Identifying Drugs & Illegal Substances
Forensic scientists are seeking to answer some basic questions when they analyze chemical evidence found at a crime scene: what substances are present in the material, are any components of the material illegal, and how much of the illegal substance is present? They use a number of methods for identifying illegal substances found at the crime scene, all of which fall under the umbrella of forensic chemistry. All matter has unique chemical characteristics due to the specific interactions of atoms and molecules within different substances. Forensic scientists use these characteristics in order to identify illegal substances, including drugs, poisons, and explosives. Substances found at crime scenes are often mixtures of a number of different chemicals. For example, cocaine powder will often contain caffeine or lidocaine within it. Therefore, forensic scientists must first separate these mixtures in order to identify the components of the mixture.
One method that scientists use to separate the components of a mixture is gas chromatography. In gas chromatography, substances are first dissolved in a liquid solvent. The solution is then injected into a superheated oven which vaporizes the liquid, changing it from a liquid to a gas. Helium or hydrogen gas is used to transport the vaporized liquid into a glass capillary tube; as it moves through the tube, the individual components of the mixture take varying amounts of time to emerge, thereby separating them.
After a mixture has been separated into its individual components, forensic scientists are then able to identify those components using a number of methods. Prior to running any tests, the mass of the substances is always measured. The testing that they complete is categorized as either presumptive testing—a less precise test which simply indicates that an illegal substance could be present—or confirmatory testing, which relies on a positive identification of the substance.
Examples of presumptive testing would be analysis under a microscope or colorimetric tests, i.e. if a specific substance is present, it will result in a color change. A microcrystalline test is another example in which scientists exploit the unique way in which atoms and molecules bond to form crystals to analyze those crystals and identify substances. Confirmatory testing is usually accomplished through the use of mass spectrometry, which uses a beam of electrons to break apart the components of a material. Each molecule breaks apart in its unique way due to its chemical structure, and the resulting fragments are mapped into a spectrum; scientists compare the spectrum against a database of known spectra to identify specific chemicals present in the evidence. Scientists also use melting point analysis—the temperature at which a substance melts—to identify illegal substances. Due to their differing chemical structures, all drugs have different melting points. During these tests, scientists must be cognizant of bias: how they expect the results to turn up versus accepted reference values for various substances.
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Identifying Hairs and Fibers
Hairs and fibers found at a crime scene can provide investigators with a wealth of information regarding the details of a crime. One of the most basic forms of analysis that forensic scientists complete on hairs and fibers is basic comparison. Scientists rely on controls to identify hairs and fibers; this type of evidence can be useless without a sample to which it can be compared, e.g. comparing the hair at the scene to the hair on the suspect. Scientists analyze fibers under a microscope to determine the material of which they are made and hair analysis can differentiate between human and animal origins.
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Fibers are small, long pieces of material that are typically woven together to make fabric or string. Fibers often stick to our clothes because they are a large part of our daily lives. When forensic scientists collect fibers, they need to be sure not to contaminate them, so picking them up with sticky tape should be avoided. Investigators can use static lift with a piece of plastic to collect fibers. Static electricity is an electric charge that can cause the fiber to stick to the piece of plastic. However, this method must also be used with caution, as microscopic electric sparks can destroy trace evidence.
Scientists use the shape of a fiber as one way to identify it. Some fibers are natural, while others are synthetic, or man-made. Synthetic fibers usually begin as a liquid and are squeezed through a nozzle to form strands, so the shape of their cross-section differs from that of natural fibers.
Spectrophotometry is a method that is used to examine a fiber’s color. Color analysis includes every way that a substance interacts with light. Humans are only able to see the visible spectrum of light, while spectrophotometers can sense a larger range. For example, humans see an apple as red due to the separation of white light. When white light hits the apple, the red part of the light is not absorbed and bounces off, and that is the color we see.
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Forensic scientists sometimes use solubility testing to identify fibers that may lack a control sample. Fibers react differently in solvents depending on the makeup of their material. Some fibers will partially dissolve in solvents while others will fully dissolve. This method, however, is destructive to the evidence. In addition, swelling, shrinking, and color changes of fibers assist scientists in identifying them. Unlike solubility testing, polarized light microscopy uses polarized light to evaluate the composition of fibers in a non-destructive manner.
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Identifying Glass & Plastics
Glass is made mostly of silicon dioxide (silica, SiO
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), which makes up most sands and is the most common compound found in the Earth’s crust. Glass is amorphous, meaning it does not have a regular, repeating crystal structure. It does flow, although so slowly that its movement is undetectable over thousands or even millions of years. Glass receives its hard and brittle properties from the inter-connecting nature of its SiO
2
network.
