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by
Adam M. Boyd and Randolph K. Larsen III
Department of Chemistry and Biochemistry
St. Mary’s College of Maryland
Gas chromatography-mass spectrometry (GC-MS) analysis is a commonly used technique in organic chemistry, quantitative analysis, and instrumental methods courses. Case studies are an effective way to introduce students to GC-MS analysis. This case study was designed for an instrumental methods course, but could also be adapted for a non-science major course.
In reading the case, students find themselves in the middle of a scenario in which they must determine both the nature of a fire and any criminal accusations which may be raised as a result of their findings. Students come to understand that a fire investigator is being accused of burning down her ex-husband’s house in a fit of rage, and it is up to them to validate or refute the suspicions of police investigators by analyzing ash samples collected from the scene. In addition, they must analyze a clothing sample collected from the suspect. Once they have determined whether or not the fire was arson, they must then determine if the allegations leveled against the fire investigator are credible. Students are presented with ash samples and cloth samples collected from the scene, and are asked to compare what they find within the samples to accelerant standards whose spectra are already known.
Use of this case depends greatly on the level of experience of the students involved. In an introductory class, this case is best utilized to introduce the principles of GC-MS analysis. In a more advanced course in which students are already familiar with GC-MS analysis, this case is a useful exercise in interpreting mass spectrometry. In any scenario, headspace analysis and the principles of forensic investigation—particularly as they relate to arson investigation—can be introduced and cross disciplinary learning emphasized.
Depending on the students’ experiences with GC-MS analysis, the instructor may want to review its principles in class, prepare an overview handout, or use the review provided in the Supplemental Material for this case. Students should read the case for homework and come prepared to discuss proposed methodologies which can address the problems presented in it. The students should also have researched and prepared answers for the questions at the end of the case.
If the size of the class is large, the instructor may want to divide the lab into smaller groups, which will allow the students to brainstorm ideas together. For this experiment, these groups may also serve as lab partners, an efficient way of organizing the students’ work given that most schools have limited access to GC-MS instruments and the amount of time it takes to complete one run.
After the students have been given about 15 minutes to discuss their ideas on how the case can best be investigated, the instructor should ask groups what ideas they have for analyzing the charred fire samples and cloth sample. The instructor should guide the groups towards the preferred methodology—using static headspace analysis coupled with GC-MS. The instructor should also discuss the advantages and disadvantages of different extraction techniques, including dynamic, passive, and static headspace analysis (see Supplemental Material). Steam distillation is not the preferred method for collecting volatile materials from charred debris because it can be time consuming and it is difficult to collect enough liquid to analyze. Solvent extraction, while useful, is destructive in nature, which may be a disadvantage with limited physical evidence.
At its core, static headspace analysis takes advantage of the high vapor pressure of accelerants to induce them into the gas phase, rendering them available to be withdrawn with an airtight syringe and subjected to GC-MS analysis. Because the analyte being analyzed is in the vapor phase, there are a few important procedural considerations that the instructor should stress in order to ensure a good result:
To conclude the discussion, the instructor may wish to give some statistics about the widespread nature of arson. According to the United States Fire Administration, 37,500 intentionally set fires resulted in 305 civilian deaths and $692 million in property damages in 2003 (see http://www.usfa.fema.gov/). The ability to characterize arson and convict those who commit it remains an important and widely used skill.
After groups have determined a correct methodology, the students should then proceed to the analysis. Groups may take turns performing headspace analysis on either standards or unknowns with each run lasting about 15 minutes. Detailed methods are outlined in Figure 1. After all of the standards and charred samples have been run, students may then pool their chromatograms so that each group has access to all of the chromatograms. Based on their findings, groups develop conclusions about the possibility of arson and the guilt or innocence of the suspect in question. They should complete lab reports in accordance with the instructor’s directions.
The entire experiment can be accomplished in one laboratory period if each group injects one unknown sample and one standard sample. The groups can then pool their chromatograms to reach individual conclusions.
