In the Laboratory
Who Set the Fire? Determination of Arson Accelerants by GC–MS in an Instrumental Methods Course
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David A. Sodeman and Sheri J. Lillard* Department of Chemistry, University of California, Riverside, Riverside, CA 92521; *
[email protected] Rationale To revitalize our Instrumental Methods laboratory, we combined forensic-based experiments with the traditional instrumentation, to achieve real-world measurements that increase student enthusiasm (1). At the University of South Carolina, Brewer et al. have taught a course on forensic analytical chemistry as a means to increase student participation in the chemistry lecture (2). At Williams College, Kaplan has developed a course for non-science majors, which incorporates forensic topics into both the lecture and the laboratory (3). Forensic experiments utilizing GC with flame ionization detection (GC–FID) have been used by Elderd et al. in the general chemistry laboratory for accelerant identification (4 ) and by Zabzdyr and Lillard in the instrumental laboratory for determination of blood alcohol content (5). Forensic-based experiments are relatively easy to incorporate into existing laboratory formats and provide an opportunity to teach students the implications of incorrect determinations (i.e., someone could be sent to jail). This approach is designed to ensure that the students perform correct analytical procedures and carefully interpret their data. This experiment simulates the steps an arson investigator would take to determine if arson was the cause of a fire. Charred samples are collected in sealed containers and presented to the students, along with five known accelerants, for analysis by headspace gas chromatography with mass spectrometric detection (GC–MS). Students analyze the standards and charred samples and, using retention times and MS data, determine which accelerant was used to start the fire. The experiment, intended for chemistry and chemical engineering majors in their junior and senior years, introduces students to (i) headspace sampling for a sample collected from an “arson” scene, (ii) chromatography, and (iii) mass spectrometry. Although other forensic experiments utilize GC (4, 5), this one gives the students hands-on experience with temperature programming in GC, introduces them to mass spectrometry, and gives them practice with deciphering and interpreting mass spectra. Incorporating mass spectrometry into the Instrumental Methods laboratory is extremely important in teaching students how to use common analytical instruments for real-world applications. Materials and Methods
Equipment A Hewlett Packard 5890 Series II gas chromatograph (GC) with a DB-2887 column and a quadrupole mass spectrometer were employed for this experiment. The following temperature program was used throughout. The column temperature was initially set at 30 °C and held for 1 min. The temperature was then ramped at 7 °C per min until the column temperature reached 80 °C. At this time, a second ramp of 35 °C per min was used until the column temperature 1228
reached 250 °C. The column temperature was held at 250 °C for 1 min, for a total run time of 13.3 min. The MS ionization potential was 70 eV. To increase the quadrupole sampling rate, no scans were performed for mass-to-charge ratios (m/z) below 40. The GC and MS were controlled by a Hewlett Packard computer, which also stored the data and was used for data analysis (using Chemstation software).
Reagents Samples of Kingsford charcoal lighter, Restore & Restyle (candle and lamp oil), and Klean Strip (lacquer thinner) were obtained from local vendors and used as received. Mobil and Chevron gasoline were obtained from local Mobil and Chevron gas stations, respectively, and used as received. Wood chips were obtained from the chemistry department machine shop. Procedure This procedure was adapted from that given by Elderd et al. (4 ). The standard liquid accelerants were placed in 25-mL Erlenmeyer flasks and capped with rubber septa. Wood pieces were cut small enough to fit into 25-mL Erlenmeyer flasks. Several wood chips were placed in the flasks and enough accelerant was added to completely cover the chips. The flasks were capped with rubber septa and the chips were left to soak for ∼ 4–6 h. The wood samples were charred in the following manner. The liquid accelerants were poured out of the flasks and disposed of in a waste container. In a hood, the wood chips were ignited with a Bunsen burner and placed in a glass container to burn. After most of the chips’ surface was burned, the fire was extinguished by blowing it out and the wood was placed back into the Erlenmeyer flask, which was immediately capped with a rubber septum. Chromatograms of the standard accelerants were obtained via headspace sampling. A 1.0-mL Hamilton gas-tight syringe was inserted into the Erlenmeyer flask containing the liquid accelerant and 0.2 mL of vapor was withdrawn, which was then diluted with 0.6 mL of air. The charred samples were also sampled via headspace. To increase the amount of accelerant in the gas phase, the charred samples were placed in a hot water bath at 80 °C for 5 min. The amount of headspace vapor sampled from the char was increased to 0.6 mL, which was then diluted with 0.2-mL of air. First the standard accelerants and then the charred samples were sampled and analyzed. The software allowed data analysis of the total ion current (TIC) chromatograms almost instantaneously after the mass spectra were stored in the computer. Once a peak was obtained, it was possible to pull up mass spectra at any retention time on the chromatogram. A library search could be performed against that mass spectrum. A list of possible compounds was given, along with the molecular weight and a quality rating of the match.
