Paint Ch11 Trace Evidence
Trace Evidence Detection • Many manufactured products and even most natural materials contain small quantities of elements, known as trace elements, present in concentrations of less than 1 percent. • For the criminalist, the presence of these trace elements is particularly useful, because they provide “invisible” markers that may establish the source of a material or at least provide additional points for comparison.
Neutron Activation • For chemists, nuclear chemistry provides a new tool for identifying and quantization of the elements. • A nuclear reactor is simply a source of neutrons that can be used for bombarding atoms, causing some neutrons to be captured to produce radioactive isotopes.
Neutron Activation • Neutron activation analysis measures the gamma-ray frequencies of specimens that have been bombarded with neutrons. • This method provides a highly sensitive and nondestructive analysis for simultaneously identifying and quantization of 20 to 30 trace elements. • Forensic analysis has employed neutron activation on find trace elements in metals, drugs, paint, soil, gunpowder residue, and hair.
Paint • Paint, in one form or another, is one of the most prevalent pieces of trace evidence. • Hit and Run cases typically involve a transfer of paint evidence. • Burglary or Breaking in with a tool typically transfers paint evidence. • Paint evidence can be left at a crime scene in a variety of ways.
Paint • Paint is composed of Binder and Pigments, as well as other additives, all dissolved in a solvent. • Pigments impart color and opacity • The Binder holds together the pigments and additives • The solvent eventually dries and leaves behind a hard polymeric coating.
Car Paint • One of the most common types of paint examined in the crime laboratory involves finishes emanating from automobiles. • Automobile manufacturers normally apply a variety of coatings to the body of an automobile. • These coatings may include electrocoat primer, primer surfacer, basecoat, and clearcoat.
Car Paint • The wide diversity of automotive paint contributes to the forensic significance of an automobile paint comparison. • Questioned and known specimens are best compared side by side under a stereoscopic microscope for color, surface texture, and color layer sequence.
Collection of Evidence • Paint chips are most likely found on or near persons or objects involved in hit-and-run incidents. • Paper druggist folds and glass or plastic vials make excellent containers for paint. • Paint smeared or embedded in garments or objects require the whole item to be packaged and sent to the laboratory. • Uncontaminated standard/reference paint must always be collected. • Tools used to gain entry into buildings or safes often contain traces of paint, requiring the tool be collected, along with reference paint samples.
Analysis • Pyrolysis gas chromatography and infrared spectrophotometry are invaluable techniques for distinguishing most paint binder formulations, adding further significance to a forensic paint comparison. • Solid materials, such as paint, may be heated or pyrolyzed to high temperatures so that they will decompose into numerous gaseous products to flow through the GC column.