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ANALYTICAL CHEMISTRY, VOL. 50, NO. 2, FEBRUARY 1978
Identification of Fuel Oils by Low Temperature Luminescence Spectrometry Scott H. Fortier and DeLyle Eastwood" U.S. Coast Guard Research and Development Center, Avery Point, Groton, Connecticut 06340
A low temperature luminescence method for identification of fuel oils was developed after optimization of experimental parameters such as solvent, sample concentration, excitation wavelength, etc. Fluorescence Instrumentation, both corrected and uncorrected, were compared. Typical emission peaks for representative fuel oils excited at 254 nm are tabulated for classification information. Low temperature luminescence spectra showed more structure than room temperature fluorescence spectra, thereby enhancing oil identification capabilities.
Low temperature luminescence of petroleum oils for identification purposes is still a relatively little explored area. Studies by previous investigators (1-3) have shown that room temperature fluorescence is a rapid and accepted analytical technique for oil identification. Earlier low temperature measurements by Parker (2) and Hornig e t al. ( 4 , 5 )showed that the increased spectral structure found a t liquid nitrogen temperatures is useful in distinguishing among closely similar oils. Also, a t 77 K, one may see phosphorescence as well as fluorescence. In the present study, petroleum fuel oils were analyzed by low temperature luminescence and the resulting procedure was optimized with respect t o experimental parameters. Preliminary application of this procedure has shown it to he a useful tool for identifying the source of an oil spill by direct comparison of spectra of spill samples with those from suspected sources. Typical wavelengths for emission peaks for representative fuel oils of each class on excitation a t 254 nm are tabulated for classification purposes.
EXPERIMENTAL Apparatus. A Baird-Atomic Fluorispec SF-100 fluorescence spectrophotometer was used to generate the majority of the spectra for this study. This instrument has double monochromators on both the excitation and emission sides, with the excitation and emission gratings blazed at 300 nm and 500 nm, respectively. A Hamamatsu R136 photomultiplier operated at 800 V was used as the detector and a Houston Instrument Model 2000 X-Y recorder was employed for graphic display. Spectra were also generated on a Perkin-Elmer MPF-4 fluorescence spectrophotometer. This instrument is equipped with a single monochromator blazed at 300 nm on each side, a Hamamatsu 446-UR photomultiplier as detector, and a corrected spectra accessory. A Perkin-Elmer Model 56 strip chart recorder was used with this instrument. Spectra generated on both the Baird-Atomic and the Perkin-Elmer instruments are uncorrected unless stated otherwise. Suprasil quartz sample tubes and Dewars were obtained from Baird-Atomic because they were of superior quality in having a low fluorescence background. A special holder was designed to adapt the Dewars to the Perkin-Elmer instrument. A Baird-Atomic phosphoroscope was used with the Fluorispec to produce a few typical phosphorescence spectra. Selection of Oils for Study. Fuel oils were chosen as representative of each class on the basis of room temperature fluorescence and infrared spectra of over 150 fuel oils which had 0003-2700/78/0350-0334$0 1.OO/O
been previously analyzed at the Coast Guard Research and Development Center. These specially selected representative oils included 19 fuel oils which were broken down as follows: four No. 1, four No. 2, five No. 4, three No. 5, and three No. 6 fuel oils. One No. 2 fuel oil was chosen for an exhaustive study on two different spectrofluorometers, varying parameters such as slit width, wavelengths, sample concentration, and solvent. After optimum experimental conditions were established, all other oils were analyzed using these parameters. Selection of Solvents for Study. Solvents evaluated included: cyclohexane, methylcyclohexane, and hexane (all spectroquality from Matheson, Coleman and Bell (MCB); heptane (spectroquality from Mallinckrodt); octane (practical grade from MCB); and tetrahydrofuran and 2-methylpentane (chromatoquality from MCB). When methylcyclohexane, cyclohexane, and heptane were used as solvents, spectra with good structure were obtained, whereas spectra of oil solutions in tetrahydrofuran showed the least structure because of the high polarity of this compound. Methylcyclohexane was chosen as the most suitable solvent because it forms a clear glass at liquid nitrogen temperatures and was found to give more reproducible spectra than either heptane or cyclohexane, which form snows. Selection of Concentration. Concentration effects with respect to changes in spectral shape for emission spectra were studied over a range from 100 to 1 ppm, and 10 ppm was selected as the standard concentration. This concentration represents a reasonable compromise in that sharp spectral structure can be achieved and reabsorption effects minimized while obtaining a good signal to noise ratio with no appreciable interference from solvent impurities. Procedure. Oil solutions were prepared at 10 A 0.3 ppm (w/w) in spectroquality methylcyclohexane in low-actinicglass volumetric flasks. An aliquot of the solution in a sample tube was lowered slowly (-30 s) into the Dewar containing liquid nitrogen. This avoids strain to the organic glass and ensures good spectral reproducibility. Instrumental Conditions. Slit widths used, unless otherwise stated, were 6 nm for the stationary monochromator and 4 nm for the scanning monochromator. Wavelengths used for excitation were 254, 290, and 340 nm, although Hornig et al. ( 5 ) as well as the present investigators checked other wavelengths. A Corning 0-52 (Corning glass No. 7380) filter, with cut-off below 360 nm, was inserted at 480 nm (readjusting gain to compensate for filter absorption) during those spectral scans when the excitation was 254 or 290 nm in order to eliminate second-order light interference. All oil solutions were used in equilibrium with the atmosphere, although other experimenters (6) have indicated that degassing can increase spectral intensity and structure for oil solutions at low temperature. Degassing would increase the complexity of the method and therefore would affect reproducibility for matching purposes because of required additional sample handling.
