Sharper ion security

Security forces aiming to prevent crime and terrorist attacks have a difficult job because their staff have to watch out for a wide range of substances.

Now UK research aims to make the task easier. A £1m project could lead to the development of a generation of chemical detectors that will sense and identify substances with a precision and selectivity far better than can be achieved with current systems.

The potential commercial benefits of the project have encouraged Smiths Detection of Watford to contribute £400,000 in equipment and expertise. The research will be based at Birmingham University’s school of physics and astronomy and led by Dr Chris Mayhew, head of molecular physics, with senior honorary research fellow Dr Peter Watts.

‘With recent threat levels in this country being raised to high, we are all aware of the importance of being able to detect agents in low concentrations,’ said Mayhew.

Timely opportunity

‘This is a unique and timely opportunity to develop a major UK initiative to greatly improve ion mobility technology, ultimately leading to a new generation of chemical detectors to increase UK security.’

Ion mobility spectrometry (IMS) is the technology used to screen people, objects and transport for traces of explosives and drugs. It works by creating positively or negatively-charged molecules (ions) at atmospheric pressure by a radioactive source or a corona discharge.

Ions of a particular polarity are selected by means of a homogeneous electric field. A sample of the ions is pulsed into a drift tube, along which there is a linear potential gradient, and they migrate under the influence of the electric field with a mean drift velocity.

This velocity is characteristic of a particular ion species and depends on the mass, shape, ion structure and the drift gas.

When a sample of air is introduced the substance being sought, if present, may react with the initial ions in the drift tube to produce further ions. These ‘product’ ions will have different mobilities than those present prior to the introduction of the air sample.

It is the measurement of these mobilities (which are effectively the changes in the drift time) that allows the identification and quantification of the substance. Selectivity can be manipulated by the presence of dopant materials (to give different ‘reactant’ ions) and the use of ions of different polarity.

Unfortunately, IMS systems have limited selectivity, which restricts how many chemicals they can detect. This means many explosives and chemical threats cannot be detected.

That is why some experts have described the present generation of IMS-based equipment — often incorporated into Sentinel portals for passenger screening at airports — as ‘first-generation’ and ‘one-dimensional’, despite their worldwide deployment and undoubted successes.

The limitations were highlighted in a recent National Research Council report on airport security, which said that ‘currently deployed systems have limitations specific to the physics and chemistry of their operation’, making them unsuitable for addressing a variety of emerging threats.

Process refinement

The Birmingham researchers hope to refine, extend and enhance the process to make it more selective and less likely to deliver false- positive and false-negative results. They will conduct detailed, fundamental research to understand the key chemical processes employed in IMS and, in particular, those occurring in the latest generation of drift tube systems being developed by Smiths.

‘There has never been anything as systematic as what we are proposing,’ claimed Mayhew.

‘We can do it because we have all the equipment in one lab. We’ve been gaining experience for 20 years and it is the right time to deploy this as a research project.’

They will investigate the ionisation chemistry, the use of chemical dopant and the mode of operation (negative or positive ion mode, high or low electric field). To do so they will couple the IMS to an Ion Trap Mass Spectrometer (ITS).

One of the advantages of the ITS is that structural information on the ions can be obtained by studying the fragmentation induced by ion collisions. By using and tailoring the ion chemistry it will be possible to enhance the operation of IMS.

Research, which has received an EPSRC grant of more than £500,000, will begin in March, and the project is scheduled to run for four years.