Oil Spill The Coast Guard high-seas oil recovery system machine is shown being readied for tests on San Francisco Bay with Coast Guard small boats in the background. The air-transportable machine has joined the Coast Guard's Atlantic Area Pollution Strike Team at Elizabeth City, N.C. Capable of recovering up to 1,000 gal of oil a minute under certain conditions, the machine is derived from the Clean Sweep, Lockheed's patented commercial oil-water separator, which is being used in many parts of the world in oil spill recovery, in refineries, petrochemical plants, and railroad maintenance facilities
During the past two decades there has been an increasing public consciousness of the effects of pollution. The Torrey Canyon disaster (1967) in the English Channel awakened the world to the international consequences of large-scale oil spills. "Supership" (i) promises an even more disastrous potential in the future. In a recent survey of the legal aspects of oil pollution, Jackson (2) points out the international efforts to control pollution by means of localized conventions addressing regional problems. He traces the history of the law of the sea up to the involvement of the United Nations through IMCO (Intergovernmental Maritime Consultative Organization). In 1959 IMCO accepted the oil pollution functions voted to the UN in the 1954 London Conference. IMCO is now trying to coordinate the various national efforts at oil pollution control. In an earlier overview of the legal problems, Mensah (3) of IMCO divided them into two categories: those relating to the prevention of pollution and those to repairing the damage caused by such pollution. The United States addressed this problem by enacting the Federal Water Pollution Control Act (Public Law 80-845) which was approved on 30 June 1948 (4). It has been extended and amended periodically in a series of ever-tightening laws relating to water pollution control and environmental quality. With impetus from Rachel Carson (5) and others of her persuasion, the United States perceived some of the potential hazards. With rare foresight the United States passed the Oil Pollution Act in
454 A · ANALYTICAL CHEMISTRY, VOL. 48, NO. 6, MAY 1976
1924 (Public Law 68-238). This law made it "unlawful for any person to discharge, or suffer, or permit the discharge of oil by any method, means, or manner into or upon the coastal navigable waters of the United States . ..". The law was updated in 1961 to "implement the provision of the International Convention for the Prevention of the Pollution of the Sea by Oil, 1954". In 1970 a major change was included which prohibited the discharge of harmful amounts of oil into U.S. waters and required reporting any accidental discharge to the United States Coast Guard. In the Federal Water Pollution Control Act amendments of 1972, a $35 million revolving fund was established for the U.S. Coast Guard to effect oil spill cleanup. Whenever the persons responsible for a spill can be identified, the cleanup costs are to be collected to replace those spent from the fund. These provisions require that the law enforcement agencies (including the U.S. Environmental Protection Agency) develop forensic oil identification capabilities (fingerprinting methods) to assess liability in "mystery" spill cases. Chemical analysis and characterization of oils have long been carried on by the petroleum industry. Unfortunately, oils which have been spilled on water undergo numerous changes which make the conventional analyses useless for fingerprinting, e.g., viscosity and flashpoint. The changes caused by weathering include dissolution of solubles, evaporation of volatiles, oxidation by exposure to sunlight, and biodégradation.
