Forensic  Science

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            Characterization / Analysis of

                    Physical Evidence         ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

Forensic science in its broadest definition is the application of science to law. As our society has grown more complex, it has become more dependent on rules of law to regulate the activities of its members. Forensic science applies the knowledge and technology of the natural sciences for the definition and enforcement of such laws. Science occupies an important and critical role in the criminal justice system – a role that is related to the scientist’s ability to supply accurate and objective information that reflects the events that have occurred at the scene of a crime. Thus, forensic science is the application of the natural sciences to those criminal and civil laws that are enforced by police agencies in a criminal justice system.

Locard's Principle of Exchange

Forensic science owes its origins first to those individuals who developed the principles and techniques needed to identify or compare physical evidence, and second to those who recognized the necessity of merging these principles into a coherent discipline that could be practically applied to a criminal justice system.

Many believe that Sir Arthur Conan Doyle had a considerable influence on popularizing scientific crime-detection methods through this fictional character Sherlock Holmes. It was indeed Mr. Holmes who first applied the newly developing principles of serology, fingerprinting, firearm identification, and questioned-document examination long before their value was first recognized and accepted by real-life criminal investigators.

The first treatise describing the application of scientific disciplines to the field of criminal investigation was written by Hans Gross in 1893. In his book Criminal Investigation, he detailed the assistance that investigators could expect from the fields of microscopy, chemistry, physics, mineralogy, zoology, botany, anatomy and physiology, and fingerprinting.

Although Gross was a strong advocate of the use of the scientific method in criminal investigation, he did not make any specific technical contributions to his philosophy. It was left instead to a Frenchman, Edmond Locard, to demonstrate how the principles enunciated by Gross could be incorporated within a workable crime laboratory. 

It was Locard’s belief that when a criminal comes into contact with an object or person, a cross-transfer of physical evidence occurs. This is known as Locard’s Exchange Principle. For example, Locard strongly believed that every criminal can be connected to a crime by dust particles carried from the crime scene. By recognizing, documenting, and examining the nature and extent of this evidentiary exchange, Locard observed that criminals could be associated with particular locations, items of evidence, and victims. The detection of the exchanged materials is interpreted to mean that the two objects were in contact. Forensic scientists also recognize that the nature and extent of this exchange can be used not only to associate a criminal with locations, items and victims, but with specific actions as well.

Identification

Crime reconstruction is the determination of the actions surrounding the commission of a crime. Careful and competent examination of the physical evidence and the detailed documentation of the crime scene allows for this determination. The systematic photographic documentation and recording of the crime scene is required for this analysis.

Conclusions regarding the circumstances and behaviors elicited from the physical evidence related to a crime can infrequently be housed within the confines of absolute certainty. But this does not suggest that crime reconstruction efforts lack investigative or legal functionality. Rather, the value of reconstruction lies more often in establishing the general circumstances of a crime, demonstrating links between victims, suspects, and offenders, corroboration of witness statements, providing investigative leads, and identifying potential suspects.

For example, the crime lab is frequently required to identify the chemical composition of an illicit drug preparation that may contain heroin, cocaine, barbiturates, and so on. It may be asked to identify gasoline in residue recovered from the debris of a fire, or it may have to identify the nature of explosive residues, such as dynamite or TNT. Also, the identification of serological fluids, such as blood, semen, and saliva, as well as hair, wood, or plant substances would, as a matter routine, include a determination for species origin. For example, did an evidential bloodstain originate from a human as opposed to a dog or a cat?  Each of these requests requires the analysis and ultimate identification of a specific physical or chemical substance to the exclusion of all other possible substances.

The process of identification first requires the adoption of testing procedures that give characteristic results for specific standard materials. Once these test results have been established, they may be permanently recorded and used repeatedly to verify the identity of suspect materials. For example, if one wants to confirm that particular suspect powder is heroin, the test results on the powder must be identical to those that have been previously obtained from a known heroin sample. Second, identification requires that the number and type of tests needed to identify a substance be sufficient to exclude all other substances. This means that the examiner must devise a specific analytical scheme that will eliminate all but one substance from consideration. Hence, if a conclusion is reached that a white powder contains elements of heroin, the examiner’s test results must have been comprehensive enough to have excluded all other drugs or, for that matter, all other substances for consideration.