Journal of Chemical Education • Vol. 78 No. 9 September 2001 • JChemEd.chem.wisc.edu
In the Laboratory
Hazards Extreme caution must be exercised when using the accelerants, owing to their flammability. It is especially important to pour excess liquid out of the flask before ignition. This preparation should be performed in a hood; gloves, eye protection, and a lab coat should be worn. Results All the accelerants have unique chromatograms except for the gasoline samples. The gasoline samples are different from the rest of the accelerants but very similar to each other. The major difference between the gasoline samples is the relative intensity of some of the peaks. Since the chromatograms are unique to the types of accelerants, it is fairly easy to decipher which accelerant was used in a charred sample. Figure 1 shows a typical TIC chromatogram of Kingsford charcoal lighter. The chromatograms of the standard accelerant (Fig. 1A) and its charred sample (Fig. 1B) are almost identical, the only major difference being the intensity of the peaks. All of the students whose charred sample contained the charcoal lighter or lacquer thinner accelerant were able to correctly determine which accelerant was used to start the fire. Moreover, the majority of students who had a gasolinecharred sample were actually able to correctly identify which gasoline was used to start the fire. Lamp oil was not used in
any of the charred samples because of its extremely simple chromatogram. Figure 2A is the mass spectrum of a charcoal lighter charred sample for the GC peak at 3.4 min. Figure 2B is the mass spectrum of nonane, which the library search gave as the most probable compound yielding the unknown mass spectrum. In each mass spectrum, the parent ion is seen at m/z = 128. The students can see that the difference between m/z = 128 and m/z = 99 is 29, which corresponds to the loss of a C2H5 group, or they notice that the difference between the other clusters is 14 mass units, corresponding to the loss of a CH2 group. Other patterns commonly seen in electron impact ionization include m/z = 77 (C6H5+) and m/z = 91 (C6H5CH2+), indicating a double-substituted benzene ring with a methyl group substituent or a monoalkyl-substituted aromatic resulting in benzyl-ion formation upon fragmentation. The students learn that mass spectrometry can help in the determination of an unknown compound by interpreting its mass spectrum. However, they also learn one of the disadvantages of MS, which is its inability to differentiate isomers. There were instances in which they would see three peaks in a chromatogram, each corresponding to an isomer. A library search on each peak would suggest the same three isomers for each chromatographic peak. This also shows that although MS alone cannot distinguish the three isomers, coupling it with GC can differentiate among the three by their different retention times.
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B
Figure 1. Total Ion Current (TIC) chromatograms. A: Kingsford charcoal lighter standard sample. B: Kingsford charcoal lighter charred sample.
Figure 2. Mass spectra. A: Mass spectrum for peak at retention time of 3.4 min in a charred sample. B: Nonane mass spectrum from the library search.
JChemEd.chem.wisc.edu • Vol. 78 No. 9 September 2001 • Journal of Chemical Education
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In the Laboratory
Figure 3. TIC chromatogram of a charred sample containing Klean Strip lacquer thinner and Kingsford charcoal lighter.
Figure 4. TIC chromatogram of a standard solution containing Kingsford charcoal lighter and Restore & Restyle candle and lamp oil.
Experiment Variations
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To create a more challenging experiment for our upperdivision students, we implemented a variation in which a few students were given unknown samples that indicated the presence of two or more accelerants. Figure 3 is such an example. It shows a chromatogram of a charred sample resulting from a mixture of Kingsford charcoal lighter and Klean Strip lacquer thinner. This chromatogram was correctly identified by the student as a mixture of the two accelerants. Another example is shown in Figure 4. This chromatogram, representing a combination of the standard liquid accelerants Kingsford charcoal lighter and Restore & Restyle candle and lamp oil, shows that a mixture of the two accelerants can be resolved. Although the examples shown in Figures 3 and 4 show little overlap of retention times, identification by MS makes overlapping retention times tolerable, as well as desirable for a more challenging experiment.
A student handout and notes for the instructor are available in this issue of JCE Online.
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Supplemental Material
Literature Cited 1. Gaenssien, R. E.; Kubic, T. A.; Deslo, P. J.; Lee, H. C. J. Chem. Educ. 1985, 62, 1058–1060. 2. Brewer, W. E.; Lambert, S. J.; Morgan, S. L.; Goode, S. R. Teaching Forensic Analytical Chemistry; ChemConf ’98, OnLine Conference on Chemical Education, 1998; http:// www.inform.umd.edu/EdRes/Topic/Chemistry/ChemConference/ ChemConf98/forensic/ChemConf.htm (accessed Jun 2001). 3. Kaplan, L. J. http://otis.cc.williams.edu/Chemistry/lkaplan/ 113syllabus.html (accessed Jun 2001). 4. Elderd, D. M.; Kildahl, N. K.; Berka, L. H. J. Chem. Educ. 1996, 73, 675–677. 5. Zabzdyr, J. L.; Lillard, S. J. J. Chem. Educ. 2001, 78, 1225–1227.
Journal of Chemical Education • Vol. 78 No. 9 September 2001 • JChemEd.chem.wisc.edu