Report
Identification Alan P. Bentz U.S. Coast Guard Research and Development Center Groton, Conn. 06340
Because of these difficulties, two schools of thought arose as to how the problem should be tackled. One was the head-on approach of direct fingerprinting (passive tagging). The other involved tagging the oil with some identifiable material (active tagging). Numerous methods of active tagging have been suggested, from organometallics identifiable by x-ray fluorescence to microglass beads identifiable by particle size. Although some foreign governments are still trying to promote active tagging, the consensus in this country appears to be against it for several pragmatic reasons such as: the obtainment of compliance from all foreign governments, the astronomical permutations involved, the possible pollution problems of adding foreign materials, and the major problem of dispersing the tags uniformly throughout a cargo—without subsequent segregation when the cargo becomes nonhomogeneous. Many analytical techniques can be used for fingerprinting oils. Since no single technique has yet been accepted by the scientific community or the courts, there is a need to use multiple analytical techniques. Although many independent groups from government, industry, and universities are working on oil spill identification, efforts in the United States are being coordinated through the American Society for Testing and Materials (ASTM) subcommittee D19.10, Identification of Waterborne Oils. This organization maintains close liaison with the Institute of Petroleum (Great Britain) (6a) and with Deutsche Gesellschaft Minerolwissen-
schaft and Kohle-Chemie (DGMK) in Germany. DGMK (the German Association of Petroleum Sciences and Coal Chemistry) is a member of the Organization of European Petroleum Technical Co-operation. The DGMK Project 4599 report (7) "was proposed as a manual for general application, as an international standard . . . to the IMCO organisation at the London meeting in the Fall of 1973". The British recommended 13 standard methods which appear in the 34th edition of "LP. Standards for Petroleum and Its Products" and are summarized under "Analytical Methods" in "Marine Pollution by Oil" (66). The American efforts in ASTM D19.10 cover diverse aspects of oil identification which will be discussed later. The following subjects are now published by the ASTM: sample preparation, sample preservation, elemental analysis, and gas chromatography (8). Although aspects of oil pollution such as quantitative analysis in the water column and tissues of marine organisms and the overall ecological impact have been studied by many workers, this article is solely concerned with oil spill identification. This means matching a spill to its source and includes, for forensic reasons, the documentation of the oil sampling and sample transmittal. Weathering Spilled oils "weather" as soon as exposed to the elements, and the changes thus introduced preclude certain standard analyses for oil characterization. However, because of extensive studies on the effects of weather-
ing on the analytical results by various techniques, the experienced analyst can identify the changes attributable to weathering and take them into account when matching a spill to a source. Small changes relative to differences between suspects present no problem. If changes are large relative to differences between suspects, the present alternative is to simulate weathering on each of the suspects. Brown et al. (21 ) devised a rapid technique (1% h) to simulate 5-7 days weathering for analysis by infrared (IR) spectroscopy. Bentz et al. (9) effectively minimized weathering effects for gas chromatography (GC) by deasphalting the oil samples and evaporating solvent with subsequent analysis of only components above C15. With the advent of microprocessors and more readily available digitizing equipment, it will soon be possible to quantify the changes due to weathering. As for the oils themselves, the changes on all oils are most pronounced during the first 24-48 h, with changes continuing thereafter at a much slower rate. Of course, lighter oils (light distillates and light crudes) change much more drastically than heavier oils. Most significant spills (even a few gallons) are discovered within the first 48 h. Thus, simulated weathering studies are generally limited to no more than 1-2 weeks. However, it is not infrequent that a leaky underground tank may have oil leach through the ground to a river some years later. A massive spill such as that by the METULA (70) off the Chilean coast will leave oil residues in
ANALYTICAL CHEMISTRY, VOL. 48, NO. 6, MAY 1976 · 455 A
