Trace evidence
Encyclopedia
Trace evidence is evidence that occurs when different objects contact one another. Such materials are often transferred by heat induced by contact friction
.
The importance of trace evidence in criminal investigations was shown by Dr. Edmond Locard
in the early 20th Century. Throughout the past century, forensic scientists have used trace evidence to reconstruct crimes, as well as to describe the people, places and things involved in them. Case studies of real life homicides have been published in the forensic science literature showing how trace evidence has been used to solve these crimes, free the innocent, and bring the guilty to justice. Trace evidence is also important in accident investigation, where movement of one part against another will often leave a tell-tale mark. Such analysis is of great use in forensic engineering
.
s, hair
s, cosmetics, plant fiber
s, mineral fiber
s, synthetic fiber
s, glass
, paint
chips, soil
s, botanical materials, gunshot residue
, explosives residue, and volatile hydrocarbons (arson
evidence). For such evidence to be useful, it must be compared to similar items from suspects, but particular care is necessary to ensure a thorough analysis.
Ladder feet often leave a trace pattern on the ground, so showing how the ladder moved and caused an accident to the user. Skid marks from tires are often critical in determining the sequence of events before and during a car crash. Vehicular accident reconstruction
relies on such marks to estimate vehicle speed before and during an accident, as well as braking and impact forces. Fabric prints of clothing worn by pedestrians in the paint and/or road grime of the striking vehicle can match a specific vehicle involved in a Hit and Run
collision.
Such traces are also known as "witness marks", especially in engineering and may be critical in understanding how a product failed. A typical witness mark could be an impact depression which broke a product, especially if that mark can be matched to the product which made the impact such as a hammer or nail. Such marks are also commonly encountered in criminal cases, and include bite marks, puncture marks, bullet holes etc.
Many different techniques are used in the protection of trace evidence from criminal investigations, although all must be photographed when fresh and still in place. Samples may be collected by shaking, brushing, taping, vacuuming, swabbing and hand picking, although great care may be needed to prevent contamination with other substances (such as natural oil and sweat on the hand of the collector). 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.
Trace Evidence is also found in much smaller amounts at crime scenes.
Analysis of trace materials most often begins with a visual examination of the evidence usually involving macrophotography. This is then usually followed by microscopic analysis
, of which a number of different types are available depending on the type of material to be analysed, such as a stereomicroscope, scanning electron microscope
(SEM) or comparison microscope
. SEM is especially useful because X-ray analysis can be conducted on selected areas of the sample, so is a form of microanalysis
. It is useful where chemical residues can show unusual elements present which may indicate chemical attack of the product. A car accident caused by a diesel fuel leak, for example, showed traces of sulfur
on the cracked tube indicative of attack by sulfuric acid
from the battery.
Gunshot residue may be identified by elemental analysis using atomic absorption or with a scanning electron microscope
equipped with an energy dispersive spectroscope. Small amounts of explosives, volatile hydrocarbons, and other chemicals are identified with the use of analytical instruments, such as gas chromatography, mass spectrometry
, and infrared spectroscopy
, all of which separate out the components of the chemicals.
Similar comments apply to damaged items from an accident scene, but care is needed in ensuring that the sample is not damaged by the testing, or sampling for testing. 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. 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.
by subsequent handling or nearby objects (e.g. mixing of blood from victim and attacker), for example, are problems owing to the presence of many common substances and the necessity of human involvement in the collection of trace evidence. Both can occur with DNA traces and fingerprints. Partial fingerprints are even more vulnerable to false positives. 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.
Friction
Friction is the force resisting the relative motion of solid surfaces, fluid layers, and/or material elements sliding against each other. There are several types of friction:...
.
The importance of trace evidence in criminal investigations was shown by Dr. Edmond Locard
Edmond Locard
Dr. Edmond Locard was a pioneer in forensic science who became known as the Sherlock Holmes of France. He formulated the basic principle of forensic science: "Every contact leaves a trace"...
in the early 20th Century. Throughout the past century, forensic scientists have used trace evidence to reconstruct crimes, as well as to describe the people, places and things involved in them. Case studies of real life homicides have been published in the forensic science literature showing how trace evidence has been used to solve these crimes, free the innocent, and bring the guilty to justice. Trace evidence is also important in accident investigation, where movement of one part against another will often leave a tell-tale mark. Such analysis is of great use in forensic engineering
Forensic engineering
Forensic engineering is the investigation of materials, products, structures or components that fail or do not operate or function as intended, causing personal injury or damage to property. The consequences of failure are dealt with by the law of product liability. The field also deals with...