State of the Art: CSI on Trial

Events surrounding the 9/11 disaster created an entirely new set of issues for forensic investigators to consider. Since that incident, first responders and laboratory personnel have been called on to identify victims in mass casualties. Investigators have been required to handle physical evidence tainted with anthrax or Sarin and are wary about future possibilities of contaminated evidence. Bioforensic experts are now called upon to determine the source of diseases in terrorist attacks. Weapons of mass destruction are now referenced concisely by the acronym: WMD. Thus, forensic scientists will need to be aware of several new areas in the public health, terrorist and military sectors. 

Another significant landmark was the O.J. Simpson trial. In this case, it became clear to many investigators in the field that there is much more to crime scene investigation than simply proper police investigative techniques, forensic scientific and technical skill. Appearance and perception as well as the ability to communicate effectively to a jury are equally important.

Thus, it would be naive for anyone planning a career in police work or forensic science to underestimate the importance of the role of the expert witness in the courtroom. An investigator with the cleverness of Sherlock Holmes or a forensic scientist with the brilliance and fundamental understanding of Albert Einstein may not be effective in a criminal investigation if he or she were unable to convince a jury made up blue collar workers whose daily lives are virtually unrelated to the specifics of a given case. In the courtroom, effective verbal communication with the members of the jury is an essential tool of the expert witness.

For example, the forensic pathologist is the medical expert who spends the greatest amount of time preparing for and giving testimony in court. Whether the death is the result of disease or violence, he or she is expected to determine the reason and testify to those findings in a court of law. Deaths caused by poisoning, drowning, or any number of trauma are routinely studied by the forensic pathologist. Also, the number of cases of battered child syndrome has increased exponentially in this country in the last decade. The forensic pathologist may also give evidence in cases dealing with rape, which often requires the positive identification of human blood and/or semen.

In the O.J. Simpson case, the criminal justice system and forensic science were also on trial. Many condemned the trial as a media circus because of the television courtroom coverage. Some believe that Mr. Simpson should have been convicted of the murders. while others proclaimed that justice was done. After listening to all of the evidence in the case, the jury proclaimed that the People failed to meet their burden of proof beyond a reasonable doubt of the defendant's alleged guilt.

The Simpson trial shifted the focus of forensic science from the laboratory to the crime scene. The crime scene investigation now plays a more important role than previously. Defense attorneys have learned that if they are able to show that the initial handling of the physical evidence at the crime scene was faulty, then the evidence can be deemed inadmissible and kept out of the trial, or at least tarnished in the eyes of the judge and jury.

In their cross-examination, Simpson's legal counsel used classical textbooks from the field to argue how physical evidence should have been handled and the proper techniques for a conducting a crime scene investigation. In the Techniques of Crime Scene Investigation (7th Edn., 2004), Barry (A.J.) Fischer emphasizes that there are few immutable rules in crime scene investigation. In his text, Fischer stresses that a set of guidelines based on common sense can be applied to most crime scenes. However, when it comes to the recovery, collection, and preservation of the numerous types of physical evidence, identical guidelines cannot always be followed. Situations may differ, and it is important to be flexible. Compromises must often be made, and as humans, we are subject to mistakes. Extensive experience allows one to minimize errors to a certain extent.    

Training and continuing education for uniformed officers, detectives, crime scene investigators, and forensic scientists, especially in the proper collection and preservation of physical evidence at crime scenes, is essential. Certification of forensic scientists: 1) Measures professional quality; 2) Enhances professional credibility; 3) Introduces a new professional standard; 4) Enhances consumer confidence. There are a number of professional certification organizations. In the criminalistics arena, it is the American Board of Criminalistics. The International Association for Identification offers an exemplary certification program in fingerprint identification and crime scene investigation. In addition, continuing education and attendance art professional seminars, conferences, and workshops are essential to the development and maintenance of professional competency and professional development.

Genetic Fingerprinting: DNA

Undoubtedly, the development of DNA testing would have to be considered as the most significant and dramatic advance in recent forensic science. Its use in many kinds of homicide, rape, and sexual assault, and other criminal cases has added an entirely new dimension to the standard protocols of crime scene investigation methods.