the area for years to come in the form of an emulsion ("chocolate mousse") buried in the sand. It is conceivable that it may be necessary to distinguish oil from the METULA from more recent spills for the next 4-5 years; thus, long-term weathering effects must be taken into account. One of the most comprehensive early weathering studies was that of Frankenfeld (11), who did a two-year study using indoor simulators. Developed originally for EPA, these simulators recycled sea water below the slick, had a fan blowing air at 15 mph, and used an ultraviolet lamp to simulate sunlight. Weathered samples were primarily analyzed by IR, GC, and thinlayer chromatography (TLC), with some selected analyses using dual-detection [refractive index and ultraviolet (UV)] high-pressure liquid chromatography (HPLC). The analyses gave a comprehensive profile of changes, due to weathering, as a function of time. One surprising discovery was that under the conditions of the experiments, the effect of air temperature was more pronounced than that of water. Another comprehensive study, now in its second year, is that by Brown et al. (12). Samples were weathered outdoors in drums on Narragansett Bay, in open chambers with circulating sea water next to the Bay, and in similar chambers on the laboratory rooftop and indoors in the laboratory. The results were affected to the greatest degree by temperature being more accelerated on the rooftop than in or near the Bay. After analyzing spectra of over 750 weathered petroleum samples, Brown et al. (13) concluded that there was a "spectral" advantage to the use of IR, in that the high boiling components weather slowly and give rise to a relatively stable IR fingerprint that can be used to identify the source of an oil. Mayo et al. (14) studied the effect of weathering by GC,and IR spectroscopy. With IR difference spectroscopy, they could estimate within approximately 24 h how long an oil had been weathered over a 1-week period. The weathering facility used was the Research Institute, Gulf of Maine (TRIGOM) in Portland, which has 2000L fiberglass tanks outdoors with circulating sea water. Four such tanks provide one of the best domestic facilities for "large-scale" weathering studies. Sampling It is imperative that representative samples be obtained from an oil slick, and all potential suspects, before a valid analysis can be made. Sampling of slicks can be complicated by emulsion formation and by deposition on
sand or other substrates. If it is collected on solid substrates, the only recourse is to extract the oil later with solvent. Sampling from the water surface depends on the thickness of the film. Numerous satisfactory techniques are available. Surface sampling by almost any method is satisfactory, provided that sufficient sample is obtained for the analyses to be performed and that no contamination of the sample occurs. A number of elaborate samplers, using polyurethane foams, are precluded by the last requirement, since the plasticizer contaminates the sample. The U.S. Coast Guard R&D Center has developed two satisfactory techniques (15). One, boat or helicopter déployable, uses an aluminum doughnut (2 ft i.d.) which is lowered into the slick. The inner rim is wiped with Shell Oil Herder, a surfactant which forces the oil to the center where it is absorbed in glass wool. The glass wool is then dropped into a sample jar. The second method uses the "rake" sampler which consists of a rake-like device with eight Teflon strips attached. The Teflon is drawn through the slick, with the oil preferentially wetting the surface. The eight strips are dropped into a wide-mouth 8-oz glass jar with Teflon (or aluminum foil) liners. Between 2-10 ml of oil will drain from the Teflon strips, depending upon the viscosity of the slick oil. Generally, the preferred sample container is a glass jar with a Teflonlined lid. For forensic reasons (which are beyond the scope of this article) it is necessary to have the sampling witnessed to insure proper labeling of the jar, to seal it so that it cannot be tampered with, and to maintain a written chain of custody until the sample is analyzed. We will assume that these precautions have been properly observed and will proceed with a discussion of the analysis. Overall Approaches As no single analytical method has yet been established as fully characterizing an oil, a multimethod approach is required. If a known spill sample is analyzed by different techniques, then a simple probability of a correct match can be determined for each technique. Assuming additivity (independence), the simple probabilities can be combined to give an overall probability of a correct match. For the degree that the methods are truly independent, the combined probability will certainly be greater than that of any individual technique. The U.S. Coast Guard R&D Center chose four different, complementary analytical procedures for their basic
458 A · ANALYTICAL CHEMISTRY, VOL. 48, NO. 6, MAY 1976
oil identification system (15). These procedures are IR spectroscopy, fluorescence spectroscopy, GC, and TLC. IR spectrophotometry examines the composite molecular vibrations of all components as an absorption spectrum; fluorescence spectroscopy indicates primarily the polynuclear aromatic components by their electronic transitions from excited states; and GC shows largely the aliphatic hydrocarbon components by physical separation. A flame photometric detector permits the simultaneous profiling of the sulfur components for an added dimension to GC fingerprinting. TLC provides a rapid separation of components that are visualized under UV light or iodine vapors. Other available methods will be discussed separately. Since it is economically unfeasible to analyze every sample by every available method, only those additional methods which are necessary to establish a match conclusively are used. The German approach suggested to IMCO involves sequential analyses in a series of screening steps. First, IR analysis is carried out. If the sample appears the same as a spill, then a sulfur analysis is carried out, followed by GC. Crude oils are