Forensic science applies science principles, techniques, and methods to the investigation of crime. A lesser known definition of the adjective forensic is any-thing argumentative or debatable. At first, this definition of forensic may seem to have no connection with the more popular crime-solving definition—but it does. Legal truth is sought through the use of the adversarial system (rather than the scientific method), and decisions are made only after each side has been given an equal opportunity to argue all the issues at hand. When one of the issues being argued is a scientific analysis (using the scientific method) of an item of evidence, the debate that ensues over the science involved could be called forensic science.
Other related definitions of forensic may include (1) the use of science to aid in the resolution of legal matters and (2) a scientific analysis for the pur-pose of judicial resolve. For example, saying that something was forensically de-termined suggests the information was scientifically determined with the intent to be presented (and debated) in a court of law.
Recently the term forensic has also been used to describe many scientific investigations—even if no crime is suspected. Often these investigations are of historical significance and may or may not have legal consequences. For example, a forensic scientist may work on the discovery of the composition of ancient pottery, the detection of Renaissance art techniques, or the identifica-tion of ancient human remains. Forensic history is the use of science to answer historical questions.
Role of a Forensic Scientist
Most forensic scientists analyze evidence in a crime laboratory and spend little time at the crime scene. The duties of forensic scientists are not exactly as they are portrayed on many popular television shows, where the crime scene inves-tigator plays the role of Sherlock Holmes and does everything from collecting the evidence to solving the crime.
In real life a team of experts does the job of television’s crime scene inves-tigators. The forensic scientists do not directly solve crimes; they simply ana-lyze the physical evidence. Physical evidence includes all objects collected and packaged at a crime scene that will be subsequently analyzed in a crime labo-ratory. This evidence is typically collected by police officers or specially trained crime scene investigators; however, the evidence of a crime is not limited to those items sent to the crime laboratory. Other evidence may include interro-gations, eye witness stories, police reports, crime scene notes and sketches, and anything else determined to aid in the investigation. Subsequently, the de-tective assigned to the case pieces together all the evidence in an attempt to solve the crime. Interpretation of all the evidence and the accompanying scientific results is also practiced by many attorneys, but typically the forensic scientist does not get involved in this aspect of the investigation. Figure 1 illustrates the role that each of these individuals plays in an investigation.
Although the service provided by the forensic scientist is central to the solving of many crimes, it is not usually required for crimes like speeding or shoplifting. In fact, most crimes do not require a forensic analysis of physical evidence. Physical evidence present at a crime scene may not even be col-lected; and if it is collected, it may not be analyzed. The decision to collect and subsequently analyze physical evidence depends on the seriousness of the crime, police department protocol, the state of the investigation, laboratory capabilities, and crime scene resources.
A large number of forensic scientists are chemists. Forensic chemists em-ploy their knowledge of chemistry to analyze evidence such as fibers, paint, ex-plosives, charred debris, drugs, glass, soil, documents, tool marks, and firearms. To a lesser extent, forensic chemists also use their knowledge for tox-icology (the study of poisons and their effects), fingerprints, footwear impres-sions, tire impressions, and hair analyses. Although many forensic analyses re-quire the expertise of a chemist, chemistry is not the only discipline that contributes to the extremely vast and truly interdisciplinary field of forensic science. Other disciplines and professions contributing to the field include engineering, computer science, entomology, anthropology, pathology, physics, nursing, and psychology, among many others. Virtually any discipline, profession, or trade that has an expertise that can aid in the solving of crimes will fall under the umbrella of forensic science. This chapter will focus on some of the many applications of chemistry in forensic science.
The Forensic Generalist and Specialist
Despite the wide variety of evidence that forensic scientists can analyze, most present-day forensic scientists are not generalists. Historically, forensic gen-eralists would analyze all types of physical evidence. Their familiarity with many forensic analysis techniques was extremely diverse, and their ability to carry out any given analysis was limited only by their knowledge and re-sources. The forensic generalist served the role of a family doctor in the field of forensic science—whatever was needed to be analyzed, the generalist could help. Today, however, forensic generalists are slowly being replaced by forensic specialists due to the ever-increasing complexity of the field of forensic science.
Forensic specialists dedicate the majority of their efforts to becoming ex-perts in only one or a few branches of forensic science. Forensic chemistry, for example, is now a specialized field of forensic science. The forensic chemist does not typically analyze biological evidence or carry out DNA analyses. These analyses are typically performed by a forensic biologist. Many argue that forensic specialization is appropriate and necessary due to the vast scope of forensic science and the diversity in analysis techniques. As is true with all other sciences, forensic science continues to evolve and develop. With such a vast body of knowledge, it is inconceivable that a single person could become an expert in all areas of science, and it is equally inconceivable that a person could become an expert in all areas of forensic science.
