Forensic Science

Forensic Science


Forensic science is a multidisciplinary subject used for examining crime scenes and gathering evidence to be used in prosecution of offenders in a court of law. Forensic science techniques are also used to examine compliance with international agreements regarding weapons of mass destruction.

The main areas used in forensic science are biology, chemistry, and medicine, although the science also includes the use of physics, computer science, geology, and psychology. Forensic scientists examine objects, substances (including blood or drug samples), chemicals (paints, explosives, toxins), tissue traces (hair, skin), or impressions (fingerprints or tidemarks) left at the crime scene. The majority of forensic scientists specialize in one area of science.

Evidence and Trace Examination

The analysis of the scene of crime or accident involves obtaining a permanent record of the scene (forensic photography) and collection of evidence for further examination and comparison. Collected samples include biological (tissue samples such as skin, blood, semen, or hair), physical (fingerprints, shells, fragments of instruments or equipment, fibers, recorded voice messages, or computer discs) and chemical (samples of paint, cosmetics, solvents, or soil).

Most commonly, the evidence collected at the scene is subsequently processed in a forensic laboratory by scientists specializing in a particular area. Scientists identify, for example, fingerprints, chemical residues, fibers, hair, or DNA left behind. However, miniaturization of equipment and the ability to perform most forensic analysis at the scene of crime results in more specialists being present in the field. Presence of more people at the scene of crime introduces a greater likelihood of introduction of contamination into the evidence. Moreover, multi-handling of a piece of evidence (for example a murder weapon being analyzed by many specialists) is also likely to introduce traces of tissue or DNA not originating from the scene of a crime. All this results in strict quality controls imposed on collection, handling, and analysis of evidence to ensure lack of contamination. For example, in DNA analysis it is essential that samples are stored at the correct temperature and that there is no contamination from a person handling a sample by wearing clean gloves and performing analysis in a clean laboratory.

Ability to properly collect and process forensic samples can affect the ability of the prosecution to prove their case during a trial. The presence of chemical traces or DNA on a piece of debris is also crucial in establishing the chain of events leading to a crime or accident.

A growing area of forensic analysis is monitoring non-proliferation of weapons of mass destruction, analysis of possible terrorist attacks or breaches of security. The nature of samples analyzed is wide, but slightly different to a criminal investigation. In addition to the already described samples, forensic scientists who gather evidence of mass destruction collect swabs from objects, water, and plant material to test for the presence of radioactive isotopes, toxins, or poisons, as well as chemicals that can be used in production of chemical weapons. The main difference from the more common forensic investigation is the amount of chemicals present in a sample. Samples taken from the scene of suspected chemical or biological weapons often contain minute amounts of chemicals and require very sensitive and accurate instruments for analysis.

Biological traces. Biological traces are collected not only from the scene of crime and a deceased person, but also from surviving victims and suspects. Most common samples obtained are blood, hair, and semen. DNA can be extracted from any of these samples and used for comparative analysis.

DNA is the main method of identifying people. Victims of crashes or fires are often unrecognizable, but adequate DNA can be isolated and a person can be positively identified if a sample of their DNA or their family's

A member of the International Commission on Missing Persons in Bosnia, inspects human remains found by forensics experts in a mass grave at the village of Kamenica, an area of Serbian-controlled Bosnia, in 2002. AP/WIDE WORLD PHOTOS.
A member of the International Commission on Missing Persons in Bosnia, inspects human remains found by forensics experts in a mass grave at the village of Kamenica, an area of Serbian-controlled Bosnia, in 2002.

DNA is taken for comparison. Such methods are being used in the identification of the remains in Yugoslav war victims, the World Trade Center terrorist attack victims, and the 2002 Bali bombing victims.

