Cdex, Inc. (fka CDEX)

[d4diddy]

The big difference between trace detection and Raman spectroscopy is that traces detect the presence of the specific substances you are looking for," said Hill. "If you don't know if a specific substance is there, but you want to know, then use trace technology. If you know [a substance] is there but you don't know what it is, then you use Raman spectroscopy to identify it."


Raman spectroscopy is an optical technique, so unless a residue is visible on the outside of a bag, the bag must be opened so that a monochromatic laser can illuminate the objects inside.


Raman libraries of recognizable substances are more extensive than trace libraries, but Raman spectroscopy requires a larger sample. Trace technology concentrates on identifying the few substances of particular concern to the Transportation Security Administration, but it works with nanograms or even picograms of the substance.


"Raman spectroscopy works on substances you can see," said Hill. "If a bag is open, screeners can use Raman spectroscopy to interrogate transparent containers to find out what is inside them."


Raman spectroscopy works similarly to gas chromatography. First the screener shines a laser on the unidentified substance or container, energizing the substance and thereby driving its electrons into higher-than-normal orbits. As the electrons drop back to their ground state, they release photons. A photodetector collects the photons, and software analyzes their colors for shifts that match substances in the Raman library.


"Depending on what frequency bands the scattered signal gets shifted to, you can develop a fingerprint for what is in there," said Hill. "Then software matches those shifts with a library of known fingerprints."

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