Trace evidence

Trace evidence 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 fingerprints, hairs, fibers, glass, paint chips, soils, 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. Fabric prints of clothing worn by pedestrians in the paint and/or road grime of the striking vehicle can match a specific vehicle involved in a Hit and Run collision.

Protection
The best form of preservation is first to photograph it in situ, and then take the object which shows key traces, protect it and analyse later under controlled laboratory conditions.

Many different techniques are used in the protection of trace evidence from criminal investigations, although all must be photographed when fresh and still in place. Samples may be collected by shaking, brushing, taping, vacuuming, swabbing and hand picking, although great care may be needed to prevent contamination with other substances (such as natural oil and sweat on the hand of the collector). 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 usually involving macrophotography. 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(SEM) or comparison microscope. SEM is especially useful because X-ray analysis can be conducted on selected areas of the sample, so is a form of microanalysis. It is useful where chemical residues can show unusual elements present which may indicate chemical attack of the product. A car accident caused by a diesel fuel leak, for example, showed traces of sulfur on the cracked tube indicative of attack by sulfuric acid from the battery.

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 positives and contamination by subsequent handling, for example, are problems owing to the presence of many common substances and the necessity of human involvement in the collection of trace evidence. Samples from accidents or crimes should therefore be protected as far as is possible by enclosure in a sealable plastic bag as soon as possible after the incident being investigated.