Specialization is not unique to forensic science and has become common-place in the medical profession. When children are sick, we take them to a pe-diatrician; when we have an ear infection, we see an ear, nose, and throat doctor; and when we need a heart operation, we see a heart surgeon. With the wide variety of evidence that may be analyzed by the forensic chemist, subspe-cialization is also quite common. It is not unusual for a forensic chemist to be given a subtitle such as firearms analyst, trace evidence analyst, fingerprint analyst, or drug chemist. Because more than 70% of all evidence is drug re-lated, drug chemists are quite common in crime laboratories. Although subspecialization is becoming widespread, many forensic chemists may still become proficient in many areas of forensic chemistry.
Forensic chemists may also be given the title of criminalist. This title, al-though less descriptive, is quite common and originates from criminalistics, the branch of forensic science heavily enriched in chemistry and biology applica-tions for the analysis of physical evidence. Criminalistics encompasses a broader area of forensic science than just forensic chemistry and includes most of the areas of forensic science practiced in a traditional crime labora-tory, for example, drugs, fingerprints, DNA, serology (biological fluid test-ing), firearms, and questioned documents.
To analyze physical evidence, forensic chemistry draws on chemistry princi-ples and concepts. Investigating the physical and chemical properties of a sub-stance is central to forensic chemistry. Without an appreciation for these prop-erties and the scientific method, forensic chemistry would not be possible.
Physical and Chemical Properties
Recall that physical properties are properties of a substance that can be de-scribed or displayed without requiring a chemical change. For example, sulfur is yellow (see Figure 2), iron is malleable (able to be hammered into sheets), cocaine is a white solid, and the density of a glass fragment broken from a win-dowpane at a crime scene is approximately 2.5 g/mL.
Chemical properties are properties of a substance that can be described through a chemical change only. Chemical changes require a chemical reac-tion to occur between reactants, generating new products. The chemical properties of a substance are described by the reaction that occurs and the products that are formed. For example, a chemical property of baking soda (sodium bicarbonate) is its reactivity with vinegar (acetic acid) to produce car-bon dioxide bubbles, as shown in Figure 3. This reaction also describes a chemical property of vinegar—its reactivity with baking soda. A chemical property of cocaine is its reactivity with cobalt thiocyanate, which produces a blue-colored product. This chemical property of cocaine, in conjunction with its physical properties (a white, fluffy powder), help investigators identify cocaine.
Scientific Method
Although the exact manner in which the physical and chemical properties are analyzed for each substance differs, the analyses are all based on the principles of the scientific method. The scientific method begins with observations. Scien-tists attempt to organize observations and look for trends or patterns. When the scientists find what appears to be a relationship among the observations, they suggest a hypothesis (an educated guess) that tentatively explains what is being observed. A plan is devised to test the hypothesis. Ultimately, the plan is carried out and further observations are made. If the new observations con-tradict the original hypothesis, a new hypothesis is suggested and tested. How-ever, if the new observations validate the original hypothesis, the scientists often choose to devise a subsequent plan to further validate the hypothesis. This cycle, as illustrated in Figure 4, continues until the hypothesis has been sufficiently validated.
For example, when an unknown substance is submitted to a crime labora-tory, the forensic chemist will first observe the properties of the substance. She may notice that the substance is a crushed and dried green-leafy material. Next, she will suggest a hypothesis as to the identity of the substance: The un-known substance is marijuana. This is an extremely crucial step because the analysis to be performed (the plan for testing the hypothesis) is different for each unknown substance. The chemist will then devise a plan to test the hy-pothesis: to view the substance under the microscope looking for properties of crushed marijuana leaves. If the microscopic observations validate the hypoth-esis, she will develop a subsequent plan to further validate the hypothesis: to react the marijuana leaves with Duquenois-Levine reagent to observe chemical properties. If the microscopic features do not validate the hypothesis, she will suggest and test an alternative hypothesis: The unknown substance is oregano.
In chemistry, physical and chemical properties are used to characterize and distinguish one compound or element from another. In forensic chem-istry, these properties aid in the identification, classification, and individual-ization of physical evidence.
THEORY OF FORENSIC ANALYSIS
After a police officer or investigator has collected evidence at a crime scene, some evidence may be brought to the crime lab for a forensic chemist to ana-lyze. The chemist follows a specific process, based on the scientific method, for analyzing evidence. Samples collected from a crime scene and brought to the lab for analysis are called questioned samples because the identities and ori-gins of those samples are unknown. In order to draw conclusions about the identity or origins of questioned samples, the forensic chemist will need known samples as a reference. A known sample might be collected as part of the evidence—for instance a hair sample collected from a suspect.