As of ~25 years ago, forensic scientists can use DNA in blood, semen, skin, saliva or hair at a crime scene to identify a perpetrator. This process is called genetic fingerprinting, or more accurately, DNA typing or profiling.

The most significant development of recent years is the universal use of Polymerase Chain Reaction (PCR) in order to replicate DNA molecules in vitro. In the primary mode of this process, a replicative amplification technique is employed in order to compare the lengths of variable sections of repetitive DNA, such as Short Tandem Repeats (STRs) and mini-satellites.

Two humans will have the vast majority of their DNA sequence in common. Genetic fingerprinting exploits highly variable repeating sequences called mini-satellites. It is possible to establish a match that is extremely unlikely to have arisen by coincidence, except in the case of identical twins, who will have identical genetic profiles. This method is usually an extremely reliable technique for identifying a criminal.

Suspects convicted of certain types of crimes may be required to provide a sample of DNA for a database. This has helped investigators solve old cases where only a DNA sample was obtained from the scene, resulting in several post-conviction exonerations of formerly convicted suspects (see: The Innocence Project ). It is also used in such applications as identifying human remains, paternity testing, matching organ donors, studying populations of wild animals, and establishing the province or composition of foods. DNA profiling can also be used to identify victims of mass casualty incidents.

In the past few years, the general public has become more familiar with the power of DNA typing as the media has covered efforts in the identification of remains from victims of the World Trade Center twin towers collapse following the terrorist attacks of 11 September 2001, the O.J. Simpson Murder Trial, the Clinton-Lewinski Scandal @ the White House in Washington, D.C., and the identification of the remains of the Tomb of the Unknown Soldier.

In addition, our perceptions of history have been changed with DNA evidence that revealed Thomas Jefferson may have fathered a child by one of his female African-American slaves. While on the big screen in Steven Spielberg's Jurassic Park, fictional scientists develop a means of bringing dinosaurs from the Jurassic Era to life using DNA taken from real dinosaur blood which has been preserved inside insects encased for centuries in organic amber.

Classification of Physical Evidence

A combination of the importance and value of physical evidence in both criminal and civil investigations and the continuing advance of the applications of science and technology has caused the role of physical evidence to grow to unprecedented levels. Various types of physical evidence may include:

1)  Transient Evidence

       Odor, temperature, imprints and markings

2)  Pattern Evidence

       Blood spatter, glass fracture, fire burn, furniture position, projectile trajectory,

       track-trail, tire or skid mark, modus operandi, clothing or article, powder  

       residue, material damage, body position, shoe or boot print.

3)  Conditional Evidence  [ Event/Action Correlated ]

        Lighting conditions (vehicle headlights, indoor)

        Smoke (color, odor, density)

        Fire (color of flames, speed & direction, temperature, condition)

        Location (weapon, bloodstains, vehicle, broken glass, wounds) 

        Vehicle (locked, windows open, radio on, radio station, ignition key, odometer)

        Body (rigor, lividity, decomposition, temperature, position, wounds) 

4)  Transfer Evidence  [ Physical Contact ]

      Classification by  Material Composition and Structure           

           A. Physical Form      

      Solid, Liquid or Vapor

                  Crystalline or Non-crystalline / Vitreous / Glassy

           B. Chemical Composition             

                  Organic compounds:

                        Blood, skin, dust, hair, semen, saliva, wood, paper, plastics                                Inorganic elements & compounds:

                        Metal, glass, minerals, plastics, paint, soil, bone, fibers                    

                  Composites:

                        Construction materials: building / aerospace              

            C. Data, Patterns & Images (Recorded and/or Printed):

                        Fingerprints, tool marks, tire tracks, printed documents,  

                        Voice prints, videotapes, digital data, computer files  

Fire Accelerants, Bombs & Explosives       

The primary examination conducted on debris collected from arson and fire scenes is the analysis at the laboratory for the presence of accelerants. These chemical substances generally fall into 2 categories:

        Petroleum Distillates

              Lighter fluids, kerosene, gas oils, fuel oils

        Non-Petroleum Distillates 

              Turpentine, alcohols, specialty solvents or

               lubricants of vegetable or synthetic origin

Explosions can cause extensive damage and destruction of property due to the force and heat produced as a result of the rapid exothermic (heat producing) reaction. Explosive residues may be encountered in numerous forms - as initial reactants as well as final products of the spontaneous chemical reaction.

