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Inquiry-Based Arson Investigation for General Chemistry Using GC-MS Marta K. Maurer,* Michael R. Bukowski, Mary D. Menachery, and Adam R. Zatorsky Department of Chemistry, Pennsylvania State University-Altoona College, Altoona, Pennsylvania 16601 *
[email protected] Most traditional experiments in general chemistry laboratory courses teach basic laboratory techniques but do little to develop students' problem-solving ability, critical-thinking skills as applied to underlying scientific concepts, or excitement about chemistry. With the introduction of real-world and inquirybased elements to experiments, students perform laboratory work that has relevance beyond the instructional laboratory and are encouraged to reach their own conclusions based on experimental results. This increases their motivation, enjoyment of learning, and enthusiasm for chemistry (1, 2). Exposing students to modern analytical instrumentation typically used in more advanced courses can further develop their enthusiasm for chemistry. Forensic-based experiments using gas chromatography-mass spectrometry (GC-MS) are of particular interest to students owing to the prevalence of television dramas based on scientific analysis of crime-scene evidence. Incorporating a GC-MS-based experiment into the general chemistry curriculum presents challenges. Chromatography is typically not covered in general chemistry, so students must be provided with the fundamental concepts. Because of the large class size of general chemistry laboratories (up to 24 students per section), the experiment must be organized in a manner that allows all students the opportunity to gain hands-on experience with the instrument while still operating within time constraints. Overview of Experiment To meet these challenges and provide general chemistry students with an enriching laboratory experience, we have developed a 2-week, inquiry-based investigation of a crime scene. In this experiment, teams of up to four students use chromatographic techniques to analyze charred wood samples from a hypothetical arson investigation (3, 4) and an ink sample from a note left at the scene. Students are provided with a handout including a memorandum from the local police describing the case, a list of the potential suspects, and facts about arson and accelerants. Background on the theory and use of a GC-MS and paper chromatography is also provided, including a discussion of polarity. Our experiment differs from the previously published experiments (3, 4) in two key aspects. First, we have incorporated inquiry-based and experimental-design concepts into our experiment. The teams of student investigators perform the first part of the experiment under different conditions, and then compare their results to determine which experimental conditions are most appropriate for the second part. In comparison, the previous experiments are prescriptive and do not allow students
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to see how changing parameters, such as sample-preparation temperatures and column-temperature programs, can affect their results. In addition, students are required to simulate the effects of changing GC-MS operating parameters using chromatographic simulation software (5). Prelaboratory questions and worksheets are designed so that the students develop and ultimately prove or disprove hypotheses regarding the impact of varying the temperature during sample preparation, using different column-temperature programs, and whether compounds other than those found in the accelerants might appear in the chromatograms. Second, the paper chromatography module included as part of the experiment provides students with a visual representation of chromatography, enabling them to better understand the process that occurs inside a GC. Students study the effect of the mobile phase (solvent) on separation of compounds in ink from a crime-scene note. Inclusion of this module also helps address the time management challenge of implementing a GC-MS experiment in a 24-student laboratory. Experimental Approach In the first week of the laboratory, students investigate and optimize experimental conditions for both the GC-MS and paper chromatography portions of the experiment. For the GC-MS component, each group evaluates a different combination of sample-preparation temperatures and column-temperature programs using a burned wood sample from the crime scene. For the paper chromatography module, each group uses an ink sample from the crime scene and evaluates separation characteristics of the ink in various solvent mixtures. On the basis of the laboratory management plans described in Illies et al. (6) and Reeves and Pamplin (7), each team of students is assigned a 25 min period to work with the GC-MS. Students carefully chisel a small piece of burned wood from the crime scene and place it in a test tube sealed with a rubber septum. Groups prepare their sample by immersing the test tube in a water bath that is either room temperature, 40 °C, or 80 °C for 5 min. During the laboratory, students are asked to identify key GC-MS components and briefly explain their function(s) to the instructor. Guided by the instructor, students load one of two possible temperature-profile programs, which differ in the GC oven-heating rate. Students obtain a 10 μL headspace sample from their sample test tube and inject the sample into a VF-5 ms column. The resulting chromatograms (Figure 1) show that an increase in sample-preparation temperature concomitantly increases
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r 2010 American Chemical Society and Division of Chemical Education, Inc. pubs.acs.org/jchemeduc Vol. 87 No. 3 March 2010 10.1021/ed800083b Published on Web 02/09/2010
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Figure 2. Crime-scene sample (wood burned with camp fuel) analyzed using GC column-temperature (A) profile 1 and (B) profile 2, with sample preparation at 80 °C. The column-temperature profiles are indicated by the temperature ranges shown on the figures. The chromatograms are cut off at 10 min, as no relevant compounds elute after this time.
