Forensic Science

Summary

Forensic science is commonly defined as the application of science to legal matters. Although forensic science incorporates numerous disciplines, ranging from accounting to psychology, in the traditional sense, forensic science refers to the scientific analysis of evidence collected at crime scenes, also known as “criminalistics.” Pattern evidence (such as fingerprints, bullets, and tool marks) is often compared visually, and chemical evidence (such as illicit drugs) and biological evidence (such as deoxyribonucleic acid (DNA), blood, and bodily fluids) are analyzed and compared using scientific instruments.

Definition and Basic Principles

Forensic science is the application of scientific principles to the analysis of numerous types of evidence, most commonly collected at a crime scene. Crime scene investigators, usually police officers or federal agents, collect evidence at the crime scene and submit it to a crime laboratory for analysis by forensic scientists.

Crime laboratories contain different sections, each specializing in a particular type of analysis, such as controlled substances, DNA, firearms and tool marks, latent prints, questioned documents, toxicology, and trace evidence. The type of analysis conducted depends on the type of evidence as well as the circumstances of the crime. A single piece of evidence may be analyzed in more than one section. For example, a firearm may be analyzed in the latent prints and DNA sections and the firearms and tool marks sections.

Following analysis, forensic scientists may be summoned to present their findings in a court of law. Forensic scientists present their analysis and interpretation of the evidence before a judge and jury, who are charged with determining the defendant's guilt or innocence. The unbiased, accurate analysis presented by the forensic scientist is an integral part of the criminal proceedings.

Background and History

Forensic science aims to identify or individualize characteristics to link people, places, and objects. In the late 1880s, French criminologist Alphonse Bertillon developed a method of identifying humans based on eleven physical measurements, including height, head width, and foot length. However, the limitations of this method soon became apparent. In 1880, Scottish scientist Henry Faulds published an article in Nature that discussed using fingerprints as a means of identification. In 1892, Sir Francis Galton published Fingerprints, proposing a system of classifying fingerprint patterns. That same year, Argentine police officer Juan Vucetich used fingerprint evidence that resulted in the arrest and conviction of a murder suspect. From 1896 to 1925, Sir Edward Henry, a police official in British India, developed the Henry Classification System for fingerprints, which was based on the pattern on each finger and the two thumbs.

In the late nineteenth and early twentieth centuries, advances were being made in other areas that have become integral to forensic science. Spanish-born French scientist Mathieu Joseph Bonaventure Orfila, often considered the pioneer of forensic toxicology, is credited with developing and improving methods for detecting arsenic, a common nineteenth-century poison, in the body. French scientist Edmond Locard developed the hypothesis that “every contact leaves a trace,” which implies that whenever two objects make contact, there is an exchange between them. This hypothesis became known as Locard's exchange principle and is the foundation of modern trace and transfer evidence analysis. In the 1920s, the comparison microscope, which analyzes side-by-side specimens, was developed by American chemist Philip Gravelle and popularized by forensic scientist Calvin Goddard. This microscope enabled significant advances in forensic science, particularly firearms, tool marks, and trace evidence.

A major scientific breakthrough in the 1980s revolutionized the field of forensic DNA analysis. British geneticist Sir Alec Jeffreys developed DNA profiling, enabling individuals to be identified from samples of blood and other body fluids left at a crime scene. In 1983, American biochemist Kary Mullis developed the polymerase chain reaction, which allowed DNA profiling to be conducted on degraded and very small samples of DNA, making it possible for forensic scientists to test a wider range of evidence.

As forensic science evolves, newly developed technologies and instrumentation allow evidence to be analyzed and compared in an increasingly rapid, objective, and reliable manner. Forensic science is a truly dynamic field that constantly seeks further improvements and advancements in its analytical methodologies.

How It Works

Forensic science incorporates numerous subdisciplines, but the most common types of analysis conducted by crime laboratories are analyzing illicit drugs, biological evidence, latent prints, firearms, footprints, tire marks, tool marks, and trace evidence. Latent prints, footprints, tire marks, and tool marks are considered pattern evidence. The patterns of an unknown sample (usually from the crime scene) and a known sample are visually compared to find similarities. Samples can also be analyzed chemically or biologically with scientific instruments. Some of the more common testing methods are infrared spectroscopy, ultraviolet/visible microspectrophotometry, gas chromatography-mass spectrometry, and electrophoresis.