.
Examples
Examples of typical trace evidence in criminal cases include fingerprintFingerprint
A fingerprint in its narrow sense is an impression left by the friction ridges of a human finger. In a wider use of the term, fingerprints are the traces of an impression from the friction ridges of any part of a human hand. A print from the foot can also leave an impression of friction ridges...
s, hair
Hair
Hair is a filamentous biomaterial, that grows from follicles found in the dermis. Found exclusively in mammals, hair is one of the defining characteristics of the mammalian class....
s, cosmetics, plant fiber
Fiber
Fiber is a class of materials that are continuous filaments or are in discrete elongated pieces, similar to lengths of thread.They are very important in the biology of both plants and animals, for holding tissues together....
s, mineral fiber
Fiber
Fiber is a class of materials that are continuous filaments or are in discrete elongated pieces, similar to lengths of thread.They are very important in the biology of both plants and animals, for holding tissues together....
s, synthetic fiber
Fiber
Fiber is a class of materials that are continuous filaments or are in discrete elongated pieces, similar to lengths of thread.They are very important in the biology of both plants and animals, for holding tissues together....
s, glass
Glass
Glass is an amorphous solid material. Glasses are typically brittle and optically transparent.The most familiar type of glass, used for centuries in windows and drinking vessels, is soda-lime glass, composed of about 75% silica plus Na2O, CaO, and several minor additives...
, paint
Paint
Paint is any liquid, liquefiable, or mastic composition which after application to a substrate in a thin layer is converted to an opaque solid film. One may also consider the digital mimicry thereof...
chips, soil
Soil
Soil is a natural body consisting of layers of mineral constituents of variable thicknesses, which differ from the parent materials in their morphological, physical, chemical, and mineralogical characteristics...
s, botanical materials, gunshot residue
Gunshot residue
Gunshot residue is principally composed of burnt and unburnt particles from the explosive primer, the propellant, as well as components from the bullet, the cartridge case and the firearm used...
, explosives residue, and volatile hydrocarbons (arson
Arson
Arson is the crime of intentionally or maliciously setting fire to structures or wildland areas. It may be distinguished from other causes such as spontaneous combustion and natural wildfires...
evidence). For such evidence to be useful, it must be compared to similar items from suspects, but particular care is necessary to ensure a thorough analysis.
Vehicular accident reconstruction
Vehicular accident reconstruction is the scientific process of investigating, analyzing, and drawing conclusions about the causes and events during a vehicle collision...
relies on such marks to estimate vehicle speed before and during an accident, as well as braking and impact forces. Fabric prints of clothing worn by pedestrians in the paint and/or road grime of the striking vehicle can match a specific vehicle involved in a Hit and Run
Hit and run (vehicular)
Hit-and-run is the act of causing a traffic accident , and failing to stop and identify oneself afterwards...
collision.
Such traces are also known as "witness marks", especially in engineering and may be critical in understanding how a product failed. A typical witness mark could be an impact depression which broke a product, especially if that mark can be matched to the product which made the impact such as a hammer or nail. Such marks are also commonly encountered in criminal cases, and include bite marks, puncture marks, bullet holes etc.
Protection
The best form of preservation is first to photograph it in situ or where the object is first located, and then take the object which shows key traces, protect it and analyse later under controlled laboratory conditions.Many different techniques are used in the protection of trace evidence from criminal investigations, although all must be photographed when fresh and still in place. Samples may be collected by shaking, brushing, taping, vacuuming, swabbing and hand picking, although great care may be needed to prevent contamination with other substances (such as natural oil and sweat on the hand of the collector). 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.
Trace Evidence is also found in much smaller amounts at crime scenes.
Analysis
Microscopy
Microscopy is the technical field of using microscopes to view samples and objects that cannot be seen with the unaided eye...
, of which a number of different types are available depending on the type of material to be analysed, such as a stereomicroscope, scanning electron microscope
Scanning electron microscope
A scanning electron microscope is a type of electron microscope that images a sample by scanning it with a high-energy beam of electrons in a raster scan pattern...
(SEM) or comparison microscope
Comparison microscope
A comparison microscope is a device used to analyze side-by-side specimens. It consists of two microscopes connected by an optical bridge, which results in a split view window enabling two separate objects to be viewed simultaneously...
. SEM is especially useful because X-ray analysis can be conducted on selected areas of the sample, so is a form of microanalysis
Microanalysis
Microanalysis is the chemical identification and quantitative analysis of very small amounts of chemical substances or very small surfaces of material...
