To airport security personnel, an opaque plastic bottle is a dangerous mystery: Does it carry shampoo or explosives? Now researchers have developed a method that uses Raman spectroscopy to detect and identify concealed hazardous materials (Anal. Chem., DOI: 10.1021/ac2018102).
Current methods for identifying explosives are either invasive, like digging through a passenger’s luggage, or require time-consuming sample preparation. X-ray imaging can hint at dangers but doesn’t provide chemical specifics.
But Raman spectroscopy can give detailed information about chemicals behind a barrier, says Freek Ariese of VU University, in Amsterdam. He and his colleagues thought time-resolved Raman spectroscopy (TRRS) would be ideal. The idea behind TRRS is that, after a laser excites a sample, the first photons back to the spectrometer are those emitted from the molecules on the sample’s surface, because they have the shortest distance to travel. Researchers avoid these signals by closing the gate in front of the spectrometer’s detector for the first few hundred picoseconds after excitation, Ariese says. Then they open the gate in time to receive the photons and chemical information from deep within the sample.
To test TRRS’s ability to detect hidden explosives, the researchers put powdered dinitrotoluene, a byproduct of TNT, in a cuvette and placed it behind a variety of common white plastics up to 5-mm thick. First, they opened the detector gate quickly to collect a spectrum dominated by signal from the plastic. Then they started delaying the opening for longer and longer, until they could see dinitrotoluene’s signature in their spectra. Both the type of plastic and its thickness affected the optimum gating delay—between 300 and 500 ps—but they successfully detected the dangerous material behind all the plastics tested.
David Moore of Los Alamos National Laboratory thinks the use of Raman spectroscopy is an “interesting experiment” but questions its applicability in the real world. The method won’t work, he says, if the explosives are hidden in metal or cardboard: “If it’s not plastic, the laser won’t go through at all.”
Ariese agrees that metals would stymie his team’s method, but he says simply tuning the laser to different wavelengths could allow researchers to look through other materials.