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SORching for hidden liquid explosives
Intensity (a.u.)
Frequent airline travelers have subtraction of the offset spectra the 3–1–1 rule down pat: only could be done in an automated _3-oz bottles of liquids in one < fashion. Contents quart-sized, clear, plastic, zip-top Important analytes include bag permitted per passenger in concentrated solutions of H 2O2, carry-on bags. The draconian which, when mixed with other SORS through jar measure stems from the fact that chemicals, can form the explothere isn’t a simple, fast, and sive compound hexamethylene CR through jar noninvasive way to detect liquid triperoxide diamine. (In the four explosives. attempted attacks on London’s Jar only In this issue of Analytical public transportation system on Chemistry (pp 8185–8189), Pavel July 21, 2005, the bombs were Matousek and colleagues at the made with chapati flour, acetone, 800 1000 1400 1200 Rutherford Appleton Laboraacid, and H 2O2.) Matousek and Wave number (cm –1) tory (U.K.) describe a technique colleagues added 30% H 2O2 to called adapted spatially offset Racontainers of lotions, creams, and man spectroscopy (SORS) that Conventional Raman (CR) and SORS spectra of a facial beverages and established that can interrogate the contents of moisturizer jar with 30% H 2 O 2 . The 6.5-mm-thick jar is made SORS could indeed detect the plastic and glass beverage, mediof semitransparent hard plastic. The dotted line shows the solution’s presence. cine, and cosmetic containers. signature H 2 O 2 band at 876 cm –1. The team also uncovered a The technique suppresses fluorespotential problem when they cence and Raman signals coming from used a different mode of photon transadded H 2O2 to wine. The key H 2O2 the walls of the container and provides port to the Raman detection region. Raman band at 876 cm–1 was partially the Raman spectrum of only the ingreThe adapted technique works on overlapped by the dominant Raman dients inside. samples with a transparent sublayer, beband of ethanol at 882 cm–1. Although SORS requires the collection of Racause the diffusely scattering container it was still possible to identify the comman spectra from regions spatially offset acts as a source of diffuse radiation for pound, the proximity of the two bands from the point of illumination on the a large area of the liquid inside. Raemphasizes the importance of collecting sample surface, followed by a scaled man photons are then generated in all a high-quality Raman spectrum. The subtraction of spectra to separate signals directions, causing the container wall problem is further reduced by collecting from individual layers within the samand the liquid directly under the Rathe Raman spectrum of the container’s ple. “We originally developed [SORS] man collection area to be effectively contents across the entire spectral region, through wider collaborative efforts for illuminated. The Raman photons then rather than at a specific wavelength. the noninvasive detection of bone dispass through the wall via diffusion to Matousek emphasizes, “It’s widely ease and quality control of pharmaceuthe surface, where they can be detected. recognized that no single technique can tical products,” says Matousek (Anal. The spatial offset ensures that the Ra[analyze] all samples. Our method is Chem. 2007, 79, 1696–1701; 1773). man and fluorescence contributions intended to be a first layer of screening. When British authorities arrested 24 from the container wall don’t drown Any objects that can’t be probed with people suspected of plotting the detona- out the weak Raman signal coming this method, such as metal containers, tion of liquid explosives on transatlantic from the liquid. The pure Raman speccan be probed by more invasive but flights in 2006, it was immediately aptrum of the liquid can then be comslower techniques as a second layer of parent that airports lacked an effective pared with a library data set of known screening.” method to quickly and noninvasively explosive ingredients. The investigators are now busy search for liquid explosives. Matousek For the technique to be useful, it converting the benchtop setup into a says it occurred to him and his colhas to be user-friendly for nonexperts. portable instrument. The researchers leagues that they could further tweak “You don’t want to reconfigure your are launching a spin-off company called SORS to address the problem. instrument when analyzing a wide range LiteThru to market this technology SORS works only on highly scatof containers,” says Matousek. The infor pharmaceutical quality control and tering media. To analyze transparent vestigators demonstrated that a single detection of liquid explosives and counor diffusely scattering containers with experimental setup worked for all kinds terfeit drugs. a transparent liquid, the investigators of containers and liquids and that scaled —Rajendrani Mukhopadhyay N o v e m b e r 1 , 2 0 0 7 / A na l y t i c a l C h e m i s t r y
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