In even the most perfect of crimes, even by the smartest of criminals, there is always a little trace evidence left behind . The reason for this is the Interchange Theory (Edmund Locard, 1920) which basically states that "every contact [between two objects] leaves a trace". Evidence left at a crime scene can be either macroscopic or microscopic evidence. A piece of a car bumper left at a hit and run scene, spent bullet shells, or a bloody butcher knife all represent macroscopic physical evidence. These can be readily observed, collected and recorded as evidence. Trace evidence, on the other hand, is evidence that is not readily apparent at the crime scene, or at least, not readily apparent to the untrained eye. Forensic crime scene investigators and the trace analysis laboratory are trained to "see" what is not readily obvious. In many cases, this trace evidence is crucial to obtaining a conviction in court or to pointing investigators in the right direction when multiple suspects may be involved.
What is Trace Evidence?
A major challenge for the forensic team is finding, analyzing and interpreting this trace evidence. Trace evidence falls into two major categories: macroscopic and microscopic. Macroscopic trace would include things like a latent fingerprint or palm print left by the perpetrator, a cast made of a shoe print or vehicle tire tread track. The majority of trace evidence is microscopic trace and requires analysis involving detailed exanimation under one or more of a variety of microscopes, or detailed chemical analysis, and in many cases, both. Many pieces of macroscopic trace eventually end up being subjected to microscopic analysis methods in the end.
Finding the Trace
Trace evidence is usually found by careful examination of clothing, rugs, car floor mats or trunk carpet, etc., using one or more of a variety of detection methods. One method of finding trace evidence is through the use of alternative light source (ALS) devices. Recent advances in LED and high intensity flashlights which can use a variety of color filters has greatly improved the ability of a CSI technician to find minute trace evidence. This evidence usually takes the form of a human hair, paint or glass chips, fiber strands, and pieces of tape materials or adhesives residue. Many times fibers and small trace materials can be removed from a rug or fabric with tweezers or with a special transparent adhesive tape strip. In vehicles where a large area, like a trunk or floor rug has to be examined, the area will be vacuumed with a small "Dustbuster" type vacuum fitted with a special filter. The filter material is them removed and any trace materials vacuumed up can be observed and processed. In this case, it is critical that the vacuum be completely cleaned of all trace evidence material from any prior case.
Gunshot residue (GSR) is trace evidence that is usually detected on a suspect's hands and fingers, or from clothing or fabric. The presence of gunshot residue on the back of the hand, thumb or the web area between the thumb and the rest of the hand is removed by using a small disc treated with an adhesive material for collecting the GSR material. These discs are then submitted to the forensic lab for microscopic analysis using a scanning electron microscope (SEM) to look for lead and organic primer materials. Older methods using coated swabs and subsequent analysis for lead by atomic absorption spectroscopy are not as widely used today. Trace metals analysis is more accurately performed by an array of more sophisticated (and expensive) analytical instrumentation. Often these instruments are too expensive for the forensic labs of smaller police departments to own and operate.