AVS: Science & Technology of Materials, Interfaces, and
Processing
Crime-scene investigators may soon have a new tool to help them
catch evildoers. Researchers have demonstrated the proof-of-principle for a new
forensic technique to identify individual fibers of cloth, which often all look
alike.
"White cotton fibers
are so common and have so few visual distinguishing features that they are
largely ignored by forensic scientists at crime scenes," says Brian
Strohmeier, a scientist at Thermo Fisher Scientific, a laboratory-instrument
company based in Massachusetts. But most of today's fabrics have gone 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 fibers. So by analyzing the chemical signature on the
surface of individual fibers, forensic scientists can, for instance, 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,
Calif., held Oct. 27 -- Nov. 1, 2013. In the new method, he and his colleagues
used a well-known technique called X-ray photoelectron spectroscopy (XPS) --
but with a twist. In XPS, the test sample is zapped with a focused X-ray beam,
which then knocks out electrons from the surface of the sample. A detector then
counts the electrons and measures their kinetic energies. The resulting
spectrum reveals the chemical signature of the surface.
XPS has been used before
to characterize the surfaces of textile fibers that don't have chemical
coatings,
Strohmeier says. But to
study the surface chemistry of treated fibers, the researchers need to go
deeper and analyze the layers just beneath the surface. To do so, the
researchers fired a beam of argon-ion clusters onto the sample fiber. The beam
drilled away a shallow hole on the surface of the fiber, revealing the layer
underneath. Each cluster contains thousands of 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
blasting the sample with the beam 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's 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 says. There was no accepted
standard for XPS methods in forensics, it often took hours to analyze 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. But, he says, XPS instruments have improved to the point that
analysis now takes minutes and you only need tens to hundreds of square microns
of sample area. And, only in the last couple years have argon-ion cluster beam
technology been able to do the kind of depth-profile analysis demonstrated that
the researchers demonstrated.
While these new results
don't yet establish a bona fide technique for forensics, Strohmeier says, it
does show great potential for analyzing fibers and the surfaces of other kinds
of evidence collected at crime scenes.
Posted by:
Gauri Shah(faculty)