Chemical reactions can be classified by the instability of their reactants and the corresponding stability of their products. The greater the difference between the two, the more likely the spontaneous chemical reaction is to occur and proceed to completion. Chemical compounds can therefore be classified according to the stability, or instability, of their chemical precursors.  

Explosives are substances which are chemically unstable in their natural form. When heated, shocked, or mechanically agitated, they are prone to rapid chemical decomposition. The resulting explosion is typically identified by the rapid liberation of large quantities of heat and gaseous chemical by-products of the reaction.

Low explosives are those whose reactants are chemically stable under standard conditions of temperature, volume and pressure (STP), but when confined and detonated will result in violent explosions. Examples of this type of explosive are black powders and many pyrotechnic materials (fireworks).

Primary high explosives, or primer, are sensitive to heat and shock and frequently used to initiate detonation of secondary high explosives. Blasting caps, detonation cord, and nitroglycerine are examples of primary high explosives.

Secondary high explosives consist of materials which are typically insensitive to heat, shock, or friction, and often used as a booster charge. Common secondary explosives are TNT, RDX, dynamite, ammonium nitrate, and monomethylamine nitrate. These materials make up the bulk of most secondary high explosive devices.

Laboratory Analysis I

Laboratory analysis includes both macroscopic and microscopic examination. Particulate matter such as soil & debris are examined for the presence of unexploded or partially burned residues. Any particulate matter should be examined microscopically using a light microscope, a scanning electron microscope (SEM) on bulk samples, or transmission electron microscope (TEM) on thin sections.   

Techniques of chemical and physical analysis of both organic and inorganic compounds are explored in detail below.

Residues & extracts are tested with various chemical reagents to in order detect characteristic chemical components. In the case of explosives, chemical reagents such as diphenylamine, J-acid, and alcoholic potassium hydroxide (KOH) act as indicators to give characteristic color reactions when the extract residues are tested. 

Optical microscopes provide an ideal means of directly observing the physical characteristics of macroscopic residue particles, hairs and fibers.  Scanning electron microscope (SEM) or a combination of SEM with Energy Dispersive X-ray (EDAX) analysis (SEM/EDX) can also be used for chemical analysis of microscopic particles of solid residue. 

Microcrystalline components of extracted residue can often be identified microscopically or buy using the technique of X-ray diffraction to identify the characteristic Bravais lattice structure of the compound and the spatial extent of the crystalline order.

Thin layer chromatography (TLC) utilizes solutions of the residue in acetone which are spotted on a plate, photographically developed, and compared to known explosives or common residues. Many liquids, such as gasoline, contain dyes which can be removed from a fire accelerant, separated on a TLC plate, and used to identify the manufacturer of the accelerant.

Spectroscopy

Spectroscopy is the study of matter and its properties by investigating light, sound, or particles that are emitted, absorbed or scattered by the matter under investigation. Spectroscopy may also be defined as the study of the interaction between light and matter. Spectroscopy is often used for the identification of substances through the frequency spectrum emitted or absorbed by them. Historically, spectroscopy referred to a branch of science in which visible light was used for theoretical studies on the structure of matter and for qualitative and quantitative analyses. The definition has broadened, however, as new techniques have been developed that utilize frequencies (or wavelengths) of electromagnetic radiation  other than those associated with the visible portion of the spectrum.

Infrared (IR) spectroscopy or mass spectra of a residue can be detected and recorded using lightwaves just outside the visible range of wavelengths. The spectral results can then be compared to known standards in order to identify a chemical compound from the specific organic components present in the residue. Of all the properties of an organic compound, the one that gives the most information about the compound's structure is its infrared spectrum. Thus, IR spectroscopy is used to determine the presence of certain functional groups. Infrared frequencies of light are used to study fundamental vibrations and associated rotational-vibrational structure via vibrational resonance and selective absorption of IR frequencies.

