Forensic chemistry is the application of chemistry and its various subfields, such as forensic toxicology, in a legal setting. A forensic chemist can assist in the identification of unknown materials found at a crime scene. Forensic specialists in this field have a wide array of different methods and instrumentation at their disposal to help identify unknown substances. Specific methods common to the field include high-performance liquid chromatography (HPLC), gas chromatography-mass spectrometry (GC-MS), ultraviolet–visible spectroscopy (UV-Vis), Fourier transform infrared spectroscopy (FTIR), and thin layer chromatography (TLC). The array of different methods is important due to the destructive nature of some instruments. If possible, nondestructive methods should always be attempted first to preserve evidence. Along with other forensic specialists, forensic chemists commonly testify in court as expert witnesses regarding their findings.
Throughout history, the availability of poisons had allowed individuals to commit murder with relative ease. Arsenic, nightshade, hemlock, strychnine, and curare were all historically used to poison individuals. With no method to accurately determine if a particular chemical was present, poisoners were frequently never punished for the crimes. It was not until the early 19th century that chemists were able to effectively detect poisons for the first time. In 1836, one of the first major contributions to forensic chemistry was introduced by James Marsh. Marsh created the Marsh test for arsenic detection which was subsequently used successfully in a murder trial. It was also during this time period that forensic toxicology began to be recognized as a distinct field. Mathieu Orfila, the father of toxicology, made great advancements to the field during the early 19th century. He helped develop tests that could determine the presence of blood and was one of the first to use microscopy in the analysis of blood and semen. Orfila was the first chemist to successfully classify different chemicals into categories such as corrosives, narcotics, and astringents.
The discovery of a valid method for arsenic detection greatly reduced the number of poisonings using heavy metals but did not eliminate poisonings altogether. Instead, poisonings using vegetable alkaloids rose in popularity as they were still undetectable. It was not until 1850 when a valid method for detecting alkaloid poisoning was successfully used in court. That year, Count Hippolyte Visart de Bocarmé was accused of murdering his brother-in-law by nicotine poisoning. Chemist Jean Stas was able to successfully isolate the alkaloid from the organs of the victim proving Count Bocarmé murdered his brother-in-law. Stas's protocol was subsequently altered to incorporate tests for caffeine, quinine, morphine, strychnine, atropine, and opium.
The wide range of instrumentation for forensic chemical analysis also started during this time period. In 1859, chemist Robert Bunsen and physicist Gustav Kirchhoff invented the first spectroscope. Their experiments with spectroscopy showed that specific substances created a unique spectrum when exposed to specific wavelength of light. Using spectroscopy, the two scientists were able to identify substances based on their spectrum providing a method of identification for unknown materials. In addition to the spectroscope, another crucial advancement in the field was invented in 1906 by botanist Mikhail Tsvet. Tsvet developed paper chromatography, an early predecessor to thin layer chromatography, in order to separate and examine the plant proteins that make up chlorophyll. The ability to separate mixtures into their individual components allows forensic chemists to examine the parts of an unknown material against a database of known products. By matching the retention factors for the separated components with known values materials can be identified. Over time, chromatography techniques have become more sophisticated with the introduction of liquid and gas chromatography.
Modern forensic chemist rely on numerous instruments in order to identify unknown materials found at a crime scene. The 20th century saw numerous advancements in technology that allowed forensic chemists to detect smaller amounts of material more accurately. The first major advancement in this century came during 1930s with the invention of a spectrometer that could measure the signal produced with infrared (IR) light. Early IR spectrometers used a monochromator and could only measure light absorption in a very narrow wavelength band. It was not until the coupling of an interferometer with an IR spectrometer in 1949 by Peter Fellgett that the complete infrared spectrum could be measured at once. Fellgett also realized that Fourier transform, a mathematical method that can break down a signal into the individual frequencies that make it up, could be used to make sense of the enormous amount of data received from the complete infrared analysis of a material. Since then, FTIR instruments have become critical in the forensic analysis of unknown material due to its nondestructive nature and extremely quick run time. On the other end of the light spectrum, the invention of the UV-Vis spectrophotometer by Arnold Beckman and Howard Cary in 1941 was the first time scientists could perform spectroscopy in the ultraviolet range leading to an entire new database of unique spectrum for identification purposes. Similar to IR spectroscopy, this type of analysis is that it is nondestructive to the material being tested allowing for verification and further examination.
