Polymer Analysis
Contributed by Ed Suzuki
Natural and synthetic polymers are ubiquitous. Natural organic polymers (cellulose, silk, protein, DNA, chitin, etc.) are predominantly from biological sources, while inorganic ones (quartz, mica, asbestos, etc.) are minerals. These materials may be analyzed in the forensic science laboratory as they occur as textile fibers; tissues, extracts and residues examined for human or animal DNA; soils; building materials; and other types of evidence.
Modern societies are also heavily dependent on synthetic polymers, which comprise materials such as plastics, rubbers, foams, composites, adhesives, films, tapes, paint binders, and man-made fibers. This importance is probably best exemplified by a scene from a 1967 popular movie, The Graduate. In it, Dustin Hoffman plays a listless individual who has just graduated from college. At his graduation party, he is approached by a guest who counsels him regarding his future job prospects: “I have just one word for you—Plastics.” The guest was referring, of course, to the increasing prominence that plastics and other synthetic polymers were assuming in our society. Reflective of this, archeologists of the future are likely to have a very definitive chronological marker when excavating sites: artifacts constructed of synthetic polymers will suddenly become prevalent.
Because so many everyday objects are constructed of synthetic polymers, it is not surprising that they are frequently encountered as evidence. Paints, fibers and tapes, discussed elsewhere, are all constructed of polymeric materials—mostly synthetic. A wide variety of other materials comprised of or incorporating synthetic polymers can also be examined as evidence, and their differentiation, identification, and comparison constitute another sub-discipline of trace evidence, usually referred to simply as “Polymer Analysis.”
Like other types of trace evidence examinations, this sub-discipline may entail either an identification of an unknown material or a comparative analysis. An unknown material is submitted as evidence to determine not only what it is, but more importantly, what significance it might have to the circumstances of the case under investigation. For an unknown material identified as a synthetic polymer, the type of polymer may provide some clues as to function of the object from which it originated. However, other properties, such color, size, texture, morphology, and structure (that is, is the material a film or laminate?) can be even more revealing. A forensic polymer analysis may therefore encompass a variety of examinations in addition to determination of the chemical makeup of the polymer. As an example, polyurethane foams are very common materials used for a variety of different purposes, but there are two types. Flexible polyurethane foam (an open cell foam) is used for packaging materials, furniture filling, seat cushions, and carpet underlays; rigid polyurethane foam (a closed cell foam) is used for thermal insulation and floatation devices. Chemically, both are polyurethanes, but their textures differ and under a microscope, they look as different as night and day: open cell foams are comprised of a network of branching filaments, while closed cell foams are comprised of interlocking miniature compartments.
More often, a comparative analysis is performed to determine if the polymeric materials could share a common origin or otherwise be used to link individuals, events, places, or objects. For example, an individual suspected of rape is found with a few tiny particles on his shirt. These are analyzed by a trace analyst, who identifies them as plastic glitter particles. At the time of the alleged incident, the victim was wearing eye shadow that included glitter particles. The analyst compares the glitter particles from the suspect’s shirt and from the victim, and finds that they are similar in color, size, shape, thickness, layer structure (each has seven layers), and chemical composition of each layer. Various commercial glitter particles are known to vary in these properties. The analyst concludes that the particles found on the suspect could have come from the victim, or some other source that happens to use this same type of glitter particle. In a real case, it is possible that the glitter particles would also include residue of the eye shadow powder, which could provide further evidence linking the suspect and the victim.
Some of the wide variety of synthetic polymer evidence that may be examined in the forensic science laboratory, as well as the various analytical tools used for their analysis, can be seen from the titles of the six references listed below.
Selected References:
1. Proceedings of the International Symposium on the Analysis and Identification of Polymers, U.S. Government Printing Office: Washington D.C., 1984.
2. Shen Z et al. A Case Study on Forensic Polymer Analysis by DIOS-MS: The Suspect Who Gave Us the SLIP®. J Forensic Sci 2004:49:1028-1035.
3. Vernoud L et al. Characterization of Multilayered Glitter Particles Using Synchrotron FT-IR Microscopy. Forensic Sci Int 2011:210:47-51.
4. Hashimoto T et al. Morphological and Spectroscopic Measurements of Plastic Bags for the Purpose of Discrimination. J Forensic Sci 2007:52:1082-1088.
5. Sarkissian G. The Analysis of Tire Rubber Traces Collected After Braking Incidents Using Pyrolysis-Gas Chromatography/Mass Spectrometry. J Forensic Sci 2007:52:1050-1056.
6. Ihms EC, Brinkman DW. Thermogravimetric Analysis as a Polymer Identification Technique in Forensic Applications. J Forensic Sci 2004:49:505-510.