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Trace evidence is found at a crime scene or accident scene in small but measurable amounts. It is important because it may show the presence of specific individuals, or specific products at the scene of a crime or accident. The trace is normally caused by objects or substances contacting one another, and leaving a minute sample on the contact surfaces. Material is often transferred by heat induced by contact friction.

Contents 1 Examples 2 Analysis 3 See Also 4 References

Examples of typical trace evidence in criminal cases include hairs, fibers, soils, botanical materials, gunshot residue, explosives residue, and volatile hydrocarbons (arson evidence). For such evidence to be useful, it must be matched against similar items from suspects, but great care is needed in analysis to ensure a good match. False positives are a frequent problem owing to the presence of many common substances. There must be distinctive matches for trace evidence to be useful. Another problem occurs when extraneous contamination occurs, for example, when an object is touched. This is why it is important to protect suspect items (using plastic bags for example) as soon as possible after an incident.

Ladder feet often leave a trace pattern on the ground, so showing how the ladder moved and caused an accident to the user. Skid marks from tires are often critical in determining the sequence of events before and during a car crash. Vehicular accident reconstruction relies on such marks to estimate vehicle speed before and during an accident, as well as braking and impact forces.

The most common instrument used in trace evidence analysis is first, the trained eye of the investigator, aided by a simple hand lens, followed by the a simple binocular stereomicroscope. Specialised microscopes such as the compound polarized light microscope are essential for examination of crime scene samples. Comparison of the item with a new or untouched sample is often very helpful.

Gunshot residue may be identified by elemental analysis using atomic absorption or with a scanning electron microscope equipped with an energy dispersive spectroscope. Small amounts of explosives, volatile hydrocarbons, and other chemicals are identified with the use of analytical instruments, such as gas chromatography, mass spectrometry, and infrared spectroscopy.

Similar comments apply to damaged items from an accident scene, but care is needed in ensuring that the sample is not damaged by the testing, or sampling for testing. Such nondestructive testing must always be used first before then considering methods which involve taking small samples from the item for more detailed tests, such as spectroscopic analysis. Use of all such methods must be done in consultation with other experts and the relevant authorities, such as lawyers on both sides of a case.

• Forensic Science
• Forensic engineering

• Forensic Materials Engineering: Case Studies by Peter Rhys Lewis, Colin Gagg, Ken Reynolds, CRC Press (2004).