Forensic fiber-color examination

The predominant techniques currently employed in forensic fiber-color examinations include microspectrophotometry [1, 2] and thin-layer chromatography (TLC) [3]. Microspectrophotometry is the most widely utilized color comparison technique in federal, state, and local forensic laboratories. To the forensic scientist, nondestructive analysis of evidence and application to extremely small sample sizes are the most attractive characteristics of this method. However, the lack of discriminatory power is an inherent limitation. Microspectrophotometry evaluates the spectral characteristics of the composite-dye mixture, but says nothing about the individual dye components. Considerable dit nostic information is therefore left unexamined. 

An important aspect of forensic fiber examinations involves the comparison of dyestuffs used to impart color on or in textile fibers. Information obtained from dyes used to color fibers can provide supporting evidence in forensic casework when comparing two fibers obtained from different locations. To determine that two fibers are of the same or in, it is necessary that they be shown to have the same dye components and that the ratio in which these components are present should be identical. Comparisons of absolute dye concentrations (i.e. nano-grams dye per mm fiber) may not be necessary - or even advisable. Dye intensity may not be distributed uniformly along different fibers from the same coloring batch, or even along the length of a particular fiber. Thus, a forensic evaluation should comprise a qualitative evaluation of dye content and a quantitative determination of the relative amounts in which those dyes are present. 

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. 

The major role of the forensic scientist is to analyze submitted evidence for the purposes of characterization and identification. For example, a blue fiber collected from the scene may be submitted to the trace evidence section, where forensic scientists characterize the fiber (for example, by its dimensions, color, cross-sectional shape) and then identify the type of fiber (for example, nylon, polyester, acrylic). Furthermore, when a known sample is available (such as fibers from the suspect s clothing), forensic scientists compare it with the unknown sample (collected from the crime scene) to determine if the two most likely originated from a common source. This process of characterization, identification, and comparison requires multiple stages of analysis, ranging from visual examination to instrumental analysis. 

Microscopy is indispensable in forensic science, where it is used to examine crime scene evidence such as blood, hair, dust, fingerprints, tiny shards of glass, and threads of fiber. Criminologists use high-powered microscopes to help them study the minute hand motions that were used to construct signatures on suspicious documents, looking for frequent stops and starts or other signs of possible foi ery. Counterfeit currency makers are often foiled by microscopes, which help scientists detect very subtle discrepancies in the color and texture of the paper fibers used to manufacture counterfeit bills. 

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