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. Optical microsocopy cannot injure the sample, so fall into this class.
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 tyre 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. Essentially the gas attacks sensitive parts of the chain molecules (especially secondary, tertiary or allylic carbon atoms), oxidising 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 srong acids, and in all these cases, care must be taken to dry the raw materials for processing at high temperatures to prevent the problem 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.
SEN. CARDIN, REP. SARBANES ANNOUNCE $141,000 IN NEW FEDERAL FUNDING FOR TOWSON UNIVERSITY'S FORENSIC CHEMISTRY INSTITUTE
Feb 04, 2008; Rep. John Sarbanes, D-Md. (3rd CD), has issued the following news release: U.S. Senator Benjamin L. Cardin (D-MD) and U.S....