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Threading together forensic evidence

Susan Langdon

Ms Langdon studied the science of fibre matching for her PhD thesis.

If you intend committing a crime you will be less likely to be caught if you are wearing black polyester rather than any other fabric, according to a new Australian study that assesses the worth of commonly worn fabrics as forensic evidence.

While researching for her doctoral thesis at the University of Technology Sydney , Ms Susan Langdon conducted a study on two commonly worn fibres to determine the probability that a match between fibres found at a crime scene and those of a suspect's clothing was coincidental. Her results provide courts with a quantifiable tool for evaluating fibre evidence.

"What we are trying to do with this study is have an idea of the Probability of Coincidental Match - how probable is it to find a match by pure chance?" explained Associate Professor Claude Roux, from the Department of Chemistry, Materials and Forensic Science who supervised Ms Langdon.

Textile fibres collected from crime scenes commonly come from clothing, but until now there has not been any Australian research into the frequency of different types of fibres worn here. The courts have had to rely on European research, and the consensus in Australia is that this data is misleading because the countries are different. "The climate is different, the fashions are different, it's a different situation," said Dr Roux.

The research aimed to gather frequency data - what sort of fibres are common, and which are unusual in the Australian context. The warmer climate here means that less wool is worn but more cotton. In addition our proximity to Asia means Australia has more fabric coming from Asia than is the case in Europe.

Ms Langdon began by researching what the most commonly worn fibres were in the populations represented most frequently by murder victims - people between 18 and 35 years old. By conferring with the Australian Federal Police and surveying male and female university students in that age range, she decided on navy blue cotton and black polyester.

Her next job was to collect fibres from the surface of car seats in all capital cities, using forensic adhesive tape. In total she collected 2109 blue cotton fibres and 2039 black polyester fibres.

For comparison Ms Langdon bought two commonly bought items from a department store - dyed blue cotton pants (not jeans) and black polyester tracksuit pants. "These are the most common clothes worn by the group - especially potential offenders," said Dr Roux.

Selecting fibres similar to those from the items she had bought, Ms Langdon ran a battery of comparative tests using fluorescence microscopy, UV-Visible microspectrophotometry and Fourier Transform Infrared Spectrometry for the black polyester fibres.

Just 6 out of 2000 blue cotton fibres were indistinguishable from the target while 26 out of 2000 black polyester fibres were similar to the target.

If the fibre from the suspect is different to that at the crime scene, they could not have come from the same source. As the likelihood of fibres matching by chance gets lower, there's an increased likelihood that the fibre is an important part of the evidence.

"This [research] reinforces that with common fibres a coincidental match is not that common. This probability is not very high," explained Dr Roux. That explains why blue cotton, with fewer indistinguishable matches, provides far more conclusive trace evidence than black polyester.

The study's results can also be used for Australian cases with less common fibres, for example orange acrylic. Given that only six out of 2000 fibres from a very common cotton pant were undistinguishable, and orange acrylic is much less common than blue cotton it is likely a match between orange fibres is a real one. "It is putting some form of quantification on it," Dr Roux said.