Thus, the same analytical scheme is used as with a fiber examination. An association between the rope or cordage in question and any known ropes submitted can be determined. Fibers can be transferred as easily as they can be lost so lack of evidence does not always indicate lack of contact. That is why collection and packaging of fiber evidence is important. Fiber-plastic fusion can occur between the clothing of the occupant and interior surfaces of the vehicle which are made of thermoplastics.
During an impact, parts of the garments can be rubbed under high pressure against the surface of the vehicle which then causes frictional heat. This, in turn, causes local melting of the thermoplastic material. During this contact, fibers from the garment can be transferred and become embedded into the softened plastic.
It is the plastic material that is actually melting and not the fibers. Media Center. Contact DPS. It looks like your browser does not have JavaScript enabled. Please turn on JavaScript and try again.
Fibers and Cordage Page Content. Figure 1 Ropes and cordage may also be submitted to the Laboratory for analysis. Fiber Plastic Fusions Fiber-plastic fusion can occur between the clothing of the occupant and interior surfaces of the vehicle which are made of thermoplastics.
Collection: Trace examiners may use magnification and light to find glass fragments on clothing, an individual or at a crime scene and extract those using tweezers. Tape may also be used to collect glass samples, but the residue left from the adhesive makes this a less desirable collection method. Paint - Painted surfaces are everywhere and the wide variety of layered colors, lusters and types often make paint high-value as evidence.
For example, paint transferred when one vehicle hits another vehicle, a pedestrian or a building can be matched to potentially identify the car in question. In a property crime where a tool is used to break into a building, paint transferred to or from the tool can connect the tool to the location. Analyzing automotive paint can identify the make, model and sometimes the year of a vehicle.
Collection: To collect paint, investigators document the scene, then peel off, or excise, small amounts of paint from the source, being careful to gather all layers. Samples as small as one square millimeter can be used for testing. For a car crash scene, paint samples from the point of contact would be photographed, collected and stored in such a way as to protect the edges for further examination.
This is particularly important when examining for fracture matches. Paint samples are typically collected by scraping small sections down to the metal or original surface or using tweezers to collect chips already dislodged. Most large laboratories or laboratory systems have a trace evidence section. Analysts have a variety of backgrounds, but most require a degree in a natural science with additional certification or additional study in chemistry, particularly if the primary degree is not in chemistry.
Certification is generally conferred on an individual who has achieved specific education, training, experience and performance on competency tests as designated by the certifying organization.
Some areas of trace evidence have individual certification programs, which are facilitated by professional associations and boards.
Since trace evidence covers a wide variety of subcategories, there is similar variety in the testing that is performed. Specialized testing may be done outside of the local laboratory at regional or national facilities.
The type of test performed and the range of information provided vary by the type of evidence tested. For example, analysis of a strand of hair may yield information on the race and general health of the donor, while analysis of a paint sample would likely yield the manufacturer of the paint and its commercial use.
Hair: Hair samples are tested primarily by microscopic comparison and chemical analysis. Microscopic comparison identifies the shape, color, texture and other visual aspects of the sample, while chemical analysis indicates the presence of toxins, drugs, dyes and other chemicals.
In some cases hair is subjected to DNA analysis. Fibers: Trace evidence analysts often have only mere strands to work with. From these strands, fiber testing is done using high-powered comparison microscopes to compare texture and wear in a side-by-side assessment. Chemical analysis can determine the chemical composition of the fibers. In the case of synthetic fabric or carpet, this information can be used to trace the product to the manufacturer using standards databases, further enhancing the probative value of the evidence.
Glass: Glass can yield valuable information through fracture marks, lines and patterns. Testing for unique characteristics such as color, optical properties and density can determine the type of glass, for example a window pane, vase or glass bottle. A detailed elemental analysis, including specific impurities, can be done using laser ablation mass spectrometry, induction-coupled mass spectrometry, X-ray fluorescence or other instruments. Glass shards can be used for sourcing the glass and also to collect potential biological evidence.
0コメント