In the Laboratory
Was the Suspect in Contact with the Victim?
W
An Instrumental Methods Experiment for the Analysis of Single Fibers Using FT-IR Microscopy Sharin Bender and Sheri J. Lillard* Department of Chemistry, University of California, Riverside, CA 92521; *
[email protected] Forensic science coupled with instrumental analysis is a logical and exciting strategy for teaching upper-level analytical chemistry (1). L. J. Kaplan’s course (Williams College, Williamstown, Massachusetts) for nonscience majors, Chemistry and Crime: From Sherlock Holmes to Modern Forensic Science (2), has demonstrated the effectiveness of this approach for lower-level students in the lecture and the laboratory. Brewer et al. (University of South Carolina) have found that their course on forensic analytical chemistry increased student interest and participation in the chemistry lecture (3). Our goal, in revitalizing our instrumental methods laboratory, is to add forensic-based experiments as a way to provide interesting examples in order to sustain interest and motivation in the upper-level laboratory. Forensic-based experiments can be easy to implement into existing laboratory formats using available instruments and usually require only minor modifications. For example, we have incorporated a new experiment into our instrumental methods course in which blood alcohol content was measured with headspace GC and the students were asked to determine whether the “suspect” had been driving under the influence (4). In addition, these experiments can be performed at all levels, as demonstrated by Elderd et al. in an introductory experiment for general chemistry students in which arson accelerants were identified using GC (5). The forensic approach requires the students to understand the implications involved with incorrect determinations (i.e., someone could be sent to jail), thus encouraging careful analytical procedures and cautious interpretations of the results. Our primary goal was to trigger student enthusiasm for the laboratory (3–5). We found that our students enjoyed applying their measurements to important forensic-based conclusions, instead of simply calculating a number. IR spectroscopy is a well-established technique that is routinely used by the FBI for polymer fiber analysis (6). The student is given a fictitious scenario of a murder case and asked to follow a procedure that is similar to the standard method used by many forensic scientists in an attempt to prove a suspect innocent or guilty. The objective is to collect and compare IR spectra of various fibers for the purpose of determining whether there is a connection between an alleged murderer and a homicide victim. The student uses an Equinox 55/S FT-IR spectrometer with a Bruker A590 microscope to analyze the fibers. Control fibers from known sources are given to the student from which spectra are taken and physical observations (e.g., color, texture) are made. Then, observations and IR spectra of fibers obtained from the victim’s clothing at the crime scene are analyzed, and both physical and spectral matches are determined by comparison with the standard fibers. A spectral match is based on peak intensity and frequency associated with functional
groups in a window of 4000 to 600 cm᎑1. From these data, the student assesses the likelihood that there was contact between the victim and the suspect. This experiment is not overly time-consuming, with preparation of the fibers taking approximately 30 min. The introduction to the instrumentation and the spectral measurements take approximately 2–3 h, depending on how many students are in a group (typically 2–3) and how smoothly their experiments are run. Thus, the entire experiment can be completed within a 4-h laboratory period. Two points to be made in this laboratory experiment are the importance of good, clean analytical technique and the critical understanding of the phrase “beyond a reasonable doubt”. When given an actual scenario, the imagination kicks in and we begin to realize that all these crimes we hear about on the news every night could happen to someone we know or maybe even ourselves. Careful and accurate measurements in the chemistry laboratory are imperative in both scientific and legal procedures that take place in determining whether someone is innocent or guilty. Contamination and mislabeling can cause misrepresentation of data and could put an innocent person in jail or set a guilty person free. The phrase “beyond a reasonable doubt” has been a dominating force within the American justice system. Attributing actual data to the prosecution or release of a suspect emphasizes the intricate relation between instrumental analysis and forensic science and how this pertains to everyday life. Experimental Procedure and Equipment Fibers were collected from a purple sweater (65% rayon, 35% nylon), a black sweater (100% wool), a black sweatshirt (50% cotton, 50% polyester), and a blue carpet (unknown composition). Gloves were worn to avoid contamination. After collection, the fibers were placed in a paper bag until used. Standard glass microscope slides were prepared by drilling a 0.25-in. hole through the center. A single fiber was isolated and mounted over the hole in the glass slide using forceps and tape. Care was taken to ensure the fiber was as flat and taut as possible to avoid interference fringes. An Equinox 55/S FT-IR spectrometer equipped with a Bruker A590 microscope and a mercury–cadmium–telluride detector was used. Spectra were taken through a 36× objective corresponding to 720× total magnification. Spectra were collected with Bruker OPUS spectroscopic software Version 2.0 over a frequency window of 4000–600 cm᎑1. Percent transmission was obtained using signal averaging over 32 scans with a resolution of 4.0 cm᎑1. A 16 K Fourier transform was collected and a Blackman Harris 3-Term apodization function with a zero filling factor of 2 was performed. The 2nd derivative method with 25 smoothing points was used for peak picking.
