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Defense & Security

Raman leaves no hiding place for liquid explosives

Raman spectroscopy could help to track down liquid explosives before they are made.
4 September 2008, SPIE Newsroom. DOI: 10.1117/2.2200809.0001

It is a common sight in airports today to see people opening their bags and throwing away large bottles of shampoo and shower gel because they have forgotten the 100ml (3 fluid ounce) limit on liquids.

Frustrating though this can be for travelers, there are good reasons for this caution. The authorities are worried that the innocent-looking bathroom products could harbor chemicals such as hydrogen peroxide needed to make improvised explosives.

Finding out who might be carrying these components is notoriously tricky. Whereas military explosives like Semtex and C4 are hard to procure and fairly straightforward to track, compounds like hydrogen peroxide are everyday ingredients in over-the-counter products such as hair dye.

Detecting them is also hard as they are in liquid form and most analytical techniques are invasive. This means they would require opening up the bottles and taking a sample, which is not a very fast or discrete way of monitoring what people are carrying. Taking samples of a liquid is even more difficult if a compound is unstable.

Help is at hand, however, in the shape of Raman spectroscopy. This non-invasive technique involves shining laser light on a sample and studying its effect on the electrons within the sample. It works particularly well for symmetrical bonds such as those between the two oxygen atoms in hydrogen peroxide and it does not require opening the bottle.

What's more, according to Robert Stokes of the University of Strathclyde in the UK, it can now be done quickly. Stokes spoke about his research at the SPIE Europe Security + Defence meeting in Cardiff (UK), 15-18 September. He has used Raman spectroscopy to successfully identify a range of hostile compounds that might have slipped through security systems in the past. "Raman is an ideal technique for identifying what's in a bottle without opening it," he explained.

Raman spectroscopy is relatively insensitive but advances in optics and detector technology have resulted in a number of low-cost systems becoming available on the market. Stokes aims to maximise throughput by matching patterns in specific regions of the spectra rather than trying to match a whole spectrum to one in a library. This is a good way to preliminarily identify many potential threats, as many compounds have common characteristic patterns in Raman spectra. These can be easily spotted and then examined in more detail if a threat is flagged.

"Fast bottle screening would be good at, for example, sports venues where thousands of people would want to bring bottles of water with them," commented Stokes.

Figure 1. Fast Raman line mapping inside glass container: Brightfield montage image 50x, Urea (blue), Potassium nitrate (red). Hyperspectral image of both components took 2-3 minutes at 5 micron resolution, recording  up to 1000 spectra per second. (Courtesy Robert Stokes)

The same principle applies to forensic analysis. Raman mapping is a technique whereby many spectra are recorded from a surface. In the past the amount of time required for each spectrum made the technique unfeasible for larger samples. However, recent advances in instrumentation and spectral matching can reduce the overall throughput time "Many of these materials are light or heat sensitive," pointed out Stokes. "This technique spreads the laser light over a larger area so it is not concentrated at one point, lessening the chance of samples damage whilst increasing scanning speed at low resolution." If something interesting is detected and matched then the system can zoom in on one spot to investigate it in more detail.

Although Stokes is enthusiastic about the potential of Raman spectroscopy, he pointed out that no one technique can do everything. He is working with Cascade Technologies, of Stirling, UK, which is also giving a presentation in the same session at the SPIE event. Cascade is working on infrared (IR)-based screening techniques using a quantum cascade laser for use in applications such as airport security. Cascade's system can rapidly detect hazardous materials and their precursors in the vapor phase. Even though it is capable of highly sensitive detection and able to pinpoint trace residue of compounds by IR absorption, it is more efficient and appropriate to address the problem of liquids in sealed semitransparent containers using Raman. Stokes is working with the company to find ways to combine his technique with Cascade's. The two techniques are great together because they work in complementary ways, he said. Whereas Raman spectroscopy predominantly identifies symmetrical bonds within molecules, IR identifies those bonds that are polar and non-symmetrical. "Pretty much no compound can slip through the net using a combination of IR and Raman," observed Stokes.

Of course, as with any new technique, there are challenges, said Stokes. One of these is to speed up the signal processing and matching algorithms to keep pace with the data acquisition, increasing the overall reliability of such systems. Stokes said that currently Raman spectroscopy is easier with some types of containers than others. Although many point and shoot Raman systems exist commercially, the method would have to be simplified before it is routinely used in applications such as airport security.

"A lot of the people who would potentially use these are not scientifically trained" pointed out Stokes, "the optical practicalities of focusing into a wide range of containers needs to be made fail-safe before this can be adopted."

Until such challenges are ironed out, the crowds at the airport bins will continue to throw away toiletries that are too large to travel with.

Siân Harris is a UK-based science and technology journalist.