Forensic analyses may be performed to (1) identify a questioned sample or (2) compare a questioned sample to a known sample for the purpose of deter-mining the source or origin of the sample (where it came from). The results of such comparisons can link a questioned sample and several known samples either to a class of samples with several possible origins (classification) or to a single origin (individualization). Thus, a forensic chemist will analyze much more than the questioned sample. A comparative analysis may require the ex-amination of several known samples for each questioned sample.
A forensic analysis follows the order of identification, classification, and in-dividualization, as illustrated in Figure 5. The challenges found during each phase of analysis are different for each item of evidence. Often, identification is straightforward and obvious to the untrained eye (for instance, hair); other times expertise and sophisticated instrumentation are required (for instance, drug analysis). We will discuss each of these phases of analysis in more detail in the sections that follow.
Identification
When a questioned sample is submitted to a crime laboratory for analysis, the first task is identification. For example, if a white powder is submitted for analysis, the primary objective will be to determine its identity. If the powder is suspected of being a controlled substance, the forensic scientist will carry out a series of analyses to identify the powder. However, because each drug has a different set of physical and chemical properties, a different series of analy-ses is required to identify each drug. As mentioned during our discussion of the scientific method, the forensic scientist must first make an educated guess as to the identity of the substance. Using known drug standards for each type of drug to be analyzed, she must develop and validate a series of analyses prior to analyzing the questioned samples. Consequently, the identification of an uncommon drug can be challenging.
Two types of analysis can be used to identify the substance: presumptive and confirmatory. Presumptive analyses look at chemical and physical prop-erties that are not unique enough by themselves for identification but that provide enough information to narrow the search. For example, the forensic scientist may guess that the questioned sample is methamphetamine. A known chemical property of methamphetamine is that it will react with sodium nitro-prusside in the presence of sodium bicarbonate and produce a very deep blue-colored product, so the scientist will carry out this test. If the reaction pro-duces a blue product, as shown in Figure 6, she can conclude that the unknown substance might be methamphetamine. However, a number of sim-ilar compounds (all containing a nitrogen atom with one hydrogen atom and two attached carbon atoms) will also produce a deep blue-colored product. This analysis does not confirm that the substance is methamphetamine, but it does reduce the number of possibilities. Now the forensic scientist can pro-ceed to more time-consuming or expensive tests knowing that she is on the right track. Presumptive analyses are usually quick and inexpensive to per-form. When presumptive analyses are negative, they exclude potential drug candidates; when they are positive, they direct the forensic scientist toward viable confirmatory analyses.
Whereas presumptive analyses only narrow the possible identities of a sub-stance, confirmatory analyses identify a questioned sample absolutely. They are required for court and must be performed to convict someone for posses-sion of an illegal substance. These analyses use the unique chemical or physi-cal properties of a substance for the purpose of identification. Typically, con-firmatory analyses require more time and expense than presumptive analyses. These analyses often require the use of sophisticated chemical instrumenta-tion to measure the unique properties that lead to identification.
One instrument used by the forensic chemist is the Fourier transform in-frared spectrophotometer (FTIR), as shown in Figure 7. With the FTIR, the forensic chemist can begin to identify the questioned sample by measuring its unique interactions with infrared light. This pattern of interaction, which is a function of wavelength, is sometimes called a chemical fingerprint. It is unique to a pure substance and allows for its identification. However, because questioned samples are typically mixtures, rather than pure substances, an ad-ditional step is often needed in the analysis.
The compounds in a mixture can be separated, and each compound can subsequently be identified by an instrument called a gas chromatograph-mass spectrometer (GC-MS). After the compounds are separated in the gas chro-matograph, the mass spectrometer breaks the separated compounds into frag-ments and measures the mass of each fragment (see Figure 8). The profile of fragment masses that is generated is exclusive to the compound and allows for identification. To understand why this works, suppose the only information you have about two people is their weight (mass). You might find it difficult to distinguish a tall and thin person from a short and stout person. However, if you could take separate weight measurements of the arms, legs, torso, head, fingers, and feet of each person, you would more likely be able to identify each individual. A system of identification like this was actually used in early investi-gations. A Frenchman named Alphonse Bertillon developed a technique called anthropometry around the year 1880; it used eleven length (rather than weight) measurements of the human body for identification. Anthropometry was replaced 20 years later by fingerprinting, which was more accurate for identification.
