Individual fibres identified by new forensic test

‘White cotton fibres are so common and have so few visual distinguishing features that they are largely ignored by forensic scientists at crime scenes,’ said Brian Strohmeier, a scientist at Thermo Fisher Scientific in the US.

Most fabrics go through various manufacturing and treatment processes – for example, to make them stain resistant, waterproof, or iron-free – leaving unique organic chemicals on the surface of the fibres. By analyzing the chemical signature on the surface of individual fibres, forensic scientists can identify the origin of scraps of fabric evidence found in crime scenes.

Strohmeier will describe this work at the AVS 60th International Symposium and Exhibition in Long Beach, California, which is held between October 27 – November. 1, 2013.

In the new method, Strohmeier and his colleagues used X-ray photoelectron spectroscopy (XPS). In XPS, the test sample is struck with a focused X-ray beam, which removes electrons from the surface of the sample. A detector then counts the electrons and measures their kinetic energies, with the resulting spectrum revealing the chemical signature of the surface.

XPS has been used before to characterise the surfaces of textile fibres that don’t have chemical coatings, Strohmeier said in statement. But to study the surface chemistry of treated fibres, the researchers need to go deeper and analyse the layers just beneath the surface.

To do so, they fired a beam of argon-ion clusters onto the sample fibre. The beam drilled away a shallow hole on the surface of the fibre, revealing the layer underneath. Each cluster contains numerous atoms, and because the clusters break up on impact, they don’t cause as much damage to the chemicals that are being measured, whereas a beam of single ions would.

With the layer underneath now exposed, the researchers used XPS to study its chemical contents. By striking the sample with the beam for longer, the researchers can scrape away deeper layers for analysis.

With this technique, the researchers were able to identify textile materials based on the surface chemistry that is the result of different manufacturing processes. They were also able to distinguish materials that had undergone different chemical treatments but were otherwise identical.

Previously, XPS hadn’t been used much in forensic science, Strohmeier said. There was no accepted standard for XPS methods in forensics, it often took hours to analyse each sample, the technique required relatively large samples with areas of several square millimeters, and XPS instruments were a lot more expensive than other forensic tools.

Strohmeier said that XPS instruments have improved to the point where analysis now takes minutes and crime scene officers only need tens to hundreds of square microns of sample area.

Strohmeier concluded that while these new results don’t yet establish a bona fide technique for forensics, it does show potential for analysing fibres and the surfaces of other kinds of evidence collected at crime scenes.