What a Crime Lab Does, It Does with Chemistry

Much as a car runs on gasoline, a crime lab runs on physical evidence. The O. J. Simpson murder trial in 1995 put a nationwide, if not a worldwide, spotlight on many aspects of what a crime lab does. Physical evidence encompasses any and all objects that can either establish that a crime has been committed or provide a link between a crime and its victim or perpetrator. Forensic scientists in a crime lab analyze this physical evidence; however, much of the analysis is in fact chemical analysis. Let’s briefly examine physical and biological analysis first. The hit-and-run death of a pedestrian can involve both. Fragments of broken headlight glass are collected from the scene. Samples of the victim’s blood are collected from the autopsy. Later, a suspect car is located. Evidence collected from the car includes the broken headlight, more fragments of broken headlight glass from the headlight well, and dried bloodstains from the undercarriage. The stains are collected by rubbing with cotton swabs moistened with purified water; the swabs are dried and submitted to the crime lab. Having first kept careful track of which fragments of glass came from the car (the “known” evidence) and which came from the crime scene (the “questioned” evidence), forensic scientists attempt to reassemble part of the headlight in jigsaw-puzzle fashion. Any jigsaw fit that includes both questioned and known fragments of glass shows that the two were originally part of the same headlight and therefore links the car to the crime scene. This is an example of physical analysis. And if DNA profiles of the victim’s blood (the “known” evidence) and the blood on one or more swabs (the “questioned” evidence) match, then the car is linked to the victim. This is an example of biological analysis. A crime lab is typically divided into several sections. Due to the degree of expertise required to perform the necessary analysis, a forensic scientist is typically hired to work in just one section. However, cross training sometimes occurs, depending upon a lab’s needs. In many labs, a single forensic scientist analyzes a given case in a given section. In some labs, scientists team up. In either instance, peer review by a scientist not involved precedes issue of the final report in many labs for quality assurance. And what is considered one “case” by an agency bringing evidence to the lab may require analysis by more than one section. A scientist in the trace-evidence section might have performed the physical analysis in the hit-and-run case mentioned earlier (whereas one in the bioscience section might have performed the biological analysis). If a jigsaw fit cannot be obtained, chemical analysis can be performed to determine whether the questioned and known glass could have had a common source. Properties compared may include ultraviolet fluorescence, refractive index, density, and elemental analysis. The more properties that agree, the more likely it is that the two at least had a common source. Conversely, a mismatch in any property rules out a common source. Trace-evidence scientists also examine fire debris for the presence of accelerants, paint chips (from auto crashes or burglaries) for common source (can compare paint vehicle itself, pigments, elemental analysis), tear gas samples (to identify the active agent), explosive residues (to identify either traces of parent explosive or products of the explosion), and headlight filaments to determine whether a broken headlight was on or off at the instant of a crash. This last analysis is actually physical (simply examination under an ordinary microscope) but is in part a result of chemistry. An intact headlight is filled with an inert gas, since air would oxidize the white-hot filament. A headlight that was off at impact has a shiny, silver-colored filament. A headlight that was on at impact has a dark blue- to black-colored filament; when the glass breaks, the admitted air forms a layer of metal oxide(s). Another feature is often evident on such a filament—microscopic globules of glass sticking to its surface. These globules arise from small glass fragments melting on contact and then freezing into position as the filament cools off. A primary function of the questioned-documents section is to determine whether the same person executed both questioned or known handwriting. For example, a murderer might attempt to conceal the crime by writing a suicide note. Comparison of handwriting in the note to known samples of both the victim’s and the suspect’s handwriting would expose the ruse. This is not chemical analysis. However, if the note was typed or computer printed, both physical and chemical analysis could compare properties, such as letter size and shape or chemical composition of the paper and ink, to see if the note matches either a typewriter, a computer printer, or paper accessible to either the victim or the suspect. If a document is suspected of being altered, modern instrumentation allows it to be examined in a matter of minutes for inks with different light absorption or emission properties. Sometimes, determining whether a document was executed on an alleged date is desired. For example, a document alleged to be executed in 1980 could not be authentic if analysis reveals that it contains either