Comparative Analysis: Classification
and Individualization
Many forensic analyses end with identification (for example, identifying an unknown substance as a drug, explosive, or accelerant used in arson), but some proceed on to comparison. The purpose of a comparative analysis is to link a questioned sample and a known sample to a common origin. The origin may be broad, resulting in a classification, or exclusive, resulting in individu-alization. In the case of a hair sample, its identification is often obvious, or may be easily established in the laboratory. In fact, the forensic value of the hair sample as evidence is not found in its identification as a piece of hair. What is more important to the investigation is the source or origin of the sample in a particular species or individual. This can be determined through comparative analysis with several samples of a known origin.
Class characteristics are properties of a substance that are shared by a group of substances, but are not unique to all substances of a single origin.
They allow for the placing of a questioned sample into a class or group of several possible origins. For example, a class characteristic of hair is its color. If a questioned hair sample is brown, it could be determined that the hair orig-inated from a person with brown hair. These properties are analogous to those used when conducting a presumptive test for the purpose of identification.
Not only are class characteristics common to other substances, but they may also vary within a substance. A class characteristic that varies within a sub-stance is called natural variation. When attempting to determine the origin of a questioned sample, the forensic chemist must know all the possible varia-tions of the class characteristics from a known sample. Consequently, he must have available several samples of the same known origin. For example, when a questioned hair sample is being compared to the scalp hairs of a suspect, often more than fifty scalp hairs are collected to determine natural variation. The forensic chemist then will compare length, color, pigment distribution, coarse-ness, and other properties of the known samples. It is likely that there will be a range in values of all these characteristics among a representative collection of a suspect’s scalp hair. If the suspect has a mullet haircut (short on top, long in back), there will be a broad range (natural variation) of scalp hair lengths. If the suspect has dark hair with light blonde highlights, there will be a broad natural variation in the individual’s scalp hair color. Figure 9 shows the natural variation of hair seen under a microscope. It is imperative that the forensic chemist identify the natural variation within all the known samples of a single origin prior to comparing these class characteristics to a questioned sample.
In the preceding example, the forensic chemist can make a connection be-tween a sample of a questioned origin and several samples of a known origin if the values of the class characteristics of the questioned sample fall within the range of natural variation. In such a scenario, he can conclude that the ques-tioned hair is found to be “consistent with” or “similar to” the known hairs based on the comparative analyses performed. However, the hairs may not share a common origin—for two reasons. First, the properties being compared (class characteristics) are not exclusive to a single origin, and, second, the natural variation of a class characteristic increases the range of possible origins. It is extremely important not to interpret more into an analysis than what is being suggested. This is a common mistake.
Virtually all physical evidence has class characteristics. These characteris-tics are more common than individual characteristics. Many items of evidence, like hair, fiber, glass, soil, and paint, routinely only have class characteristics. In other words, classifications are more common than individualizations. This does not, however, suggest that comparative analyses of items only containing class characteristics are unimportant. The ability to exclude is a very powerful aspect of class characteristics. If, for example, a comparative analysis excludes a questioned hair from originating from the suspect’s head, this information is just as important as individualization—the suspect may be exonerated (set free of guilt). Also, if the class characteristics of many questioned items of evi-dence are similar to those of many samples of known origins, each additional link (even if tentative) further incriminates the suspect. For example, if a pubic hair, a scalp hair, a glass fragment, a cat hair, and two fibers found on a suspect were all consistent with those found at the crime scene, although no single item offers an exclusive link, the composite becomes highly significant. Fibers were the key to solving a series of child murders in Atlanta, Georgia, when between 1979 and 1981 over twenty African-American children were killed. This infamous child murder case was ultimately solved by linking 19 sources of fibers found in the personal environment of the suspect Wayne Williams to several of his victims. Wayne Williams was only tried and found guilty for two murders, although many attribute all the murders to him.
Individual characteristics are properties of a substance that are unique and can be used to establish origin. For example, if the brown hair sample
contained enough DNA in its root for a DNA analysis, the DNA would be con-sidered an individual characteristic that would exclusively link the hair sample to a single origin (person). DNA found in semen on Ms. Monica Lewinsky’s dress was used to exclusively link President Bill Clinton to her dress. Figure 10 shows the results of a DNA analysis. These properties are analogous to those exploited when conducting a confirmatory test for the purpose of identifica-tion; however, during a comparative analysis these properties are used to de-termine the origin of a substance rather than simply to identify it.