RESULTS AND DISCUSSION The value of low temperature luminescence as an analytical tool is illustrated in Figure 1 which shows a comparison between the room temperature and low temperature emission spectra of a No. 2 fuel oil excited a t 254 nm. The room temperature spectrum exhibits a single broad peak (- 355 nm) while the low temperature emission spectrum for the same oil shows much sharper spectral structure and the emission extends t o longer wavelengths because of phosphorescence. C 1978 American Chemical Society
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254 nm. Figure 3 depicts a spectrum of a No. 1 fuel oil and Figures 4,5 , and 6 show spectral comparisons of two No. 4, two No. 5, and two No. 6 fuel oils, respectively. For each class, the spectra shown are typical of the class and represent the limits of spectral variability within the class. For No. 1 fuel oils, relatively little variability occurs and therefore only one example was included. Progressing from the lighter to the heavier fuel oils, the changes in the spectra are readily apparent. The spectrum of a No. 6 fuel oil, for example, is considerably different from a No. 2 fuel oil excited at the same wavelength and, in general, is shifted to longer wavelengths. Even for a No. 6 fuel oil, sometimes difficult to identify by
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ANALYTICAL CHEMISTRY, VOL. 50, NO. 2, FEBRUARY 1978
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Table I. Characteristic Luminescence Peaks for Fuel Oil Types a t 254 nm Excitationa'b Fuel oil N o . studied Wavelengths No. 1 5 326, 339, 369, 384, 407 No. 2 6 328. 341. 353. 369. 388. 410. 432. 465, 496 ' No. 4 6 356. 372.' 384.' 435. 465 No. 5 5 380: 390; 418,433, 468, 482,494 No. 6 5 434, 465, 497' a Wavelength reproducibility is i 1 nm. Wavelengths of all peaks are i.2 nm except where noted otherwise. The wavelength variability represents the maximum variWavelengths of these peaks are ability within a class. *3 nm.
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room temperature fluorescence, there is considerable structure to characterize the oil. Varying Excitation Wavelength. Examples of the effect of varying excitation wavelength on spectra of different fuel oil types are shown in Figures 7-10; Figures 7 and 8 for two different No. 2 fuel oils, Figure 9 for a No. 4 fuel oil, and Figure 10 for a No. 6 fuel oil. Useful information can be obtained a t all three wavelengths (254, 290, and 340 nm) and, by utilizing selected wavelengths for excitation, advantages may be gained in terms of avoiding interference from impurities or in displaying only certain peaks for greater contrast. Excitation a t 254 nm excited a larger proportion of aromatic
hydrocarbons and produced more structured emission. Wavelengths Corresponding t o Characteristic Luminescence Peaks for Fuel Oil Types. Data on the spectra of 27 fuel oils of various types are summarized in Table I, which lists the wavelengths a t which principal peaks occur on excitation a t 254 nm. Although the primary purpose of this paper is to present a technique useful for identification, Table I lists data which are useful for classifying fuel oils. Table I was constructed using spectral data from the following fuel oils: five No. 1; six No. 2, six No. 4, five No. 5 and five No. 6. For each class of oils, Table I indicates the wavelengths corresponding to major peaks common to all oils within a class as well as the maximum variation for each peak position. Therefore, additional peaks which were found only for some oils within a class are not included. The wavelengths are grouped in a pattern suggesting common classes of compounds in the oils. Minor shifts (e.g. between No. 1 and No. 2 fuel oils 339 to 341 nm or 384 to 388 nm) could possibly be ascribed to different luminescent backgrounds or to different members of the same class of compounds. Additional information useful for identification is supplied by the peak ratios, additional structure not common to all oils within a class and differences in peak shapes or luminescence envelopes. Ranges of peak ratios could also have been tabulated, but for uncorrected spectral data would have been of value only to those using the same instrumentation. Data generated on 17 fuel oils excited at 290 nm (not tabulated) showed that several, but not all, of the same peaks appear. For example, for a No. 2 fuel oil excited a t 290 nm, peaks were observed only a t 320, 342, and 353 nm (Figures 7 and 8 and Table I). In addition, excitation a t 290 nm produced peaks which would not be characteristic of that fuel oil type on excitation a t 254 nm. A No. 4 fuel oil, when excited at 290 nm exhibited peaks a t 331 and 343 nm which were not
