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Trace evidence is material found at a Crime Scene or Accident scene in small but measurable amounts. This is important as it can definitively link an individual or object to the scene. Trace evidence is based on Locard's Exchange Principle which contends that every contact no matter how slight will leave a trace. 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 . EXAMPLES Examples of typical trace evidence in criminal cases include Hair s, Fiber s, Glass , Paint chips, Soil s, Botanical materials, Gunshot Residue , Explosives residue, and volatile Hydrocarbons ( Arson evidence). For such evidence to be useful, it must be compared to similar items from suspects, but particular care is necessary to ensure a thorough analysis. 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. COLLECTION Many different techniques are used in the collection of trace evidence including shaking, brushing, taping, vacuuming, swabbing and hand picking. In some cases, such as with oil or grease, a solvent extraction can be used to collect the evidence for analysis. The method used for collection is generally dependent on both the type of evidence and from where or what sort of object it is being collected from. ANALYSIS Analysis of trace materials most often begins with a visual examination of the evidence. This is then usually followed by Microscopic Analysis , of which a number of different types are available dependent on the type of material to be analysed such as a Stereomicroscope , Scanning Electron Microscope or Comparison Microscope . 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. PROBLEMS False Positive s and contamination are frequent problems owing to the presence of many common substances and the necessity of human involvement in the collection of trace evidence. SEE ALSO
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