Brief Introduction Of Forensic Evidence
Forensic evidence plays a vital role in an investigation. During an investigation, multiple forensic evidences are collected at the crime scene, analyzed in a laboratory and often prensented in court. These evidence provide a lot of essential clues to the investigators, such as in tracing an illicit substance, identify remains or reconstruct a crime.
According to the National Institute of Justice (NIJ), USA, evidence refers to information or objects that are admitted to the court for judges and juries to consider when hearing a case. In other words, evidence is the documentary or oral statements and the material objects admissible as testimony in a court of law.
In general, evidence can come from various sources and divided into a few categories. One of the major type of forensic evidence is chemical evidence. Chemical evidence can be any chemicals found on people, various objects, or in solutions, generated as part of a crime and recovered at the scene or a related locations. With proper handling and preservation, chemical evidence is able to optimize its value in providing objective and reliable information about the crime.
Forensic Analysis Of Chemical Evidence
Various types of forensic analysis can be perfomed towards chemical evidence. The sample for analysis can be obtained from a of objects and often contain only trace amounts of chemicals. Basically, there are two main goals to be achieved in the analysis of chemical evidence:
- identify the present of targeted chemical (Qualitative Analysis);
- quantitate the chemical (Quantitatve Analysis).
- Drug And Forensic Toxicological Analysis
In forensic toxicological analysis, various kinds of samples collected from crime scene or deceased are being analysed for the present of toxic substances or toxicant, including drugs. There are two key information that need to be included in the toxicology report:
- type of toxic substances present; and
- amount of concentration
These information can be used to make inferences when determining a sustances’s potential effect on individual’s death, illness, or metal or physical impairment. For example, the results of a blood analysis from a driver involved in a car accident can be used to determine if the individual was under the influence of drugs or alcohol.
According to Department of Chemistry Malaysia (JKM), toxicant can be of any substances that can bring harmful effects to the health of lived organisms when intentionally or unintentionally intake into body. Quantity or dose is the main indicator to differentiate a chemical substance of whether it is toxin or a therapeutic drug.
Detection of drug and toxic substances in forensic samples are usually done by an initial screening and followed by a comfirmation of the compounds. The screening and confirmation are usually, but not necessarily done with different analytical methods. The choice of method for testing is mostly depends on what kind of substance that toxicologist expects to find and the material on which the testing is performed. There are a variety of analytical techniques used in forensic toxicology laboratory, such as gas-liquid chromatography for examination of volatile organic compounds, atomic absorption chromatography for analysis of heavy metal, and high liquid performance chromatography (HPLC) for analysis of toxicant or poison.
In general, forensic toxicological analysis is a challenging field. Since most of the samples for toxicological analysis are biological samples, determination of the substance is often complicated as the substance had been go through various mechanisms inside the body and is seldom remain in its original form. At the sametime, the substance may also have been diluted by its dispersal through the body. The difficulty of analysis is further increased as the biological samples is more vulnerable to contamination.
- Fire Accelerant and Its Detection
In the context of fire investigation, accelerant can be referred as any substance or mixture that “accelerates” the development of fire. When accelerant is applied into a fire, more heat is produced, the reactant is consumed more quickly, and the spread of fire is increased as well. Fire accelerant plays a essential role in fire investigation as it provides valuable hint in determining the cause of fire. The accelerant used in arson cases can be presented in various form, such as liquid (gasoline or kerosene) and gases (propane or natural gas). In general, many accelerants are hydrocarbon-based fuels.
As in other investigation, the collection and packing of fire debris collected from fire scene must be carried out properly and carefully in order to avoid the destruction of evidence. Debris collected from a fire must be packed in tight, secure containers as the compounds to be analysed are often volatile. The packed sample is then sent to laboratory for further analysis.
There are multiple techniques in detecting the accelerants from the fire debris. The analysis of fire debris basically can be divided into two parts:
- extraction or isolation of volatile compounds which could be accelerants
- identification of the volatile compounds which are completely isolated from the fire debris.
For the first part, there are few common methods which are being used in forensic laboratory, including solvent extraction, headspace extraction, and adsorption extraction. Once the isolation is completed, the volatile compounds are detected using a combined gas chromatography-mass spectrometry (GCMS).
Forensic analysis of accelerant is never an easy task. Recovery of fire debris containing accelerant at fire scene is a tough job as in most of the cases, the accelerant might be completely burnt off. Besides that, the fire debris also need to be handled properly as most of the accelerants are volatile. Correct analytical method must be chosen in order to obtain a good analysis result.
- Gunshot Residue (GSR) And Detection of Primer Particles
In a crime involving shooting activities of firearm, it is important for an investigator to determine who may have shot the firearm. In this circumstance, gunshot residue (GSR) is the most critical evidence in helping the investigator to reconstruct the crime.
According to Association of Firearms and Toolmark Examiners (AFTE), GSR is defined as the total residues resulting from the discharge of a firearm. It includes both gunpowder and primer residues, plus metallic residues from projectiles, fouling, etc. In general, gunshot residue can be found on the skin or clothing of the person who had fired the gun, on an entrance wound of a victim, or on other target materials at the scene.
Basically, the major primer elements are lead (Pb), barium (Ba), or antimony (Sb). In most of the cases, all the three elements are present. In forensic analysis, the major method for detection of primer residues are analytical and qualitative. Analytical methods include atomic absorption spectrophotometry (AAS) and inductive coupled plasma-mass spectroscopy (ICP-MS), while qualitative methods include scanning electron microscopy with energy dispersive analysis (SEM-EDA) and atomic force microscopy (AFM).
The major problem with GSR analysis is the issue of possible contamination. There are many environmental sources of particles that can yield a GSR-like element and able to affect the analysis result when GSR is contaminated with these particles. Therefore, evaluation of GSR in the forensic laboratory need to be carried out in an extra careful manner in order to avoid the false positive analysis result. Beside that, GSR can also be removed easily from the surfaces they land on. Basically, regular activities such as putting hands in pockets and rubbing hands together can wipe them away. Thus, collection of GSR need to be carried out as soon as possible in order to prevent the loss of the evidence.
Although, there are a lot of limitations in GSR analysis, however, it still plays an important role in the forensic investigation as an useful tool in putting the pieces of the puzzle together, especially when GSR results are interpreted in conjunction with other evidences.
Every incident, be it a crime, accident, natural disaster or armed conflict, leave traces at the scene. The objective of the subsequent investigation is to correctly interprete the facts, reconstruct the event, and understand what had happened. However, since these traces are fragile and transient in nature, forensic analysis of these forensic evidences become more challenging nowadays. Therefore, a good practice in sample handling at crime scene and forensic laboratory is essential in order to obtain an accurate and reliable analysis result.
- Dennis L. McGuire, M.S., “The Controversy Concerning Gunshot Residues Examinations”, Forensic Magazine, January 2008
- AFTE Glosary, Fourth Edition, 2001
- Phil Locke, “Gunshot Residue Evidence – What It Really Takes”, March 2012
|Last Reviewed||:||21 October 2014|
|Writer||:||Phang Hui Lee|
|Accreditor||:||Khoo Lay See|