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The Essence of Forensic Science

Physical Evidence1

No two crime scenes are alike; each is unique in location, persons involved, activities, and the objects present at the scene. However, scenes share one common thread: when a crime or incident occurs, the status of ordinary common objects present at, or brought to the scene is suddenly elevated as they become crucial physical evidence which need to be carefully collected, preserved and methodically examined to extract information of the incident. 

Do not underestimate or overlook these silent witnesses as they may have “recorded” objective information that human witnesses missed. They may provide the answers you seek. Recognising2 and recovering relevant physical evidence at crime scenes is a key step towards criminal justice. This evidence becomes the working material for the forensic laboratory. The role of the forensic scientist is to examine the evidence, extract the essence of the “recordings”, interpret each “recording”, and stitch them together as a coherent whole. The value of the forensic findings can be better appreciated and brought to bear when different stakeholders in the judicial system are aware of their significance and limitations. 

This article provides a glimpse of the basis of forensic science. We will discuss the key scientific principles, methodologies commonly used, and the significance and evidential value of the physical evidence. The types of forensic analyses applicable to various crime and incidents are listed for quick and easy reference.

Types of Physical Evidence

Fingerprints, footprints and DNA can associate an individual with a crime, identifying the “who” of a case. Trace evidence such as paint, fibres, ignitable liquids, explosives, gunshot residues, glass, toxic substances and soil can be useful in linking evidence from the scene to its possible source, and in narrowing down or eliminating suspects. Impression evidence such as shoeprints, tyreprints and toolmarks can be associated to the specific source of the impression. 

Injuries and bloodstains commonly occur in a violent scene. In such crimes, bloodstain pattern analysis (“BPA”) supplements the above disciplines, revealing the “what”, “when”, “where” and “how” of the crime. BPA examines bloodstains present at the scene and on individuals and objects. It has the potential to shed light on the nature and sequence of events occurring, the weapons used, the area of origin of the bloodshed, and the relative positions, the movements and interactions of objects and individuals during and after the crime.  

In drug consumption and trafficking cases, identifying and quantifying the illicit drugs, as defined in the Misuse of Drugs Act is a key element in proving the crime. Chemical identification is also integral to toxicological analysis; biological samples from the victim of poisoning and evidence collected at the scene are screened for toxic chemicals, alcohol, pharmaceutical and illicit drugs, agricultural chemicals, household and industrial products.   

Underlying Principles

Forensic science is an applied science, drawing on sciences such as chemistry, physics, biology, mathematics, engineering and specific domain knowledge pertaining to each forensic discipline. Take paint evidence as an example: besides being well-versed with the physical and analytical techniques employed in the analysis of paint fragments, the scientist has to understand the types of paint coatings, the chemical ingredients used and their properties, the mechanisms of a paint transfer, the collection and recovery of paint evidence, the significance and limitation of the evidence. 

Forensic science seeks to uncover the truth regarding past events. It is grounded on the Scientific Method, an objective and reliable method of inquiry that is structured, systematic, iterative and self-correcting (See Figure 1). It starts off with defining and scoping the investigative problem in a form that can be scientifically interrogated and tested using accepted laboratory methods. Preliminary empirical data is gathered through scene observation, and recognition of potential physical evidence. The nature of the crime, facts gathered from investigators, and the available physical evidence lead to the formulation of a hypothesis regarding the crime and the suspect. The hypothesis consists of questions that can be answered by conducting examinations, analyses, experiments and comparisons, and interpreting the results using deductive reasoning. If the hypothesis is not supported by findings, it is rejected (excluded). In such circumstances, findings may prompt the formulation of a new hypothesis which is tested by additional inquiry and forensic examinations. The process, raw data, interpretations and basis for the conclusion are documented and can be independently reviewed by another competent scientist.

Figure 1: The scientific method

For forensic disciplines such as illicit drugs, toxicology and DNA profiling where the problem is essentially target analysis to identify controlled substances and poisons, or to obtain and compare the DNA profile. The formulation of the hypothesis here is pre-defined. The complexity of problem definition and hypothesis development and testing increases for non-routine forensic examinations, especially those involving reconstruction. In such cases, the problem for the case is likely to be broken down into several parts and the hypothesis is developed for each part.      

At this juncture, it must be emphasised that it is empirical data that determines the conclusion. The forensic scientist must never form the conclusion first, and then try to collect suitable data to support it.  

