Trace Chemistry

The trace chemistry section is called upon to conduct scientific examinations on a variety of different types of physical evidence such as hairs, glass, wood,  fire debris and flammable liquids.  The trace evidence examiner can be requested to analyze totally unknown substances for identification purposes.  Due to the fact that trace materials are characteristic of their source, and easily transferred between objects and persons, and transfer is difficult to prevent, trace evidence has special significance in forensic science.  This is the basis for Edmond Locardís Exchange Principle Ė when a person comes in contact with an object or another person, a cross-transfer of evidence occurs.  Although trace evidence such as hairs and glass fragments typically do not contain unique individual characteristics like a fingerprint, trace evidence commonly contains class characteristics which can also be used for comparison purposes.  Class characteristics are defined as properties of evidence that can be associated with a group but never to a single source.  Individual characteristics are properties of evidence that can be attributed to a common source with an extremely high degree of certainty.

Arson evidence being extracted in an oven

The most common type of analysis request is the examination of fire debris, clothing from

 suspects and victims, and unknown liquids for flammable liquid identification.  Fire debris is collected at suspicious fire scenes by investigators and packaged into vapor tight metal evidence containers with tight fitting metal lids.  Clothing evidence is also packaged in the same manner.  This type of evidence packaging helps to retain any traces of volatile substances and/or flammable liquids that may still be present.  Various extraction procedures are employed to isolate any traces of flammable liquid.  A portion of this extract is then inserted into a gas chromatograph-mass spectrometer or GC-MS, for chemical analysis.  The examiner then interprets the analytical data printouts and identifies various classes of flammable liquids present in the sample.  The identification is confirmed by comparing sample data to data produced from the GC-MS analysis of known flammable liquids under identical conditions.



Human and animal hairs

 Hair evidence is commonly encountered at crime scenes.  Before a comparison is conducted to help link a suspect to a crime scene, questioned hairs must first be examined under a microscope.  The following may be answered from such an examination:  Is this a hair or fiber? Is the hair human or animal?  What part of the body did the hair originate from?  What is the race of the hair donor?  Has the hair been exposed to cosmetic treatment?  Has the hair been damaged in some manner?  The next step is to perform a hair comparison with the aid of a comparison microscope which consists of two compound microscopes combined into one unit with the aid of an optical bridge.  The examiner can then visually compare questioned hairs against known hairs.  If a crime scene hair is determined to be microscopically similar to the hair standards of a particular suspect, it may be possible to obtain additional forensic information to support a hair association through DNA analysis.  On occasion animal hairs are also of forensic value.  It may be possible to determine from what species of animal the hair originated.


In the process of breaking a window or other glass objects, some of the glass fragments will shower those people in close proximity to the breaking glass.  These tiny glass fragments can become lodged in the suspectís shoes, clothing and in the tools or other objects used to shatter the glass.  The physical properties of refractive index and density are used most successfully for characterizing and comparing glass fragments.  If a piece of flat glass is available containing both the outer and inner surfaces, the thickness of the glass is also measured.  It may be possible to determine from which direction a force was applied to break the glass and the sequence of events if more than one object went through the glass.