1

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

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are dimensionless and can be determined rapidly without having to calibrate a

fluorometer (Genders, 1988; Hurford et al., 1989; Chapman et al., 2007) with

independent measurements of oil concentration.

In response, we have reapplied the fluorescence intensity ratio (FIR) technique to thirteen

reference oils with a wide range of dynamic viscosities at three dispersant-to-oil ratios

(1:10, 1:20, and 1:40). These treatments, applied to suspensions of oil in seawater

maintained in baffled flasks at constant agitation (equivalent to an eddy diffusivity of 0.1

m2 s-3 under spilling breaking waves) have been shown to be more representative of

mixing at sea than other techniques (Venosa et al., 2002; Kaku, et al., 2006). This results

in reproducible determinations of dispersion efficiency (Sorial, et al., 2004) and better

approximations of natural wave conditions than those employed by Bugden et al, (2008).

One of the objectives of the work reported here has been to determine whether the FIR

concept holds true for oils under these more realistic regimes of turbulent energy and

over a range of dispersant to oil ratios. More importantly, the objective has been to

determine if fluorescence intensity ratios (FIRs) are accurate indices of dispersion

efficiencies. This would allow FIRs to be used in a first response (SMART, Tier II or III)

scenario to provide immediate feedback on how well a spill has been dispersed.

Methods

Thirteen reference oils from the Emergencies Science and Technology Division of

Environment Canada were selected to cover a wide range of viscosity (Table 1), and

include approximately equal numbers of low, medium and high-viscosity oils. The oil-

dispersant treatments were randomly organized into a block of experiments such that

each treatment (with oil and dispersant) and each oil-only control was included once in

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the block. Individual blocks of experiments were repeated three times, with the

treatments within each block randomly shuffled.

Approximately 100-µL of oil was added to a 250-mL baffled flask containing 120 mL of

0.45-µm filtered seawater from Bedford Basin, Nova Scotia (salinity 30-32 ppt), and

placed, prior to the addition of the oil/dispersant, on an OS-500 orbital shaker with an

orbital diameter of 2cm set at 200 rpm (VWR International/Henry Troemner LLC,

Thorofare, NJ). Approximately 300-µL of oil was aspirated into a 1-mL syringe (Becton

Dickinson, Franklin Lakes, NJ), and the plunger displaced to the 100-µL mark. If the oil

was of low viscosity, a 26 gauge needle was placed on the end of the syringe to prevent

oil from leaking from the tip. The oil and syringe were then weighed and the contents of

the syringe (approximately 100 µL of oil) dispensed onto the surface of the seawater in

the baffled flask; the syringe was then re-weighed. The volume of oil added was

calculated from the measured weight and density (available from the Environment

Canada database). This allowed the correct amount of dispersant (Corexit 9500) to be

dispensed and provide the required dispersant to oil ratios (DOR) of 1:10, 1:20 and 1:40.

Following the procedure of Soriel, et al. (2004), Corexit was then added as a drop to the

surface of the oil using a 20-µL Pipet-Plus Pipetman (Gilson). The

oil/dispersant/seawater was allowed to mix for ten minutes at an agitation rate equivalent

to an eddy diffusivity of 0.1 m2 s-3 under spilling breaking waves. Approximately 3 mL

of the dispersed oil/seawater mixture was then dispensed through a spigot located near

the bottom of the flask into a 4.5-mL UV-grade quartz cuvette (10 mm light path -

Hellma (Canada) Limited, Concord, Ontario) for ultra-violet fluorescence spectroscopy

(UVFS) scans on a Shimadzu 5301-PC UV-fluorometer (following the procedure

described by Bugden et al., 2008).

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About 30 mL (the exact volume being recorded) was then drawn into a 50-mL graduated

cylinder fitted for Total Petroleum Hydrocarbon (TPH) analysis (Li et al, 2008a). Care

was taken to prevent the transfer of non-dispersed (slick) oil through the flask spigot.

The sample was then transferred into a 100 mL amber bottle with a tin-foil-lined screw

cap. The graduated cylinder was rinsed three times with di-chloro-methane (DCM); two

times with 10 mL and once with 20 mL. The graduated cylinder was stoppered after each

rinse, the contents shaken (with the stopper removed after every few shakes to release

pressure built up in the cylinder), and the contents transferred to an amber bottle which

was then placed in a refrigerator at 4oC; DCM was then allowed to settle for a minimum

of 24 hours. After this extraction period, the oil-in-DCM extract was transferred to a 40

mL glass vial using a lime glass pipette and glass syringe, and adjusted to a final volume

of 10 mL with DCM. The samples were scanned on a Genesys 20 spectrophotometer at

340, 370 and 400nm (Thermo Fisher Scientific Inc., Waltham, MA, U.S.A), using the

same quartz cuvette employed for UVFS measurements.