analyzed for V and Ni; lubricating oils for Ca, Ba, and Zn by x-ray fluorescence. Column chromatography is used to separate oil into a hydrocarbon fraction (to be examined by GC) and a polar fraction (to be examined by IR). The ASTM approach (8) is to optimize each individual method prior to publishing it, with the user choosing those methods within the capability of his own laboratory. The Institute of Petroleum, Great Britain (IP) (6b), recommends an elaborate scheme of analysis which begins with a preliminary GC examination followed by TLC/UV examination. This permits classification and identification of oil type. A toluene distillation and a filtration are used to remove water and debris. GC, neutralization number, saponification number, TLC/UV, etc., are carried out on the sample. Distillation to 343 °C yields a distillate and residue which can be analyzed by GC. The residue is also analyzed for Ni, V, S, asphaltenes, and specific gravity. Notably, IP does not recommend IR spectroscopy. Infrared Spectroscopy IR spectroscopy has been used for petroleum analysis for over 25 years. A review of recent papers up to 1963 is available from Perkin-Elmer (16). Rosen and Middleton (1955) suggested that IR spectra might be sufficiently characteristic to identify oils. During the past three years the use of IR spectroscopy for oil spill identi-
fication has been improved to a high degree by numerous workers. An ex tensive review has been written by Brown et al. (17). Westervelt has developed and re fined the Coast Guard infrared tech nique (15, 48). Recently, she has adapted this method for field use by relatively untrained personnel. Identifying a wet oil poses prob lems, since water interferes with the IR spectrum. It is best removed, with out the addition of solvent, by centrifuging the oil sample, warming the higher viscosity oils if necessary, re moving water with a pipet, and recentrifuging in the presence of anhydrous MgS04. In some cases, if the sample is deposited on a solid substrate, it is necessary to extract the sample with solvent (generally pentane) and to dry and evaporate the solvent. The solvent removal must be done carefully to avoid interference in the final spec trum. The actual analysis is straightfor ward, but the development of a good technique is required for reproducible results. A sealed liquid cell may be used, but a sealed demountable cell is preferred in most cases—especially for oils too thick to flow in a syringe for filling. Excellent reproducibility can be achieved with careful procedures. The windows commonly used are KBr or AgBr with a spacer of 0.05 mm. Note that in a given spill set, all sam ples must be analyzed by the same procedure for valid comparisons. A Wilks Mini-cell, useful for small samples down to 10 mg, is used with AgBr windows with a 0.025-mm cavi ty. Very small samples that appear as a sheen on water can be sampled with the Wilks micro multiple internal re flectance sampler to provide a useful spectrum. This consists of a small (8 X 10 mm) zinc selenide crystal that is dipped three times through the sheen to build up a film on the surface. It is then placed in a beam-condensing mirror system. Baier (1972) first used multiply attenuated internal reflec tance to study the fate of very thin oil films on water. Mattson (1969,1970) pursued this technique for water pol lution and oil analysis. Mattson was able to distinguish natural seeps from platform produc tion samples in the Santa Barbara area using IR spectroscopy. Eastwood and Grant developed a field IR meth od (unpublished and now in use in Santa Barbara) which utilizes a com pact, single-beam instrument and is capable of distinguishing weathered or unweathered production oils and seeps. The most difficult part of the analy sis is the interpretation. The simplest approach is a manual overlay of two spectra on a light box for peak-by-
peak comparison. There is no problem when the curves match exactly. How ever, problems do arise when an oil slick is weathered on water, causing changes to occur which result in the generation of a carbonyl peak at 1710 cm - 1 , a general lowering of the base line from about 1350 to 900 cm - 1 , and the dimunition of certain absorption peaks due to loss of lighter compo nents. Comparison to known weath ered curves or simulated weathering of suspect oils can be used to determine whether differences in curves are at tributable to weathering or different sources. The magnitude of changes on the weathering of oils is a function of oil type. Owing to the complex nature of the mixtures constituting oils, a systemat ic comparison of spectra is necessary to establish that comparable pathlengths are being examined and that observed differences are significant.
Table I. Peaks of Major Significance for Oil Identification ( c m - 1 ) Chris Brown
Jim Mattson
695
870
672
720*
890
698
870 888"
725 s
915°
723"
963"
740
955
743
1033"
765"
1030
766"
1165"
780"
1070
780"
1306"
790"
1145
792"
1376 e
810
1160
810 6
1456 e
835
831
1604°
845
847
1710 6 ''
• Brown digitizes 720 and 725 separate ly; Mattson digitizes them together at 723 c m - 1 . * Peaks considered most important by Mattson. °The 915 c m - 1 peak remains stable to weathering. d These peaks be come weak after weathering. e Peaks con sidered nonsuitable. 'The 1710 c m - 1 is useful to monitor the degree of oxidative weathering, not fingerprinting per se.
