The artificial nose - Laser IMS outperforms Nature itself

Researchers have developed a device which is not only better at scenting explosives than a dog, but can also be programmed to detect cancer or to clean contaminated air.

By the window stands a bottle of “Eternity”. This Calvin Klein fragrance combines the scents of mandarin, lavender and a balanced blend of essences of grasses, supplemented at the core by aromas of sage, basil, jasmine and geranium. But no one here is particularly interested in perfume poetry. Nor even in this liquid of the highest quality. Eternity’s scent fades without effect. Perfume does not explode.

“Such essences are only a diversionary manoeuvre,” says Johann Göbel, project manager in the Sensors for Safety and Security department at EADS Innovation Works, the central Corporate Research Centre of aerospace company EADS. The scientist’s nose is expert at sniffing out explosives against any cloud of perfume. That is, not his own nose, but the one this Electrical Engineering graduate has invented. Its name is actually the Laser Ion Mobility Spectrometer – or Laser IMS for short – and it has a competence in sniffing out chemical substances that is globally unique. But at Göbel’s laboratory in Munich, people talk only of the “artificial nose”.

The development goal is clearly defined: sniffing in the service of safety and security in the air. “As the manufacturer of passenger aircraft, we have a vital interest in the maximum possible air safety and security,” says Göbel. The artificial nose is therefore one of the sub-projects of the EU’s Security of Aircraft in the Future European Environment (SAFEE) programme (see box).

The artificial nose is likely to be a hit with the passengers: besides holding out the prospect of safer flying, because it is so quick the laser IMS will make life easier in the airport terminal as well. Long queues and time wasted at security checkpoints do not just tax the nerves of passengers – they are also a nuisance for airports and airlines. “At the moment, depending on the type of checkpoint, we can process between 110 and 160 passengers per hour,” calculates Jürgen Friedmann, head of Operational Planning and Projects at Frankfurt Airport Services Worldwide. His team is responsible on behalf of the Federal Ministry of the Interior for security checks at Frankfurt Airport. Since limits on liquids allowed in hand luggage were introduced, checks have become even more critical in terms of the time taken. “We try to keep the queue times at traffic peaks, of which we have four to five a day, to a tolerable ten minutes. Customers won’t accept any longer than that.”

The dilemma is that they can still only search for explosives by random checks. That is why security professionals want “devices with short analysis times and low false alarm rates, which take up little space and are capable of searching every single passenger for explosives in the shortest possible time”. Reliability in particular is fundamental: if security staff should lose faith in the artificial nose because it gives too many false alarms, confidence will drop and so will use of the device.

It is here that the artificial nose developed by the EADS scientists comes into its own: it fulfils virtually all these requirements as no other system to date. “In 2,000 measurements with ten substances we established a recognition rate of 98.5 percent,” says Göbel.

The laser IMS works in the same way as the most sophisticated system of all, the biological nose. “The aim was that, starting from the natural model, the electronic version developed at EADS should be able to imitate nature as closely as possible,” Johann Göbel explains. What he means is that it should possess three of the most important characteristics. Firstly it should be able to categorise and recognise different substances in a simple way. Secondly, it should be sufficiently sensitive and immune to “noise” to be able to pick up slight traces as well. And thirdly it should work fast enough to detect dangerous substances reliably and independently.

Designed in the course of evolution, the human olfactory mucosa, i.e. the region of the nose responsible for scenting odours, measures about 5 cm² on either side of the nose. It has 347 receptors, each of which is geared to a different scent. For example, one milligram per thousand cubic meters is a sufficient concentration for the human nose to detect the scent of vanilla, but to identify it the concentration needs to be about 50 times higher.

But vanilla is not our concern any more than perfume is: explosive substances liberate a lot less gas. With TNT, for example, there is just one molecule for every thousand million air molecules, while with plastic explosives the ratio is as weak as one to a billion – that’s a number with twelve zeros. Or, as Johann Göbel puts it, “It’s like looking for a postage stamp in an area the size of Paris.”

The canine nose therefore provides a better model. After all, as everyone knows, our four-legged friends are able to sniff out explosives. The edge they have over us humans is that their olfactory mucosa is about five times as big as ours. On top of that, the canine organ contains over 1,000 receptors for sniffing out all manner of scents. Hence biologists talk of the human perceptual space as being the size of a shoe box, whereas that of the dog is more like a barn. Expressed in mathematical terms, a dog’s olfactory sense is about 1,000 times better than that of the human. Yet it has the disadvantage of all living creatures, namely, the sniffer dog’s attention declines significantly after about half an hour.

This is not the only respect in which the EADS artificial nose is superior. For certain substances, the electronic sniffer is also far more sensitive than its model. This is how it works. In the first filter stage the explosive molecules we are looking for are ionised. This is accomplished by feeding the air sample through a pulsed laser beam, in the course of which the light is beamed in between two concave mirrors and reflected up to 36 times. As Göbel explains, “In our prototype, this means that we can achieve a laser distance of 3.6 m inside a 10 cm wide case.” In this way a fine screen composed of light is created. Because the fixed laser is set to one particular frequency, only the molecules that the device is looking for respond when they cross through the light beam: the light energy shoots an electron out of the atomic compound and turns the molecule into an ion – an electrically charged molecule.

“Every substance, i.e. including every explosive compound, has its own ionisation potential at which it becomes electrically charged,” explains Göbel. This ionisation potential is measured in electron volts (eV). The explosive trinitrotoluol (TNT), for example, has an ionisation potential of 10.59 eV. To electrically charge a TNT molecule, the laser beam must have a wavelength of 117.02 nanometres (nm). This is an inverse relationship: if the wavelength is doubled, the energy halves and electrifies molecules with an ionisation potential of 5 eV. In this way, depending on the laser light, a different molecule can be electrically activated.