Calibration curves were generated from stock solutions by adding 1 mL of oil (dispensed

by mass) to a 10 mL volumetric flask containing DCM. Dispersant was added to achieve

the appropriate DOR using a 20 !L Rainin pipet"plus with a 20 !L tip. For each

dispersant-to-oil ratio (DOR), 7 standards were made up in a 10 mL volumetric flask with

DCM at the following stock volumes: 10, 20, 40, 100, 200, 400, and 500 !L. The

standards and samples were run on the same day, and under the same conditions to

eliminate variation due to changes in UV-bulb intensity over time. Absorbance and

percent transmission were recorded and the data were entered into a spreadsheet to

calculate sample concentration. Some samples needed to be diluted by up to 100 mL to

obtain a signal that could be read on the spectrophotometer. Dispersion efficiency was

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calculated by dividing the concentration of oil extracted from the water column (the

dispersed fraction) by the total amount of oil added to the flask, and recorded as percent

dispersed.

Results

Three dimensional ultra-violet fluorescence spectroscopy (also known as excitation-

emission matrix spectroscopy - EEMS) provides clear evidence of the effect of chemical

dispersant on oil fluorescence in seawater (Bugden, et al, 2008, with references). In

Figure 1, this effect was apparent in the EEM fingerprints of the fuel oil, IFO 180. As the

dispersant to oil ratio (DOR) increased from 0 (no dispersant) to 1:10 (10% dispersant),

the prominent emission peak centred at 340nm decreased in intensity, and a broader peak

centered at 445nm increased.

By comparing the intensity of the two emission peaks, the effect of increasing dispersant

on each fluorescence signature could be quantified in terms of a fluorescence intensity

ratio (FIR). Previous work (Bugden, et al, 2008) has shown that an excitation

wavelength of 280nm and an emission range of 300 to 550 nm best captures the

variations in emission intensity of the two peaks. The 280nm excitation wavelength is

indicated by a dashed white line in Figure 1, and the white boxes in the same figure

indicate the locations of the two emission wavelengths of 340nm and 445nm that were

used to calculate FIRs.

The changes in the two respective peak intensities in the results for IFO 180 (Figure 1)

can be simplified by extracting two-dimensional emission spectra (Figure 2) at an

excitation of 280 nm. As DOR increased from 0 (no dispersant) to 1:10 (10%

dispersant), the peak at 340 nm decreased from 140 to 27 (Figure 2). In contrast, the

broader peak at 445 nm increased from 0 to 50 over the same range of DOR.

8

The fluorescence intensity ratios (FIRs) calculated from intensity at 340 nm divided by

intensity at 445 nm (Table 1) suggest that that the action of dispersant on FIR had a

number of effects: First, in the case of low viscosity oils, FIR was decreased by at least

98%. Second, the decrease was less pronounced (between 85 and 99%) in the medium

viscosity oils and finally, in the high viscosity oils the reductions in FIR were more

variable (64 - 98%) as DOR decreased.

There was also a distinct increase in the fluorescence intensity ratio (FIR) as viscosity

increased (Figure 3); this increase was approximately the same over the entire range of

dispersant to oil ratios (DORs). However, FIR remained largely unchanged (Figure 3)

once the dynamic viscosity was greater than 200 cP, again regardless of the DOR. Some

of the variations obtained in the fluorescence intensity ratios (FIRs) in Figure 3 were

probably related to different methods employed during two series of experiments, ie., the

pre-mix of oil and dispersant and high dispersion energy utilized by Bugden et al. (2008)

compared to oil added to the surface of seawater at the more realistic, lower dispersion

energies recorded here. This can result in different contour plots, but also result in very

little change in how FIRs could be applied to oil dispersion.