ANALYTICAL CHEMISTRY, VOL. 50, NO. 2, FEBRUARY 1978
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characteristic of this oil type a t 254 nm excitation (Figure 9 and Table I). The wavelengths for prominent peaks can be correlated with aromatic ring systems as comparisons with spectra obtained from model ring systems show. For example, the peak at 326 f 2 nm for a No. 1 fuel seems to be attributable to naphthalene-type compounds ( 7 , 8 ) ,and the peak at 384 i 3 nm may be ascribed to pyrene-type compounds ( 7 ) . Phosphorescence. Although most measurements were made as luminescence (fluorescence and phosphorescence), some measurements were needed to determine the phosphorescence separately. These phosphorescence measurements were made using a delay time of approximately 1 ms. Varying the delay time over the available range 0.5-30 ms (9) did not change the shape of the spectra appreciably, which is reasonable since many aromatic compounds have lifetimes of a t least 0.1 s. Figure 11 shows the low temperature luminescence spectrum of a No. 2 fuel oil as well as the phosphorescence spectrum of the same oil. Recording the phosphorescence spectrum required the use of wider slits for instrumental reasons. I n s t r u m e n t a l Comparison. Figure 12 compares uncorrected emission spectra generated on instrumentation from different manufacturers. The uncorrected low temperature emission spectrum of a No. 2 fuel oil generated on the double monochromator Baird-Atomic Fluorispec is contrasted with the corresponding uncorrected spectrum obtained on the single
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monochromator Perkin-Elmer MPF-4. The detected signal appeared weaker on the MPF-4 and therefore a wider excitation slit width (16 nm) was used. The spectra differ partly because the excitation slits are different and partly because use of a filter (Spectro-Film Incorporated with cut-off a t 265 nm) was necessary to eliminate first-order scatter. Also, the Perkin-Elmer grating is blazed at 300 nm while the gratings in the Baird-Atomic double monochromator are both blazed a t 500 nm, which makes the peaks near 500 nm relatively much stronger as measured on the Baird instrument. Considerably more structure was obtained on the Fluorispec, partly because of the difference in the excitation slits. For low temperature measurements, the double monochromators in the Baird proved of value in reducing scatter; more scatter was noticed with the Perkin-Elmer MPF-4 with single monochromators. Excitation Spectra. In addition to luminescence emission spectra, luminescence excitation spectra were also obtained detecting a t major emission peaks. In some cases, closely similar oils could be more easily distinguished by examining excitation spectra. To compare corrected and uncorrected spectra, and to compare instruments further, excitation spectra were generated on the Baird-Atomic Fluorispec and the Perkin-Elmer MPF-4. This is illustrated in Figure 13 for a No. 2 fuel oil. The apparent rapid drop-off in intensity at shorter wavelengths, observed for uncorrected spectra run on both instruments, is an instrumental artifact, especially noticeable for a double monochromator instrument such as the Fluorispec. The relative intensity of the short wavelength peak (-230 nm) increases considerably in the corrected spectrum obtained with the MPF-4. Spectral Correction-Corrected vs. Uncorrected Ins t r u m e n t s . For all spectrofluorometers, neither the source/excitation monochromator combination nor the emission monochromator/photomultiplier combination is flat in spectral response. Therefore, for uncorrected instruments, relative luminescence intensity is not independent of wavelength and the correct peak ratios are not obtained. As long as all data are taken on the same instrument and under the same experimental conditions, uncorrected spectra may be compared to identify oils. For corrected spectrofluorometers, at least in principle, it should be possible to compare spectral data and peak ratios taken on different instruments. Useful information may be overlooked by using uncorrected spectra, principally on the short wavelength end of excitation spectra and on the long wavelength side of emission spectra.
