Difference between revisions of "Forensic chemistry"

Latest revision as of 09:10, 20 September 2010

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 useful method is the gas chromatograph-mass spectrometer (GCMS), which is actually two instruments that are attached. The gas chromatograph is essentially a very hot oven holding a hollow coiled column. A drug sample is diluted in a solvent (e.g., chloroform, methanol) and is injected into this column, the solvent will evaporate very quickly leaving the drug to travel through the column. Different substances are retained in the column for different amounts of time. The retention time, as compared to a known standard sample using the same method(same column length/polarity, same flow rate, same temperature program), can help to provide a positive identification for the presence of a compound of interest. The column eluent is then fed into a mass spectrometer. A mass spectrometer bombards the eluant with electrons, causing it to fragment 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 fragmentation pattern, which functions as a sort of fingerprint for each compound, and is compared to a reference sample.

Spectroscopy

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.

Thermoplastics can be analysed using infra-red spectroscopy, UV spectroscopy, as NMR and ESEM. 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 much 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 cannot injure the sample, so it falls in this class.

Examples

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 only ppb of ozone will 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.

See also

• Applied spectroscopy
• Environmental stress cracking
• Forensic biology
• Forensic engineering
• Forensic polymer engineering
• Polymer degradation
• Polymer engineering
• Stress corrosion cracking
• Trace evidence

References

1. ↑ 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).cs:Forenzní chemie

pt:Química forense