X-ray diffraction
X-ray diffraction (XRD) is a powerful analytical technique used to study the crystal structure of materials by deflecting X-rays off atomic planes within a substance. Each crystalline material produces a unique X-ray diffraction pattern, akin to a "fingerprint," making it particularly useful in forensic science for analyzing trace evidence such as fibers, hair, minerals, and various substances found at crime scenes. The method can analyze small samples and minute traces, providing insights that can link suspects to victims or crime scenes, or conversely, eliminate individuals from suspicion.
The principle behind XRD lies in the interaction of X-rays with the crystal lattice, where most X-rays pass through but some are scattered, forming a distinctive interference pattern. This technique not only allows the examination of large quantities of materials but is also adept at analyzing small amounts, often requiring samples to be converted to powder form. One of its key advantages is its nondestructive nature, preserving valuable forensic evidence while identifying the material’s composition and characteristics.
Furthermore, advancements in computer automation have enhanced the speed and ease of interpreting X-ray diffraction patterns, with extensive databases available to assist in the identification process. Overall, XRD serves as a crucial tool in forensic investigations, facilitating a detailed analysis of crystalline materials that are often encountered in crime scene evidence.
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Subject Terms
X-ray diffraction
DEFINITION: Technique for studying crystal structure by deflecting x-rays off the atomic planes of a substance.
SIGNIFICANCE: Because each crystalline substance has a unique x-ray diffraction pattern, this technique allows forensic scientists to analyze the “fingerprints” of evidence found at crime scenes and related locations.
X-ray diffraction (XRD) is a significant tool for the analysis of solid materials encountered in forensic science. Analysis of trace amounts of materials, including fibers, hair, minerals, metals, dust, pollens, blood, drugs, dyes, polymers, explosives, firearms discharge residues, soil, and paint, can help establish links between suspects and victims and between suspects and crime scenes; conversely, such analysis can eliminate some persons from suspicion.
Crystal structures serve as three-dimensional diffraction gratings for x-rays. Most x-rays impinging on a crystal pass straight through, but some are scattered from the crystalline planes and form an interference pattern on exposed film that is uniquely related to the atomic arrangement and subsequent identity of the substance. In addition to enabling the analysis of large amounts of a material, XRD can be employed to analyze small samples, smears, and minute contact traces of a substance. The way a sample is prepared for XRD analysis and the XRD method employed are determined by the type, amount, and consistency of the trace specimen; the involved surface where the specimen was found; and the forensic questions raised by the criminal offense.
The majority of physical materials recovered at crime scenes and related locations are typically crystalline or semicrystalline in nature; thus forensic scientists can analyze and identify them using XRD methods. Other evidence can be converted to crystalline form. With blood evidence, for example, iodine is used for this purpose, and other compounds are used to convert other kinds of specimens to crystalline form. Samples of recovered evidential materials are typically converted to powder form for study using x-ray powder diffraction. Non-powder samples can also be analyzed using XRD.
Forensic scientists use XRD patterns as a screening tool to examine fiber trace evidence and to sort the evidence into groups based on type of fiber. This discriminates the type of fiber and preserves the fabric, which may contain stains of evidential value. The nondestructive nature of XRD analysis is a very important advantage of this technique when the forensic evidence must be preserved.
Among other advantages of XRD methods in are their ability to identify the unique character of patterns produced by crystalline material, their ability to distinguish between elements and their oxides, and their ability to identify chemical compounds, polymorphic forms, and mixed crystals in a nondestructive manner. In many cases, XRD methods are the only methods that allow detailed differentiation of materials under laboratory conditions.
With advances in computer automation, the analysis of x-ray diffraction patterns has become increasingly easy and fast. The International Center for Diffraction Data has more than forty thousand digitized diffraction patterns in its database that forensic scientists can use to help identify critical specimens found at crime scenes and related locations.
Bibliography
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Fisher, Barry A. J., and David R. Fisher. Techniques of Crime Scene Investigation. 9th ed. CRC Press, 2022.
Houck, Max M., ed. Forensic Chemistry. San Diego, Calif.: Academic Press, 2015.
James, Stuart H., Jon J. Nordby, and Suzanne Bell, eds. Forensic Science: An Introduction to Scientific and Investigative Techniques. 4th ed. Boca Raton, Fla.: CRC Press, 2014.
Kobilinsky, Lawrence F., ed. Forensic Chemistry Handbook. Hoboken, N.J.: John Wiley & Sons, 2012.
Mozayani, Ashraf, and Carla Noziglia, eds. The Forensic Laboratory Handbook: Procedures and Practice. Totowa, N.J.: Humana Press, 2006.
Robertson, James, and Michael Grieve, eds. Forensic Examination of Fibres. 2d ed. Philadelphia: Taylor & Francis, 1999.