PaintCh11 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.

Pyrolysis gas chromatography