For this lab, the instructor will need to create three charred samples and one cloth sample to present to students for analysis. To prepare the charred samples, the instructor should obtain pine wood chips or a piece of pine wood from a local hardware store. The pine board can then be finely divided into pieces small enough to fit inside 125 mL Erlenmeyer flasks. After about 10 small pieces of pine have been added to the flask, the instructor should add three different accelerants to each of the three Erlenmeyer flasks. For the purposes of this lab, gasoline, lighter fluid, isopropyl alcohol, and lamp oil are ideal accelerants to use because all would be easily accessible to potential arsonists and yet, if found within the proper environment of a home, the presence of each could be logically explained. Detailed explanations for the preparation and ignition of the charred samples and the preparation of the cloth sample can be found in Figure 1. The preparation of the charred samples is modeled after the procedure given by Eldred et al. (1996) and Sodeman et al. (2001). The total preparation time is two hours.
GC chromatograms and mass spectra obtained from two standards and charred unknowns are presented in Figures 2 through 4. All of the accelerants used had a distinctive chromatogram which makes identification of unknowns an easy task. The major difference between the chromatograms obtained from the standard accelerants and those obtained from the charred samples is the relative intensity of the peaks—yet, owing to their unique character, each is easily matched.
The analysis of the charred samples should be planned to reveal whether or not the mock fire was the result of arson. To lend a greater sense of realism to the experiment, and to aid students in their elucidation of the truth, we have included a crime scene schematic in the case showing the precise locations from which the charred samples were collected; this schematic can be modified by other instructors to serve their own purposes regarding the guilt or innocence of Marie Stanforth.
The instructor may wish to remind students that the detection of an accelerant from any charred sample does not necessarily constitute arson, due to the fact that there could be a reasonable explanation for the presence of many accelerants in the common household. For upper division classes, the instructor may wish to allow students to make this conclusion on their own.
For example, in the crime scene schematic provided, the detection of isopropyl alcohol in sample 2 would not lend much weight to an arson case because it is possible that isopropyl alcohol would have been stored in the bathroom of the victim’s house. Likewise, the detection of lighter fluid in sample 1 would not clearly prove arson, because it is not uncommon for lighter fluid to be stored in garages. In order to make a strong case for arson, students will need to show that an unnatural accelerant can be detected near the point of origin. For our purposes, we chose sample 1 to correspond to lighter fluid, sample 2 to correspond to isopropyl alcohol, and sample 3 to correspond to gasoline.
Armed with the information that the detection of accelerants at or near the “point of origin” is a strong case for arson, students should pay particular attention to unknown #3, or whichever sample the instructor chooses to correspond to the point of origin. In the provided crime scene schematic, the detection of gasoline in unknown #3 makes a strong case for arson because there are no real reasonable explanations for the presence of gasoline in the victim’s living room.
In order to make a conclusion about the guilt or innocence of Dr. Marie Stanforth, students should be provided with a fourth unknown which should be introduced as a swatch of clothing recovered from a glove found in the trunk of Dr. Stanforth’s automobile on the day of the crime. Again, the lab is sufficiently flexible that the instructor has the option of spiking the clothing any number of different ways. To give the impression that Dr. Stanforth is guilty, you might choose to spike the clothing sample with the same accelerant you chose for the point of origin. Or, you might choose not to spike the clothing with anything at all. An interesting cross disciplinary discussion might arise if you chose gasoline as your “point of origin” accelerant and as your clothing accelerant. Though this correlation might clearly seem to indicate Dr. Stanforth’s guilt, some students may recognize that gasoline is sometimes stored by motorists in the trunks of their automobiles. Some students may argue, then, that the presence of gasoline on the swatch of clothing recovered from Dr. Stanforth may not adequately establish her complicity in any arson charges, and consequently, may not be enough to charge Dr. Stanforth with arson.
The overall flexibility of the experiment makes it appropriate for use in many different settings. For a higher level chemistry analysis class, the instructor may choose not to provide the students with standards but instead require them to respond based on their knowledge of mass spectrometry and their familiarity with chemical literature to uncover the composition of the accelerants. With the standards in place and the analysis qualitative, the experiment can also be tailored for an introductory course in forensic analysis or a forensic analysis class for non-science majors.
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Acknowledgements: This case was developed with support from the National Science Foundation under CCLI Award #0341279. Any opinions, findings and conclusions or recommendations expressed in this material are those of the author(s) and do not necessarily reflect the views of the National Science Foundation.
Date Posted: 10/19/05 nas
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