Those portions of the spectra which are of most importance in character ization of oils become evident with ex perience. A list of the wavenumbers considered important was compiled by Mattson and Brown at an Infrared Pattern Recognition Seminar held at the U.S. Coast Guard R&D Center in May 1975 (Table I). These peaks are important for manual curve compari son, but even more important for com puter pattern recognition programs and for use with digitizing spectropho tometers. Kawahara (1969, 1970) applied IR to distinguish heavy residual fuels from asphalts by means of six absorbance ratios. He later refined this dis tinguishing capability by applying sta
460 A · ANALYTICAL CHEMISTRY, VOL. 48, NO. 6, MAY 1976
tistical discriminate function analysis (1974). Mattson (1971) selected 11 wavenumbers and normalized to one (1456 c m - 1 ) giving 10 ratios. He deleted the 1375 c m - 1 because of small variation in the normalized area and the 1309 c m - 1 because of its large relative stan dard deviation and small range of values. The remaining eight ratios were used to generate eight integers (each one representing the relative area at that wavenumber) which gave an eight-character number as a "fin gerprint" for each oil. He obtained dif ferent numbers for each of the 40 se lected crude oils. Only two crudes, both from Texas oil fields, had as many as five coincident characters in the eight-digit string. Mattson has since interfaced a Perkin-Elmer 180 IR spectrophotometer to a Nova 1200 computer to digitize spectra taking 1350 data points per curve. He discussed general problems of computerized IR in a recent paper (1975) and did extensive pattern rec ognition work on oil spectra, deter mining analytical variance and popu lation variance for over 200 oils. He applied multivariate statistical analy sis to the pattern recognition of in frared spectra (18). Other workers such as Clark and Jurs (19) and Curtis and Starks (20) are applying pattern recognition tech niques to IR spectra (as well as other techniques) for oil identification. In the near future, we expect to see great strides in objective curve matching with known degrees of confidence (21). Brown determined the absorptivities at 21 frequencies and compared Absor privities of U n k n o w n
Absorptivities of K n o w n
«/
Ν Ratios
!1 !
I Log-Patios
·,'•£>
i Average l o t } Ratio
Î3)
i ί oy Ratios
Average Lou Ratio I
i4?
*
List N o of R a t i o s W i t h i n 5. 1 0 2 5 % o f A v e r a g e
(5>
Figure 1 . Outline of c o m p u t e r m e t h o d for r a t i o i n g a b s o r p t i v i t i e s of a n oil f r o m a n u n k n o w n s o u r c e to absorptivities of all oils s t o r e d in c o m p u t e r data bank
peak for peak between curves. An out line of his "log-ratio" method is shown in Figure 1. An advantage of this ra tioing technique is its automatic com pensation for possible differences in cell pathlength. Figure 2, a histogram for a comparison of two oils by this method, shows clearly the magnitude of changes, with weathering, in the
Figure 2. Histogram of weathered (7 days) vs. unweathered crude oil (#177) as obtained by log-ratio method
various IR peaks examined for a crude oil. Brown et al. analyzed about 950 weathered oil spectra among over 1200 oil spectra. Fluorescence Spectroscopy Fluorescence spectroscopy, a powerful tool in fingerprinting oil spills, is extremely sensitive to the naturally fluorescing components of oil and to additives such as tank cleaners which might be difficult to detect by other means. Many of the naturally fluorescing materials present, such as the polynuclear aromatic hydrocarbons, are not particularly volatile, nor soluble, so that fluorescence spectra are relatively stable to weathering. In a brief survey of fluorescence applied to water pollution studies, Anacreon pointed out that water does not interfere and that all emulsions with water may be readily treated with cyclohexane to extract the oil. It is also possible to measure emulsions or dissolved oil directly. Parker et al. (1960) were among the first to show the feasibility of fluorescence for quantitative oil measurements and characterization. A spectrofluorometer can be used in a variety of ways. At ambient temperatures, the fluorescence can be examined at one or more excitation wavelengths, or the