paper or ink that did not exist until 2000. A primary function of the firearms section is to determine whether a questioned bullet (for example, removed from the body of a shooting victim) was fired from a known (suspected) weapon. Rifling in a weapon’s barrel cuts impressions into a bullet’s surface, but fine striations unique to a given barrel are superimposed on these impressions. Analysts fire the suspected weapon two or more times into a water tank (for handguns) or a cotton waste recovery box (for rifles), which stops the bullets without damaging them. If the impressions and striations around the entire circumference of these recovered bullets match those on the questioned bullet (as viewed under a comparison microscope), then the same weapon fired all the bullets. This analysis is physical. But chemistry also enters into the firearms section; one example is serial-number restoration. Often weapons (or other metal parts such as motor-vehicle engine blocks) arrive at the lab with their serial numbers obliterated. Serial numbers are usually die stamped; this process cold-works the metal in the area immediately surrounding and a short distance below the penetration of the die. The cold-worked metal is less resistant to chemical attack than the base metal. Polishing the obliterated area, then etching the area with a chemical suitable for the particular metal (sometimes with an applied voltage to speed up the attack), can make the numbers visible again. Serial number restoration also works with plastic parts; cold worked plastic is less heat-resistant than the base plastic, so one can replace etching with a high-intensity lamp. The drug-chemistry section analyzes evidence believed to contain illegal or controlled substances. Evidence is most often tablets, capsules, powder, crystals, or vegetation. Scientists isolate the active ingredient(s) using solvent extraction and then perform several chemical tests. Final identification is accomplished by one or more techniques considered to produce a “chemical fingerprint” for a substance: infrared (IR) spectroscopy, nuclear magnetic resonance (NMR) spectroscopy, or mass spectrometry (MS). Rarely is the mass spectrometer used alone; it is linked instead to either a gas chromatograph or a liquid chromatograph, hence the names GC-MS and LC-MS, respectively. The hyphenated techniques are especially useful if solvent extraction yields a mixture rather than one pure compound. The chromatograph separates the mixture; one by one, each compound is identified as it passes into the mass spectrometer. The author is assigned to the toxicology section, which will provide the final examples of chemical analysis. Toxicologists analyze body fluids (such as blood and urine) and body tissues (such as liver, brain, and stomach contents, although the last item is not strictly a tissue) for the presence of (and often the amount of) drugs and poisons. These items are submitted in two main types of cases. The first is in arrests of motorists for driving under the influence of alcohol and/or drugs. The second is in unattended deaths, where a drug or poison overdose might be the cause of death. Occasionally, non-biological specimens will be submitted. One example is suspected alcoholic beverages in cases involving violations of open-container laws (open-container laws prohibit the possession of any open alcoholic beverage container and the consumption of any alcoholic beverage in the passenger area of a motor vehicle). Toxicologists isolate drugs, poisons, and their by-products (metabolites) from blood, urine, and tissue specimens using solvent extraction or solid-phase extraction. A complex mixture normally results; identification by IR or NMR is impossible, since these techniques require a pure substance. Hence GC-MS and LC-MS are employed. Toxicology can reveal the unnatural nature of what might at first appear to be a natural death. For example, a murderer might attempt to conceal the crime by setting a fire. But a fire victim normally has a high blood carbon monoxide level. A murder victim, dead before the fire began, couldn’t inhale any carbon monoxide. In another example, the author analyzed autopsy specimens from a 43-year-old female found dead in bed. Lethal levels of chloroform (an industrial solvent) were found. When confronted by this evidence, the woman’s estranged husband admitted entering the house while she slept and covering her mouth and nose with a chloroform soaked tissue. Other times a death really is natural. The author analyzed autopsy specimens from Sergei Grinkov, the Olympic pairs figure skating champion who collapsed and died during a practice session in Lake Placid, New York. Only lidocaine and atropine (given during resuscitation attempts) were found. Occasionally the toxicology section analyzes unusual items. The author examined both fighters’ boxing gloves for foreign substances after the 1996 Madison Square Garden heavyweight bout in which Evander Holyfield defeated Bobby Czyz—and found none. Czyz’s camp argued that he could not continue because his eyes were burned from a foreign substance on Holyfield’s gloves.

—Harry K. Garberis a forensic scientist in the toxicology section of the New York State Police Forensic Investigation Center in Albany, New York.


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