Biological traces, investigated by forensic scientists come from bloodstains, saliva samples (from cigarette buts or chewing gum) and tissue samples, such as skin, nails, or hair. Samples are processed to isolate the DNA and establish the origin of the samples. Samples must first be identified as human, animal, or plant before further investigation proceeds. For some applications, such as customs and quarantine, traces of animal and plant tissue have to be identified to the level of the species, as transport of some species is prohibited. A presence of a particular species can also prove that a suspect or victim visited a particular area. In cases of national security, samples are tested for the presence of pathogens and toxins, and the latter are also analyzed chemically.

Chemical traces. Forensic chemistry performs qualitative and quantitative analysis of chemicals found on people, various objects, or in solutions. The chemical analysis is the most varied from all the forensic disciplines. Chemists analyze drugs as well as paints, remnants of explosives, fire debris, gun shot residues, fibers, and soil samples. They can also test for a presence of radioactive substances (nuclear weapons), toxic chemicals (chemical weapons) and biological toxins (biological weapons). Forensic chemists can also be called on in a case of environmental pollution to test the compounds and trace their origin. The samples are obtained from a variety of objects and often contain only minute amounts of chemicals.

The identification of fire accelerants such as kerosene or gasoline is of great importance for determining the cause of a fire. Debris collected from a fire must be packed in tight, secure containers, as the compounds to be analyzed are often volatile. An improper transport of such debris would result in no detection of important traces. One of the methods used for this analysis involves the use of charcoal strips. The chemicals from the debris are absorbed onto the strip and subsequently dissolved in a solvent before analysis. This analysis allows scientists to determine the hydrocarbon content of the samples and identify the type of fire accelerator used.

Physical evidence. Physical evidence usually involves objects found at the scene of a crime. Physical evidence may include all sorts of prints such as fingerprints, footprints, handprints, tidemarks, cut marks, tool marks, etc. Analysis of some physical evidence is conducted by making impressions in plaster, taking images of marks, or lifting the fingerprints from objects encountered. These serve later as a comparison to identify, for example, a vehicle that was parked at the scene, a person that was present, a type of manufacturing method used to create a tool, or a method used to break in a building or harm a victim.

An examination of documents found at the scene or related to the crime is often an integral part of forensic analysis. Such examination is often able to establish not only the author, but more importantly identify any alterations that have taken place. Specialists are also able to recover text from documents damaged by accident or on purpose.

Identification. The identification of people can be performed by fingerprint analysis or DNA analysis. When none of these methods can be used, the facial reconstruction can be used instead to generate a person's image. TV and newspapers then circulate the image for identification.

Other Fornsic Scientists

Pathologists and forensic anthropologists play a very important part in forensic examination. They are able to determine the cause of death by examining marks on the bone(s), skin (gunshot wounds), and other body surfaces for external trauma. They can also determine a cause of death by toxicological analysis of blood and tissues.

A number of analytical methods are used by forensic laboratories to analyze evidence from a crime scene. Methods vary, depending on the type of evidence analyzed and information that needs to be extracted from the traces found. If a type of evidence is encountered for the first time, a new method is developed.

Biological samples are most commonly analyzed by polymerase chain reaction (PCR). The results of PCR are then visualized by gel electrophoresis. Forensic scientists tracing the source of a biological attack could use the new hybridization or PCR-based methods of DNA analysis. Biological and chemical analysis of samples can identify toxins found.

Imaging used by forensic scientists can be as simple as a light microscope, or can involve an electron microscope, absorption in ultraviolet to visible range, color analysis or fluorescence analysis. Image analysis is used not only in cases of biological samples, but also for analysis of paints, fibers, hair, gunshot residue, or other chemicals. Image analysis is often essential for an interpretation of physical evidence. Specialists often enhance photographs to visualize small details essential in forensic analysis. Image analysis is also used to identify details from surveillance cameras.

