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Anal. Chem. 1986, 58,834-036
Arson Analysis by Second Derivative Ultraviolet Spectrometry Larie Meal Chemical Technology Department, University of Cincinnati, Cincinnati, Ohio 45210
Second derivative uitravloiet spectrometry is an excellent method for arson analysls. The second derivative spectra of accelerants are relatlvely uncompilcated even In the case of many-component mixtures such as gasoline. This simpliclty allows good vlsual comparlsons of standards to samples and generally facliltates accelerant ldentlflcatlon. I n addition, the spectra of weathered and unweathered accelerants do not differ greatly. The flre debris (matrix materials) are extracted with cyclohexane. Few matrlx materials give second derlvatlve spectra when treated In this manner. Those that do, give spectra that do not complicate interpretation or lead to false positlve results. The method is both sensitive and rapid.
Gas chromatography is the analytical technique preferred by many arson analysts. Although this technique has proven to be reasonably successful, there are still attendant problems. The complexity of the chromatograms sometimes makes visual examination and comparison to standard chromatograms unreliable (I). This is particularly serious when petroleum products have been used as accelerants (2). In addition, chromatograms of weathered samples may appear quite different from standard, unweathered samples. This may render exact identification impossible (2). In other instances interpretation is not possible at all. For example, nonpetroleum products contain few components and do not give chromatograms with enough peaks to establish a pattern (1). The method can be time-consuming. Retention times are often long and it is sometimes necessary to use more than one type of column (1).It is a particularly lengthy process when tandem methods are used (GC/MS or GC/IR). Background peaks from matrix materials (wood, carpet, etc.) complicate the interpretation of the chromatogram (I).This seriously limits the methods that can be used for removal of the accelerant from the matrix material. The most efficient method for accelerant recovery is solvent extraction ( 3 ) . However, the appearance of background or contaminant peaks often precludes the use of this method. Background peaks can interfere even when headspace samples are taken. Second derivative ultraviolet-visible spectrometry has been established as a valuable qualitative method in some areas of forensic chemistry (4-6).As long as recording parameters were kept constant, good comparisons were obtained between standard spectra and the sample spectra. This paper will show that second derivative ultraviolet spectrometry is an excellent alternative method for arson analysis. Most of the problems encountered in using GLC may be eliminated or minimized through the use of this method.
EXPERIMENTAL SECTION Reagents. Cyclohexane was obtained from Eastman (Spectro ACS). The accelerants studied were commercial products. Their brand names are identified in the captions accompanying the figures. Apparatus. Spectral data were recorded with a Pye-Unicam, Model 8-100,recording UV-vis spectrophotometer with first and second derivative accessory. Quartz cells of 1-cmpath length were used. The spectra were obtained with the following instrumental
parameters: bandwidth, 1 nm; wavelength speed, 1 nm/s; chart speed, 5 s/cm; absorbance, 2. The derivative accessory was set for second derivative measurement at gain 3. Procedure. Cyclohexane was used as solvent for all scans. Solvent extraction with cyclohexane was used for recording spectra of matrix materials. These materials were soaked in cyclohexane for 1min or less with the exception of soil, concrete, and heavily charred wood, which were soaked for about 12 h. Concentrations of accelerants used in running standard accelerant spectra were approximately 0.1 mg/L. Accelerants were weathered in two ways: to simulate weathering due to exposure at normal temperatures over a period of time and to simulate weathering from subjection to heat. The former was accomplished by saturating a tissue with accelerant and allowing it to stand at room temperature for various lengths of time. For the latter, a tissue was moistened with accelerant, ignited, and extinguished with water after varying time intervals. This same procedure was used to obtain spectra of accelerants from other matrix materials. The ultraviolet region was scanned from 245 to 320 nm in all cases.
