The fibres that look like they match those from the crime scene are individually removed and cleaned for a more detailed analysis. There are four phases of detailed analysis: microscopy, microspectrophotometry (MSP), thin layer chromatography (TLC), and infrared spectroscopy (IR) for man-made fibres.
In the first phase, microscopy, the images of the fibres to be compared are brought together optically and examined side by side to see if they are showing the same colour and morphological characteristics, such as grooves, furrows, pitting and cross-section of the shape of the fibre, of those from the crime scene. The fibres are examined under white light and ultraviolet to induce the fibres the fibres to fluoresce. Fibres are then compared by the colour it fluorescence, any that appear different from the original source are rejected.
MSP measures the colour of the matching fibres using a micro spectrophotometer. This is done by recording, by using a spectrum graph, the different wavelengths (colours) of light that are absorbed as they are passed through the fibres. Recovered fibres with different colour characteristics are also rejected.
TLC is used to identify pigments from fibres. This is done by the dye being stripped from the fibre. The dye is then concentrated and placed at the bottom of a plastic sheet which is dipped into another solution which slowly creeps up the plastic sheet, carrying the dye with it and depositing the various components at different levels to form a line of coloured spots. This way the dye mixture can be separated out and compared to the original fibre. TLC can not be used on small, or colourless, or dye-fast fibres.
IR is for man-made fibres such as polyester, nylon, acrylic, ect to confirm their chemical identity. This is done by individual fibres fragments being squashed into a film which infrared light is passed through. The chemical structure of the film reacts with the infrared light is recorded in a graph which shows various peaks to determine the chemical identity of the material.
IR can not be used on natural fibres, fibres taken from plants or animals.
Bibliography
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http://www.chemsoc.org/exemplarchem/entries/2003/hull_barry/AG%20Fibres.htm#Coll Friday 14th October
- Crime scene to court: the essentials of forensic science. Edited by Peter White. Published by The Royal Society of Chemistry 1998. ISBN 0-85404-539-2
Fibres South Downs College
