Forensic chemistry is the application of principles of chemistry to law enforcement and criminal justice system. There are various analytical methods which may be used to find out the chemical changes that occurred during any incident, and in this way it helps in the reconstruction of crime scene. Forensic chemistry meets the need of both the scientific and the legal communities by innovations in its research, practice, and presentation.
History
The history of Forensic Chemistry began with analysis of poisons. In early times, there are evidences of the use of poisons by early Egyptians, Greeks and Romans. At that time, poisons were used both for murder and for the execution of death penalty; the philosopher Socrates was sentenced to death by drinking hemlock. Before the development of any systematic scientific procedure of criminal investigation, circumstantial and hearsay evidences were usually employed to determine the guilt of a person. Arsenic was a commonly used poison during Roman times. The first instance of an actual chemical test for poison is the Blandy trial conducted during 1752.
ROLE OF A FORENSIC CHEMIST
The ability of forensic professionals to establish the presence of a link between accused and crime by matching physical evidence collected from the scene of crime and those which are obtained from the victim or accused (Law of Comparison), strengthen the case of prosecution.
Forensic chemists perform different analyses to spot materials so as to work out the character and composition of such evidence. A trained forensic chemist is capable of determining the composition and nature of materials and predicting the source also matching evidences collected from the crime scene with the control sample. Modern chemistry employs the analytical techniques hand in hand with much younger methods of study.
Physical evidence collected from the scene of crime is properly collected, packaged and sealed in special containers to order to maintain its integrity and to prevent contamination. A chain of custody should be maintained for each evidence which is sent to forensic laboratory for examination. At the laboratory, the evidence is examined by trained personnel to reach up to a definite opinion.
Forensic chemists have a crucial role in forensic science laboratories and when required they are often called on in the courts, to provide testimony as expert witnesses. In these cases, the chemist may compare questioned evidence to control samples and may be asked to give an expert opinion in court on the basis of grounds on which such opinion is based. Their academic credentials, along with years of experience to deal with such analysis, render their opinion valuable and
believable.
The fact that most samples that are submitted for analysis are contaminated with dirt or debris and
several other contaminants, this poses a major challenge before forensic chemist. Every substance that is collected from the scene of crime has a unique composition that can ultimately be identified. Arsonists, for example, often use accelerants such as kerosene or gasoline to accelerate the rate of combustion and each accelerant has its own unique composition.
A forensic chemist may look for and collect burnt and unburnt material samples, extract the volatile hydrocarbons from them, and should use gas chromatography to separate the components of these samples.
Cases Encountered in Forensic Chemistry
Forensic Chemistry deals with the qualitative analysis of physical evidences. These include samples of suspected accelerants from noxious chemicals, arson debris, propellants, lubricants, gunshot residues, etc. The cases that are typically examined include, homicides, arsons, assaults, robberies, etc. Some of them are discussed below:
Gun Shot Residue: In addition to arson debris, this section also analyzes evidences associated with shooting cases. When ammunition is discharged from a weapon, gases are generated which contain burned and
unburned components of primer and propellant cartridge which may deposit on the hands and cloths of the person firing the weapon and is referred to as Gun Shot Residue (GSR). Scanning Electron Microscope (SEM) coupled with an Energy Dispersive Spectrometer (EDS) is used to examine tape lifts taken from the hands of suspected persons. If elements like barium, antimony, and lead is found in the sample, it is classified as primer residue.
Toxins: Forensic chemists examine a vast range of evidences such as blood stains, urine, etc for traces of
drugs or poisons. Many businesses including sports now require the drug screening of employees
and sports persons; it is the chief task of the forensic chemist to distinguish between the external addition of illegal drugs and metabolites from certain food materials like poppy seeds. Such tests may be as simple as various color tests or may involve different instrumental techniques ranging from simple to complex one. Paper or thin-layer chromatography or complicated techniques like gas chromatographic or electrophoresis could be used for the analysis of different toxins.
In suspected death, samples of the victim’s blood, lungs, urine, stomach content and vitreous humour are examined for traces of poisons. Insects, in some cases, found on the dead bodies are also examined; as they may absorb drugs or poisons from the body in trace amounts, which sometimes are found long after, even when the concentrations in the body have fallen
down.
Noxious Chemical: Noxious chemicals like capsaicin spray and tear gas can be both qualitatively as well as
quantitatively analyzed by using the Gas Chromatograph-Mass Spectrometer (GC-MS). Tear gas is a component in dye packs that are used by banks to identify the suspects as they may be found on the clothing of the person involved, after the dye pack has exploded.
Arson Investigation: Arsonists may use a number of accelerants to set a fire. During the examination of any arson scene and its background cause, investigators must look for the fire debris that they believe to
contain residues of the accelerants used in arson. The debris is collected and packed in airtight
containers to avoid loss of its volatile components and is then sent to Forensic lab for analysis. Gas Chromatography-Mass Spectrometry is employed to identify and quantitatively analyze the traces of ignitable liquid residue in collected samples.
Along with this the forensic chemists often concentrate on several other minute residue traces. They do this by using activated charcoal strips to adsorb the accelerant residues which are then eluted off the strip
by dissolving in a solvent.
Soil: The composition and character of soil not only varies with depth, but also laterally. In most of the cases, generally the superficial layers of soil are examined, unless there is a crime like digging of a grave. However, the changes of texture and color of soils are not that much morphologically visible along the ground, but its chemical composition can show a variety changes even in a very short distance range, and it helps in accurately determining the source of the soil sample. Therefore, in order to determine the normal distribution of soil of a particular crime scene, sufficient soil samples should be submitted. Two or three soil samples of crime scene are generally sufficient.
IMPRESSIONS – Insure that impressions in soil such as footprints, tire marks or any other patterns are photographed by keeping a scale nearby to take the dimensions of that print and a cast of plaster of paris is made before in any manner disturbing the footprint. Photographs should be taken with the camera keeping it perpendicular to the impression. This will prevent the photograph from angle distortion. If the soil is firmly affixed to any object, it should not be removed.
Such objects must be AIR DRIED and properly be packed in paper bag or other appropriate container, along with proper sealing and labeling. Clean paper envelope should be wont to pack loose sand or soil sample, which may later be sealed in suitable container and labeled as to source.
Miscellaneous Analyses: The Forensic Chemistry can also assist in investigation of sexual assault cases by examining the lubricants (even when present in traces) used by suspects in rape cases.
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