excitation curve can be determined at a given fluorescence wavelength. All permutations of emission intensity as a function of both excitation and emission wavelengths yield a three-dimensional response surface which can be visualized as a small mountain range. To depict the shape in two dimensions, Freegard et al. (1971) proposed a contour plot in which contours of equal fluorescence intensity are plotted against excitation and emission wavenumbers. [See review by Adlard (22).] Hornig et al. (23, 24) are pursuing this procedure in which they repetitively scan with a spectrofluorometer to generate luminescence data which are taken directly into a minicomputer
for correction and reduction to contour graphs (Figure 3). [These data were uncorrected and, as a result, the energy (excitation and emission) is low at the short and long wavelengths.] Frank (25) proposed an approach which can be carried out manually (without recording the spectrum) in a relatively short time. The emission peak maxima from excitation at 15 wavelengths between 220-500 nm (in 20-nm intervals) are plotted vs. excitation wavelengths as points. He introduced a novel presentation of the data by connecting the isolated data points with straight lines to derive silhouette profiles used to compare oils. Simpler yet is the emission spectrum at a fixed excitation wavelength. Thruston and Knight (1971) excited at 340 nm, measured the emission at 386 and 440 nm, and calculated the ratio at three concentrations (100, 50, and 10 ppm) to yield three ratios. Freegarde et al. (1971), in addition to their contour plots, analyzed four
crude oils using 250 nm as their excitation wavelength. Coakley (26) excited oil in cyclohexane (1000 ppm) at 295 nm, applied his technique to two oil spills, and found his results in agreement with those obtained by GC. Jadamec examined a large variety of oils for their emission at various exciting wavelengths. Confirming the maximum absorption at 238 nm as reported by Parker, he found a secondary absorption maximum at 255 nm for all oils screened but found little or no absorption at 286 nm for some oils screened. By selection of the 254-nm region as a possible excitation wavelength for all petroleum samples, he found that it gave more structured emission spectra than 290 nm (Figure 4). Extension of the use of excitation at 254 nm to over 200 varieties of crude and refined petroleum oils and its application to eight spill cases was reported in 1974 (27). Since then, the U.S. Coast Guard has employed this technique in the laboratory and field (Captain of the Port Offices) in several hundred spill cases along with other techniques. Two techniques can be used to enhance this already sensitive method: low-temperature analysis and doublebeam difference spectrofluorometry. The low-temperature approach was carried out by Hornig and Eastwood under contract to EPA and was reported at the Pacific Conference on Chemistry and Spectroscopy (1972) [see the review of oil identification methods by Gruenfeld (28)]. Hornig et al. (24) and Fortier and Eastwood (29) are continuing to examine the advantages of low-temperature (77 K) luminescence (phosphorescence and fluorescence). The analysis is carried
Figure 3. Computer-generated contour plot of total luminescence spectrum from Louisiana crude oil (100 ppm in methylcyclohexane). Effective bandwidth: 10 nm; contour interval: 50 Courtesy of A. W. Hornig, Baird-Atomic
462 A · ANALYTICAL CHEMISTRY, VOL. 48, NO. 6, MAY 1976
out in methylcyclohexane at 77 Κ with the advantage of increased spectral structure which in certain instances can distinguish closely similar oils. Eastwood and Hanks (30) examined the low-temperature phosphorescence alone and successfully applied it to real-world oil spills. Double-beam spectrofluorometers are just becoming available. Two of these were evaluated by Sheridan and Jadamec (31). Twenty-five represen tative fuel oil types were examined. In the No. 2 fuel oil class, 30 different samples were compared. Several opti mum operating parameters were the same as those previously found for single-beam spectra, e.g., an excitation wavelength of 254 nm, with the excita tion/emission bandpass being large and the emission bandpass narrow. Oil type determines optimum concentra tion: heavy fuel oils and crudes