Infrared Spectroscopy. In infrared spectroscopy, the chemical structure of a sample is determined based on how the sample interacts with infrared radiation. Chemical bonds can absorb infrared radiation of a specific energy, which causes the bond to vibrate. Additionally, each bond can vibrate in different ways. Therefore, when infrared radiation is introduced, chemical bonds within the sample absorb different energies, and the results are shown in the form of an infrared spectrum. The spectrum is a graph of radiation transmitted versus wave number, which is related to the energy of the radiation. Additionally, transmission can be mathematically converted to absorbance such that the spectrum can be displayed as absorbance versus wave number. The infrared spectrum of a sample displays numerous absorptions, each corresponding to a particular type of chemical bond and a particular type of vibration. The infrared spectrum of a sample is unique to that sample, and therefore, this technique can be used to definitively identify compounds.

Ultraviolet/Visible Microspectrophotometry. Infrared spectroscopy and ultraviolet/visible microspectrophotometry are based on the principle of the interaction of radiation with a sample. However, ultraviolet/visible microspectrophotometry is typically used to compare the dye or pigment composition of samples. The technique is used to determine the color of a sample and identify subtle differences in color that cannot be seen with the naked eye.

A microspectrophotometer consists of a microscope with a spectrometer attached, which allows the analysis of microscopic pieces of evidence. The sample is viewed under the microscope, and ultraviolet and visible radiation is introduced. Depending on the chemical structure of the sample, wavelengths of light will be absorbed, reflected, or transmitted. The transmitted light is collected in the spectrophotometer, and the intensity of each wavelength is measured. Results are displayed in the form of a spectrum that is a graph of transmittance (or absorbance) versus wavelength. Subtle differences in color between two samples are observed as differences in wavelengths of light transmitted or absorbed in the corresponding spectra. Such differences are caused by differences in chemical composition between the two samples. Therefore, a comparison of the resulting spectra can be used to determine if the two samples are similar in color.

Gas Chromatography–Mass Spectrometry. In any chromatography technique, sample mixtures are separated based on differences in interaction between a mobile phase and a stationary phase. In gas chromatography (GC), the mobile phase is a gas, and the stationary phase is a liquid coated on the inner walls of a very thin column. Liquid samples are typically introduced into the system and carried, in the mobile phase, through the stationary phase. Sample components with a stronger attraction for the stationary phase spend longer in that phase, and components with less attraction spend less time in that phase and move more quickly through the system. The time it takes for sample components to travel through the system and reach the detector is known as the retention time.

In gas chromatography-mass spectrometry (GC-MS), the detector is the mass spectrometer, which contains three major components: the ion source, the mass analyzer, and the detector. On emerging from the GC column, sample components enter the ion source, where each component is first ionized. The resulting ion is known as the molecular ion. This ion is unstable because of its high energy, so it breaks down, or fragments, into smaller ions. Molecular ions and fragment ions then enter the mass analyzer, where the ions are separated according to their mass-to-charge ratio. The separated ions enter the detector, where the number of ions of each mass-to-charge ratio is counted. Results are displayed as a mass spectrum, which is a graph of intensity versus the mass-to-charge ratio. Because molecules break down, or fragment, in a predictable manner, the mass spectrum can be used to determine the structure of the original sample component. Furthermore, because the fragmentation pattern is unique to a molecule, the mass spectrum can be used to definitively identify the component.

When analyzing a sample by GC-MS, two pieces of information are obtained. First, a chromatogram is obtained from gas chromatography, which is a graph of detector response versus retention time. Each separated component in the sample mixture is shown as a peak on the chromatogram. Components that take longer to reach the detector are more attractive to the stationary phase and have longer retention times. Additionally, for each separated component, the mass spectrum is obtained, which can be used to identify the component definitively.

Electrophoresis. Although electrophoresis is also used to separate sample mixtures, the technique is not considered a chromatographic technique because no mobile phase is involved. Instead, sample mixtures are separated based on differences in migration under the influence of an applied electrical potential. Therefore, electrophoresis is used to analyze samples with an electric charge.

Although there are different types of electrophoresis, capillary electrophoresis is most commonly used for DNA profiling purposes. In this technique, a capillary column is filled with a polymer, and the ends of the column are immersed in reservoirs containing a buffer solution. The reservoirs also contain electrodes to allow the application of the electric potential. The sample is introduced to one end of the column, and the sample components move through the column under the influence of the applied potential. Separation occurs based on differences in the migration rate of the components through the column, which depends on size and charge. Separated components pass through a detector at the other end of the column, producing an electrophoretogram. The electrophoretogram shows the migration time of the separated components. Smaller components move more quickly, reaching the detector before larger components, and they have shorter migration times.