Figure 1. Crime-scene sample (wood burned with camp fuel) analyzed using GC column-temperature profile 1 with sample preparation at (A) room temperature, (B) 40 °C, and (C) 80 °C.
overall signal intensity, especially for higher molar mass compounds. Chromatograms (Figure 2) are also obtained using the two temperature-profile programs. As the sample elutes, students identify the most intense chromatographic peaks using the National Institute of Standards and Technology (NIST) database. Students also answer worksheet questions geared toward evaluating different sample-preparation conditions and demonstrating understanding of chromatographic principles, including the roles of stationary and mobile phases, polarity, and the effects of molar mass, boiling point, and vapor pressure on the separation of components in a sample. The data from the first week are shared with the class, and students review the chromatograms to determine which samplepreparation conditions and GC oven-temperature profile result in the best peak resolution. These conditions are used the second week to determine which accelerant was used in the arson based on comparison to a genuine standard. The GC-MS portion of the experiment, including sample preparation and GC-MS analysis, takes approximately 40 min for each group. During the remainder of the first laboratory period, students perform paper chromatography using four different solvent mixtures and an ink sample extracted from a note left at the crime scene. Both polar and nonpolar solvents are used, including water, acetone, ethanol, hexane, and ethyl acetate. With the use of a chromatographic developing chamber, students spot an ink sample from the crime-scene note on a piece of chromatography paper and develop it in the one of four solvent combinations. After the paper is removed from the solvent and dried, students calculate retention factors for each spot. The paper chromatograms are shared with the entire class, so that each group can determine which solvent mixture provides the 312
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best separation of the components in the crime-scene ink. The selected solvent mixture is used in the second week of the experiment to analyze ink from four pens that could have been used by the arsonist to write the note. In the second week, each student group is provided with wood chips soaked in one of four suspected accelerants (camp fuel, lighter fluid, paint thinner, and paint remover), each of which yields distinct chromatograms. The students ignite the wood sample with a Bunsen burner, and decide the extent to which they should burn the sample in order to reproduce the degree of charring in the crime-scene sample analyzed the previous week. After extinguishing the flame, the sample is placed in a septum-sealed test tube and analyzed using the optimum experimental conditions from the first week. The resulting chromatograms are shared with the class, so that the students can determine which accelerant was most likely used for the arson. In addition, each team develops paper chromatograms of ink components from four pens using the optimum mobile phase determined the previous week. Students then compare the resulting chromatograms with their results from the previous week to determine which of the four pens was used to write the crime-scene note. Each pen, as well as each accelerant, correlates to a suspect identified in the laboratory handout. As part of the final report, students are asked to use the evidence to build a circumstantial case against one of the suspects. Students must revisit their original hypotheses and explain the impact of the sample-preparation temperature and column-temperature profile and solvent composition on the chromatographic results. Students must also compare and contrast the two different types of chromatography included in this experiment and evaluate how polarity and molar mass affect separation. Hazards The accelerants are composed of aromatic and aliphatic hydrocarbons that can cause irritation to the eyes, skin, and digestive tract. They may also cause nervous system depression. Acetone, ethanol, hexane, and ethyl acetate are flammable and
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are harmful if swallowed or inhaled. They can irritate the skin, eyes, and respiratory tract. Wood chips should be burned in a fume hood, and goggles must be worn at all times. Conclusion Students responded enthusiastically to the new experiment, with 48% of surveyed students identifying it as their favorite experiment of the course, nearly double the next most-popular experiment. They appreciated the direct, real-life application of chemistry principles and the realism that came with determining how to prepare the samples, rather than being given a complete prescriptive procedure. Students also enjoyed using modern instrumentation that they would use in a commercial analytical laboratory. By including the paper chromatography module, students develop a visual representation of separation science. This enables them to better understand gas chromatography and intermolecular interactions as they can draw direct comparisons between the two chromatographic methods, and student knowledge of GC-MS fundamentals shows a marked increase after performance of the experiment. Performing this experiment in the first-semester general chemistry course provides the students with fundamental chromatographic knowledge and hands-on use of a GC-MS, allowing subsequent chemistry courses to expand on this knowledge.
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Acknowledgment Funding for purchase of instrumentation and development of the laboratory experiment was provided by the National Science Foundation CCLI program (Grant #0633646) and Pennsylvania State University-Altoona College. Literature Cited 1. Perkins, D. Smart Schools: From Training Memories to Educating Minds; Free Press: New York, 1992; pp 43-72. 2. Ram, P. J. Chem. Educ. 1999, 76, 1122–1126. 3. Elderd, D. M.; Kildahl, N. K.; Berka, L. H. J. Chem. Educ. 1996, 73, 675–677. 4. Sodeman, D. A.; Lillard, S. J. J. Chem. Educ. 2001, 78, 1228–1230. 5. Fundamentals of GC/MS, version 1.0; Academy Savant: Fullerton, CA, 2005. 6. Illies, A.; Shevlin, P. B.; Childers, G.; Peschke, M.; Tsai, J. J. Chem. Educ. 1995, 72, 717. 7. Reeves, P. C.; Pamplin, K. L. J. Chem. Educ. 2001, 78, 368–370.
Supporting Information Available The prelaboratory handouts; experimental procedure and instrument operational parameters; a summary of the project evaluation data. This material is available via the Internet at http://pubs.acs.org.
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