. It is useful where chemical residues can show unusual elements present which may indicate chemical attack of the product. A car accident caused by a diesel fuel leak, for example, showed traces of sulfur
Sulfur
Sulfur or sulphur is the chemical element with atomic number 16. In the periodic table it is represented by the symbol S. It is an abundant, multivalent non-metal. Under normal conditions, sulfur atoms form cyclic octatomic molecules with chemical formula S8. Elemental sulfur is a bright yellow...
on the cracked tube indicative of attack by sulfuric acid
Sulfuric acid
Sulfuric acid is a strong mineral acid with the molecular formula . Its historical name is oil of vitriol. Pure sulfuric acid is a highly corrosive, colorless, viscous liquid. The salts of sulfuric acid are called sulfates...
from the battery.
Gunshot residue may be identified by elemental analysis using atomic absorption or with a scanning electron microscope
Scanning electron microscope
A scanning electron microscope is a type of electron microscope that images a sample by scanning it with a high-energy beam of electrons in a raster scan pattern...
equipped with an energy dispersive spectroscope. Small amounts of explosives, volatile hydrocarbons, and other chemicals are identified with the use of analytical instruments, such as gas chromatography, mass spectrometry
Mass spectrometry
Mass spectrometry is an analytical technique that measures the mass-to-charge ratio of charged particles.It is used for determining masses of particles, for determining the elemental composition of a sample or molecule, and for elucidating the chemical structures of molecules, such as peptides and...
, and infrared spectroscopy
Infrared spectroscopy
Infrared spectroscopy is the spectroscopy that deals with the infrared region of the electromagnetic spectrum, that is light with a longer wavelength and lower frequency than visible light. It covers a range of techniques, mostly based on absorption spectroscopy. As with all spectroscopic...
, all of which separate out the components of the chemicals.
Similar comments apply to damaged items from an accident scene, but care is needed in ensuring that the sample is not damaged by the testing, or sampling for testing. Such nondestructive testing
Nondestructive testing
Nondestructive testing or Non-destructive testing is a wide group of analysis techniques used in science and industry to evaluate the properties of a material, component or system without causing damage....
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. 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.
Problems
False positives and contaminationContamination
Contamination is the presence of a minor and unwanted constituent in material, physical body, natural environment, at a workplace, etc.-Specifics:"Contamination" also has more specific meanings in science:...
by subsequent handling or nearby objects (e.g. mixing of blood from victim and attacker), for example, are problems owing to the presence of many common substances and the necessity of human involvement in the collection of trace evidence. Both can occur with DNA traces and fingerprints. Partial fingerprints are even more vulnerable to false positives. 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.
See also
- Forensic chemistryForensic chemistryForensic chemistry is the application of chemistry to law enforcement or the failure of products or processes. Many different analytical methods may be used to reveal what chemical changes occurred during an incident, and so help reconstruct the sequence of events...
- Forensic engineeringForensic engineeringForensic engineering is the investigation of materials, products, structures or components that fail or do not operate or function as intended, causing personal injury or damage to property. The consequences of failure are dealt with by the law of product liability. The field also deals with...
- Forensic materials engineeringForensic materials engineeringA branch of Forensic engineering, the subject focuses on the material evidence from crime or accident scenes, seeking defects in those materials which might explain why an accident occurred, or the source of a specific material to identify a criminal...
- Forensic polymer engineeringForensic polymer engineeringThe study of failure in polymeric products is called forensic polymer engineering. The topic includes the fracture of plastic products, or any other reason why such a product fails in service, or fails to meet its specification...
- Forensic photographyForensic photographyForensic photography, sometimes referred to as forensic imaging or crime scene photography, is the art of producing an accurate reproduction of a crime scene or an accident scene using photography for the benefit of a court or to aid in an investigation. It is part of the process of evidence...
- Forensic profilingForensic profilingForensic profiling refers to the study and the exploitation of traces in order to draw a profile relevant to the supporting of various security tasks, mostly in the criminal justice system. The term forensic in this context refers to "information that is used in court as evidence" . The traces...
- Forensic Science
- Locard's exchange principleLocard's exchange principleThe Locard exchange principle, also known as Locard's theory, was postulated by 20th century forensic scientist Edmond Locard.Locard was the director of the very first crime laboratory in existence, located in Lyon, France...
, which states that when two objects come into contact, there is an exchange of material - Skid marks
- Digital traces