Human hair trace material is often critical evidence in rape, murder and abduction cases. Hairs found in bedsheets, floor rugs, or in the clothing of a rape victim can often lead investigators to a potential suspect. Hair structure is determined microscopically and can be indicative or certain nationalities. If the hair is artificially colored, it is often possible thru detailed chemical and spectrographic analysis to determine the type of hair dye used, and this can be compared to samples obtained from potential suspects. If the hair is a pubic hair and has a root still attached, then it is possible to submit this trace evidence to the lab for DNA analysis.
Fibers and Other Trace Evidence
Fibers represent another area of trace evidence that can often be critical in developing a sound case. Fibers from the sweater of an abducted child found in the trunk of a suspect's car potentially place the victim in that individual's car. Likewise, unique automobile rug fibers found on the clothing or body of a murder victim disposed of in a wooded area or shallow grave provide an indication that the victim was in the vehicle of the suspect (or other suspect with similar carpeting). Fibers can be either natural, like cotton or wool, or synthetic like nylon or rayon. Under the comparison microscope these fibers can be matched to known fibers of synthetic or natural material. In a similar manner, animal hair fibers can be identified to the species of origin. With synthetic fibers, it is often necessary to degrade the fibers and break them into their chemical constituents, which can then be analyzed by a variety of analytical organic chemical methods. Variations in chemical composition can sometimes be used to match fibers to specific manufacturers or manufacturing lot of a fiber sample. For colored fibers, a variety of spectroscopic analyses are available to determine the type of dye used to color the fiber. Again, variations in the chemical makeup of the dye can be used to match dye lots and manufacturers to an evidence sample, thus linking it to a particular piece of clothing or other material sold at a specific store or company. Many fiber dyes are complexes of organic chemicals and various metals. Variation in metal content of a fiber material is another indicator often used to match fibers and sources.
Paint, glass, and plastic trace materials can also be analyzed microscopically and compared chemically to known material. Paint chips from a hit-and-run accident can be compared to the Paint Data Query (PDQ) database operated by the FBI and the Royal Canadian Mounted Police. Like the AFIS system, PDQ contains data on tens of thousand paint materials to match the trace to known vehicles of specific makes, model and years.
As new technologies continue to evolve in the forensic laboratory, newer, more sensitive methods of analysis will become available to CSIs and better trace evidence will be available to investigators and prosecutors.