The scene of a crime often yields a large amount of trace evidence that has come from contact between the perpetrator and his or her surroundings. The importance of collecting and analyzing trace evidence comes from Edmond Locard's Exchange Principle , which states that every contact leaves a trace. That is, criminals leave something of themselves, such as hair or clothes fibers , behind at the crime scene, and they also take something away with them from their contact with people and objects there. Often criminals are not aware of this, because traces of contact evidence are, by their very nature, difficult to detect with the naked eye. It is precisely this property that makes trace evidence so valuable to the forensic investigator. Try as he might, the perpetrator cannot clear away all forms of trace evidence from the crime scene.
The most common forms of trace evidence are bloodstains, hair, textile fibers, paint, and glass fragments. The forensic scientist will be on the lookout for microscopic particles collected from the scene of the crime, to distinguish what is part of that environment and what is linked to the crime that took place there.
At the scene of a crime, the investigators will make an initial assessment that will tell them where to start looking for trace evidence. For instance, if a window has been broken, this might be a good source for textile fibers belonging to the perpetrator that might be matched in an examination of the clothing worn by a suspect. The investigator can probably work with a minute amount of trace evidence, given the power and sensitivity of modern analysis techniques. However, the investigator will be interested in how much material might be available for analysis. Rough surfaces will capture more trace evidence than smooth ones will, for example.
The persistence of the trace evidence is also an issue. Small particles persist longer than larger particles, as do those with irregular surfaces like broken glass or with rough surfaces like wool fibers. Trace evidence landing on a rough surface stays longer than that on a smooth surface. If a suspect's garments are worn between committing a crime and its investigation, there is a high chance that any trace evidence on the clothing may be lost.
However, material forming a smear on the surface through contact persists for longer than particle contact. This applies to blood or paint that might be found on a suspect's clothing. Washing may remove it, but otherwise it is more likely to be detected than particulate evidence, which can be brushed off.
Forensic scientists have a range of techniques for detecting invisible trace evidence. Much depends upon the contrast between the trace evidence and its background. Deeply dyed fibers are clearly easier to detect than pale ones, for example. One of the simplest methods of recovering trace evidence is to shake an item into a test container. This works for collecting glass and paint from garments. Some particulate trace evidence will not be dislodged by shaking but can be collected by brushing with a new toothbrush or paintbrush. Taping trace evidence can also be useful in the case of fibers and hairs. Strips of clear sticky tape are applied to surfaces like garments, car seats, and window ledges to pick up any trace evidence that might have been deposited there. Other methods of collection are vacuuming, swabbing, and hand picking.
There are many different techniques in use for the comparison and characterization of trace evidence, depending on its nature. The physical and chemical properties of glass fragments can easily be measured and compared with control samples . Textile fibers are often found at the scene of a crime. Typically, these are tiny, broken, and fragmented fibers that are not usually visible to the naked eye. Tapings of these fibers are usually first examined under a low power microscope. Further analysis involves a number of techniques such as thin layer chromatography or infrared spectroscopy .
Paint is another important kind of trace evidence. It is particularly important in the case of hit and run accidents, where fragments of paint might have been transferred to a victim's clothes or to another vehicle. If a match can be made between paint chips and the missing flakes of paint using microscopy, then a suspect may be either eliminated or convicted.
Hairs collected from the scene of a crime can also be a significant source of trace evidence. The inner layer, known as the cortex, is where the pigment granules lie, and this determines the color of the hair. However, on its own this kind of evidence is not really individualizing. If the hair has fresh roots, however, it may be a source of blood grouping or DNA profiling , in which case it may well establish identity. Other sources of trace evidence that can prove significant include: oils and waxes, as found in car accidents; soil, which may be trodden in all types of crime; and invisible bloodstains, which can be visualized with luminol . Taking care of the trace evidence may be the key to the investigator helping secure the right result in a crime investigation.
see also Crime scene investigation; Hair analysis; Paint analysis.