Call it CSI: Abracadabra. A camera that can make invisible substances reappear as if by magic could allow forensics teams to quickly scan a crime scene for blood stains without tampering with valuable evidence.
The prototype camera, developed by Stephen Morgan, Michael Myrick and colleagues at the University of South Carolina in Columbia, can detect blood stains even when the sample has been diluted to one part per 100.
At present, blood stains are detected using the chemical luminol, which is sprayed around the crime scene and reacts with the iron in any blood present to emit a blue glow that can be seen in the dark. However, luminol is toxic, can dilute blood samples to a level at which DNA is difficult to recover, and can smear blood spatter patterns that forensic experts use to help determine how the victim died. Luminol can also react with substances like bleach, rust, fizzy drink and coffee, causing it to produce false positives.
The camera, in contrast, can distinguish between blood and all four of these substances, and could be used to spot stains that require further chemical analysis without interfering with the sample.
To take an image of a scene, the camera beams pulses of infrared light onto a surface and detects the infrared that is reflected back off it. A transparent, 8-micrometre-thick layer of the protein albumin placed in front of the detector acts as a filter, making a dilute blood stain show up against its surroundings by filtering out wavelengths that aren't characteristic of blood proteins.
By modifying the chemical used for the filter, it should be possible to detect contrasts between a surface and any type of stain, says Morgan. "With the appropriate filter, it should be possible to detect [sweat and lipids] in fingerprints that are not visible to the naked eye," he says. "In the same way you could also detect drugs on a surface, or trace explosives."
Journal reference: Analytical Chemistry, DOI: 10.1021/ac101107v