Trace Evidence

Crime scene examiners are looking for anything, no matter how minute, that appears to be foreign to the place.  Aside from hair and fiber, along with other bodily secretions, there are more items that can come under scrutiny.  Any object or substance becomes evidence if it can be identified as not naturally belonging to a scene and then linked with a suspect.  Databases for comparison are maintained in most large forensic labs, with the most extensive collection for many types of trace evidence located at the FBI.

Among the most common substances for trace analysis are:

• Glass:  When product tampering made national news in the 1980s, some consumers placed glass fragments in jars of baby food in the hope of a monetary settlement.  Analysis of the fragments indicated that each type of contaminant was from a different source, so this "crime spree" was unlikely to have been the work of a lone tamperer.  Glass is a solidified liquid that has unusual properties.  If a perpetrator smashes glass, some tiny slivers will adhere to his clothing---even after it has been cleaned.  Glass identification involves a complex microscopic examination that measures the "refractive index," calculated from the angle at which a ray of light hits the surface.  Pieces of glass may also be analyzed through spectography or neutron activation, and glass shatter patterns have provided important clues about how an event took place.
• Dirt/dust:  Particles picked up on a suspect's clothing can sometimes reveal where he or she has been, and the same goes for where a corpse has been, which helps to determine whether a murder victim has been moved.  Plant spoors, insects, and other microorganisms that are revealed under the microscope provide clues.  Dust particles generally yield something about their origins, whether from a concrete floor, bricks, cement, or a particular room.  They might offer leads about where someone lives or works.  The 1960 murder of 8-year-old Graeme Thorne was partly solved by traces of pink mortar found on his clothing.  A house with pink mortar was located, and further evidence was found to close the case and convict the killer.
• Palynology:  The is the study of palynomorphs, or pollen data, trapped in or found on materials associated with a crime.  Because of their predictable production and dispersal rates in specific regions, they can help to link a suspect to the scene of a crime.  In fact, palynologists believe that both legal teams missed a bet in the O. J. Simpson case by failing to look into pollen.  If Simpson was present at the crime scene, and had hidden in the bushes as was suggested, his clothing might have picked up pollen spoors.  If not, then this would have been important evidence in his defense.  The earliest cases in this field were in the sixties.  A 1969 murder investigation in Sweden used the presence of pollen in the dirt found on the body to show that she had been killed in a spot other than where she was found.  In an Austrian case, mud on a murderer's boots linked him to a crime scene, and he confessed.  One detective even found pollen in the grease of a killer's gun, and another found pollen in the ink of a document that demonstrated that it was a forgery.
• Paint:  The techniques used for glass analysis are also employed for paint chips.  Chips from cars can be compared to samples in the National Automotive Paint File, which holds more than 40,0000 samples.  Undercoats help to narrow down the possible manufacturers.  Also the shape of a chip can be matched to an area where a chip is missing, and its chemical constituents can be analyzed via releasing the gases and using gas chromatography.  That creates identifying characteristics for each layer and establishes points of comparison.  A trace of yellow paint was found in a spot where a rapist had hidden his car was traced to a specific model.  When a suspect was located through a computer database that included those cars, his vehicle showed the scrape at the appropriate height.  With the police on his trail and evidence accumulating, he confessed and was sent to prison.
• Seeds:  In the 1960 murder of Graeme Thorne, seeds from a rare type of cypress told investigators that the body had been moved from the murder site.  A cypress tree found in a garden of a house that also matched mortar found on the body pointed the police in the right direction.  Further evidence built a solid case against Stephen Bradley, who was convicted.  Like pollen analysis, knowledge of plant life peculiar to certain areas can offer important information for making decisions in criminal investigations.

Other types of evidence and modes of analysis may be lumped under the category of "trace evidence," but the information presented here shows the basic principles:  Collect apparently foreign matter from a crime scene or body, and use the best method for measuring the most clearly identifying characteristics.  That information, when compared with similar substances associated with a suspect, can corroborate other types of evidence and help to build a case.  Right or wrong, juries have convicted on trace evidence alone, and responsible forensic investigators will try to make sure that it truly proves what they claim.