The ease with which TV's intrepid crime scene investigators employ science to analyze evidence and catch the perps belies the reality of most criminal investigations. Although fingerprints, gunshot residue and other forensic evidence is critically important in connecting a suspect to a crime, the very methods used to analyze such samples often corrupt or destroy them.
In an attempt to spare delicate evidence, researchers at the University of Surrey in England are experimenting with a technique that uses ion beams to detect trace elements contained within particles found on a suspect's body or clothing without contaminating or ruining the evidence.
"Forensics is a really interesting discipline and something [an] ion beam is suited for," says Melanie Webb, a fellow at the university's Ion Beam Center who is leading its forensic analysis research. "You can see which elements you have, even trace elements. At a forensics level, residues and soils are usually studied using electronic microscopy, which has a lower level of sensitivity than [an] ion beam."
Ion-beam analysis allows investigators to compare the evidence found at a crime scene with evidence found on a suspect—or in a suspect's home or car—at the atomic level. "If you compared two samples of gunshot residue, you would check the elemental composition of these samples to see if they match," Webb says. "Ultimately, the advantage of the technique from a forensic point of view is that it's not destructive." Investigators do not have to use any chemicals on a sample to prepare it for analysis, which means the same evidence can be preserved and studied multiple times.
Ion-beam analysis was first discovered about a century ago after Lord Ernest Rutherford's experiment with backscattering spectroscopy, which measures the number and energy of ions backscattered from atoms in the sample being tested. In addition to the Rutherford backscattering (RBS) spectrometry approach, other ion-beam techniques include particle-induced X-ray emission (PIXE), during which X-rays are emitted that are characteristic of the elements in the sample, and secondary ion mass spectrometry (SIMS), a process that uses a low-energy ion beam directed at atoms in a sample's outermost layer. In SIMS, these secondary ions are then analyzed in a mass spectrometer.
In many police investigations, soil samples are compared in an effort to link a suspect to a specific location. "Many soils in the U.K. contain a mixture of commonly occurring minerals (such as quartz, calcite, feldspar, biotite) and, therefore, mineralogy studies can fail to uniquely distinguish soils from different locations," Webb said last week during a presentation to The Royal Society—the U.K.'s national academy of science—describing how ion-beam technology could be used to aid law enforcement. "The high sensitivity of ion-beam techniques to trace elements can establish similarities or differences between the samples to a greater degree of certainty than conventional methods."
Other clues, such as fingerprints, have likewise been limited by traditional analysis, which takes into account the shape and spacing of the ridges on a suspect's fingers. In many cases, the fingerprint may be incomplete or too faint to resolve, Webb says. Many fingerprint tests also rely on chemical interactions between a developer and the proteins, fats or amino acids found in a print, but these interactions vary depending upon the chemical composition of the surface—glass, wood, etcetera—from which the prints are lifted. Recent studies have demonstrated that ion beams can be used regardless of the surface to provide additional chemical information by picking up traces of drugs, gunshot residue and other incriminating evidence.
The Surrey police department is assisting and supporting Webb's research, "which may offer exciting opportunities for the future," says Martin Hanly, the department's acting scientific support manager.
Hanly makes clear that, even though Webb's work is worth exploring, this does not mean that current investigative techniques and technologies are lacking. "To find evidence, you may use techniques such as looking for evidence with the naked eye, high-intensity light sources, chemicals, tapings, powders and many others," he says, adding that a crime scene examiner may recover fingerprints, glass, tool marks, DNA swabs, blood, footprints, fibers and other exhibits from a crime scene that each require separate analysis, techniques and skills. "There are many techniques for recovery of many types of forensic evidence," he notes.
Still, Webb's work represents a possible advance in the field of forensics, which "is constantly evolving," Hanly says. It is impossible to say yet whether ion-beam analysis will significantly take a bite out of crime, because it has yet to be used in many real-life investigations. But researchers hope the nascent technology will prove itself a useful law enforcement tool, one day becoming a routine part of criminal investigations.