Trace evidence
The Trace Impressions and Materials Section performs analysis on trace evidence using a wide variety of instrumentation, microscopes and chemical testing techniques. Trace evidence refers to evidence that is transferred from one person to another person or place. This concept of transfer is known as Locard's Exchange Principle. The identification or characterization of these unknown materials is accomplished by the examination of their physical properties using wet chemistry techniques or analytical instrumentation.
Trace impressions
Footwear and tire tracks
Trace impressions are often times recovered at crime scenes. These impressions can be compared to the suspect’s footwear to see if the shoes could have made that impression. Tire tracks can be found in snow, mud, dirt, or sand or even on a victim at a crime scene. Tire tracks from the scene can be compared to tires or known tire impressions from the suspect's vehicle. Impressions at scenes can be collected in several different ways. They can be collected by photographing, casting, lifting, or collecting knowns from the victim or scene.
Make and model determination
When unknown footwear impressions or tire tracks are found at a crime scene, the make and model of the shoes or tire that made those impressions may be identified using various databases. This information could help generate investigative leads. Unknown footwear impressions are searched through the internal BCA Footwear Reference Collection and Solemate® FPX databases or by conducting online searches. Similarly, unknown tire track impressions are searched using Tire Tread Design Guides®. These guides are published annually and provide an accurate, current collection of technical tire information.
Trace materials
Chemical unknown analysis
The analysis of chemical unknowns involves the identification or characterization of unknown materials by examination of physical properties, wet chemistry or using analytical instrumentation. This type of evidence can be encountered at a variety of crime scenes. Evidence may be in the form of a solid, liquid, mixture or gas. Submissions may be for identification or comparison to a known sample. Commonly encountered evidentiary items are pepper spray, tear gas, bank dyes, acids, bases and bleach.
Glass
Glass is extremely prevalent in our lives and used in a variety of applications including windows, bottles, containers, light bulbs and even our cell phone screens. When glass breaks, it easily disperses onto nearby surfaces and may be useful to connect a suspect to a crime scene. Several types of examinations can be performed on glass, such as categorizing the type of glass (for example, window, eye glass or container glass). Questioned glass, for instance from a suspect's clothes or around the body of a hit and run, can be compared to a known source of glass from a crime scene or perhaps a suspect's car. These comparisons first look to see if there is a physical fit between the questioned and known glass (think of a large, clear jigsaw puzzle). This exam can conclude that the two glasses (questioned and known) were once a single unit. Without finding a physical fit, the chemical and physical properties are compared to see if the glasses share the same characteristics and therefore, are likely from the same source.
Paint
Paint is found on the surface of many things. It is used mainly to protect the material it is placed on and for decorative purposes. Paint formulations are specific for what the paint will be used for. There are automotive paints, architectural paints, tool paints, bike paints, cosmetic paint (fingernail polish), boat paints, etc. Through examination of paint obtained at the scene of a crime a link could be made between people, places, and/or objects. The examination of paint usually involves the comparison of paint from the crime scene with a sample of paint that has been taken from a known source. The trace evidence section also has the ability to examine automotive paint left at a crime scene and potentially develop a list of cars in which the paint could have come from. This is done through a database called Paint Data Query (PDQ).
Physical fit
Broken, torn or cut materials can be fit back together directly, indirectly or both. A physical fit is determined when two pieces of a material are physically fit back together using pattern of the broken edges and surface features. An indirect physical fit occurs when two pieces of a material are linked together using surface features such as scratches or extrusion marks. A physical fit or an abundance of matching surface features is sufficient to establish an association between the pieces. A physical fit may be established within many types of materials. Possible types of evidence include tape, plastic, metal, glass, paint, wood, matches and plastic bags.
Tape
Tape is commonly encountered in our environment. Often times we see tape as evidence in crimes committed against individuals such as kidnappings and homicides. The analysis of tape includes a physical fit examination; determining if torn or cut pieces of tape could be directly associated back to a known roll of tape and then possibly followed by an examination of the physical and chemical characteristics of the tapes if a physical fit is not determined. When comparing two tapes the various components that make up the tape are examined including the adhesive and backing film. If the tape is a cloth reinforced tape, like duct tape, then a fiber examination would also be part of the tape examination.
Trace collections
Searches can be conducted on items for the presence of trace evidence such as paint, glass, fibers and hair. A visual search is performed using natural or alternate light sources to find trace materials that may be on a piece of clothing, footwear, a knife or other items from the scene or suspect. In addition, specialized microscopes can be used to further examine items for very small particles. The materials are collected using gloved fingers, scraping or by the use of tools such as tweezers or post-its, tape lifts or vacuuming.