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By correction, the intensities in these spectral regions are amplified to produce the correct relative intensity of all of the fluorescence peaks, which allows more accurate comparison of peak ratios. Also, in comparing data taken on different instruments, results may be incompatible unless suitably corrected spectra are used. Spectrally corrected spectrofluorometers are now commercially available. Even with corrected instruments, however, unless adequately dilute solutions (-0.05 p\ a t excitation wavelength) are used, geometric effects may prevent the comparison of spectra run on different instruments. Identification Procedure. Presently, identification is made by using visual overlays to compare spectra. Two samples may be considered to have originated from a common source if their emission spectra match to within f4% of full scale over the spectral range from 300 to 600 nm. In-house weathering studies, which will be reported later, showed that light oils weathered more than two days or heavy oils weathered more than one week fall outside of these tolerances. Figure 14 shows an example of oil identification by this low temperature luminescence method taken from a real-world spill case. Using the criteria stated above, the weathered spill sample of No. 2 fuel oil was compared to two possible source samples: one of which proved to be a definite match (both by physical evidence and by experimental results from other analytical methods such as infrared spectroscopy and gas and thin-layer chromatography) and the other a non-match.
CONCLUSION Fuel oil identification capabilities have been enhanced by the use of low temperature luminescence. A considerable increase in spectral structure is obtained by low temperature luminescence for fuel oils at 77 K as compared to conventional room temperature fluorescence. At room temperature, fuel oils of a given type may yield similar broad and featureless signatures with only minor spectral differences which could
be confused by inexperienced operators. These subtle spectral differences are enhanced and sharpened in low temperature luminescence spectra. The sharp structure observed with low temperature luminescence is invaluable when comparing spectra of spilled fuel oil and suspect source samples to identify the correct origin of an oil spill. The rigid matrix formed at liquid nitrogen temperature is responsible for the spectral sharpening ( 2 , 1 0 , I I )because of a lack of collisional broadening and reduction of other broadening and quenching mechanisms. Optimum conditions determined for the low temperature luminescence method were excitation at 254 nm, methylcyclohexane as solvent, and 10 ppm as a standard concentration. If all measurements are performed on the same instrument under identical experimental conditions, both corrected and uncorrected instrumentation can be used. The low temperature luminescence method described in this paper is presently being used as an auxiliary technique for oil identification. I t is part of a multimethod approach for oil spill investigations which includes room temperature fluorescence and infrared spectroscopy as well as gas and thin-layer chromatography and is most successfully employed for oil identification dealing with closely similar fuel oils. The low temperature luminescence method extends the power of room temperature fluorescence where uncertainty occurs in fuel oil identifications. I t is also evident, from the data presented, that spectral patterns exist which can be correlated with fuel oil type and may therefore be utilized to classify these oils.
ACKNOWLEDGMENT The authors thank Arthur W. Hornig for stimulating technical discussions.
LITERATURE CITED "Oil Spill Identification System", Chemistry Branch-US. Coast Guard Research and Development Center, Report No. CGD-4 1-75, October 1974. (Available to the public through the National Technical Information Service, Springfield, Va. 22151. No. ADA003803.) C. A. Parker, "Photoluminescence of Solutions", Elsevier Publishing Company, New York, N.Y., 1968. A. D. Thruston and R. W. Knight, Environ. Sci. Techno/., 5 , 64 (1971). A. W. Hornig and D. Eastwood, "Development of a Low Temperature Molecular Emission Method For Oils", Program No. 16020GBW prepared for EPA Water Quality Office by Baird-Atomic, Inc., 15 October 1971. A. W. Hornig, D. Eastwood, and J. Guiifoyie, "Development of a Low Temperature Molecular Emission Method For Oils". Program No. 16020GBW DreDared for EPA Water Qualii Office by Baird-Atomic, Inc., 18 August 197'1. A. W. Hornig. D. Eastwood, J. Guilfoyle, and F. Kawahara, "Molecular Luminescence Studies of Petroleum Oils at 77 OK", presented at the Pacific Conference on Chemistrv and SDectroscoDv. . . San Francisco. Calif. 16-20 October 1972. J. F. McKay and D. R. Latham, "Fluorescence Spectroscopy in the Characterization of High-Boiling Petroleurn Distillates", presented at the 163rd National Meeting of the American Chemical Society, Boston, Mass., April 1972. I. B. Betiman, "Handbook of Fluorescence Spectra of Aromatic Molecules", Academic Press, New York, N.Y.. 1971 J. J. Hanks, "Oil Fingerprinting by Phosphorescence-A Feasibility Study", Academy Scholar's Report-US. Coast Guard Academy, 13 May 1975. J. B. Birks, "Photophysics of Aromatic Molecules", Wiiey-Interscience, New York, N.Y. 1970. S.P. McGiynn, T. Azumi, and M. Kinoshita. "Molecular Spectroscopy of the Triplet State", Prentice-Hall. Inc., Engiewood Cliffs, N.J., 1969.
RECEIVED for review May 16, 1977. Accepted November 21, 1977.
