- Applied spectroscopy
Applied spectroscopy is the application of various spectroscopic
methods for detection and identification of different elements/ compounds in solving problems in the fields of forensics, medicine, oil industry, atmospheric chemistry, pharmacology, etc.
Among the more common spectroscopic methods used for analysis is
FTIRspectroscopy, where chemical bonds can be detected through their characteristic infra-red absorption frequencies or wavelengths. UV spectroscopyis used where strong absorption of ultra-violet radiationoccurs in a substance. Such groups are known as chromophoresand include aromaticgroups, conjugated systemof bonds, carbonyl groups and so on. NMR spectroscopydetects hydrogen atoms in specific environments, and complements both IR and UV spectroscopy. The use of Raman spectroscopyis growing for more specialist applications.
There are also derivative methods such as Infrared microscopy which allows very small areas to be analysed in an
One method of
elemental analysiswhich is important in forensic analysisis EDXperformed in the environmental scanning electron microscope, or ESEM. The method involves analysis of back-scattered X-rays from the sample as a result of interaction with the electron beam.
In all three spectroscopic methods, the sample usually needs to be present in solution, which may present problems during forensic examination because it necessarily involves sampling solid from the object to be examined.
FTIR: Three types of samples can be analyzed, a solution (KBr), a powder, or a film. A solid film is the easiest and most straight forward sample type to test.
Analysis of polymers
polymer degradationmechanisms can be followed using infra-red spectroscopy, such as UV degradationand oxidation, amongst many other failure modes.
Many polymers are attacked by
UV radiationat vulnerable points in their chain structures. Thus polypropylenesuffers severe cracking in sunlightunless anti-oxidants are added. The point of attack occurs at the tertiary carbon atompresent in every repeat unit, causing oxidation and finally chain breakage. Polyethyleneis also susceptible to UV degradation, especially those variants which are branched polymers such as LDPE. The branch points are tertiary carbonatoms, so polymer degradationstarts there and results in chain cleavage, and embrittlement. In the example shown at left, carbonyl groups were readily detected by IR spectroscopyfrom a cast thin film. The product was a road conewhich had cracked in service, and many similar cones also failed because a anti-UV additive had not been used.
Polymers are susceptible to attack by atmospheric
oxygen, especially at elevated temperatures encountered during processing to shape. Many process methods such as extrusionand injection mouldinginvolve pumping molten polymer into tools, and the high temperatures needed for melting may result in oxidation unless precautions are taken. For example, a forearm crutchsuddenly snapped and the user was severely injured in the resulting fall. The crutch had fractured across a polypropyleneinsert within the aluminium tube of the device, and infra-red spectroscopyof the material showed that it had oxidised, possible as a result of poor moulding.
Oxidation is usually relatively easy to detect owing to the strong absorption by the
carbonyl groupin the spectrum of polyolefins. Polypropylenehas a relatively simple spectrum with few peaks at the carbonyl position (like polyethylene). Oxidation tends to start at tertiary carbonatoms because free radicalshere at more stable, so last longer and are attacked by oxygen. The carbonyl group can be further oxidised to break the chain, so weakening the material by lowering the molecular weight, and cracks start to grow in the regions affected.
The reaction occurring between double bonds and ozone is known as
ozonolysiswhen one molecule of the gas reacts with the double bond:
The immediate result is formation of an
ozonide, which then decomposes rapidly so that the double bond is cleaved. This is the critical step in chain breakage when polymers are attacked. The strength of polymers depends on the chain molecular weightor degree of polymerization, the higher the chain length, the greater the mechanical strength (such as tensile strength). By cleaving the chain, the molecular weight drops rapidly and there comes a point when it has little strength whatsoever, and a crack forms. Further attack occurs in the freshly exposed crack surfaces and the crack grows steadily until it completes a circuit and the product separates or fails. In the case of a seal or a tube, failure occurs when the wall of the device is penetrated.
The carbonyl end groups which are formed are usually
aldehydesor ketones, which can oxidise further to carboxylic acids. The net result is a high concentration of elemental oxygen on the crack surfaces, which can be detected using Energy-dispersive X-ray spectroscopyin the environmental SEM, or ESEM. The spectrum at left shows the high oxygen peak compared with a constant sulphurpeak. The spectrum at right shows the unaffected elastomer surface spectrum, with a relatively low oxygen peak compared with the sulphur peak. The spectra were obtained during an investigation into ozone crackingof diaphragm seals in a semi-conductorfabrication factory.
Infrared spectroscopy correlation table
Forensic polymer engineering
*"Forensic Materials Engineering: Case Studies" by Peter Rhys Lewis, Colin Gagg, Ken Reynolds, CRC Press (2004).
*Peter R Lewis and Sarah Hainsworth, "Fuel Line Failure from stress corrosion cracking", Engineering Failure Analysis,13 (2006) 946-962.
* [http://materials.open.ac.uk/mem/index.htm Museum of failed products]
* [http://openlearn.open.ac.uk/file.php/2980/formats/print.htm New Forensic course]
* [http://www.elsevier.com/wps/find/journaldescription.cws_home/30190/description#description The journal Engineering Failure Analysis]
* [http://www.forensic-courses.com/wordpress/?p=42; Forensic science and engineering]
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