The examination of chemical traces often requires very sensitive chromatographic techniques or mass spectrometric analysis. Four major types of chromatographic methods used are: thin layer chromatography (TLC) to separate inks and other chemicals, atomic absorption chromatography for analysis of heavy metals, gas chromatography (GC), and liquid chromatography (HPLC). GC is most widely used in identification of explosives, accelerators, propellants, and drugs or chemicals involved in chemical weapon production, while liquid chromatography (HPLC) is used for detection of minute amounts of compounds in complex mixtures. These methods rely on separation of the molecules based on their ability to travel in a solvent (TLC) or to adhere to adsorbent filling the chromatography column. The least strongly absorbed compounds are eluted first and the most tightly bound last. By collecting all of the fractions and comparing the observed pattern to standards, scientists are able to identify the composition of even the most complex mixtures.

New laboratory instruments are able to identify nearly every element present in a sample. Because the composition of alloys used in production of steel instruments, wires or bullet casings is different between various producers, it is possible to identify a source of the product.

In some cases chromatography alone is not an adequate method for identification. It is then combined with another method to separate the compounds even further and results in greater sensitivity. One such method is mass spectrometry (MS). A mass spectrometer uses high voltage to produce charged ions. Gaseous ions or isotopes are then separated in a magnetic field according to their masses. A combined GC-MS instrument has a very high sensitivity and can analyze samples present at concentrations of one part-per-billion.

As some samples are difficult to analyze with MS alone, a laser vaporization method (imaging laser-ablation mass spectroscopy) was developed to produce small amounts of chemicals from solid materials (fabrics, hair, fibers, soil, glass) for MS analysis. Such analysis can examine hair samples for presence of drugs or chemicals. Due to its high sensitivity, the method is of particular use in monitoring areas and people suspected of production of chemical, biological or nuclear weapons, or narcotics producers.

While charcoal sticks are still in use for fire investigations, a new technology of solid-phase microextraction (SPME) was developed to collect even more chemicals and does not require any solvent for further analysis. The method relies on the use of sticks similar to charcoal, but coated with various polymers for collecting different chemicals (chemical warfare agents, explosives, or drugs). Collected samples are analyzed immediately in the field in by GC.

A number of instruments used are smaller than ever before, allowing them to be used directly in the field with rapid results. For example, a combined GC-MS analysis device can analyze a sample within 15 minutes directly in the field. The standard laboratory instrument is large with a weight over 100 kilograms, while the portable version is only 28 kilograms. A number of government agencies (for example the FBI) are now armed with the portable instruments and can perform rapid forensic analysis in the field in a time shorter than it would take to transport samples to a forensic laboratory. United States troops are equipped with similar instruments on board some tanks and trucks, in order to quickly determine the presence of chemical or biological weapons on the battlefield

Applications of forensic science. The main use of forensic science is for purposes of law enforcement to investigate crimes such as murder, theft, or fraud. Forensic scientists are also involved in investigating accidents such as train or plane crashes to establish if they were accidental or a result of foul play. The techniques developed by forensic science are also used by the army to analyze the possibility of the presence of chemical weapons, high explosives or to test for propellant stabilizers. Gasoline products often evaporate rapidly and their presence cannot be confirmed, but residues of chemicals, such as propellant stabilizers, are present for much longer indicating that an engine or missile was used.


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Saferstein, Richard. Criminalistics: An Introduction to Forensic Science. New York: Prentice-Hall, 2000.


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Forensic Science Center, University of California Lawrence Livermore National Laboratory, 7000 East Ave., Livermore, CA 94550–9234. (925) 423–1189. < .> (7 February 2003).

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Chemistry: Applications in Espionage, Intelligence, and Security Issues
DNA Recognition Instruments
Document Forgery
Gas Chromatograph-Mass Spectrometer
Isotopic analysis
Polymerase Chain Reaction (PCR)
Thin Layer Chromatography

User Contributions:

I appreciate the explanation of using several tests at once like choma... and mass spectrometric analysis.
It's hard to believe analysis can be accurate at 1 part per billion!
Wow. Very logically and understandably written. Good references too.

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