Fibers and Forensic Chemical Analysis

Raman spectroscopy has been used to characterize organic fibers and films since the 1960s. Initially, Raman spectroscopy was used primarily to identify the material in the same way that infrared (IR) spectroscopy was used. Chemical identification and quantification are still used extensively to determine the type of polymer, the type and amount of comonomers, and the type and amount of pigments, dyes, or other additives. This has been used in forensic science, archaeology, competitive analysis, and quality control. The techniques are nearly identical to those used for the identification of other solids and liquids, with minor modifications required by the fibrous or filmlike nature of the materials. This application will be discussed in Section II,... 

Flynn, K., R. O Leary, C. Roux, and B. J. Reedy, Forensic Analysis of Bicomponent fibers using infrared chemical imaging, J. Forensic Sci., 51, 586 (2006). 

Grieve, M.C., and Kearns, J.A. (1976). Preparing samples for the recording of infrared spectra from synthetic fibers. J. Forensic Sci., 21, 307-314. Krause, A., Lange, A., Ezrin, M. (1983). Plastic Analysis Guide Chemical and Instrumental Methods, Hanser Publishers, NY. 

The fact that Raman measurements can usually be made through glass and plastic packaging, eliminating the need to prepare samples prior to analysis, makes Raman spectroscopy very attractive for forensic science. The availability of commercial portable instramentation and extended fiber optic probes makes Raman suitable for on-site forensic use, minimizing the risk of exposure of investigating personnel to potentially hazardous chemicals. Eor identification of explosives the SERS method has proved to be very useful. A tiny amount of explosive, diluted... 

Other uses of an IR microscope in forensic analysis include the examination of fibers, drugs, and traces of explosives. For example, oxidation of hair can occur chemically or by sunlight oxidation of cystine to cysteic acid can be seen in hair fibers by FTIR microscopy (Robotham and Izzia). Excellent examples in full color of FTIR imaging microscopy can be found on the websites of companies like PerkinElmer and Thermo Fisher Scientific. Our limitations in use of gray scale make many of the examples unsuited for reproduction in the text. A novel IR microscope combined with atomic force microscopy, the nanoIR platform from Anasys Instruments (www.anasysinstruments.com), permits nanoscale IR spectroscopy, AFM topography, nanoscale thermal analysis, and mechanical testing. 

Chemical imaging is described, including confocal Raman imaging. UV and visible spectroscopy includes innovations such as flow-through sample holders and fiber-optic probes, as well as instruments for analysis of submicroliter volumes and nondestructive analysis for nucleic acid and protein determinations. UV absorption spectral interpretation for organic molecules is covered in depth. Applications described include nucleic acid and protein measurements, spectrophotometric titrations, and new applications in forensic chemistry. Nephelometry, turbidimetry, fluorescence, and phosphorescence are described in detail, including instrumentation and applications. The measurement of color using the CIE system is described with examples. 

Polymers can be classified in different ways. From the forensic perspective, a reasonable starting point is to divide pol)Tmers into biologically derived polymers (biopol3rmeis) and synthetic organic polymers. Biopolymers are extracted from natural sources such as plants or animals. Even though proteins and DNA are biopolymers of unquestioned importance in forensic science, their analysis resides in the context of forensic biology. The biopolymer we will concentrate on is cellulose, the base material in paper and cotton fibers. Historically and chemically, semisynthetic polymers fall between naturally derived and synthetic polymers. Rayon and cellophane are made from regenerated... 

Several characteristics of fibers are targeted by forensic analysis. As shown in Figure 14.8, the chemical composition of the fiber is just one of many important characteristics. The diameter and cross section are useful for determining how a fiber is used. For example, carpet fibers are relatively thick and often have hollowed-out cross sections compared with those of fibers used in clothing. Cotton fibers have a characteristic flat ribbon geometry. Fiber color and how it... 

Next to PLM, IR spectroscopy (principally, micro-IR) makes up the most important family of techniques in fiber analysis. As with PLM, IR spectroscopy is minimally destructive or nondestruchve and is useful on even the smallest fiber fragments. Microspectrophotometry (MSP) probes the chemical identification of the synthetic fiber, colorants, and other treatments. MSP is indispensable in the comparative analysis of questioned and known fibers, the central task of most forensic fiber analysis cases. The caveat is that such comparisons require knowledge of typical inter- and intrasample variation. Consider, for example, a pair of blue jeans typically, fibers from along the seams and hems are worn compared with fibers to other portions of the garment. Also, the source from which a sample is obtained will clearly affect any visible spectra and color analysis. Along a single fiber, characteristics of a dye or colorant will vary as well. Such inherent variations must be factored into any conclusions drawn from a comparative analysis, be it of dye or chemical composition. 

The analysis of many classes of materials is undertaken with the purpose of discovering the chemical and physical history of the object. This is certainly the case in forensic analysis where substances collected from a crime scene can provide vital information that may incriminate suspected criminals. The exchange principle enunciated by Edmund Locard (Thornton, 1997) is also applicable to fibers in the sense that every stage in the synthesis or processing of the fibers or filaments, every thermal or mechanical treatment and every subsequent contact with chemicals leaves an indelible mark on the material. The challenge is to detect, characterize, and correctly interpret the information. Thermal analysis provides a set of tools that, effectively applied, can provide a wealth of information from minute samples. 

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