JChemEd.chem.wisc.edu • Vol. 80 No. 4 April 2003 • Journal of Chemical Education
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In the Laboratory
Figure 1. IR spectrum of a single fiber obtained from a black sweatshirt composed of 50% cotton and 50% polyester. Conditions as described in experimental procedure.
Figure 2. IR spectrum of a single fiber obtained from a purple sweater composed of 65% rayon, 35% nylon. Conditions as described in experimental procedure.
Table 1. IR Peak Data from a Black Sweatshirt Fiber (Figure 1)
Table 2. IR Peak Data from a Purple Sweater Fiber (Figure 2)
Wavenumber/cm᎑1
Abs. Intensity
Rel. Intensity
Wavenumber/cm᎑1
Abs. Intensity
Rel. Intensity
3852
0.801
0.011
3851
0.840
0.019
3743
0.802
0.009
3742
0.839
0.014
3628
0.775
0.032
3626
0.791
0.048
0.010
3357
0.803
0.015
0.684
0.146
3363
0.787
2939
0.665
0.145
2937
2242
0.616
0.197
2869
0.760
0.012
1733
0.580
0.219
2358
0.736
0.062
1635
0.739
0.021
2243
0.636
0.178
1585
0.721
0.045
1733
0.612
0.153
1531
0.754
0.014
1633
0.722
0.027
1454
0.602
0.168
1585
0.702
0.043
1363
0.674
0.067
1539
0.722
0.016
1248
0.678
0.022
1454
0.606
0.213
1172
0.657
0.108
1362
0.643
0.051
902
0.740
0.018
1170
0.638
0.119
765
0.718
0.012
1072
0.701
0.053
694
0.717
0.024
667
0.654
0.011
Hazards Care must be taken when using liquid nitrogen—appropriate gloves, lab coat, and eye protection should be worn when filling the detector. No other significant hazards are present with this experiment. Results Spectra of two different fibers are shown in Figures 1 and 2, with corresponding peak tables shown in Tables 1 and 2, respectively. Functional groups were identified by matching frequencies and intensities of peaks to those of literature values. Spectra are representative of functional groups, such as alkanes or alcohols, that construct compounds that make up the fiber. The peaks are not representative of the individual compounds, such as acrylic or cotton, therefore, similar 438
peaks can be observed for fibers composed of entirely different materials. For example, the black sweatshirt composed of 50% cotton and 50% polyester contains a triple bond associated with a nitrile or an alkyne, represented by the peak at 2242 cm᎑1 with a relative intensity of 0.197, as shown in Figure 1. Likewise, the purple sweater composed of 65% rayon, 35% nylon also contains a triple bond associated with a nitrile or an alkyne, represented by the peak at 2243 cm᎑1 of 0.178 relative intensity, as can be seen in Figure 2. It is important to distinguish these as having similar functional groups although they are composed of very different compounds. Therefore, matching fibers entails interpreting the entire spectrum, not just a few peaks. Figures 3 and 4 (and corresponding Tables 3 and 4) show spectra and peak tables for the two remaining fibers, the black sweater composed of 100% wool and the blue carpet of unknown composition, respectively.
Journal of Chemical Education • Vol. 80 No. 4 April 2003 • JChemEd.chem.wisc.edu
In the Laboratory
Figure 3. IR spectrum of a single fiber obtained from a black sweater composed of 100% wool. Conditions as described in experimental procedure.
Figure 4. IR spectrum of a single fiber obtained from a blue carpet of unknown composition. Conditions as described in experimental procedure.