Advancements in the field of chromatography arrived in 1953 with the invention of the gas chromatograph by Anthony T. James and Archer John Porter Martin allowing for the separation of volatile liquid mixtures whose components have similar boiling points. While nonvolatile liquids could be measured with liquid chromatography, substances with similar retention times could not be resolved until the invention of high performance liquid chromatography by Csaba Horváth in 1970. Modern HPLC instruments are capable of detecting and resolving substances whose concentrations are as low as parts per trillion.
One of the most important advancements in forensic chemistry came in 1955 with the invention of the GC-MS by Fred McLafferty and Roland Gohlke. The coupling of a gas chromatograph with a mass spectrometer allowed for the identification of a wide range of substances. GC-MS analysis is widely considered the "gold standard" for forensic analysis due to its sensitivity and versatility along with its ability to quantify the amount of substance present.
One particularly useful method for the simultaneous separation, identification, and quantitation of one or more individual components of an unknown substance or mixture is the use of a gas chromatograph-mass spectrometer (GC-MS). A GC-MS is actually two instruments that are attached together physically, and together comprising one of the so-called "tandem" or "hyphenated" techniques.
The gas chromatograph (GC) is essentially a hot (150-350°C), temperature-controlled oven holding a bent or coiled, specially packed or coated glass column between one and a few dozen meters long. A small volume (typically a few microliters) of a drug sample or other unknown substance that has been dissolved in an organic solvent (such as chloroform or methanol) is quickly injected into the hot column. Volatile components in the sample are vaporized by the heat of the oven and are forced toward the end of the column by the flow of an inert "carrier gas" (typically helium). The special chemical component(s) within the column bind to substances contained in the moving vaporized sample mixture with slightly different force. As a result, different substances eventually are "eluted" (i.e. emerge from the end of the column) in differing amounts of time, which is known as the "retention time". The retention time of various components so eluted can then be compared to those of known standard molecules eluted using the same method (column length/polarity, flow rate of carrier gas, temperature program). While this comparison provides (presumptive) identification of the presence of a particular compound of interest in the unknown sample, in general the GC portion of the technique is used as a separation and quantitation tool, not an identification tool.
To provide positive identification of the sample components, the column eluent is then fed into a mass spectrometer ("MS"). These highly complex instruments use one or more methods (bombardment with electrons, high heat, electrical force) to break apart molecules into ions. These ions are separated by their mass, commonly with the use of a quadrupole mass analyzer or quadrupole ion trap, and detected by an electron multiplier. This provides a distinctive fragmentation pattern, which functions as a sort of "fingerprint" for each compound. The resulting patterns are then compared to a reference sample for identification purposes.
Another instrument used to aide in identification of compounds is the Fourier Transform infrared spectrophotometer (FTIR). The sample is bombarded with infrared radiation. Polar bonds found in organic compounds have a natural frequency of vibration similar to the frequency of infrared radiation. When the frequency of the infrared radiation matches the natural frequency of the bond, the amplitude of the vibration increases, and the infrared is absorbed. The output of an infrared spectrophotometer charts the amount of light absorbed vs. the wavelength, typically with units of percent transmission and wavenumbers(cm−1). Because both the frequency and the intensity of absorption are dependent on the type of bond, a skilled chemist can determine the functional groups present by examining the infrared spectrum.
As with the GCMS the FTIR spectrum can be compared to that of a known sample, thus providing evidence for the identification of a compound. Spectroscopy can also help to identify materials used in failed products, especially polymers, additives and fillers. Samples can be taken by dissolution, or by cutting a thin slice using a microtome from the specimen under examination. Surfaces can be examined using Attenuated total reflectance spectroscopy, and the method has also been adapted to the optical microscope with infra-red microspectroscopy
Ultraviolet-visible-near infrared spectroscopy is used to test for certain drugs of abuse. UV-visible-NIR microspectrophotometers are instruments able to measure the spectra of microscopic samples. The UV-visible-NIR microspectrophotometer is used to compare known and questioned samples of trace evidence such as fibers and paint chips. They are also used in the analysis of inks and papers of questioned documents and to measure the color of microscopic glass fragments. As these samples are not altered, UV-visible-NIR microspectroscopy is considered a non-destructive technique.
Thermoplastics can be analysed using characterization techniques such as infra-red spectroscopy, ultraviolet–visible spectroscopy, nuclear magnetic resonance spectroscopy, and an environmental scanning electron microscope. Failed samples can either be dissolved in a suitable solvent and examined directly (UV, IR and NMR spectroscopy) or be a thin film cast from solvent or cut using microtomy from the solid product. Infra-red spectroscopy is especially useful for assessing oxidation of polymers, such as the polymer degradation caused by faulty injection moulding. The spectrum shows the characteristic carbonyl group produced by oxidation of polypropylene, which made the product brittle. It was a critical part of a crutch, and when it failed, the user fell and injured herself very seriously. The spectrum was obtained from a thin film cast from a solution of a sample of the plastic taken from the failed forearm crutch.
