Few processes are more important to society than solving crimes, both to protect the public from criminals and to protect the innocent from unjust punishment. Very often, the strength of a prosecution rests on the ability of law enforcement personnel to connect the accused with the victim by matching physical evidence from the crime scene or victim with trace evidence found on or about the person accused of the crime. Forensic investigators consult a wide range of experts who analyze evidence collected at crime scenes and brought to the crime laboratory for examination. Forensic chemists perform specialized analyses to identify materials and learn the nature of such evidence. A highly trained forensic chemist can determine the composition and nature of materials and predict the source as well as matching sample against sample. Modern chemistry employs a wide range of analytical techniques along with traditional methods of analysis.
Physical evidence collected at crime scenes is sealed in special containers to prevent contamination and degradation and is catalogued carefully. A chain of custody is established and documented as the evidence is sent to a forensic laboratory. At the laboratory, the evidence is examined by personnel trained in one of several fields: Forensic serologists examine body fluids, forensic pathologists examine human remains, firearms technicians classify and test firearms and explosives, and forensic chemists determine the composition and identity of materials.
The History of Forensic Chemistry
Poisons were employed by early Egyptians and ancient Greeks and Romans. Democritus was probably the first chemist to study poisons, and he communicated some of his findings to Hippocrates . Poisons were used both for murder and as a means of execution; the philosopher Socrates was condemned to death by drinking hemlock . Ancient Roman civilization had laws against poisoning in 82 B.C.E. Before the development of systematic, scientific criminal investigation, guilt was determined largely by circumstantial evidence and hearsay. Arsenic was a popular poison in Roman times. It was referred to as inheritance powder in early France. The Blandy trial of 1752 was the first instance of an actual chemical test for poison, and the Marsh test, developed in 1836, was the first reliable analysis that could show scientifically that arsenic was present in the body of a victim.
Every chemist is schooled in general, organic, and analytical chemistry, but forensic chemists also specialize in specific areas of expertise. For example, an inorganic chemist may examine traces of dust by using microchemistry to identify the chemical composition of tiny particles. Another chemist might employ thin-layer chromatography during the analysis of
blood or urine for traces of drugs, and still another might use chemical reactions in test tubes to identify larger samples of compounds.
What Do Forensic Chemists Do?
Forensic chemistry encompasses organic and inorganic analysis, toxicology , arson investigation, and serology . Each method of analysis uses specialized techniques and instrumentation. The process may be as simple as setting up a density gradient column to compare soil samples or as complicated as using a mass spectrometer or neutron activation analysis to characterize an unknown substance.
A wide array of laboratory techniques and instrumentation is used in forensic studies. This includes ultraviolet, infrared, and visible spectrophotometry; neutron activation analysis; gas chromatography and mass spectrophotometry; high pressure liquid chromatography; and atomic absorption spectrophotometry. The techniques and instrumentation chosen depend on the type of sample or substance to be examined.
The fact that most samples examined are not pure substances, but are often mixed with dirt or debris, presents a major challenge to the forensic chemist. This may also be an advantage, as every substance collected at a crime scene is a unique mixture of chemical compounds that can ultimately be identified. Arsonists, for example, often use accelerants such as gasoline or kerosene to speed combustion and spread flames in the interior of a building. A forensic chemist may collect samples of burned and unburned materials, extract the volatile hydrocarbons, and separate the components for analysis by gas chromatography.
Analytical Techniques and Instrumentation
The gas chromatograph (GC) separates volatile substances into separate components by passing the volatile materials through a long absorbent column. The technique is highly reproducible and reliable; since each sample is likely to contain a definite number and type of impurities, it may provide a close match of the unknown accelerant to a known source such as a gasoline tank or hardware store. It may be necessary to vaporize a tiny sample of the material to provide volatile gases for analysis. The components of the vapor are then passed through the column and separated into a number of components, each of which can be captured and analyzed. In most cases, the accelerant will be mixed with portions of burned paint or building materials, making simple identification by examination impossible, but the ability of the GC to separate tiny portions helps in identification.
The GC is often connected to a mass spectrometer. Mass spectrometry (MS) breaks samples apart and separates the ionized fragments by mass and charge. Vast libraries of comparison fragments make computer-aided identification of materials possible even when the sample is very small. Most forensic laboratories have access to a combined gas chromatograph/mass spectrometer (GC/MS). High pressure liquid chromatography (HPLC) separates many types of drugs and may also be combined with MS.