The total petroleum hydrocarbon (TPH) results for dispersion efficiency (Table 2)

indicate that all of the oils were poorly dispersed in the absence of dispersant (DOR 0),

with a mean efficiency of 7.6 ± 5.6%. As the amount of dispersant increased (Table 2),

there was an associated increase in mean efficiency, from 71.0 ± 15.8% at a DOR of 1:40

to 73.6% ± 16.4% at 1:20 and 80.0% ± 9.7% at 1:10. Within this overall increase, low

viscosity oils displayed a high efficiency of dispersion, with at least 79% dispersed,

regardless of the DOR. The results for medium viscosity oils were more variable (Table

2), but the efficiencies remained greater than 40%. High viscosity oils displayed similar

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variations (Table 2) but only IFO 300, the highest-viscosity oil tested (with a DV of

14470 cP), dispersed with an efficiency of less than 47% (at a DOR of 1:10).

When the fluorescence intensity ratio (FIR) was plotted versus dispersion efficiency

(Figure 4), the effect of dispersant was readily apparent, with two distinct clusters of

results: Dispersion efficiencies of 40% or more were associated with the application of

dispersant, and efficiencies of 20% or less were evident in the oil-only treatments (Figure

4A). In order to establish a threshold FIR that could define “effective” versus “non-

effective” dispersion, the FIR scale of 0 – 70 in Figure 4A was expanded to 0 -10 (Figure

4B). In this expanded cluster plot, a FIR of four delineated between well dispersed and

poorly dispersed oils, with a ratio less than 4 associated with sufficient (> 40 %)

dispersion and a ratio greater than 4 characteristic of poor (< 20%) dispersion.

Discussion

Even though dispersion efficiency is normally determined with bench-scale tests based on

solvent extraction to measure dispersed oil concentration (Sorial et al., 2004a), new

methods are needed to provide rapid feedback on the dispersion of oil slicks during a

spill. Bugden et al, (2008) have demonstrated that a 3-D excitation-emission matrix

(EEM) can be simplified down to a fluorescence intensity ratio (FIR). The major

advantage of this approach is that results can be obtained quickly. All that is required is a

fluorometer that can excite at 280nm and read emission peaks at 340 and 445 nm. As

Bugden et al, (2008) have already pointed out, FIRs calculated from the two emission

peak intensities can provide rapid feedback on how well a slick has been dispersed. This

would give first responders operating under the monitoring protocols of the SMART

program (www.response.restoration.noaa.gov/smart) the chance to make informed

decisions early on in a spill remediation.

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The effect of dispersant on fluorescence intensity ratio (FIR) is summarized in Figure 3

and Table 2. The FIRs of low viscosity oils dropped by at least 98%, regardless of the

dispersant to oil ratio (DOR). This decrease in FIR was more variable as viscosity

increased (Figure 3), but was still 85 - 94% for the case of medium viscosity oils and 64 -

98% for high viscosities (Table 2). However, the results obtained with high viscosity oils

provide the clearest indication of the effect of DOR on FIR. When mean percent

reductions were calculated from the data in Table 2, the values for FIR reduction

increased from 71% at a DOR of 1:40 to 74% at 1:20 and 80% at 1:10.

As can be seen in Figure 4, < 20% of a given oil was mixed into the water column in the

absence of chemical dispersant, yet a wide range of FIRs was measured (Figure 4A), with

all of the FIRs greater than 4 (Figure 4B). These dispersant-free FIRs did not correlate in

any discernible way with dynamic viscosity which is not in agreement with earlier work.

However, the earlier studies were not carried out at 280 nm excitation, and utilized either

oil of non-defined composition (Ryder, 2002) or oil that was pre-dissolved in an organic

solvent (Thruston and Knight, 1971; Horvitz, 1986).

Addition of dispersant to suspensions of oil resulted in a drop in the fluorescence

intensity ratio (FIR) and an increase in dispersion efficiency by 40 - 99% (Figure 4A),

resulting in more oil mixed down into the seawater as small droplets (Lessard and

Demarko, 2000; Page et al, 2000). Even though the data in Figure 4A can be interpreted

as a curve of decreasing FIR and an exponential function can be fitted to the data, the

equation derived from the fit does not accurately define dispersion efficiency in relation

to FIR, especially at low ratios. Instead, it is probably more realistic to treat the data in

Figure 4A as a cluster plot. This means that FIRs may be better utilized in terms of a

threshold index of whether oil has been dispersed sufficiently (Figure 4B). With a FIR

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above 4, additional treatment with dispersant would be needed to ensure the dispersion

was above 40% and if the FIR was below 4, the oil would be sufficiently dispersed (at an

efficiency already > 40%).