Raman spectroscopy is also finding a wider niche in forensic applications, given that it provides information that is complimentary to absorption IR spectroscopy and is generally non-destructive. A Raman spectrum is generated by directing a laser onto the sample and observing the patterns of lightwaves which are scattered at higher and lower wavelengths relative to that of the incident laser beam. Thus, inelastic (Raman) scattering consists of scattering on either side of the primary frequency band. In inelastic scattering, photons are absorbed by the sample and then reemitted. The frequency of the reemitted photons is shifted up or down in comparison with original frequency, which is called the Raman Effect.

Raman spectroscopy can be used to study solid, liquid and gaseous samples. In a report last year, a team of scientists said it could be used to identify "substances beneath surfaces" and foresaw its use to analyze the internal composition of bones and tissues, jewelry and industrial materials. Furthermore, a study published recently in the Journal of Analytical Chemistry said the new laser Raman technique could be used examine and correctly identify the contents of paper and plastic packages and bottles used to contain and transport counterfeit drugs and medications.

Nuclear Magnetic Resonance (NMR) spectroscopy is one of the principal non-destructive techniques used to obtain physical, chemical, electronic and structural information in hydrocarbon compounds. This method identifies different nuclei (& elements) from their magnetic properties, or their magnetic moment. This is the most important and commonly used spectroscopic technique for organic chemists, often permitting complete assignment of atom connectivity and stereochemistry.

Thus, like electrons, the protons of certain atoms are considered to have spin. The spinning of these positively (+) charged particles generates a magnetic moment along the axis of spin, so that these nuclei act like tiny bar magnets. Thus, if a proton is placed in an external magnetic field, its magnetic moment can be aligned in either of two ways: with or against the field. Alignment with the field is more stable, and energy must be absorbed in order to "flip" the tiny proton magnet over to the less stable alignment (against the field).

NMR can be extremely useful for non-destructive analysis of samples. Radio waves and static magnetic fields easily penetrate many types of matter and anything that is not inherently ferromagnetic. For example, various expensive biological samples, such as nucleic acids, including RNA and DNA, or proteins, can be studied using NMR for weeks or months before using destructive biochemical experiments. This also makes NMR a good choice for analyzing dangerous samples or explosives.
 

Physical Properties

Qualitative analysis and identification of unknown chemicals is a complex and integral part of a forensic scientist’s duties. Observing whether the unknown is a solid or a liquid reduces the number of compounds under consideration.

Physical properties of elements and compounds which provide conclusive evidence of chemical composition include odor, color, volume, density (mass / volume), melting point, boiling point, heat capacity, physical form (solid, liquid or gas), pH, hardness, porosity, and index of refraction. Physical properties which constitute the study of the science of materials in the solid state include mechanical, thermal, electrical and optical properties.

Mechanical properties, important in structural and building materials as well as textile fabrics, include mechanical responses such as elasticity / plasticity, tensile & compressive strength, fracture toughness & ductility (low in brittle materials, high in metals), shear strength, and indentation hardness.      

Thermal properties focus on the stability of a material at elevated temperatures. Also important is the capacity of a material to store energy in the form of heat (or thermal vibrations). In the aerospace industry, high performance materials used in the design aircraft exteriors must have a high resistance to thermal shock. New age polymers and ceramic /metal composites are now being designed with this purpose in mind.

Electrical properties include conductance, resistance, impedance and capacitance. Electrical conductors (metals & alloys) are contrasted with electrical insulators (glasses & ceramics). Semiconductors (Si, GaAs) behave somewhere in between, showing electrical conduction only beyond a threshold applied voltage. Alternatively, ionic superconductors are at the extreme end of the conductivity spectrum, with highly mobile ions acting as charge carriers moving thru a crystalline lattice with virtually negligible resistance (@ low temperatures)

Optical properties focus on the response of a material to incoming lightwaves of a range of wavelengths. Frequency selective optical filters can be utilized to alter or enhance the brightness and contrast of a digital image. Guided lightwave transmission involves the emerging field of fiber optics and the ability of certain glassy compositions to transmit a range of frequencies simultaneously (thus “multi-mode”) with little or no interference between competing waveforms. This resonant mode of energy & data transmission, though low powered, is virtually lossless.

