Trace Evidence by Kathy Reichs download in iPad, pdf, ePub
Many techniques are used in the protection of trace evidence from criminal investigations, although all must be photographed as soon as possible, and while still in place. Trace Evidence is also found in much smaller amounts at crime scenes. Such marks are also commonly encountered in criminal cases and include bite marks, puncture marks, bullet holes etc. Samples may be collected by shaking, brushing, tapping, vacuuming, swabbing and hand picking. The droplets are round and show no splattering, indicating they impacted relatively slowly, in this case from a height of two feet.
Such nondestructive testing must always be used first before considering destructive methods which involve taking small samples from the item for more detailed tests, such as spectroscopic analysis. 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. 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.
Great care may be needed to prevent contamination with other substances such as natural oil and sweat on the hand of the collector. Samples from accidents or crimes should therefore be protected as much as possible by enclosure in a sealable container as soon as possible, after an incident is under investigation. Gunshot residue may be identified by elemental analysis using atomic absorption or with a scanning electron microscope equipped with an energy dispersive spectroscopy. Partial fingerprints are even more vulnerable to false positives. Analysis of trace materials most often begins with a visual examination of the evidence usually involving macrophotography.