At this point it should be stressed that, even though these experiments have been

performed under well controlled conditions in baffled flasks, there are limitations

inherent in this approach. The data obtained from flask experiments are more

representative of the dispersion of relatively high oil concentrations early on during a

spill, but they do not take the transport and dilution of oil into account as the spill

progresses (NRC, 2005). Recent experiments in a wave tank have been reported which

include the effects of oil transport and dilution on measurements of dispersion efficiency

(Li et al., 2008; Li et al., 2012 in press). More work of this type is required to better link

FIR and dispersion using the transport and dilution effects that are more typical of late

spill conditions.

In conclusion, fluorescence intensity ratios (FIRs) could be used as a rapid, on site

method to provide information on the state of oil slick dispersion. The calculation of a

FIR would provide a basic, dimensionless assessment of the extent of oil dispersion

without having to measure oil concentration. However, instead of being used to estimate

a discrete value of dispersion efficiency, the FIR would be better utilized as a screening

tool at the high oil concentrations found early on during a spill. Employed in this way, a

FIR value of less or greater than a threshold of 4 would act as a “yes/no” index of

sufficient oil dispersion.

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References

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fingerprint oil and chemically dispersed oil in seawater. Marine Pollution Bulletin.

56, 677-685.

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Lessard, R. R., and G. Demarco. 2000. The significance of oil spill dispersants. Spill

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Li, Z., K. Lee, T. King, M.C. Boufadel, and A.D. Venosa. 2012. Evaluating crude oil

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Ryder, A. G. 2002. Quantitative analysis of crude oils by fluorescence lifetime and

steady state measurements using 380-nm excitation. Appl. Spectrosc. 56 (1), 107-116.

Special Monitoring of Applied Response Technologies (SMART).

http://response.restoration.noaa.gov/smart

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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

Low -- Dynamic Viscosity between 1 and 50cPFederated 4 4.515 0.050 98.9 0.050 98.9 0.061 98.6Brent Blend 6 9.398 0.070 99.3 0.088 99.1 0.104 98.9Gullfaks 13 5.609 0.106 98.1 0.091 98.4 0.102 98.2Terra Nova 22 9.733 0.057 99.4 0.064 99.3 0.059 99.4Hibernia 49 8.510 0.060 99.3 0.073 99.1 0.076 99.1

Medium -- Dynamic Viscosity between 51 and 400cPVasconia 72 24.081 0.192 99.2 0.244 99.0 0.399 98.3Lago 153 9.767 0.155 98.4 0.178 98.2 0.209 97.9Maya 280 27.836 1.653 94.1 3.446 87.6 3.313 88.1

Santa Clara 304 10.523 0.604 94.3 1.007 90.4 1.616 84.6High -- Dynamic Viscosity between 401 and 14470cP

Hondo 735 10.050 1.030 89.8 2.760 72.5 3.150 68.7IFO 180 2324 31.860 0.510 98.4 0.900 97.2 2.560 92.0Belridge 12610 7.300 1.080 85.2 1.850 74.7 2.650 63.7IFO 300 14470 60.081 1.163 98.1 2.086 96.5 2.990 95.0

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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.

Oil DOR%

Dispersed%

Dispersed Oil DOR%

Dispersed%

Dispersed(Mean) (St.Dev.) (Mean) (St.Dev.)

Federated 10 88.9 3.2

LowViscosity

Oils

Brent 10 91.0 7.6Federated 20 89.0 7.8 Brent 20 93.4 1.7Federated 40 91.8 3.4 Brent 40 80.3 12.7Federated 0 6.6 2.6 Brent 0 9.6 5.2

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

Hibernia 10 79.3 6.7Hibernia 20 85.5 2.6Hibernia 40 79.4 10.3Hibernia 0 9.6 2.3

Medium

ViscosityOils

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

High

ViscosityOils

Hondo 10 78.3 5.9 IFO180 10 80.6 4.5Hondo 20 73.5 2.3 IFO180 20 68.5 11.3Hondo 40 63.4 0.9 IFO180 40 77.6 3.0Hondo 0 1.2 0.9 IFO180 0 3.3 0.8

Belridge 10 73.8 5.1 IFO300 10 56.7 4.4Belridge 20 69.0 7.8 IFO300 20 51.3 9.9Belridge 40 61.5 9.5 IFO300 40 47.2 15.4Belridge 0 17.3 5.6 IFO300 0 1.4 0.6

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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