How hair and fibre aid in the investigation of crime.

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How hair and fibres aid in the investigation of crime

Locard’s exchange principle ‘every contact leaves a trace’. Most of the forensic science revolves around this principle - it states that with every contact between two items, there will always be an exchange. This famous quote was used since the beginning of the twentieth century and basing on this theory, many crimes were able to be solved.1 Trace evidence, which the principle refers to, could be from crime scenes or accident scenes as a form of recoverable materials, marks or impressions. Although trace evidence on its own may sometimes not be enough to solve the crime, it could support other evidence or even pressurize a suspect to confess.2 This essay focuses on recoverable trace evidence such as hair and fibres and how they are analysed in the investigation of crimes.

Textile fibres as trace evidence

The term fibre is used to describe ‘any solid object that is thin, flexible and elongate, having a high length to transverse cross section area ratio’ .3 Fibres are the basic units in fabrics and they are classified into two classes: synthetic and natural. Natural fibres originate from plants and animals. Plant fibres used in the production of texile materials include flax (linen), ramie, sisal, jute, hemp, kapok, and coir. Animal fibers such as wool, cashmere and mohair are commonly found as well. Synthetic fabrics include nylon and polyester , which can be identified more easily as the cross section of the fibre may be manufacture specific.5 In the past, fabrics were used to be made with pure natural fibres until the manufacturers mixed and matched the natural and man made fibres to create variety of different fabrics. This made the identification of the fibres more difficult; however,                                  figure 1 Top: cotton, bottom: wool 4

 properties (such as how they are woven) and laboratory tests enable one to differentiate between them. Figure 1 shows a microscopic view of a transverse section of cotton (top) and wool (bottom). The diagrams show clearly the different structures and appearances between these two fibre types. Wool appears to be rougher on the walls and has a circular transverse section. Cotton has a smoother side with a curved transverse. On the other hand, the cross section of a man made fibre may be manufacturing specific and some shapes are only produced for a short period of time.6


When natural and synthetic fibres are compared, it is with much difficulty to analyse man-made fibres because (despite it could be manufacture specific) most of them have smooth surfaces. So unlike natural fibres, they cannot be analysed just by looking under the microscope- further tests would have to be performed (discussed later).

Transfer of fibre

According the Locard’s exchange principle, when there is physical contact between two fibre materials, transfer from one to another would occur. A given fibres may be transferred more than once, however, the amount of fibres transferred, depends on the duration of contact (between suspect and the victim or crime scene) and the nature of the fibres. Fibres are made in several ways as some are woven, knitted or spun to form yarn. Tightly woven fabric shed less often than loosely woven. The age of fabric also contribute to the degree of transfer: new fabrics tend to shed more readily than old. And damaged fabrics during physical contact increase the likelihood of fibre transfer.6,4

Forensic scientists would search for these transferred fibres from the scene and on victims and suspects: the more number of fibres found (and matched), the more likely that there was physical contact between victim and suspect or suspect and crime scene.7

Collection and analysis of textile fibres

When fibres are collected at a crime scene, hand retrieval and tape lifting technique would usually be followed to remove the evidence which could be found as a transferred material or may be loosely attached to objects at the crime scene. For surfaces that are difficult to tape lift, would require a vacuum.9 Collections of the fibres are done with extreme care as cross contamination from different objects would destroy the credibility of the evidence. At the laboratory, the tape would be analysed under the microscope in search of similar colour, texture and structure as the fabric of the supposed source. Colour comparisons of found fibres to the original source are done using a micro spectrophotometer which can produce an absorption spectrum and view the fibre under the microscope at the same time. Infrared spectrometers are also used in fibre analysis as the obtained spectrum can tell the chemical composition of the material. Figure 2 shows the IR spectra of acylic fibre and cotton. IR spectrum for natural fibres would result in braoder peaks compared to synthetic fibres. As shown in the figure 2b, cotton has a larger and broader stretch.

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 A more traditional method would be using thin layer chromatography to separate the dye (which is first extracted using solvents) into its components.10  

As mentioned previously, man made fibres are more complicated to analyse: they are constructed in such a way that it is thicker in the centre and thinner on the walls. This property is caused by the alignment of molecules in the polymer with the length of the fibre during the          

manufacturing process. Due to this                                figure 2a (top) acylic fibre, 2b (bottom) wool 15

characteristic,        refractive indices are found in synthetic ...

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