Applications and Products

The major role of the forensic scientist is to analyze submitted evidence for the purposes of characterization and identification. For example, a blue fiber collected from the scene may be submitted to the trace evidence section, where forensic scientists characterize the fiber (for example, by its dimensions, color, and cross-sectional shape) and then identify the type of fiber (for example, nylon, polyester, or acrylic). When a known sample is available, such as fibers from a suspect's clothing, forensic scientists compare it with the unknown sample collected from the crime scene to determine if the two most likely originated from a common source. This process of characterization, identification, and comparison requires multiple analysis stages, ranging from visual examination to instrumental analysis.

Infrared Spectroscopy. The technique of infrared spectroscopy is commonly used in the controlled substance and trace evidence sections of the crime laboratory. This technique can identify illicit drugs present in unknown samples, the type of fiber found at a crime scene or on a person, the polymer present in a paint chip, or the organic compounds present in explosive residues. The evidence is prepared for analysis in several ways, depending on the type of sample.

Solid samples of illicit drugs can be mixed with potassium bromide and pressed into a pellet, which is then placed in the spectrometer. Infrared radiation is passed through the sample, which will absorb at characteristic energies depending on its chemical structure. The transmitted radiation is collected, and the infrared spectrum is generated. Because potassium bromide does not absorb infrared radiation, the subsequent infrared spectrum shows only contributions from any drug present in the sample.

For opaque samples, such as fibers or paint chips, attenuated total reflectance-infrared (ATR-IR) spectroscopy is more commonly used. The sample is positioned over a crystal, and pressure is applied to ensure good contact between the sample and crystal. Infrared radiation is passed through the crystal, and because of the close contact, the radiation penetrates a small depth into the sample. Certain energies are absorbed depending on the chemical bonds within the sample, resulting in the characteristic spectrum of the sample.

The infrared spectrum of the questioned sample can be compared to a database containing infrared spectra for known standards (drugs, fibers, paints, and so on) to identify the unknown sample. However, care must be taken when comparing a spectrum to spectra in a database. Although the spectrum of a given compound is unique, it can vary slightly depending on the instrument used to analyze the sample and standard. Rather than relying on a database search, it is often preferable to analyze the unknown sample and known standards on the same day, using the same instrument, to allow for a direct comparison of spectra.

Although infrared spectroscopy can rapidly analyze samples, the technique works best for relatively pure samples. If impurities are present in the sample and they also absorb infrared radiation, the resulting spectrum contains contributions from both the sample and the impurities. This can complicate the spectrum's interpretation and subsequent identification of the sample.

Microspectrophotometry. The comparison and analysis of colored samples are often undertaken using microspectrophotometry. This technique is used in the trace evidence and questioned documents sections to compare the dye or pigment composition of fibers, paints, and inks.

Methods for sample preparation vary depending on the type of sample to be analyzed. Fibers are flattened and mounted on a microscope slide with a drop of immersion oil. Paint samples require more involved preparation, particularly for transmission spectra. The paint chip must be cut into a section so thin that light can be transmitted through it. Spectra of inks can be obtained directly if the paper is sufficiently thin to allow transmission. Otherwise, the ink must be removed from the document. This can be done by removing a small sample of the paper containing the ink and immersing the paper in a solvent to extract the ink. The resulting ink solution is placed on a microscope slide, and the solvent is allowed to evaporate, leaving a residue of ink for analysis. However, this is a destructive procedure because the document is damaged when the sample is removed. Alternatively, a piece of clear tape can be placed on an area of the document that contains the ink. When the tape is lifted off, particles of ink adhere to the tape. These particles can be removed from the tape and transferred to a microscope slide for analysis. The document is minimally damaged using this procedure.

Although microspectrophotometry offers a rapid means to investigate the dye or pigment composition of certain samples, no extensive spectral databases are readily available. Therefore, the technique is more useful when known samples are available, and the color of the unknown and known samples can be compared directly based on spectral interpretation.

Gas Chromatography–Mass Spectrometry. As with infrared spectroscopy, gas chromatography-mass spectrometry is commonly used in the controlled substances and trace evidence sections, as well as in the toxicology section, to determine if drugs and poisons are present in body fluids.