Analysts may use several types of spectrophotometry. A typical spectrophotometer consists of a light source that provides light of a known wavelength; a holder to position solid, liquid, or gaseous samples; and a system of lenses and photocells that compare light shining on the sample with light passing through. A decrease in the intensity of light passing through the substance indicates the presence of materials that absorb light at that wavelength; the absorbance is quantitative and a measure of the concentration of material, and the wavelengths of maximum absorbance are characteristic of the type of material. Infrared spectrophotometry is especially useful for the identification of organic compounds, as bonds between certain atoms readily absorb infrared radiation (IR).
Ultraviolet (UV) spectrophotometry helps distinguish between samples of proteins and nucleic acids such as deoxyribonucleic acid ( DNA ). Atomic
absorption spectrophotometry provides ways of determining absorption and emission spectra, useful tools in the analysis of metals such as bullet fragments.
Nuclear magnetic resonance spectrophotometry (NMR) makes use of the fact that nuclei of some molecules absorb radio frequency radiation in strong magnetic fields. Nuclei in certain molecules absorb radiation at characteristic frequencies, making the identification of even tiny or impure samples possible. X-ray analysis allows the forensic investigator to visualize foreign objects within the body.
In neutron activation analysis, a beam of neutrons from a nuclear reactor is directed at a sample of test material. The material becomes temporarily radioactive, emitting γ -rays that are characteristic of the composition; analysis of the γ -radiation provides a highly accurate and reproducible determination of the content of the sample. This technique has made possible the determination of arsenic in the hair of corpses buried for hundreds of years. In one case, the body of an Arctic explorer who had died under suspicious circumstances during the 1870s was found buried in a coffin surrounded by ice. Neutron activation analysis of hair from the body showed that hair that was several centimeters long contained little arsenic, but that shorter hair closer to the scalp (which had grown in the few days before death) contained high levels of arsenic, indicating that death was probably caused by arsenic poisoning.
Often, the presence of very small impurities makes comparison possible. For example, cars are painted with paints prepared to certain specifications of color and composition, and pigments and binders used vary from one manufacturer to another and even between models from the same distributor. A small sample of paint left at the scene of an accident may be checked for color by spectrophotometry and then analyzed for composition. Perpetrators of many hit-and-run crimes have been convicted on the basis of combined GC/MS analysis of paint chips.
Residues left by burning powder from firearms consist of patterns of particles that have both characteristic physical and chemical properties. Burned powder, for example, usually contains traces of nitrites that yield chemical reactions and traces of metals such as barium that are often present in primers. Both chemical reactions and microscopic analysis (including electron microscopy) are employed in the identification of powder residues on clothes and skin. An early method for detecting gunshot residue on the hands of suspects involved coating the hand with melted paraffin , allowing the paraffin to cool, and then stripping it off. Gunshot residues transferred from skin to paraffin turned blue or green in the presence of diphenylamine, but many common substances such as urine gave a false positive test. The Greiss reagent is much more definitive, and additional tests can identify traces of lead around bullet holes. Even microscopic particles are found to have definite compositions and can be unequivocally identified.
Toxicologists examine a wide range of materials such as blood stains, urine, and blood gases for traces of poisons or drugs. Many businesses now require the drug screening of employees; it is the responsibility of the technician to distinguish between the presence of illegal drugs and metabolites from foods such as poppy seeds. Such tests may be as simple as paper or thin-layer chromatography or as complicated as gas chromatographic or electrophoretic and serological analysis of a blood sample. Following death by unknown cause, samples of the victim's lungs, blood, urine, vitreous humor, and stomach contents are examined for traces of poisons or medication. Insects found on or near corpses are also collected and examined; they may actually absorb traces of drugs or poisons from the body, and in fact, traces of poisons sometimes are found in the surrounding insects long after concentrations in the body have fallen below detectable limits.
Forensic biochemists perform blood typing and enzyme tests on body fluids in cases involving assault, and also in paternity cases. Even tiny samples of blood, saliva, or semen may be separated by electrophoresis and subjected to enzymatic analysis. In the case of rape, traces of semen found on clothing or on the person become important evidence; the composition of semen varies from person to person. Some individuals excrete enzymes such as acid phosphatase and other proteins that are seldom found outside seminal fluid, and these chemical substances are characteristic of their semen samples. The presence of semen may be shown by the microscopic analysis for the presence of spermatozoa or by a positive test for prostate specific antigen.