A physical match is the classic example of an individual characteristic. A physical match, or jigsaw fit, is what occurs when a questioned and a known sample fit together like puzzle pieces. For example, a diamond might be chipped as it is being forcibly removed from a ring during the commission of a crime. If the chipped piece is located at the crime scene with the ring, and the diamond is recovered from the suspect, all that may be needed to link the diamond to the same origin as the chipped piece (the victim’s ring) is a phys-ical match seen under a microscope.
Other common items of evidence having individual characteristics include fingerprints, footwear impressions, tool marks, and bullets. Comparative analyses of all these items of evidence often result in a linkage to a single ori-gin when the questioned sample is compared to several known samples. In the case of a DNA or fingerprint analysis, a link can be made exclusively to a sin-gle person. In the case of a footwear or tool mark impression, if individual characteristics are present (which is not always the case), a link can be made to a single shoe or tool, respectively. Figure 11 shows a court display that com-pares the footwear impression found at a crime scene to one from a known source. In addition to class characteristics like tread pattern and size, the im-pression displays individual characteristics (marked in yellow in the figure) that link it to a specific origin. However, as mentioned earlier, many items of evidence do not have individual characteristics that can be used for individu-alization so investigators must rely solely on class characteristics. Table 1 gives examples of class and individual characteristics for common physical evi-dence.
FINGERPRINT DEVELOPMENT
Suppose a burglar enters your home while you are away and steals your plasma television. Examining your home, you see the burglar has left nothing behind but cut wires, a broken window, and a few holes in your wall. Is there anything the police can do to catch the thief? Could hidden clues have been left behind that you missed? If so, how can these hidden clues be discovered?
Among the most common items of evidence collected at a crime scene are fingerprints. The ridged-skin patterns at the end of our fingers contain indi-vidual characteristics that make them highly unique. When perspiration on the hands and fingers combines with oils, dirt, or other substances, these fin-gertip ridges can leave an impression on surfaces that are touched. Finger-prints are useful in investigations because an individual’s fingerprints are con-sistent over time, and no two fingerprints have ever been found that are exactly alike. Even identical twins have unique fingerprints. Fingerprints col-lected at a crime scene can be compared to fingerprints collected from sus-pects and from individuals who had legitimate reasons to be at the crime scene. They can be checked against databases of prints collected by law en-forcement agencies. Figure 12 shows a court display comparing a print found at a crime scene to a print from a known source, with individual characteristics marked. The ability to compare fingerprints is an art that requires skill and training; fingerprint analysts spend years perfecting these skills.
Fingerprint development is the process by which hidden fingerprints can be found, visualized, and examined. There are three different types of finger-prints: latent, plastic, and negative. Latent (hidden) fingerprints are those most common to a crime scene. These prints are produced by touching a sur-face and leaving behind fingerprint residue (oils, dirt, perspiration) in the pat-tern of the ridges. Because the prints are invisible to the naked eye, investiga-tors must use development techniques to find them. Development techniques use the chemical and physical properties of the fingerprint residue to produce contrast so the hidden prints can be observed. To develop a latent fingerprint, investigators must understand the potential composition of the residue. Fin-gerprints typically not requiring development include plastic fingerprints made into soft surfaces such as silly putty, butter, or clay and negative finger-prints created as the skin ridges of a finger remove transferable material from a surface leaving behind a pattern of the ridges (e.g., a person touches a dusty chalkboard or a greasy wrench).
Virtually all fingerprint residues of latent fingerprints contain perspiration because our hands and fingers contain sweat glands. The composition of per-spiration is slightly different for each individual and changes as a function of diet and throughout the day. Perspiration comprises water and any water-soluble salts (sodium chloride and potassium chloride), acids (lactic acid and acetic acid), and proteins composed of amino acids. Skin cells are continually being shed from the fingers and may also be present in fingerprint residue.
In addition, our hands are very active during the day and come in contact with many items. Without thinking about it, we may scratch our backs or necks, rub our noses, touch our ears, or massage our foreheads. All these ac-tivities put our fingers in locations that contain sebaceous glands, which are found at the base of hair follicles and exude fats and oils. Contact with these locations will transfer sebum, a mixture composed of fatty acids, triglycerides, squalene, and wax esters. Cosmetic products may also be transferred to our fingers. In addition, we put lotion on our hands, touch dirty surfaces, use household cleaners, pick up food, and even occasionally forget to wash our hands after using the restroom. All these activities potentially add to the com-position of our fingerprint residue. Because the composition of one person’s fingerprint residue may be considerably different from that of another, many development techniques have been established. A technique that works well for one fingerprint residue may not work well for another. In addition, some development techniques work better on fingerprints found on certain sur-faces. Several fingerprint development techniques will be discussed in the fol-lowing sections. The properties of the components allowing for development and the surfaces on which the techniques work best will be identified.