GC-MS is advantageous over infrared spectroscopy in that samples containing impurities can still be identified because of the separation abilities of gas chromatography. For example, gas chromatography analysis of a drug mixture containing methamphetamine and caffeine separates the two components. In the resulting chromatogram, two peaks are observed: one for methamphetamine and one for caffeine. The mass spectrum of each peak is also obtained, which can be used to definitively identify each component.

In most cases, samples must be in liquid form for GC-MS analysis. This is achieved by adding a suitable solvent to the sample and analyzing the resulting solution. For body fluid or tissue samples, solid phase or liquid-liquid extraction is necessary to isolate any drugs and poisons from additional components in the fluids or tissues.

Solid samples can be analyzed using pyrolysis GC-MS. In this case, a pyrolysis unit is attached to the gas chromatography inlet. Solid samples (for example, paint chips or fiber fragments) are placed in a small quartz tube and introduced into the pyrolysis unit, rapidly heating the sample to a very high temperature. The sample is broken down and vaporized in the pyrolysis unit and then carried in the flow of carrier gas onto the gas chromatography column, where the sample components are separated.

Before analyzing the sample, it is important to demonstrate that the GC-MS system is free from contamination. This is usually done by injecting a volume of the solvent used to prepare the sample. If the solvent and instrument are not contaminated, the resulting chromatogram should show no peaks. For pyrolysis GC, the empty quartz tube is analyzed to demonstrate no contamination in the tube or instrument.

Because the mass spectrum rather than the retention time is unique to a sample component, the spectrum of an unknown sample is compared to a suitable database of spectra. However, there may be slight differences between the database spectrum and the spectrum obtained for the unknown sample, depending on the instrument used to collect the spectra. It is often preferable to prepare and analyze a known standard like the unknown sample and then compare the corresponding mass spectra.

Electrophoresis. DNA profiling makes the most use of electrophoresis. Typically, blood, semen, saliva, or another body fluid from the crime scene is used to generate a DNA profile, which is compared with profiles generated from known samples. If known samples are not available, the generated DNA profile can be compared to a database of profiles. The Federal Bureau of Investigation (FBI) maintains a database of DNA profiles submitted by crime laboratories across the United States (US). The Combined DNA Index System (CODIS) contains profiles from crime scenes, convicted criminals, and missing persons.

Modern DNA profiling is based on the characterization of short tandem repeats (STRs) or regions (loci) on the chromosome that repeat at least twice within the DNA. For profiling, the number of repeats at each location on the chromosome is determined. To do this, the DNA is first amplified via the polymerase chain reaction (PCR) method, in which the double-stranded DNA is split into two single strands, and a mixture of enzymes and primers is used to replicate specific STR regions of the DNA. In the US, STRs at thirteen loci are typically considered. The reaction is repeated many times, generating exact copies of the STRs. Because of this amplification procedure, profiles can be obtained from very small samples of DNA.

The STRs are analyzed using electrophoresis, most commonly capillary electrophoresis, which allows rapid and automated analysis. The STR mixture is separated based on differences in migration rate through the capillary column, which is related to the size of the STR. The resulting electrophoretogram displays a series of peaks corresponding to the STRs at each locus. Additionally, for each STR, there are two variants, one inherited from the mother and one from the father. Therefore, the electrophoretogram shows a pair of peaks at each locus. A match in the number of STRs for both variants at all loci is considered strong evidence that the unknown and known samples originate from the same person. Because DNA is unique to an individual, this is one type of evidence considered individualizing rather than class evidence.

Massively Parallel Sequencing (MPS). MPS is an advanced method used to determine a portion or all of an individual’s genome sequence. It can sequence many STR markers in complex DNA mixtures, including nearly tenfold more genetic loci with higher accuracy. MPS is also helpful in trace or highly degraded DNA found at crime scenes.

Laser Ablation Inductively Coupled Plasma Mass Spectrometry (LA-ICP-MS). LA-ICP-MS is used to analyze solid materials, such as glass, paint, documents, and fibers. A high-power laser irradiates solid materials to create an aerosol of super-fine fragments that can then be analyzed using this technology.

Careers and Course Work

At a minimum, a forensic scientist must have a Bachelor's degree in a natural science, such as biology or chemistry, or in a related discipline, such as microbiology or biochemistry. It is also possible to obtain a Bachelor's degree in forensic science. However, care should be taken to ensure that the degree meets minimum credit hour requirements in either biology or chemistry. Successful completion of the Bachelor's degree ensures that students have a strong background in the appropriate science for their future field of work.

