Document Title: The Analysis of Trace Forensic Evidence Using Isotope Ratio Mass Spectrometry: Differentiating Fibers

Fibers are often found at crime scenes and current methods can determine the chemical composition and color of the fiber as well as physical characteristics. Unless the fiber contains DNA, there is no way to compare a fiber found at a crime scene with fibers found in a suspect’s possession. This study aims to determine if isotope ratio mass spectrometry (IRMS) can be used to add an additional level of information about a fiber, potentially allowing two fibers to be compared for common characteristics. This could provide additional evidence that two fibers may have come from the same original source. Between 10 μg and 200 μg of fiber was used for each analysis with an elemental analyzer used to measure the Carbon and Nitrogen isotope ratios, or a high temperature conversion elemental analyzer for the analysis of Hydrogen and Oxygen isotope ratios. A variety of types of non-colored fibers, including cotton, wool, silk, and a number of different synthetics were analyzed. Using a combination of all four isotopes, all fibers were able to be distinguished from other fibers of the same chemical composition but from different manufacturers, or from the same manufacturer but from different years. Homogeneity within a commercial shirt was studied to determine if sampling location is important when comparing two different fibers. All fibers from the same clothing panel were found to be isotopically homogenous, but depending on manufacturer some shirts were found to have different isotope ratios in the shirt body compared to shirt sleeves. This could be the result of different shirt panels being cut from different bolts of cloth and then sewn together. White and colored fibers were compared to determine if the coloring has an effect on the isotope ratio. The manner in which the shirt was constructed was found to determine the effect of color on the fiber isotope ratios. For shirts that were constructed of distinct fibers of different colors, differences in the isotope ratios of differently colored fibers were observed. For shirts woven from a single thread, with different regions colored differently, fibers of different colors exhibited consistent isotope ratio values. Thus, the fiber material itself, and not the coloring, appear to control the isotope ratios. Fibers were stained with blood, grass, or dirt to investigate whether common stains can change the isotope ratio values. Grass and dirt stains were not found to result in a significant change in isotope ratio values likely because these are surface stains. Blood stained fibers, however, did exhibit a change in isotope ratio values, most likely because blood can penetrate the fiber. Since fibers are routinely analyzed for DNA, preand post-DNA processed fibers were compared to determine if isotope ratio analysis can still be used after fibers have been treated to extract any DNA that might be present. Carbon and nitrogen isotope ratios were not found to change, but in one-third of the fiber samples, oxygen and/or hydrogen values were affected by the DNA processing protocol. Based on the results of this study, isotope ratio mass spectrometry appears to be a potentially useful forensic technique for the analysis and differentiation of a wide range of fiber types, both natural and synthetic. Fibers of the same chemical composition but from different manufacturers or production batches can be differentiated using IRMS whereas fibers from the same clothing item have statistically indistinguishable isotope ratio values, provided that the fibers are sampled from the same region of cloth. Thus, fibers from a crime scene and those from a different source will likely be able to be distinguished from one another.