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Emerald Group Publishing Limited
Copyright © 2000, MCB UP Limited
Revolutionary new detector for toxic gases
Keywords Engineering, Environment, Technology
British scientists have developed a revolutionary new method for detecting toxic gases in the environment. The technique, which is based on novel optical sensor technology, enables several gases to be monitored simultaneously and at a distance from the sensor, something that most conventional gas-monitoring systems cannot do.
The work has been carried out by physicists at St Andrew's University led by Dr Miles Padgett, who has now moved to the University of Glasgow, together with a consortium of industrial collaborators. The project was part of the government's LINK photonics programme, funded by the Engineering and Physical Sciences Research Council.
One of the commercial partners, Siemens Environmental Systems Ltd, has put the new instrument on the market recently.
Because gases can absorb light at specific wavelengths, it is possible to identify a given gas by shining a beam of light through it and measuring which wavelength has been absorbed - a technique termed spectroscopy.
"For example neon absorbs red, and sodium yellow", says Dr Padgett. "Every gas has a particular absorption fingerprint which you can recognise".
Most gas detection systems currently on the market measure the gas concentration only at a single point. For this reason many sensors may be necessary at a given site, such as on an oil rig. The advantage of an optics-based system on the other hand is that it can detect over a distance, meaning that fewer detectors are necessary.
A number of optical systems exist already. These measure absorption in the infrared end of the spectrum, which for technical reasons limits the range of gases that can be detected.
Dr Padgett's team was interested in developing a system that could measure absorption at the ultraviolet end of the spectrum. This would give it greater sensitivity than infrared detectors and enable a wider range of gases to be detected. However, conventional spectroscopic technology is too fragile for such use.
In most spectrometers the incoming beam of light passes through a small slit and is then split into its constituent colours by a grating or prism. These instruments are simple to design and operate but the narrowness of the slit means that most of the light is lost. Alternatively "Fourier Transform" spectrometers have no slit and therefore waste no light. These devices use a semi-reflecting mirror to split the light into two beams. One beam is directed onto a moving mirror before being recombined with the other, creating a characteristic pattern from which information about the light's wavelength can be calculated. However, part of the process requires the mirror to be moved tiny distances - of the order of thousandths of a millimetre.
"The slightest vibration upsets the whole thing, so these types of spectrometer are traditionally used only in the lab - if you want to use them in the field they have to be built like battleships", says Dr Padgett.
The key to the St Andrew's system was to make the moving mirror redundant. To do this the team adopted a different approach. Rather than rely on a semi-reflecting mirror to split incoming beams into different paths before re-combining them, it was decided to direct the incoming beam onto wedges of crystalline quartz. The effect is to polarize the beam in two planes, vertically and horizontally.
So two beams of light emerge, one whose waves are vibrating horizontally and another vibrating vertically. The thickness of the wedge is equivalent to the position of the moving mirror and a camera allows the whole pattern to be recorded without the need to move any of the components.
Crucially, the beams follow the same path. "Because of this, any vibration affects both beams identically", says Dr Padgett. "The problem of mechanical robustness is therefore solved".
By recombining the beams a pattern is formed which, when subjected to appropriate mathematical analysis called Fourier Transform, or FT, can yield information about the wavelengths of light present in the original incoming beam of light. Any "missing" parts of the spectrum indicate that the light has been absorbed.
"We succeeded in building an FT spectrometer which could operate in the ultraviolet", says Dr Padgett. "That was technically a new idea and was unique."
"Because the system works across a range of wavelengths in the ultraviolet, it can deliver information about several gases simultaneously", says Dr Padgett, "and the absence of moving parts makes it extremely well suited to use in the field."
"The collaboration was a very important aspect of the project", says Dr Padgett. "It was a model of how universities can work with industry."
For further information contact: Dr Miles Padgett. Tel: 0141 3305389; Fax: 0141 330 2893; E-mail: email@example.com