Security questions

The last decade has seen a sharp rise in airport security to try and thwart those who attempt to smuggle explosives onto planes, either in luggage or about their person. Ideally, every piece of luggage and every passenger should be screened to ensure that they are clean, although not all airports are geared up to achieve this at present.

In some locations, individuals are spot checked by examining their carryon bags with special probes for detecting traces of explosives but this it too time consuming to apply to every passenger. The only practical way forward is the installation of automated equipment that will examine each person individually and some countries are well advanced in their implementation.

One of the main advances is the use of portal machines, also known as puffer machines, which can “sniff out” explosive vapours that are blown off the passenger by a puff of air. Detection systems have been based on gas chromatography, mass spectrometry or laser spectroscopy. The same systems can be set up in the luggage areas to scan every item that is destined for the aeroplane hold.

Of course, security is not restricted to airports. Fast reacting portals can be established at any venue where crowds congregate or pass through, such as railway stations and sporting venues like the Olympic Games.

In the early 21st century, a team of Japanese scientists published details of a trace explosives detector based on mass spectrometry with atmospheric pressure chemical ionisation (APCI). Their system involved wiping objects with a sheet which was inserted into a heating unit for desorption and analysis. So, although it could screen up to 200 people an hour, it was still too slow for large numbers of people.

Now, they have designed a walkthrough portal system, still based on APCI, which is faster and can approach the target throughput of 1200 people per hour. The system was described in Rapid Communications in Mass Spectrometry by Yasuaki Takada and colleagues from Hitachi, Ltd. and WDB Co., Ltd, Tokyo.

Wire in the trap for better sensitivity

The design was based on a push-pull air sampler which blew warm air across the person from a vertical slit providing a uniform air curtain. The air was trapped on the opposite side in an equal sized slit and drawn through a heated tube to the detector.

The APCI source was the same as that developed earlier, in which the sample vapours were introduced in a counterflow mode into the secondary ionisation region before encountering the corona discharge where ionisation occurred.

The ions were transmitted to a linear ion trap, which had been designed in-house and reported in 2009. It consisted of 4 parallel quadrupole rods and two wire electrodes positioned between the rods and before the extraction electrode.

The wires were at right angles to each other, one acting to confine ions within the trap and the other extracting excited ions in an axial direction. The measurement sequence comprised ion accumulation, cooling by collision with buffer gas, scanning and emptying.

The wire-linear ion trap was used to replace the quadrupole ion trap in a commercial mass spectrometer in an attempt to improve sensitivity. It was positioned as the detector in the walkthrough portal system and its performance was measured with an explosive compound with which most people will be familiar.

Tracing TATP

Triacetone triperoxide, also known as TATP, is a popular explosive with terrorists. It was used, luckily without success, by the shoebomber Richard Reid who had packed one of his shoes with 8-10 ounces of TATP and another explosive, pentaerythritol tetranitrate (PETN).

TATP has been dubbed Mother of Satan due to its inherent instability but its popularity is based on its transparency to many types of security scanners which are designed to detect nitrogen-containing explosives. So, PETN would be detected but TATP would not. This is not the case with mass spectrometry.

In tests with TATP on a swab placed at the centre of the sampling portal for 10 seconds, the characteristic peak at m/z 77 was monitored. The wire-linear ion trap gave a promising improvement in the signal-to-background ratio from 17 to 170 compared with the original quadrupole ion trap.

The portal was then tested in its intended role as a walkthrough explosives detector. One of the authors walked through carrying a cotton swab with 100 µg of TATP powder on it.

A signal at m/z 77 appeared within 2 seconds and the signal noise ratio was still good under real conditions, at a value of 47. Takada declared that this testing speed is sufficiently quick for high throughput screening operations at airports, railway stations, stadia and the like.

The researchers have taken the system into real settings at a railway station and an airport to determine the proportion of false positives that arise from fluctuations in the background signal and these results will be published in due course.

If it proves successful onsite, the walkthrough portal will contribute to increased security at locations to which the general public has access. It will help to prevent terrorist operations involving TATP and could easily be extended to the detection of traces of other explosives.

Related links

• Rapid Communications in Mass Spectrometry 2011, 25, 2448-2452: “High-throughput walkthrough detection portal for counter terrorism: detection of triacetone triperoxide (TATP) vapor by atmospheric-pressure chemical ionization ion trap mass spectrometry”