Four key principles2 are associated with forensic science: 

1. Principle of individuality;

2. Principle of divisible matter;

3. Principle of exchange; and

4. Principle of causality.

The principle of individuality states that “no two objects are identical”. The principle of divisible matter states that “matter divides into smaller components when sufficient force is applied. Some of the features are unique to each piece while some of the features are common to both. The passage of time may affect the two pieces differently because they are subjected to different environments and influences”. 

These two principles have a huge bearing on the individualisation of an object, even more so, when it ages and changes due to wear and tear, or when it breaks into smaller components. They form the basis of toolmarks, manufacturing marks, impressions, physical comparisons and association to a source. The manufacturing processes of tools such as firearms, pliers and cutters, will impart specific manufacturing characteristics and random and incidental defects to the tools. With use, these objects will acquire additional features such as wear characteristics and other random characteristics. 

The third principle commonly known as the Locard’s principle of exchange states that “every contact leaves a trace”. When two objects come in contact, they will add something or remove something from each other, usually unintentionally and sometimes unknowingly. This principle provides the basis for transfer evidence such as DNA, fibres, paint, glass and soil. In a traffic collision where vehicles come into forceful contact, or in a fight where there is body contact amongst persons, trace evidence is transferred from one object or person to the other. In the former, we examine for transferred paint and glass evidence, and in the latter, fibre evidence.   

The principle of causality states that every action, condition or event has something that preceded it (a cause), and something that follows after it (an effect). In other words, nothing just happens. This forms the basis of scene reconstruction and the selection of physical evidence. When physical evidence is collected, we seek answers for the following questions:

1. Q1: What is this object and what function does it serve?

2. Q2: Is this object associated with other objects at the scene?

3. Q3: What does this object tell us about the sequencing of events?


The workflow for the forensic analysis of physical evidence typically involves five stages as shown in Figure 2. 

The purpose of the examination can usually be classified into three main categories: identification, comparison and reconstruction:

1. Identification is the process of determining physical and/or chemical identity of a substance to the exclusion of other possible substances. 

2. Comparison is undertaken to associate persons, objects and the scene.

3. Reconstruction further elevates the evidential value of physical evidence by exploring the activities that resulted in the presence or state of the physical evidence. It identifies the possible events and sequence in relative time and space.

Figure 2: Workflow


Examples of identification include the analysis of flammable liquids, explosive residues, unknown stains and liquids, adulterants, toxic substances and illicit drugs. The analysis process typically involves preliminary screening tests, followed by confirmatory tests. Quantitative analysis may be performed to determine the concentration or amount of the analyte(s) of interest. The equipment, techniques and procedures used are validated for reference materials, before applying them to ascertain the identity of questioned materials.

The techniques can be classified into physical techniques and analytical techniques:

1. Physical techniques look into the macroscopic, microscopic appearances, texture, and morphology of substances. Features such as dimensions, weight, pH, densities, melting point, boiling point, viscosity and optical properties are measured. 

2. Analytical techniques assist in determining the chemical composition of a substance by providing structural information of the compound. In forensics, the major analytical techniques used are Chromatography, Capillary Electrophoresis (“CE”), Mass Spectrometry (“MS”) and Spectroscopy. 

a. Chromatography and CE are separation techniques. In both techniques, the sample is introduced into a separation column or capillary. Chromatography works on the physical-chemical interactions of the analytes with the stationary phase in the column and the mobile phase that flows continuously through the column, whereas CE relies on the differential migration of charged particles when a high voltage is applied across the capillary. CE is used in the analysis of DNA where the fragments are separated based on their size. 

b. MS is an identification technique. It identifies a substance based on the mass fragmentation pattern that is unique to each compound. Due to its high discriminating power, it is commonly used as a detector which is coupled to chromatographic equipment to confirm the presence of a compound. The bulk of forensic analyses such as drugs, toxic substances, flammables, explosives analyses uses chromatographic-mass spectrometric-equipment.

c. Spectroscopy studies the interaction between the substance and electromagnetic radiation such as ultraviolet light, infrared light and X-rays.  

Identification is commonly based on multiple complementary techniques. For an identification, the number and type of techniques used must be sufficient to exclude other substances. The choice of the technique depends on the chemical characteristics of the substance, and its expected concentration in the sample. Techniques which are more sensitive and discriminating are usually the methods of choice for confirming the identity of a substance. 