Thermal Radiation. Also of value to the forensic scientist is the sensitivity of materials to radiation in the thermal (IR) infrared portion of the electromagnetic spectrum. This heat-seeking ability is responsible for such diverse phenomena as night vision and IR luminescence. Some inks, when exposed to blue-green light, will absorb the radiation and will re-radiate infrared light. This technique utilizes infrared-sensitive film to record the light emanating from the surface of an illegally altered document. IR luminescence has also been used successfully to reveal writing that has been erased. Another important application of infrared photography arises from the differences in IR absorption by different inks. Techniques of IR photography, as well as oblique lighting, have also been used successfully in order to reveal the contents of a document that has been charred in a fire.

Laboratory Analysis II    

The proper selection of analytical techniques that will allow the forensic scientist to identify or compare matter can best be understood by categorizing all substances into one of two broad groups: organics (body fluids, fibers, pharmaceuticals) and inorganics (tools, explosives, poisons, metal scrapings).

Chromatography. Although spectroscopy is exceedingly useful in the chemical identification of many organic materials, its optimum utilization requires that a material be in a relatively high state of chemical purity. Because the impurity levels of any given sample is often beyond the control of the analytical chemist, this criterion is often not met. For this reason, the analytical technique of chromatography is widely applied for the analysis of physical evidence. Chromatography is a means of separating and tentatively identifying the components of a chemical mixture. It involves passing a mixture dissolved in a "mobile phase" through a stationary phase, which separates the analyte to be measured from other molecules in the mixture and allows it to be chemically isolated.

Mass spectrometry characterizes organic molecules using fragmentation patterns created by collisions with high-energy electrons. Gas Chromatography (GC) combined with Mass Spectroscopy (MS), e technique known as GC/MS is extremely effective. Thus, many of the uncertainties associated with GC have been overcome by linking the gas chromatograph to a mass spectrometer to yield an extremely powerful combination known as gas chromatography / mass spectrometry (GC / MS).

High Performance Liquid Chromatography (HPLC), capillary electrophoresis, and ion chromatography also separate and aid in the detection and identification of explosive residues.

Atomic absorption, atomic emission, x-ray fluorescence, and neutron activation analysis (as well as infrared spectra) can aid in the detection and identification of inorganic substances present in the residues. Emission spectroscopy, inductively coupled plasma, and atomic absorption spectrophotometry are three techniques available to the forensic scientist for determining the elemental composition of inorganic materials. An emission spectrograph vaporizes and heats samples to a high temperature so that the atoms present in the material achieve an excited state. Under these conditions, the excited will emit light (or fluoresce) as they release energy. In inductively coupled plasma, the sample is introduced into a hot plasma, creating charged particles that emit light of characteristic wavelengths for that particular element.

X-ray diffraction is the study of long-range atomic structure in solid crystalline materials. As the X-rays penetrate the crystal, a portion of the beam is reflected by each of the atomic planes. As the reflected beams leave the crystal’s planes, they combine with one another to form a series of light and dark bands known as a diffraction pattern. Every mineral compound has its own unique diffraction pattern, depending on the atomic sizes, the interatomic spacings, and the type of long-range crystal structure.

In summary, virtually any chemical substance found at a crime scene can be identified by chemical or physical techniques and may be considered as physical evidence.

Summary

Analytical techniques which rely on the basic physical properties of chemical elements & compounds have been used extensively in the laboratory for the identification of fire accelerants, bombs & other explosives, serological fluids such as blood, semen and saliva, genetic material including DNA, computer files & other documents, controlled substances, hair & fibers, fingerprints, firearms, metals, glass, plastics, paint, GSR (gunshot residue), imprints & other impressions, patterns, tape, toolmarks, video and voice print identification. 

The technical support provided by crime laboratories can be assigned to five basic services. The physical science unit incorporates the principles of chemistry, physics and geology to identify and compare physical evidence. The biology unit applies the knowledge of biological science in order to investigate blood samples, body fluids, hair, and fiber samples. The firearms unit investigates discharged bullets, cartridge cases, shotgun shells, and ammunition. The document unit provides the skills needed for handwriting analysis and other questioned-document issues. Finally, the photographic unit applies specialized photographic techniques for recording and examining physical evidence. In addition, some crime laboratories may offer the optional services of toxicology, fingerprint analysis, voiceprint analysis, evidence collection, and polygraph administration.