Enhancing the Forensic Value of Dyed Cotton Trace Evidence through the Application of Novel Techniques in Fiber Discrimination

This thesis examines the capability of current techniques in fiber classification such as UV-visible microspectrophotometry (MSP) (for dye in situ and/or extracted) to discriminate between fibers from sources known to be different. When these methods fail to adequately distinguish the fibers, novel alternative techniques, such as pulsed pumped laser-induced fluorescence spectroscopy (LIF) and liquid chromatography-mass spectrometry (LC, MS), are utilized to provide definitive forensic evidence. The FBI Dye Extraction Classification and Chromatography Schemes: Forensic Fiber Examination Guidelines provides the methodology used by the majority of crime labs across the United States (Fong, 1984). In the case of cotton fibers, the most frequently encountered fiber form of trace evidence, the scheme fails to produce adequate evidence to establish a questioned/known match (Grieve & Wiggins, 2001). In fact, in many criminal investigations the protocols indicate a false positive association (Cheng, 1991). New methods of discriminating between dyed cotton fibers are needed to promote the evidential value of trace fibers. The preliminary data confirm unique identification of all the fibers using these enhanced investigative tests, a task not possible by conventional analysis alone. Analysis by multiple techniques greatly enhances the probative value of trace fibers in criminal investigations by providing fiber discrimination at a higher degree of certainty. This study demonstrates the benefit of applying new techniques in the forensic investigation of fibers to reduce the chance of an incidental match. Sixty percent discrimination was achieved by employing current protocols; discrimination was improved to one-hundred percent by applying the methods outlined in this paper. The application of liquid chromatography-mass spectrometry (LC-MS) and ultraviolet fluorescence spectroscopy to the analysis of cotton fibers is shown in this paper to greatly increase their evidentiary value by providing highly specific chemical and structural information about the dyes and brighteners. Republication not permitted without written consent of the author. THE UNIVERSITY OF CENTRAL FLORIDA UNDERGRADUATE RESEARCH JOURNAL 59 www.URJ.ucf.edu The cellulose chains contain sporadic crystalline regions separated by amorphous arrangements of molecules. The degree of crystallinity affects the ability of each fiber to absorb water and dyes. The hydrogen bonding and crosslinking between chains responsible for these crystalline regions increase fiber strength and account for the ability of cotton to absorb as much as 70% of its own weight in water while maintaining structural integrity (Cotton, 2003). Cotton Processing and Textile Manufacture Wrinkle Resistance When cotton is immersed in water, the water molecules penetrate between cellulose chains. The fibers swell to accommodate the extra molecules and wrinkle to release structural tension. This characteristic is undesirable to consumers and led to the development of permanentpress processing. Wrinkle resistant cotton has been altered chemically to form crosslinks not broken by water. Often the substitution of formaldehyde derivatives for hydroxyl groups on the pyranose ring is the method by which a durable press is achieved. However, the heat and the acid needed to catalyze this reaction can shorten fabric life; often, synthetic fibers are added to the yarns to increase textile strength. The most common synthetic used in this manner is polyester; such fabrics are referred to as a polycotton blend (Cellulose, 2003). Mercerization Mercerization is a method textile manufacturers employ to decrease twists in the fibers, thereby increasing the luster of the fibers and their ability to absorb dyes by 25%. Mercerization is achieved by the application of caustic soda (sodium hydroxide) to make the fiber lumen swell, become round, and straighten out. Subsequently tension is applied to the fiber to stretch the fibers straight. When the fibers dry and the lumen collapses there are fewer twists, creating a round, smooth surface that reflects light creating a lustrous sheen (Beaudet, 1999). Long staple cottons (Sea Island, Egyptian, and Pima) naturally have fewer twists and the highest dye adhesion characteristics, so they are the preferred breeds used in textile manufacturing. Mercerization in combination with direct dye can optimize dye absorption and dye-fastness. Fiber maturity and micronaire also influence dye absorption. Fibers in a given textile are of varying degrees of maturity and thickness at the time of harvest. Often all types of fibers are mixed to homogenize yarn quality (Mogahzy, 1998). INTRODUCTION AND BACKGROUND