Painted surfaces are everywhere, so it is not surprising that paint is an important source of trace evidence . Typically, paint chips are transferred in car accidents, either from one car to another or, in the case of a hit-and-run, from the car to the victim. If there is wet paint at the scene of a crime, the perpetrator may also get it on their clothing. When tools like a crowbar are used in a breaking and entry crime, they may end up with microscopic flakes of paint on them. Analysis of paint evidence can therefore make an important contribution to an investigation.
Paint is a complex mixture consisting of pigments, modifiers, extenders, and binders. The pigments give the paint its color. Blue and green pigments tend to be organic compounds , while reds, yellows, and whites are often inorganic compounds . The modifiers control the properties of the paint such as gloss, flexibility, toughness, and durability. An extender adds bulk and covering capacity and is usually inorganic in nature. Some substances, such as titanium oxide, which is white, may act as both a pigment and an extender. A binder is a natural or synthetic resin that helps stabilize the mixture and form a film when it is spread. Topcoat, primer, and undercoat all have different types of chemical composition. The sample may also have been exposed to dirt, rain, and other contaminants, which can complicate the analysis.
Paint samples can be difficult to collect from the scene of a crime. They can be found on a variety of objects, including clothing, vehicles, and tools. Often the paint is mingled with other materials such as dirt or grease, and its removal may well be a specialist task. In the case of paint chips on cars, it is often the undermost layer of the surrounding paint that is most informative; great care has to be taken to preserve it. Matching chips with flakes of paint that have been knocked off a vehicle can be important individualizing evidence, so great care must be taken not to disturb any features of the surface during evidence collection to allow an accurate match.
Because paint has both organic and inorganic components, a variety of different chemical analysis techniques may be used to find out its actual composition. Micro-spectrophotometry in its reflectance mode will help determine the nature of the pigments, while infra red spectrometry will determine its organic components. X-ray powder diffraction is useful for determining the identity of any microcrystalline components. Because paint in the form of a chip is solid, a specialized technique called pyrolysis gas chromatography might be used to determine its composition. Pyrolysis involves heating the sample until it turns into a vapor. This is then injected into a gas chromatograph that separates the components. These can be identified by molecular weight using mass spectrometry, which creates a chemical fingerprint that can be compared to reference samples.
If the paint is in the form of a flake, then information on the number of layers can be obtained by various microscopic techniques. The forensic investigator compares the sample to known paints orcontrol samples , by whatever techniques are most appropriate, to see if they came from the same source. The most individualizing type of paint evidence consists of flakes whose fractured edge can be matched to an area of paint loss. Thus, if a paint flake is found on the clothing of the victim of a hit and run accident, then the perpetrator's car should show a chip whose edge exactly matches that of the flake. The investigator uses a light microscope, a stereomicroscope, and perhaps even a scanning electron microscope to look for a jigsaw-like fit of the edge of the chip and the flake. Analysis of a paint can narrow down a sample to this kind to the make, model, and maybe even the year of a car, making it easier to catch the driver.
Paint analysis was used to help convict British serial rapist Malcolm Fairley, also known as "The Fox," in 1985. After one attack, investigators found minute specks of yellow paint on a tree branch around 45 inches (114.3 cm) from the ground. The paint was analyzed and identified as a type of car paint used on a single model, the Austin Allegro, between 1973 and 1975. Other evidence accumulated and the police went to an address in North London to interview a suspect. A young man was cleaning a yellow Austin Allegro outside. Examination revealed scratches on the paintwork about 45 inches from the ground that matched the paint flakes found at the scene of the crime. On this, and other evidence, Fairley was convicted on several accounts of indecent assault, rape, and burglary and given six life sentences.
see also Gas chromatograph-mass spectrometer.