Forensic fiber analysis practices are supported today by the succinct body of knowledge on fibers and their origin. Because of this, through laboratory testing of practically every sample found, experts can easily identify the material present and link it to the same material somewhere else. Adding to this collective understanding can only help. As stated in ASTM E2225-10 - Standard Guide for Forensic Examination of Fabrics and Cordage, gaining an understanding of “the construction, composition, and color of a textile can aid the examiner in including or excluding a textile for consideration in a forensic examination.”
The first step of the analysis of fibers of interest is their extraction. This part of the process is fairly simple, but efforts should be made to prevent contamination of the sample. ASTM E2228-10 - Standard Guide for Microscopic Examination of Textile Fibers
After careful extraction, fibers should be first examined with a stereomicroscope, with which physical features, such as crimp, length, color, relative diameter, luster, apparent cross section, damage, and adhering debris, should be noted. Initial observations of these can help to classify the fiber samples into broader groups, such as synthetic, natural, or inorganic.
Pursuing all possibilities in a broad sense is a necessity for forensic fiber analysis, simply due to the variety of fibers present in the world. Quickly narrowing down all of the originating options for a fiber prevents the forensic specialists from pursuing any false conclusions. For example, one can classify a fiber as a strand of animal hair if it carries its common morphological features: the root, medulla, cortex, and cuticle. Experts can then determine the species of the animal through additional features on the hair shaft.
After basic microscopic examination, it is often important for fibers to be examined through a comparison microscope. This side-by-side, point-by-point examination technique is highly valuable, as it can discriminate between two fibers that appear identical when observed under a single microscopic lens. Comparisons are incredibly important for attaining accurate evidence, since they can link the fibers found on a suspect to those found at the scene of a crime.
Other procedures for analysis can help this process. For the comparison of textile fibers, observations in dye coloration can simplify the link between two fibers from the same source. One common method for observing this is through thin-layer chromatography (TLC), “a separation technique in which the flow of solvent causes the components of a mixture to migrate differentially from a narrow initial zone over a planar, thinly-applied porous adsorptive medium.” This method is addressed in ASTM E2227-13 - Standard Guide for Forensic Examination of Non-Reactive Dyes in Textile Fibers by Thin-Layer Chromatography.
Another complementary method for identifying fibers is microscopic infrared analysis, in which an infrared microscope is paired with an infrared spectrometer to interpret absorption spectra. Fourier transform infrared (FT-IR) is the standard for mid-infrared spectrometer used in this method. However, infrared spectral analysis is only applicable for synthetic, manmade fibers. For natural fibers, as noted in ASTM E2224-10 - Standard Guide for Forensic Analysis of Fibers by Infrared Spectroscopy, “no additional discriminating compositional information of the fiber is provided over that yielded by light microscopy.”
Reliable forensic fiber analysis is important because it cannot only provide leads for certain investigations, but it can also grant investigators the key evidence needed to solve major crimes. From 1996-2002, a serial killer was at large in Spotsylvania, Virginia, and an assembled taskforce pursued 12,000 leads. When they eventually found the killer, the police and FBI agents on the taskforce were able to link him to all of the crime scenes through hair remains and particular blue fibers. In cases like this, forensic fibers are a major component of the total evidence to complete investigations.
Forensic Fiber Analysis Standards are available on the ANSI Webstore.