For non-routine analysis involving an unknown material, the forensic scientist relies on knowledge gained through education, training and experience to select the battery of techniques to use, and to determine at which point he/she is satisfied that the analysis was comprehensive and sufficiently complete. After interpretation of the findings, the scientist makes a conclusion which is appropriate and relevant to the findings, one that is substantiated beyond any reasonable doubt in Court. Over or understating findings is one of the common pitfalls in forensic science. 


Comparison goes beyond identification, and is the process of determining whether two or more objects share a common origin. For example, glass fragments found on a suspect may be compared with the broken windows of several vehicles involved in suspected serial car thefts. Insofar as instrumental capabilities and sample quantity permit, as many chemical and physical characteristics as possible are compared. Factoring in sampling and measurement variations, the scientist concludes whether the objects are indistinguishable in characteristics and, therefore, originate from a same source. 

Evidence that possess individual characteristics* in agreement with each other are very strongly associated. In contrast, evidence with only class characteristics* can be narrowed down to a group only and not to a single source. Thus, if we compare a single yellow polyester fibre on the suspect’s shirt with the deceased’s yellow polyester shirt, the chances of the questioned polyester fibre originating from the shirt are not as high as when we compare and associate four different types of questioned fibres with four types of fibres constituting the shirt fabric. 

Note: *Individual characteristics are unique and random features acquired with use of the tool or object or in the manufacturing process. Class characteristics are features shared by two or more objects. They are usually intended manufacturer’s specified features such as the design and size. 


Reconstruction is the determination of human actions, environmental factors, events and causes in a crime or incident. It uses physical evidence to identify and order events in relative space and time. The forensic scientist relies on logical reasoning and broad experience to assess and integrate findings on various physical evidence, facts of case established by police investigators, autopsy and medical reports. Reconstruction specifies the inter-relationships between persons, objects and the scene. The reconstructionist evaluates and interprets the overall significance of the findings in the context of a scene, and presents the likely scenarios, locations and sequence of events before, during and after the incident. Critical success factors in reconstruction include the scientist’s breadth and depth of expertise and experience, the quality and quantity of physical evidence, and the availability of information and expert report findings. 

Significance and Evidential Value

Forensic findings based on physical evidence enhance the outcome for prosecuting criminal and civil cases, and provides intelligence and investigative leads to law enforcement agencies. The evidential value of the physical evidence is closely related to the purpose of the examination. 

At the identification level, evidential value depends on the discriminating power and the number of independent techniques used. If the techniques used have high discriminating power and provide structural information of a specific compound, the scientist will be able to identify the compound with a high degree of certainty. Otherwise, the conclusion will not be definitive.

At the comparison level, evidential value hinges on its commonness in the context of the crime or incident. For example, finding a yellow vehicle paint fragment is significant in our local context as the majority of the vehicles in Singapore are silver, white or black in colour. In contrast, in Mexico where yellow cars are more common, the evidential value of the yellow paint fragment will be lower. 

Totality of Evidence

Ever since its widespread use from the 1990s, DNA evidence has been an indispensable forensic science tool for identifying the suspect – of answering the question “Who?” Blood is a very obvious form of physical evidence. By virtue of its colour and appearance, blood is easily recognised and usually present at a scene where a victim and/or suspect is injured. In contrast, trace evidence, the most diverse form of physical evidence present in practically every scene, is often overlooked and not collected because it is very small and not readily visible to the naked eye. This is an interesting insight because something not obvious to the suspect is more likely to be unaltered and retained at the scene. 

Trace evidence, unknown materials, marks, impressions and bloodstain patterns complement DNA evidence by providing answers on the “What”, “When”, “Where” and “How” of a crime. Although some of these physical evidence cannot be linked definitively to a single individual or object, the chances of finding them are far greater. The rich diversity of class evidence in our environment makes their comparison significant in the context of an investigation. They are also able to corroborate events and provide useful investigative leads. 

In a nutshell, by harnessing the wide variety of physical evidence, we can determine the various aspects of the crime/incident, and thus reduce gaps in the completeness of the story. We come closer to the truth when the totality of physical evidence is considered in a holistic manner. 