Forensic chemists usually perform their analytical work in a sterile laboratory decreasing the risk of sample contamination. In order to prevent tampering, forensic chemists must keep track of a chain of custody for each sample. A chain of custody is a document that stays with the evidence at all times. Among other information, contains signatures and identification of all the people involved in transport, storage and analysis of the evidence.
This makes it far more difficult for intentional tampering to occur, it also acts as a detailed record of the location of the evidence at all times for record keeping purposes. It increases the reliability of a forensic chemist's work and increases the strength of the evidence in court.
A distinction is made between destructive and non-destructive analytical methods. Destructive methods involve taking a sample from the object of interest, and so injures the object. Most spectroscopic techniques fall into this category. By contrast, a non-destructive method conserves the integrity of the object, and is generally preferred by forensic examiners. For example, optical microscopy and microspectroscopy cannot injure the sample, so they are considered non-destructive techniques.
A method frequently used in forensic chemistry is that employing luminol (as preemptory test), a derivative of phthalic acid, which reacts with metal cations and hence to detect traces of blood. The process involves mixing luminol with a polar solution dependent upon the method used to create the luminol base, which is spread carefully in places where it is thought that there are remnants of blood after all other evidence has be collected due to its destructive properties.
Thus, typically the iron-shaped cation found in the heme group of hemoglobin reacts with luminol observing a blue luminescence of the reaction itself is carried out. However, due to the nature of luminol there are other metal ions that it can react with to produce false positives, for this reason alone that is why it is used only to determine the possibility of blood being present.
In this process, the final product is the 3-aminophthalate anion which is in an excited state. Upon returning to the ground state (or basal) releases energy in the form of light, which is known as blue luminescence.
The reaction described has a very slow cinétic. In fact it is the iron in the heme group of hemoglobin, which catalyzes the process. Another note to keep in mind when working with luminol is that the reagent is only viable for a maximum of ten minutes and the darker the room the better the test.
Polymers for example, can be attacked by aggressive chemicals, and if under load, then cracks will grow by the mechanism of stress corrosion cracking. Perhaps the oldest known example is the ozone cracking of rubbers, where traces of ozone in the atmosphere attack double bonds in the chains of the materials. Elastomers with double bonds in their chains include natural rubber, nitrile rubber, and styrene-butadiene rubber. They are all highly susceptible to ozone attack, and can cause problems like car fires (from rubber fuel lines) and tire blow-outs. Nowadays, anti-ozonants are widely added to these polymers, so the incidence of cracking has dropped. However, not all safety-critical rubber products are protected, and, since it only takes a few parts per billion of ozone to start attack, failures are still occurring.
Another highly reactive gas is chlorine, which will attack susceptible polymers such as acetal resin and polybutylene pipework. There have been many examples of such pipes and acetal fittings failing in properties in the USA as a result of chlorine-induced cracking. In essence, the gas attacks sensitive parts of the chain molecules (especially secondary, tertiary or allylic carbon atoms), oxidizing the chains and ultimately causing chain cleavage. The root cause is traces of chlorine in the water supply, added for its anti-bacterial action, attack occurring even at parts per million traces of the dissolved gas.
Most step-growth polymers can suffer hydrolysis in the presence of water, often a reaction catalysed by acid or alkali. Nylon for example, will degrade and crack rapidly if exposed to strong acids, a phenomenon well known to those who accidentally spill acid onto their shirts or tights. Polycarbonate is susceptible to alkali hydrolysis, the reaction simply depolymerising the material. Polyesters are prone to degrade when treated with strong acids, and, in all these cases, care must be taken to dry the raw materials for processing at high temperatures to prevent the problem from occurring.
Many polymers are also attacked by a powerful UV radiation. Vulnerable points in their chain structures can be seen. Thus, polypropylene suffers severe cracking in sunlight unless anti-oxidants are added. The point of attack occurs at the tertiary carbon atom present in every repeat unit, causing oxidation and finally chain breakage.
- Applied spectroscopy
- Environmental stress cracking
- Forensic biology
- Forensic engineering
- Forensic polymer engineering
- Polymer degradation
- Polymer engineering
- Stress corrosion cracking
- Trace evidence on line 4
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