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Forensic scientists use density and color to test glass evidence. The density of a substance is its weight per volume. Because forensic scientists usually only have small pieces of glass to test, they need to use very accurate and sensitive instruments. They look to see if the density of glass particles found on a suspect match glass found at the scene of a crime. If it does, the evidence suggests the suspect was present at the scene.
Scientists also try to match samples of glass by testing the refractivity of glass, which is the degree to which light is bent by the glass. When light passes from one transparent substance to another, i.e. from air to water, it is refracted, or bent. Some materials refract light more than others. For example, some types of glass bend light more than other types of glass because they have a higher refractive index. Scientists can use this property of glass to determine if two samples match each other.
Forensic scientists have a clever way of testing refractivity with unusually small pieces of glass. Typically, glass samples used as evidence are too small to bend narrow beams of light (not to mention that they are irregularly shaped). However, if glass is placed in a transparent liquid that refracts light exactly the same way, the glass will disappear. The light treats the glass and liquid as the same substance with respect to how it bends. This property enables scientists to analyze extremely small pieces of glass found at crime scenes. Scientists use clear silicone oil for this testing because when the oil is warmed or cooled, its refractive properties change with temperature. When a glass sample disappears, the investigator can conclude that it has the same refractive index of the oil at that specific temperature. If two glass samples disappear at the same time, then they refract light in the same way. If their densities also match, then the scientist can conclude that the glass samples are likely from the same source.
Plastics are materials that can be molded and made from high molecular weight polymeric compounds. Forensic scientists use the physical and chemical properties of plastics for identification purposes. For example, density and refractive index are two physical properties commonly used to identify plastics. As for chemical properties, scientists must understand the formation of polymers from their individual monomer components to identify plastics.
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Identifying Soil
Soil analysis is typically accomplished using its density. Although forensic scientists are usually not able to pinpoint soil evidence to a specific location, they can determine the type of soil found which can provide further information about a crime. When soil is added to a density-gradient tube, the particles in the soil will sink to the portion of the tube that has the same density and remain suspended there. Scientists can study the soil density distribution patterns to analyze the crime scene. For example, they are able to determine if two samples of soil are from the same area.
Understanding Explosions & Identifying Explosives
Fire is defined as the rapid oxidation of substances through combustion reactions with the release of energy in the form of heat and light. Atmospheric oxygen typically acts as the oxidant in fires with hydrocarbons commonly acting as the fuel source.
Explosions can be classified as physical or chemical explosions. A physical explosion is characterized by the rapid release of gasses from a highly pressurized container, but does not contain the release of energy in the form of heat or light. A chemical explosion is characterized by an extremely rapid chemical reaction that releases heat, and it is the most common type of explosion in forensic investigations. Chemical explosive reactions are too fast for atmospheric oxygen to deliver the oxidizer quickly enough to sustain the reaction, so explosives contain both fuel and oxidizer components together. Nitroglycerin is a major component of dynamite and is an example of a substance that contains both the fuel and oxidizer components together so that atmospheric oxygen is not needed for an explosion to occur.
Forensic scientists investigate explosive materials by first separating mixtures using gas chromatography followed by mass spectrometry to identify individual components of the mixture. Each type of explosive will produce different spectra, enabling scientists to identify the specific explosive used in a crime. In addition, forensic scientists can used electron microscopic analysis for identifying explosives, as explosives leave residues with distinctive structures and shapes.
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Identifying Blood
Blood is a bodily substance consisting of solids within a liquid. Blood, being a mixture, can be separated into its individual components: plasma, red blood cells, and platelets/white blood cells. Plasma is the liquid portion of blood and consists of 90% water. The volume in a single drop of blood is about 0.05 milliliters (mL) – it would take 20 drops of blood to equal a milliliter.
Many times a crime scene will contain traces of blood, but it is possible that someone may have tried to clean it up. Investigators rely on chemical reactions to reveal faint traces of blood. If luminol, a chemical, is sprayed on a crime scene, it will react with iron in the blood and cause a faint blue glow for up to half an hour.
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The Kastyle-Meyer test has been used since the early 1900s to quickly test a sample for the presence of blood. The test uses a chemical indicator, phenolphthalein, and hydrogen peroxide. The hydrogen peroxide reacts with hemoglobin, the protein found in blood, to create water and a highly reactive form of oxygen. The oxygen subsequently reacts with the indicator molecule changing it from colorless to pink. Because the test can also detect peroxidase, an iron-containing enzyme found in plants, a positive test means the sample could be either blood or plant cell material. A negative test means the sample is definitely not blood.
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