But the laser beam also has a downside. As the scientist points out, “With this so-called single-photon ionisation, however, every substance that possesses a smaller ionisation potential is also ionised. To filter these out, the EADS engineers employ a trick from nuclear physics. Every molecule has distinctive, stable, inner energy levels to which electrons can be energised. These inner patterns are also unique. Hence the ionisation energy is not conveyed into the substances in a single step, but uses the inner level as an intermediate stage. First of all, with half the laser energy, the electron is raised to this intermediate state, then it is removed from the molecule and thus ionised with the second energy charge – like a key that must be turned twice in the lock to open the door.

The hit probability of the laser ionisation does not actually rise to 100% as a result, but the number of other substances also ionised is reduced to a fraction. “The more accurately and selectively the gaseous particles can be ionised, the lower the cross-sensitivities to be expected,” says Göbel. The major advantage of the laser procedure lies here. Classic ionisation procedures work predominantly with radioactive sources, whose energy radiation is of such a wide bandwidth that virtually all ionisable substances are electrically charged as a result. Moreover, radioactive substances are subject to strict controls and require extensive precautions.

Ionisation is actually only the filtering precursor to the actual analysis of the electrically charged particles. For this purpose the ions are sent into a race through an electrical field. “Ion mobility spectroscopy (IMS) is an analytic procedure that works under environmental conditions and requires neither the vacuum needed by the mass spectrometer, nor time-consuming sample preparation, like gas chromatography,” explains Göbel. The ions are drawn by the collector to the other end of the electrical field as if by a magnet. The molecules accelerate and, depending on their mass and surface area, attain different speeds, known as drift time.

To accelerate the ions as smoothly as possible for the best results, drift rings with graduated electrical charges, alternating with insulators, are placed along the flight path. The ions head for a Faraday plate which functions as a collector. When the ions strike this, they give off their electrical charge and in so doing generate a current flow. What is critical now is how long the particles spent en route: in practice, the drift time between laser ionisation and arrival at the collector constitutes a unique fingerprint for each substance. And the higher the charge registered at the collector, the more ions were travelling simultaneously and therefore the higher concentration of the substance.

The laser IMS actually comes extremely close to its model in nature. Like the canine nose, the artificial sensor can be trained for particular substances, which it can then sniff out from virtually any mixture of gases to a high degree of accuracy. “We just have to enter the characteristic ionisation energy and drift time of a given substance into the control and evaluation system, and we will know immediately whether it is contained in the sample.” Immediately means virtually in real-time: the electronic nose needs just 50 milliseconds for a single scan operation, i.e. ionisation and recording of times. Moreover, on account of this speed and precision it is able to compete with another speciality of the four-legged creature: the search for clues. “Because we also know the concentration of a substance in the air at any time, we can sniff out its origin like a dog,” says Göbel.

As well as its reliability, another feature that is especially important as regards practical use is the fact that the EADS nose can be fitted into a case smaller than a shoe box. The laser compartment and drift housing together are less than 15 cm high in the prototype. “Thanks to the consistent use of miniaturised components, these measuring devices are much smaller than conventional sensors and are highly suitable for installation in portable equipment, or for use as an integral element of measurement and monitoring systems,” Göbel adds. Because the laser uses less than 10 watts, the power supply can be kept low too.

The EADS explosives nose can be used as either a mobile sniffer dog or a fixed security gate. Johann Göbel’s team has developed a procedure whereby all the air in a space the size of a phone box can be checked out in only five seconds. How it works he is not yet able to divulge. “We have applied for a patent but it has not yet been granted,” he says. This would make it possible, for example, to screen passengers at a checkpoint for metal objects, explosives, drugs and any number of other substances all at the same time.

Its variability makes the EADS researcher’s artificial nose a potential aid for other applications as well, for example, in medicine. Certain types of cancer such as skin and lung carcinomas, diabetes and tuberculosis leave certain decomposition substances in the human breath which can be detected with the laser IMS. In this way it is possible to detect the fire before the first flame flares up. Again, mines can be searched for with robots and located precisely.
The technology offers some novel perspectives in decontamination, as Göbel explains. “With this procedure we could also filter poison gases or radioactive substances such as polonium from the air.” Moreover, the military options won him and his team the Defence Industry Technology Prize awarded by the Federation of German Industries in 2005 for development of the artificial nose.

The first electronic nose could be deployed for practical purposes in only 18 months. The scientists are currently in the process of refining the evaluation system so that all the substances detected can be displayed to the user by name on a screen. In the laboratory these data are still in the form of curves which only a scientist can understand at first glance. “We will also be able to tell what perfume a passenger is wearing,” says Johann Göbel with a smile. But that might lead to misunderstandings, for example, if the perfume being worn happened to be Theany’s cosmetic product TNT.

To protect against terrorist attacks in air transport, the European Union inaugurated the Security of Aircraft in the Future European Environment (SAFEE) programme in 2004. As well as development of the artificial nose, the consortium is working on five other important technologies:

The SAFEE project, which is funded to the tune of € 36 million, is part of the 6th Framework Programme of the EU and will extend over four years. A joint test of the functionality of the SAFEE systems is scheduled for February 2008. Depending on the results, extension of the project by a further seven years is under discussion.

European Aeronautic Defence and Space Company EADS N.V.
Mendelweg 30 · 2333 CS Leiden · The Netherlands

EADS Deutschland GmbH · 81663 Munich · Germany
EADS France S.A.S. · 37, boulevard de Montmorency · 75781 Paris Cedex 16 · France
EADS CASA · Ava. de Aragón, 404, 28022 Madrid · Spain