Microscopy and Microanalysis of Solution Dyed Fibers
On Tuesday, September 10th, 2024, Microtrace Senior Research Microscopist Kelly Brinsko Beckert presented a talk to the Department of Forensic Science at Virginia Commonwealth University. “Microscopy and Microanalysis of Solution Dyed Fibers” is part of the weekly Forensic Science Seminar presentations offered by the department. Ms. Beckert’s presentation explored the usefullness of pigmented fibers in a forensic investigation, as well as the chemistry of colored fibers and techniques for analysis.
Abstract
Fibers are one of the most common types of trace evidence encountered. The physical, optical, and chemical properties of these fibers can be very different from each other, but one common feature they share is that they are comprised of long chains of molecules that form the basic structure of the fiber. The molecular chemistry of a fiber has a significant effect on its numerous attributes, from the formation of the fiber itself, and the subsequent manufacture into yarns and textiles, to the gross appearance and handle of the woven fabric. It is the chemistries of the polymers, colorants, and finishes that are responsible for these differences, many of which may be tested and exploited during a forensic fiber analysis. Fibers can be classified based on their morphology, optical properties, and molecular chemistry using a combination of microscopy, microchemistry, and infrared microspectroscopy.
Color is a primary feature of nearly every fiber comparison. Color is utilized at nearly every stage of analysis: from the initial search for target fibers and initial microscopical comparison of questioned and known samples, to late-stage objective comparisons by microspectrophotometry. In contrast to dyed fibers, pigmented fibers (i.e., solution dyed fibers) are colored by the addition of insoluble pigment to the liquid polymer prior to extrusion. Such fibers are becoming increasingly common due to a variety of factors, including their ability to withstand harsh cleaning agents, inherent resistance to fading, and environmentally friendly manufacturing methods. The number of pigment types, their identity, particle size, and density of pigmentation represent unexploited properties that can be used to evaluate fiber associations or provide investigative information during a fiber analysis. However, there have been no systematic studies of pigmented fibers; and therefore, no practical guidance is available to the bench-level analyst to identify, characterize, or interpret pigmented fiber evidence in forensic fiber cases.
This talk will explore the various laboratory analyses that may be performed during a forensic examination of a fiber, and how a fiber’s chemistry can impact everything from its cross section to its color and end-use. This research also fills this knowledge gap through a systematic study of pigmented fibers. The selected fiber samples span major manufacturers, include various applications of pigmented fibers, and represent the variety of colors and polymers that are produced. This presentation will provide an overview of results arising from the critical study of samples from this population of pigmented fibers by polarized light, oil immersion, and fluorescence microscopy. This study of fibers is supported by research into sample preparation techniques, including longitudinal whole mounts and cross sections, that have been optimized to maximize the resolution of individual pigment particles that often approach, or at times exceed (are smaller than) the resolution limits afforded by light microscopy. The range of information that has been obtained and tabulated from these samples includes methods of recognizing pigmented fibers, estimates of the number of different pigment types present in a fiber, and tabulations of the color, size, and morphology of individual pigment grains detected in a given sample.
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