RESULTS AND DISCUSSION Many products that are commonly used as accelerants contain significant concentrations of aromatic hydrocarbons. Gasoline, kerosene, fuel oil, diesel fuel, charcoal lighter fluid, Coleman fuel, many paint and lacquer thinners, lubricating oils, and mineral spirits are among these. Each of the listed products gives a unique and easily recognizable second derivative ultraviolet spectrum of the mixture of aromatic compounds present. Figures 1 and 2 show the second derivative spectra of some selected products. Diesel fuel and fuel oil no. 2 are easily distinguished from the others by noting the strong minimum at 251 nm and strong maximum at 261 nm. Diesel fuel can be differentiated from fuel oil no. 2 by the minimum at 263 nm and the maximum at 264 nm, which appear in the diesel fuel spectrum only. Diesel fuel, fuel oil no. 2, and kerosene all show a strong minimum at 276 nm, which none of the other products have. However, kerosene lacks the strong minimum at 251 nm and the strong maximum at 261 nm and can easily be distinguished from diesel fuel and fuel oil no. 2. The mineral spirits spectrum shows a strong minimum at 274 nm and a prominent maximum a t 272 nm. Neither the gasoline nor the paint thinner spectra have these characteristics. Paint thinner cannot be confused with gasoline because paint thinner has fewer maxima and minima in the 300-275 nm region and those between 260 and 250 nm are at different wavelengths from those of gasoline. There are some maxima and minima common to all the spectra, but their ratios are different in each spectrum. Of the approximately 150 accelerants studied, none showed identical spectra-all were distinguishable from the others. In addition, spectra of the same accelerant but from different manufacturers showed little variation. Many were identical, while others presented only slight differences in the ratios of the maxima and minima. The wavelengths at which the peaks occurred did not vary. In using this technique, identification is made from the spectra of only the higher boiling aromatic fractions of each accelerant. The lower boiling components are not of concern. Since these spectra are later used as standards for comparison, this offers two advantages. Firstly, the spectra are relatively uncomplicated, Secondly, the higher boiling fractions are
0 1986 American Chemical Society 0003-2700/86/0358-0834$01.50/0
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Figure 5. (A) Uncharred wood. (6) Uncharred plywood. (C) Uncharred particle board. I
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Figure 1. (A) Gasoline standard, Sohio Oil regular. (6) Kerosene standard, Gulf Oil 1K. (C) Fuel oil no. 2 standard, Este Oil. B
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Figure 3. (A) Gasoline, weathered, air, Sohlo Oil regular. (6) Gasoline, weathered, heat, Sohlo Oil regular. (C) Kerosene, weathered, air, Gulf Oil 1K. A
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Figure 4. (A) Diesel fuel, weathered, air, Sohio Oil. (6) Paint thinner, weathered, air, Cortland Specialty Co. usually all that persist after weathering due to heat, fire, or exposure to air. Therefore, the standard spectra and the spectra of weathered accelerants are not significantly different (Figures 3 and 4). Furthermore, weathering of the aromatic fractions due to heat or fire is very similar to weathering from exposure to air. This can be noted from Figure 3A,B for gasoline and was also found to be the case with the other accelerants mentioned. Therefore, it is relatively easy to make visual comparisons of standard spectra to spectra of samples weathered in either manner. Both types of spectra may be used as standards. Spectra of unweathered accelerant are used when liquid samples are submitted. Spectra of weathered products are compared to spectra of samples removed from fire debris.