The popularity of forensic science has made the field highly competitive, and many forensic scientists have a Master's degree in forensic science. Although more than one hundred American universities offer undergraduate and graduate degree programs in forensic science, in 2019, only approximately twenty-eight undergraduate programs and twenty-one Master's programs were accredited by the Forensic Science Education Programs Accreditation Commission (FEPAC). Accreditation ensures that rigorous standards have been met regarding the content and quality of the degree program. In most cases, the degree obtained is a Master of science in forensic science with a concentration in a specific discipline, such as forensic biology, forensic chemistry, or forensic toxicology. Other courses include criminal and forensic science, forensic pathology, analytical and forensic science, and analytical and forensic chemistry.

Forensic scientists are not limited to careers in local and state crime laboratories. Many federal agencies, such as the Federal Bureau of Investigation (FBI), the Drug Enforcement Administration (DEA), and the Bureau of Alcohol, Tobacco, Firearms, and Explosives (ATF), employ forensic scientists. In addition, forensic scientists can be employed by private forensic laboratories (such as paternity testing or sports testing laboratories) or as independent consultants. Professionals, such as crime scene supervisors, forensic analysts, forensic fingerprint experts, and forensic psychologists, can work in private firms.

Social Context and Future Prospects

Although significant advances have been made in forensic science, many more have yet to be achieved. In 2009, the National Research Council published Strengthening Forensic Science in the United States: A Path Forward, a report on forensic science in the US. The report highlighted several deficiencies in the field. It recommended improving forensic science education, training, and certification and developing standardized procedures and protocols for evidence analysis and reporting. Additionally, the report recommended research into the reliability and validity of many procedures used for evidence analysis. The report concluded that more research is necessary to improve existing practices and develop new technologies that can be implemented in forensic science laboratories. It called for the development of a national institute of forensic science that would have many objectives, including developing standards for certification for forensic scientists and accreditation of forensic laboratories, along with improving education and research in the field. As the twenty-first century progressed, innovations continued to be made in forensic science. Advancements in technology, such as using artificial intelligence to better analyze fingerprints, allowed for more efficient and accurate execution of forensic science, signaling not only scientific progress but also an acknowledgment that the work was responsible for bringing justice to victims of crimes. 

Bibliography

Bertino, Anthony J., and Patricia Nolan Bertino. Forensic Science: Fundamentals and Investigations. South-Western, 2009.

Brettell, Thomas A., et al. “Forensic Science.” Analytical Chemistry, vol. 79, no. 12, 2007, pp. 4365–84.

Butler, John M. "Recent Advances in Forensic Biology and Forensic DNA Typing: INTERPOL Review 2019–2022." Forensic Science International: Synergy, vol. 6, 2023, doi.org/10.1016/j.fsisyn.2022.100311. Accessed 5 Jun. 2024.

"8 New Forensic Science Techniques and Technology." Forensic Digest, 2021, forensicsdigest.com/8-new-forensic-science-tools-and-technology. Accessed 6 June 2024.

Embar-Seddon, Ayn, and Allan D. Pass, editors. Forensic Science. Salem, 2008. 3 vols.

"Forensic Research News." ScienceDaily, 5 June 2024, www.sciencedaily.com/news/matter‗energy/forensics. Accessed 5 June 2024.

Gustafson, Kimmy. “10 Modern Forensic Technologies Used Today.” Forensics Colleges & Universities, 9 Feb. 2024, www.forensicscolleges.com/blog/resources/10-modern-forensic-science-technologies. Accessed 5 June 2024.

Houck, Max M., and Jay A. Siegel. Fundamentals of Forensic Science. 2nd ed., Academic, 2010.

James, Stuart H., et al., editors. Forensic Science: An Introduction to Scientific and Investigative Techniques. 4th ed., CRC Press, 2014.

Kobilinsky, Lawrence, et al. DNA: Forensic and Legal Applications. John Wiley & Sons, 2005.

National Research Council. Strengthening Forensic Science in the United States: A Path Forward. National Academies Press, 2009.

Primorac, Dragan, and Moses Schanfield, editors. DNA Applications: An Interdisciplinary Perspective. CRC Press, 2014.

Rudin, Norah, and Keith Inman. An Introduction to Forensic DNA Analysis. 2nd ed., CRC Press, 2002.

Saferstein, Richard. Criminalistics: An Introduction to Forensic Science. 11th ed., Prentice Hall, 2015.