In cases of sexual assault, tiny samples of DNA in blood, semen, skin, or hair found on the victim may be purified and the amount of DNA increased by the use of a polymerase chain reaction to produce quantities large enough to analyze. Since DNA is as specific to a person as fingerprints, matching the DNA of a perpetrator to a sample found on a victim is considered to be proof of contact. The Federal Bureau of Investigation (FBI) is currently in the process of establishing a national Combined DNA Index System (CODIS) that will collect data from many states and law enforcement agencies and index it so that particular DNA patterns from evidence collected at many crime scenes can be compared and matched. Many perpetrators of crimes have been convicted and many innocent persons set free after years in prison as a result of DNA analysis.
Testing for Alcohol
Accidents caused by intoxicated drivers kill nearly 15,000 persons a year in the United States alone (almost half of fatal auto accidents are alcohol-related), so a Breathalyzer kit is standard equipment in most police squad cars or state patrol vehicles. Breathalyzers are used to estimate the blood alcohol content of drivers suspected of being intoxicated; the driver may appear sober, but still have a blood alcohol level above the legal limit. Although it is impractical to take blood samples on the highway, research has shown that the concentration of ethanol in the breath bears a definite relationship to its concentration in blood. Many communities have now set a legal limit of 0.08 percent (meaning that 100 milliliters [3.38 fluid ounces] of blood would contain 0.08 grams [0.0028 ounces] of ethanol). In fact, authorities now consider that a person's driving ability is probably impaired at a blood ethanol level of 0.05 percent.
Several types of analytic devices are available to administer Breathalyzer tests. One test makes use of a portable infrared spectrophotometer, another uses a fuel cell, and the most common test employs several glass or plastic tubes and some common chemical reagents. The person being tested blows through a tube, which bubbles the breath through a solution of chemicals containing sulfuric acid, potassium dichromate, water, and silver nitrate. Oxidation of the alcohol results in the reduction of dichromate ion to chromic ion, with a corresponding change in color from orange to green. An electrical device employing a photocell compares the color of the test solution with a standard solution, giving a quantitative determination of the alcohol content. The test provides a quick and reproducible determination of the amount of alcohol in a person's breath and is a numerical measure of the amount of alcohol in the bloodstream. Use of a chemical test helps to avoid subjective opinions of sobriety and provides reliable evidence for court proceedings. The test can be readily and quickly administered by trained law enforcement personnel, but forensic chemists test and calibrate the equipment and testify to its accuracy.
Fingerprints on smooth surfaces can often be made visible by the application of light or dark powder, but fingerprints on checks or other documents are often occult (hidden). Occult fingerprints are sometimes made visible by the use of ninhydrin, which turns purple due to reaction with amino acids present in perspiration. Fingerprints or other marks are also sometimes made visible by exposure to high-powered laser light. Some fingerprints can be treated with chemical substances, resulting in a pattern that fluoresces when exposed to light from lasers. Cyanoacrylate ester fumes from glue are used with fluorescent dyes to make the fingerprints visible.
Forensic chemists are usually employed by laboratories in law enforcement agencies or by private testing laboratories and are often called on to provide testimony in court proceedings as expert witnesses. In these cases, the chemist may compare evidence in the case in question to a large number of similar cases he or she has examined and is often asked to give an expert opinion as to the quality of the evidence. Since forensic chemists usually have both a bachelor's degree in chemistry and an advanced degree in forensic science, their academic credentials, along with years of experience and the ability to compare the case in question with a large number of other cases, renders the testimony both valuable and believable.
Ho, Mat H. (1990). Analytical Methods in Forensic Chemistry. New York: Horwood.
Inman, Keith, and Inman, Norah (1997). An Introduction to Forensic DNA Analysis. Boca Raton, FL: CRC Press.
Saferstein, Richard (1998). Criminalistics: An Introduction to Forensic Science. Upper Saddle River, NJ: Prentice Hall.
Consulting and Education in Forensic Science. Available from http://www.forensicdna.com/ .
Forensic Chemistry Network. Available from http://www.geocities.com/CapeCanaveral/4329/ .
Forensics: Back-Calculating the Truth. Available from http://www.facstaff.bucknell.edu/mvigeant/univ_270_03/Derek/ .