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.

Spectroscopic methods

Among the more common spectroscopic methods used for analysis is FTIR spectroscopy, where chemical bonds can be detected through their characteristic infra-red absorption frequencies or wavelengths. UV spectroscopy is used where strong absorption of ultra-violet radiation occurs in a substance. Such groups are known as chromophores and include aromatic groups, conjugated system of bonds, carbonyl groups and so on. NMR spectroscopy detects hydrogen atoms in specific environments, and complements both IR and UV spectroscopy. The use of Raman spectroscopy is growing for more specialist applications.

There are also derivative methods such as Infrared microscopy which allows very small areas to be analysed in an optical microscope.

One method of elemental analysis which is important in forensic analysis is EDX performed 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.

Sample preparation

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

Many polymer degradation mechanisms can be followed using infra-red spectroscopy, such as UV degradation and oxidation, amongst many other failure modes.

IR spectrum showing carbonyl absorption due to UV degradation of polyethylene

UV degradation

Many polymers are 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. Polyethylene is also susceptible to UV degradation, especially those variants which are branched polymers such as LDPE. The branch points are tertiary carbon atoms, so polymer degradation starts there and results in chain cleavage, and embrittlement. In the example shown at left, carbonyl groups were readily detected by IR spectroscopy from a cast thin film. The product was a road cone which had cracked in service, and many similar cones also failed because a anti-UV additive had not been used.

Oxidation

IR spectrum showing carbonyl absorption due to oxidative degradation of polypropylene crutch moulding

Oxidation is usually relatively easy to detect owing to the strong absorption by the carbonyl group in the spectrum of polyolefins. Polypropylene has a relatively simple spectrum with few peaks at the carbonyl position (like polyethylene). Oxidation tends to start at tertiary carbon atoms because free radicals here 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.

Ozonolysis

The reaction occurring between double bonds and ozone is known as ozonolysis when one molecule of the gas reacts with the double bond:

A generalized scheme of ozonolysis

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 weight or 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.

EDX spectrum of crack surface

EDX spectrum of unaffected rubber surface

The carbonyl end groups which are formed are usually aldehydes or 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 spectroscopy in the environmental SEM, or ESEM. The spectrum at left shows the high oxygen peak compared with a constant sulphur peak. 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 cracking of diaphragm seals in a semi-conductor fabrication factory.

See also

• Absorption spectroscopy
• Infrared spectroscopy correlation table
• Infrared spectroscopy
• Forensic chemistry
• Forensic engineering
• Forensic polymer engineering
• Polymer degradation
• Polymer engineering
• Spectroscopy

References

• 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.

External links

• Museum of failed products
• New Forensic course
• The journal Engineering Failure Analysis
• Forensic science and engineering

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