Forensic chemistry

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Forensic chemistry is the application of chemistry to law enforcement or the failure of products or processes. Many different analytical methods may be used to reveal what chemical changes occurred during an incident, and so help reconstruct the sequence of events. "Forensic chemistry is unique among chemical sciences in that its research, practice, and presentation must meet the needs of both the scientific and the legal communities. As such, forensic chemistry research is applied and derivative by nature and design, and it emphasizes metrology and validation."^[1]

Methods

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.

Spectroscopy

IR spectrum showing carbonyl absorption due to oxidative degradation of polypropylene

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.

Sample integrity

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.

Luminol

A method frequently used in forensic chemistry is that employing luminol, a derivative of phthalic acid, which reacts with metal cations and hence to detect traces of blood. The process involves mixing luminol with a dilute solution of hydrogen peroxide, which is spread carefully in places where it is thought that there are remnants of blood.

Thus, 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.

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.

Examples

Ozone cracking in Natural rubber tubing

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 UV radiation at vulnerable points in their chain structures. 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.

References

↑ Bell S (2009). "Forensic Chemistry". Annual Review of Analytical Chemistry 2 (1): 297–319. doi:10.1146/annurev-anchem-060908-155251. PMID 20636064.

Lewis,P R, Gagg, R and Reynolds, K, Forensic Materials Engineering: Case Studies CRC Press (2004).
Lewis, P R and Hainsworth S, Fuel Line Failure from stress corrosion cracking, Engineering Failure Analysis,13 (2006) 946-962.
Ezrin, Meyer, Plastics Failure Guide: Cause and Prevention, Hanser-SPE (1996).
Lewis, Peter Rhys, and Gagg, C, Forensic Polymer Engineering: Why polymer products fail in service, Woodhead/CRC Press (2010).

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