Powder Dusting
Powder dusting involves the use of fine powders to visualize latent finger-prints. It works well on smooth nonporous surfaces such as glass, certain plastics, and ceramics but is less effective on porous surfaces such as paper or cardboard (the residue tends to absorb into the fibers over time) or on wet or sticky surfaces. Among the many components of fingerprint residue, sebum and perspiration tend to adhere to powder particles. This physical property of fingerprint residue, in conjunction with the fact that many smooth, non-porous surfaces do not adhere well to powder particles, allows for fingerprint development. The contrast developed between the adhered powder and the surface allows for visualization. The same concept is illustrated when spilled flour sticks to residue on the counter or when beard and mustache shavings stick to the toothpaste stains in the sink.
Investigators use many different types of powders. Most black powders are made from fine carbon or iron. Light-colored gray or white powders can be made of any number of substances, such as finely divided aluminum. There are also fluorescent powders in red, green, yellow, or orange, some of which may also contain iron particles. Any powder containing iron may be applied with a magnetic applicator. The magnetic applicator, a small cylinder the size of a marker or pencil, contains a sliding magnet that can be moved up or down the inside of the cylinder. When the magnet is positioned at the tip of the applicator inside the cylinder, powder containing iron will adhere to the tip and provide a collection of powder for application. Sliding the magnet away from the tip will release any excess powder. Other powders are typically applied with a variety of fine brushes made of animal hair or synthetic fibers.
Whether magnetic or nonmagnetic powder or black or fluorescent pow-der is used depends on personal preference and the contrast needed. Some forensic scientists prefer magnetic powders because they believe brush bristles damage the fingerprint; others think magnetic powders are too messy.
Once the powder has been applied and contrast can be seen, the finger-print can be lifted and preserved using fingerprint tape, a high-quality trans-parent tape typically at least an inch wide. The lifted fingerprint can then be placed onto a fingerprint lift card that offers the greatest contrast (black for white-powder lifts and white for black-powder lifts). Identifying information such as the name of the investigator, date and time of collection, location of fingerprint, and case number all should be recorded on the card. Figure 13 shows a fingerprint lift of a black powder impression.
Ninhydrin Reaction
For years, biochemists have used the ninhydrin reaction for both qualitative and quantitative determination of ?-amino acids. There are approximately 20 ?-amino acids that comprise proteins. Proteins are natural polymers (mol-ecules composed of repeating monomer units) containing ?-amino acid monomers. Ninhydrin is known to react with alfa-amino acids and produce a purple-colored product called Rhuemann’s purple, named after Siegfried Ruhemann who discovered the reaction in 1910. The reaction is sensitive enough to be used on the development of the small amounts of ?-amino acids found in fingerprint residue. It became popular with forensic scientists in the 1950s and is still used frequently by most fingerprint examiners. Although the reaction is relatively slow (24 hours for development), it can be accelerated by the use of heat or moisture. Special ninhydrin chambers, which provide a hot and humid environment, allow for ninhydrin development in 20 minutes or less. When used for fingerprint development, the reaction works best on porous surfaces such as paper, and because amino acids are relatively stable, ninhydrin development works considerably well on old fingerprints. Figure 14 shows prints developed with ninhydrin.
Silver Nitrate Reaction
Chloride salts like sodium and potassium comprise a significant percentage of perspiration, and thus fingerprint residue. When silver nitrate reacts with any soluble chloride salt, the insoluble salt silver chloride is produced. The reac-tion occurs almost immediately. The silver chloride produced is a white solid that does not offer much contrast for fingerprint development. However, as the silver chloride remains exposed to ultraviolet light, it decomposes pro-ducing silver and chlorine gas. This produces a purple-black product that of-fers contrast for fingerprint development, as shown in Figure 15. Silver nitrate development works best on porous surfaces like paper.
Iodine Fuming
Iodine, in much the same way as solid carbon dioxide, undergoes a phase tran-sition from solid to gas, skipping the liquid phase. This phenomenon is known as sublimation. Iodine is a purple solid under ambient temperature and pres-sure. When iodine crystals are heated, they will sublime, producing iodine vapors. These vapors are thought to be absorbed by the fingerprint residue so that they produce a transient amber-colored product, shown in Figure 16a.
Over time, the amber color will fade. Techniques have been devised to fix the developed print. One technique employs the reaction of iodine with starch to produce a stable dark purple product, shown in Figure 16b. Iodine fuming is one of the oldest fingerprint development techniques; it works well on porous surfaces.
