oil ratios: 0, 1:40 (2.5% dispersant), 1:20 (5% dispersant) and 1:10(10% dispersant). The 280nm excitation wavelength (white dashed line), and the340nm and 445nm emission wavelengths (white boxes) used for fluoresnceintensity ratio (FIR) calculations are shown for reference.
IFO 180 OilDOR 1:20
Excitation Wavelength (nm)
240 260 280 300 320 340
EmissionWavelength(nm)
300
350
400
450
500
550
600
IFO 180 OilDOR 1:10
Excitation Wavelength (nm)
240 260 280 300 320 340
YData
300
350
400
450
500
550
600
IFO 180 OilDOR 0
240 260 280 300 320 340
EmissionWavelength(nm)
300
350
400
450
500
550
600
0102030405060
IFO 180 OilDOR 1:40
240 260 280 300 320 340
300
350
400
450
500
550
600
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Figure 2. Fluorescence emission spectra of IFO 180 crude oil at an excitationwavelength of 280nm for various dispersant to oil ratios (DOR). The arrowsindicate the position of the 340 and 445 nm emission peaks used in the fluorescenceintensity ratio (FIR) calculation.
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Figure 3. Log plots of mean fluorescence intensity ratio (FIR) versus dynamic viscosityof thirteen reference oils in seawater. Three Corexit 9500 to oil ratios of 1:40 (2.5%dispersant), 1:20 (5 % dispersant) and 1:10 (10 % dispersant) were compared toresults obtained with only oil dispersed (DOR 0).
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Figure 4. Cluster plot (A) of fluorescence intensity ratio (FIR) versus the percentdispersion efficiency in seawater (as determined from total petroleum hydrocarbonmeasurements) for the 13 reference oils. The lower graph (B) expands the 0 to 10scale of FIR, and the black dashed line indicates the threshold ratio (FIR = 4),below which oil is dispersed with an efficiency of at least 40%.
A
B
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Acknowledgements:
We gratefully acknowledge Encana Corporation for their financialsupport of this research