Any method can be utilized in removing the accelerant from the debris or matrix material. The most efficient method, and the one used in this laboratory, is solvent wash or extraction (3). This technique involves soaking the debris for various lengths of time in solvent. Cyclohexane was used in this work. Most samples require soaking times of less than 60 s. The only exceptions to this are soil, concrete, and heavily charred wood. These materials are soaked at least 12 h to ensure maximum removal of accelerant. If high concentrations of accelerant are present, much shorter soaking times are required. Turbid solutions, which sometimes result, are not a problem in derivative spectrometry. Absorbance due to turbidity gradually increases as the wavelength decreases. This gradual change in absorbance does not cause any significant change in the derivative spectrum and is essentially eliminated. The appearance of background peaks is not a limitation with most matrix materials, burned or unburned. The second derivative UV spectra of the following show no peaks in the 320-245 nm range: concrete, charred wood, cardboard, metal, glass, ceramic tile, fiberglass insulation, drywall, marble, cotton, orlon, wool, kapok, nylon, paper without print, and anything that is completely charred. Soil gives no peaks as long as it has not been treated with pesticides in an aromatic hydrocarbon solvent. Rubber from tires does not produce background peaks when the soaking time is kept under 10 s. Some matrix materials exhibit peaks in the 320-245 nm region, but they do not interfere and cannot be confused with accelerant peaks. Standard spectra of these are recorded for future comparison. These materials include uncharred wood, uncharred plywood, and uncharred particle board (Figure 5). It is apparent that these spectra show no similarities to accelerant spectra. Except for a minimum at 274 nm, which also appears on the weathered gasoline and the mineral spirits spectra, the wood products have no peaks that are common to accelerant peaks. These wood extract spectra are typical of the approximately 3000 that have been recorded in this laboratory. Some, but not all, uncharred carpet and foam padding, are the only materials which showed several peaks that corresponded with accelerant peaks. These are the minima at 266, 263,260, and 251 nm and the maxima at 268, 264, and 261 nm (Figure 6). However, this does not prevent the detection and identification of accelerants. The accelerant spectra shown in Figures 1-4 and all others that have been recorded (about 150) have strong peaks at wavelengths higher than 268 nm. Minima occur at 285,281,271 nm and either 276 or 274 nm in all accelerants containing aromatic hydrocarbons with
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Figure 9. (A) Kerosene, Gulf Oil l K , from charred carpet. (B) Kerosene, Gulf Oil l K , from partly charred wood. (C) Kerosene, Gulf Oil l K , from concrete.
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Figure 8. (A) White rum, Bacardi. (B) Whiskey, Windsor.
maxima occurring at 282 and 278 nm. These peaks are unique to accelerants and have not been noted in the spectra of any matrix materials. Weaker maxima and minima at wavelengths above 285 nm are also used to distinguish accelerants from matrix materials. It should be mentioned that the appearance of the carpet and padding extract peaks between 268 and 251 nm may make it impossible to distinguish fuel oil from diesel fuel. Vegetable oils also give second derivative UV spectra (Figure 7A). These are quite different from those of accelerants containing mixtures of aromatic hydrocarbons. Furthermore, they show no similarity to spectra of matrix materials. The vegetable oil absorption is probably due to the presence of oxidation products. Ketones also show second derivative spectra (7), which differ markedly from other ac-
celerant spectra and matrix spectra. Turpentine gives a second derivative UV spectrum only at high concentrations (1mg/L). This is probably due to the presence of a low concentration of p-cymene. The spectrum (Figure 7B) does not resemble the spectra of other accelerants or matrix materials. Alcohols, of course, cannot be detected by UV spectrometry. Whiskey and rum, however, do give distinctive spectra (Figure 8). The spectra of accelerants removed from matrix materials are essentially identical with the standard spectra of weathered accelerants. The only differences arise from the appearance of matrix peaks from those particular materials that were discussed previously. Figure 9 shows spectra of kerosene that has been extracted from charred carpet, partly charred wood, and concrete. These materials were moistened with kerosene, ignited, and extinguished with water prior to extraction. Comparison of these spectra to Figure 3C demonstrates that the accelerant spectrum is not altered by the matrix material. This was found to be the case with all accelerants and all matrixes that have been studied. Second derivative spectrometry offers a relatively simple, sensitive, and inexpensive method for arson analysis. Background peaks do not complicate interpretation or lead to false positive results.
LITERATURE CITED (1) Camp, M. J. Anal. Chem. 1980, 52, 422A-426A. (2) DeHaan, John D. "Kirk's Fire Investigation", 2nd ed.; Wiley: New York, 1983; Chapter 14. (3) Davles, Geoffrey "Forensic Science", 1st ed.; American Chemical Society: Washington, DC, 1975; Chapter 12. (4) Gill, R.; Bai, T. S.;Moffat, A. C. J . Forensic Sci. SOC. 1982. 22 (2), 165-17 1. (5) Lawrence, A. H.; MacNeil, J. D. Anal. Chem. 1982, 54, 2385-2387. (6) Lopez, A,; Mazzeo, P.; Quaglia, M. G.; Segnalini, F. Farmaco, Ed. R a t . 1982, 37(11), 371-376. (7) Meal, L. Anal. Chem. 1983, 55, 2448-2450.
RECEIVED for review August 29, 1985. Accepted November 12, 1985.