Superglue Fuming
In the late 1970s, it was discovered that superglue fumes, composed of cyano-acrylate monomers, would selectively polymerize (form polymers) on finger-print residue found on smooth nonporous surfaces. The technique was first employed by the Criminal Identification Division of the Japanese National Police Agency in 1978. The technique was later introduced to the U.S. Army Crime Laboratory of Japan and was soon adopted by many crime laboratories nationally. It is presently one of the most popular fingerprint development techniques.
The polymerization of superglue monomers results in adhesion, and cyanoacrylates are commonly used for bonding purposes. The polymerization process is typically initiated by negatively charged water-soluble species (an-ions), which are found in fingerprint residue and are thought to preferentially initiate polymerization on their surface. This preferential initiation allows for the white–gray superglue polymer to form first on the fingerprint residue. The polymer not only offers modest contrast for fingerprint development but also aids in fingerprint preservation.
Items containing fingerprints to be developed are placed into a superglue fuming chamber, shown in Figure 17. Liquid superglue is poured into a con-tainer and slowly heated within the chamber on a hot plate to produce super-glue fumes. The fumes saturate the air within the chamber and begin to poly-merize on the fingerprint residue. Often the chamber is humidified. If the items containing fingerprints are not removed from the superglue fuming chamber in a timely fashion, polymerization may additionally occur on the surface resulting in “overfuming” and so jeopardize the development. Alternatively, superglue fumes may be produced by reducing the pressure in-side the chamber.
Typically, analysts use fingerprint powders or dyes to enhance the contrast on the developed fingerprints. Most of the dyes used after superglue fuming are fluorescent dyes, which require the use of ultraviolet light for visualiza-tion. Superglue fuming played a role in the capture of the infamous Night Stalker of California, Richard Ramirez. Ramirez, suspected of having gone on a true murder spree in the mid-1980s, killed both young and old with no pre-ferred murder weapon, location, or technique. His victims ranged in age from mid-twenties to mid-eighties. He was known to have beaten, shot, and/or stabbed his victims in addition to sexually assaulting them. From an abandoned car known to have been driven by the Night Stalker, police used superglue fuming to develop a single fingerprint that matched the finger-print of Richard Ramirez. Ramirez was ultimately convicted of thirteen counts of murder, five attempted murders, eleven sexual assaults, and four-teen burglaries.
Phenolphthalin Reaction
Often fingerprints contain a trace amount of blood. Many reactions can be catalyzed by the heme portion of hemoglobin found in blood (shown in Fig-ure 18) and have been used for the presumptive identification of blood. When used for fingerprint development of blood-containing fingerprints, the reac-tant molecule is converted to a colored product resulting in contrast. Because these reactions require heme as a catalyst, blood is not consumed, and the reactions are extremely sensitive. They have been used to develop latent fin-gerprints containing the slightest amount of blood.
Phenolphthalin is a molecule chemically related to phenolphthalein (a common chemical used in chemistry for acid–base reactions). Under ideal conditions, and in the presence of hydrogen peroxide and blood, colorless phenolphthalin will be converted to pink phenolphthalein. The reaction is often called the phenolphthalin or Kastle–Meyer reaction. Because com-pounds other than heme may also catalyze the reaction (potassium perman-ganate, rust, and some plant enzymes), the test is only presumptive for the presence of blood.
Reactions with chemicals other than phenolphthalin (leucomalachite green, tetramethylbenzidine, and ortho-tolidine) can also be catalyzed by heme and used to develop blood-containing fingerprints. All these chemicals produce a green-blue product in the presence of blood. The use of luminol is a popular test to locate trace amounts of blood, but it is not typically used for fingerprint development. It is a chemiluminescent reaction, producing light that can be seen in a dimmed room where blood is located. This technique can be used to determine hidden bloodstain patterns. Bloodstain pattern analysis is the examination and study of bloodstains (hidden or visible) for the purpose of crime scene reconstruction. Bloodstains can suggest where a crime occurred, what occurred, how it occurred, and a potential sequence of events.
PRESUMPTIVE DRUG ANALYSISPRESUMPTIVE DRUG ANALYSIS
A police officer pulls over a car for speeding. While proceeding to the car, she sees the driver hurriedly put a plastic bag in the glove box. Suspecting that the driver has drugs in the car, she asks the driver to step out of the vehicle. The officer searches the vehicle and finds the plastic bag with nine other, similar bags, all containing a white powder. When asked what is in the bags, the driver responds, “Powdered sugar.” Suspicious that the driver is lying, but wise enough not to taste the unknown substance, she retains the bags for analysis.
What techniques are available to her to identify the substance quickly and pre-sumptively on the scene? How can the identity of the substance in the bags ul-timately be confirmed in the crime laboratory?
