|
It may not seem much like it at the moment, but the real bedrock of the global economy is not the availability of credit but rather the much more mundane cargo container. The use of standard-sized cargo containers that can be transferred directly from ships to trains to trucks is what allows the efficient transport of goods all over the world. Indeed, almost every manufactured product consumed by humans has spent some of its life in a container. But what has proved a boon to the global economy has also provided a great opportunity for smugglers, drug traffickers and terrorists. Around 18 million cargo containers are transported around the world each year and only a small fraction of those (around 4% in the US) tend to be checked when they arrive at a port. So contraband such as drugs, firearms and explosives hidden in cargo containers currently have a good chance of remaining undiscovered by the authorities. The problem is that it's clearly impractical to open up and physically check each of these containers when they're stacked at a port. So the authorities need to use less intrusive forms of detection, such as sniffer dogs or chemical detectors. But sniffer dogs can usually only be trained to detect a limited set of contraband, while the current generation of chemical detectors, such as gas chromatography-mass spectrometry (GC-MS), tend to be bulky and expensive. Now, however, a new portable option is available, which should allow a greater proportion of containers to be tested. Developed by Edward Staples from the US measurement technology company Q&A Solutions and Shekar Viswanathan from the National University in San Diego, this new option takes advantage of a phenomenon known as surface acoustic waves (SAWs). As their name suggests, SAWs are sound waves that travel along the surface of a material. First discovered back in 1885, SAWs have subsequently been employed in electronic circuits, where the propagation of these waves forms the basis for filters, oscillators and transformers. SAWs can also be utilised to detect chemicals. This involves inducing a SAW in a quartz crystal and then monitoring changes to the frequency of this SAW as molecules absorb onto the crystal. 'Electrical signals in the piezoelectric crystal generate surface waves or elastic vibrations bound to the surface of the crystal,' explains Viswanathan. 'Material interacting with the crystal surface changes the frequency of the resonator by slowing down the surface waves. The change in frequency is a direct measure of the amount of material on the crystal surface.' Although SAW detectors have been around since the early 1990s, Staples and Viswanathan have managed to develop an advanced version that is a thousand times more sensitive than previous detectors. One of their advances has been to attach a thermoelectric heater to the crystal. This not only allows them to control which molecules adsorb onto the crystal, as different molecules adsorb at different temperatures, but also means they can evaporate off any adsorbed molecules by simply raising the temperature of the crystal. By connecting this SAW detector to a capillary-based gas chromatograph column, the researchers came up with a portable sensor for sensitively detecting contraband in cargo containers. The idea is to take a sample of air from a cargo container, which can be done by simply pumping the air from the container's ventilation duct. Following a concentration step, the volatile compounds in this air sample are separated by GC and then detected by the SAW detector. The end result is a standard chromatogram, in which chemical markers of specific drugs or explosives can be identified based on their retention times. Testing this GC-SAW system on samples of cocaine hidden in a cargo container, Staples and Viswanathan found that their system could detect characteristic chemical markers for this drug in around 20 seconds at concentrations as low as parts per trillion. According to Viswanathan, this system is now being employed by government agencies in several countries around the world, including the US. But detecting contraband is just one of many potential applications for this GC-SAW system. Viswanathan is currently testing its ability to analyse emissions from land-fill sites and to detect volatile biomarkers for diseases such as cystic fibrosis and diabetes. Related links:
The views represented in this article are solely those of the author and do not necessarily represent those of John Wiley and Sons, Ltd. |
 |
|
|
|
|