Table 3. IR Peak Data from a Black Sweater Fiber (Figure 3)
Table 4. IR Peak Data from a Blue Carpet Fiber (Figure 4)
Wavenumber/cm᎑1
Abs. Intensity
Rel. Intensity
Wavenumber/cm᎑1
Abs. Intensity
Rel. Intensity
3749
0.878
0.010
3302
0.442
0.287
3628
0.870
0.011
3081
0.504
0.067
3565
0.871
0.006
2931
0.447
0.152
3430
0.860
0.021
2860
0.458
0.031
3345
0.871
0.008
2362
0.679
0.030
3054
0.857
0.014
1637
0.494
0.247
2969
0.804
0.080
1559
0.503
0.037
2907
0.832
0.020
1474
0.515
0.037
2502
0.871
0.008
1373
0.533
0.042
2360
0.846
0.035
1282
0.548
0.036
2107
0.869
0.010
1200
0.569
0.030
1959
0.855
0.020
1146
0.611
0.027
1717
0.729
0.124
1043
0.653
0.036
1615
0.835
0.014
935
0.615
0.094
1578
0.814
0.035
669
0.607
0.070
1506
0.764
0.075
1456
0.774
0.042
1410
0.731
0.114
1371
0.757
0.013
1339
0.751
0.017
1307
0.741
0.041
1240
0.745
0.033
1092
0.731
0.076
1019
0.737
0.061
972
0.770
0.045
873
0.745
0.076
793
0.781
0.029
727
0.714
0.094
668
0.783
0.023
Conclusions Infrared spectroscopy has been incorporated into routine analysis used by the FBI for fiber comparison. In this experiment, students used FT-IR microscopy with a mercury–
cadmium–telluride detector to analyze and compare fibers that were (supposedly) collected at a homicide scene. The main goal is to provide hands-on experience in forensic analysis to upper-level chemistry students. The importance of proper analytical technique is portrayed by associating the experiment to a fictitious, yet realistic scenario. If the students were to imagine that someone’s life actually depended on the results of their experiment, the severity of proper technique becomes a life-threatening issue, not just a good grade. The students voiced their enthusiasm and provided creative conclusions on the fate of the victim and suspect. Creativity has been shown to increase both participation and the desire to learn about analytical and writing techniques. Also, the students were asked to state whether or not the suspect could be linked beyond a reasonable doubt to the victim. This caused the students to think on a realistic level about how instrumental analysis is incorporated into judiciary relations.
JChemEd.chem.wisc.edu • Vol. 80 No. 4 April 2003 • Journal of Chemical Education
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In the Laboratory
Many students are concerned with what kind of career a degree in chemistry can provide. Since this course is designed for upper-level chemistry students, many are preparing to graduate and enter the workplace. This experiment presents an opportunity to explore employment options while maintaining a learning environment. Acknowledgment The authors thank Dan Borchardt for his assistance with the FT-IR instrumentation. W
Supplemental Material
Instructions for the students and notes for the instructor are available in this issue of JCE Online.
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Literature Cited 1. Gaenssien, R. E.; Kubic, T. A.; Deslo, P. J.; Lee, H. C. J. Chem. Educ. 1985, 62, 1058–1060. 2. Kaplan, L. J. http://otis.cc.williams.edu/Chemistry/faculty.html (accessed Jan 2003). 3. Brewer, W. E.; Lambert, S. J.; Morgan, S. L.; Goode, S. R. ChemConf ’98, On-Line Conference on Chemical Education 1998, http://www.infor m.umd.edu/EdRes/Topic/Chemistr y/ ChemConference/ChemConf98/forensic/ChemConf.htm (accessed Jan 2003). 4. Zabzdyr, J. L.; Lillard, S. J. J. Chem. Educ. 2001, 78, 1225–1227. 5. Elderd, D. M.; Kildahl, N. K.; Berka, L. H. J. Chem. Educ. 1996, 73, 675–677. 6. Forensic Science Communications. April 1999, vol 1, no 1. http:// www.fbi.gov/programs/lab/fsc/backissu/april1999/houcktoc.htm (accessed Jan 2003).
Journal of Chemical Education • Vol. 80 No. 4 April 2003 • JChemEd.chem.wisc.edu