Most confirmatory analyses employed for drug identification are moder-ately time-consuming and require the use of expensive instrumentation such as a gas chromatograph-mass spectrometer or a Fourier transform infrared spectrophotometer. To save time and money, before conducting a confirma-tory analysis (potentially resulting in inconclusive information), quick and in-expensive presumptive drug analyses are performed. These analyses direct the forensic scientist toward an appropriate confirmatory analysis that will yield the desired results the first time.
Color tests, sometimes called spot tests, are examples of presumptive drug tests used to probe questioned drug samples for their chemical properties. If chemical properties consistent with a known drug are discovered, a ques-tioned drug sample can be presumptively identified. When conducting a color test, chemicals known to produce a colored product in the presence of a sus-pected drug are added to a small amount of the questioned sample. If the questioned sample contains the suspected drug, a colored product, having a color representative of the suspected drug, will be produced. The measured chemical property for drug identification is both its reactivity with the chemi-cals and its ability to produce the color-indicative product. For instance, co-caine produces a blue product when allowed to react with the chemical cobalt thiocyanate, and LSD produces a red-violet product when allowed to react with p-dimethylaminobenzaldehyde.
Many substances other than drugs will react with the chemicals used for presumptive drug identification and produce products of varying colors, or no color at all. The differing color (or lack of color) suggests that the ques-tioned sample does not contain the suspected drug. Yet, many other sub-stances will produce a product with the same representative color as the suspected drug. For example, in the presence of cobalt thiocyanate, lidocaine, benzocaine, and procaine all yield a blue-colored product similar to that of co-caine. For this reason, a positive color test is not confirmatory and simply di-rects the forensic scientist toward identification by limiting the possible num-ber of drug candidates. For example, if a blue product is formed with cobalt thiocyanate, methamphetamine, heroin, and LSD can be excluded as possible identities for an unknown substance.
Color test reactions are commonly preformed in crime laboratories using spot plates. Spot plates are small ceramic or plastic dishes that contain several wells. Because most drug color tests are quite sensitive, requiring only a few micrograms of the sample being tested, the small wells of a spot plate are ideal for analysis. In addition, neighboring wells can be used to conduct positive (drug standard) and negative (no drug) controls. Side-by-side comparison of colors generated from the questioned sample and the control samples assist in positive identification.
Presumptive color tests, in addition to being performed in the crime labo-ratory, are commonly performed by police officers on the street in little plas-tic bags called Narcotic Field Test Kits. This is done to determine quickly on the scene if the police officer has enough probable cause for an arrest. The offi-cer places the questioned substance in a bag containing ampoules of chemical reagents necessary for the presumptive identification of a particular drug and then breaks the ampoules within the bag, initiating the chemical reaction. The color of the product is observed. If the test is positive for the presumptive identification of a suspected drug, the test is typically performed a second time in the crime laboratory with controls for verification.
Many different presumptive color tests for drugs are available. Typically, different chemicals are used for each drug to be tested; therefore, the forensic chemist must propose a hypothesis regarding the identity of the substance prior to performing the presumptive test. Some chemicals, however, are used for more than one class of drugs. For example, the Marquis test, consisting of two chemicals (concentrated sulfuric acid and formaldehyde) yields a purple product for opiates like morphine, heroin, codeine, and oxycodone and an orange-red product for amphetamine and methamphetamine. Figure 19 shows a spot plate setup for the Marquis test to identify opiates. The center well has the negative control (no drug); the right-hand well has the positive control (codeine mixed with the Marquis reagent). The unknown substance will be mixed with the Marquis reagent and placed in the left-hand well for a color comparison with the other two wells.
Because the chemistry of most presumptive drug tests is complex, many re-actions are not completely understood. In fact, most presumptive tests were developed by chance, when a certain chemical was added to a given drug and a colored product was generated without knowing exactly why. Subsequently, validations were performed to determine what else could yield a colored prod-uct when combined with that same chemical. Table 2 lists various drug color tests.
Microcrystalline tests, a special class of presumptive drug analyses, produce solid products. Typically the solid forms slowly, producing representative crys-talline structures that may be viewed under the microscope with transmitted il-lumination (see Figure 20). To a trained expert, these tests are practically con-firmatory. However, some controversy still exists regarding the applicability of microcrystalline tests. Many forensic chemists are not trained to recognize the characteristic crystalline structures. Identification of the target product is more complicated than simply observing a color. Consequently, even though well-developed microcrystalline tests are available for cocaine, amphetamine, heroin, and other drugs, many crime laboratories do not perform these tests.
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