Types of Forensic Analyses

The table below lists the types of forensic analysis which can be conducted for various types of crime/incident, to provide answers for law enforcement officers, prosecutors, defense counsel and Judges. It highlights the potential of physical evidence to be silent witnesses of a crime/incident, providing associations between the scene, persons and objects.


Types of Crime/Incident

Types of Forensic Analysis


Asphyxiation & gas poisoning

Identification of noxious or foreign gases, oxygen level, functionality of life jackets and diving gear



Identification of chemical composition, determination of authenticity and origin (source) based on design and manufacturing marks


Counterfeiting (Imitation products)

Comparison of printing on packaging with authentic sample, identification, quantitation and authentication of chemical ingredients of drug and device


Drug trafficking

Identification of controlled substances, association of packaging materials from different sources, association of fibres between suspects and the scene   



Damage analysis of clothing, analysis of electrical circuit



Identification of precursors and explosive residues, flammable gases, damage analysis of burnt/exploded parts, functionality of adaptors, regulators and tubings, cause of explosion


Fall from a height

Identification of transferred material from the victim (paint, soil, fibres, shoeprints, DNA), damage analysis of clothing, print comparison, fall dynamics 


Firearm discharge

Association of fired bullet and cartridge case with the firearm, bloodstain patterns on firearm, victim and suspect, analysis of transfer evidence and gunshot residues, sequence of shots, firing distance for trajectory reconstruction



Identification of ignitable liquids, cause of fire



Handwriting/signature comparisons, examination of fraudulent documents, stamp impressions, ink and toner analysis, printing analysis 


Gang fight/riot

Cause of damage to clothing, weapons and other objects, type of bloodstain patterns, association of fibres on weapons and clothing, comparison of shoeprints, footprints and toolmarks



Association of fibres, comparison of strips of tapes with the roll of tape, construction and function of knots, comparison of prints, DNA analysis, type of bloodstain patterns 


Poisoning and hurt using chemicals

Identification of poisons, adulterants and corrosive materials


Robbery, housebreaking and theft

Chemical composition of substances, toolmark comparison, transferred evidence on tools, print comparison, DNA analysis



Damage analysis of clothing and cut items, fibre association, DNA analysis of blood on weapons, bloodstain pattern analysis of clothing, weapons and objects at the scene


Strangulation / hanging

Hanging mechanism, transferred evidence (eg dirt, paint, fibres) from cordage/clothing to neck/clothing, construction and function of knots


Traffic collisions

Association of transferred materials, vehicle dynamics, video evidence, trajectories and speeds of vehicles and pedestrians, impact point, sequence of traffic events, line of sight, collision avoidability, human factors


Vandalism and graffiti

Chemical composition of inks, paint, flammable liquids, association of toolmarks to a specific tool, comparison of handwriting

What’s Next?

What is the science behind a forensic discipline? What does the examination process entail, what are the common methods of choice and what are their limitations? Look out for our next article which will focus on “Questioned Document Examination”, a forensic discipline that relates to the examination of handwritten, printed and other machine-generated documents. It involves the analysis, comparison of handwriting and signatures, printing, writing instruments and other document-related items in order to determine the authorship of the writing, detect tampered and counterfeit documents, reveal handwriting impressions, or establish links between documents by comparison of their printing, inks and rubber stamp impressions.

The Forensic Experts Group*

* The Forensic Experts Group (“TFEG”) is Singapore’s first one-stop private and independent provider of forensic consultancy, analysis, research, training and education for legal and law enforcement agencies, forensic and tertiary institutions, and private organisations. It comprises a team of accomplished forensic scientists (former leading forensic scientists in a government forensic laboratory in Singapore), who are combining more than 70 years of specialised knowledge, unique experience and skillsets to deliver high quality forensic services both locally and overseas.


1 W. G. Eckert, S.H. James (1992) “Introduction to Forensic Sciences”.

2 K. Inman, N. Rudin (2001) “Principles and Practice of Criminalistics”.

3 T. Bevel, R.M. Gardener (2008) “Bloodstain Pattern Analysis with an Introduction to Crime scene Reconstruction”.

4 R. Saferstein (2007) “Criminalistics – An Introduction to Forensic Science”.  


1 Physical Evidence may be generally defined as any material either in gross or trace quantities that through scientific examination and analysis can establish whether a crime was committed.

2 If you don’t recognise it, you cannot collect it; if you don’t collect it, you cannot analyse it; if you don’t analyse it, you cannot interpret it.