TRACE vs VAPOUR

TRACE vs VAPOUR

In the rapidly changing world of aviation security, a key element of the screening process has been steadily developing: explosive and narcotics detection. Eugene Gerstein offers an in-depth analysis of the differences between trace chemical detection and vapour detection, looking at their various pros and cons, applications in different environments, as well as the methods of equipment utilisation and new technological developments.

On 10 October 1933, a United Airlines flight from Newark to Oakland crashed as a result of a deliberate on-board explosion, marking the beginning of an era in which the (sometimes successful) attempts to bring aircraft down with explosives slowly picked up their pace. Whilst that first act of deliberate sabotage in commercial aviation is a lesser known case, we all know about the Lockerbie disaster, the failed Bojinka plot, Richard Reid – the infamous ‘shoe bomber’ – and about Umar Farouk Abdulmutallab, better known as the ‘Underpants Bomber’. Some of us also remember the story of Anne-Marie Murphy, duped by Nezar al-Hindawi to carry explosives on board an El Al flight from Heathrow.

“…ICAO is clear on ETD systems not being the defining factor in a decision regarding clearing baggage …”

All of the above stories, and many more, gave rise to new technologies and, later, a bit of a competitive race between different manufacturers. Different sample acquisition methods and thought processes led to the emergence of different approaches, as well as technologies. Whilst this article will touch upon what is commercially available, the key point is to discuss trace and vapour as the most widespread sample acquisition methods.

I have to admit to having had some personal bias towards vapour detection in the past, as this method of sample acquisition provides limitless possibilities not offered by proper trace chemical detection (TCD) systems. It allows, for example, the screening of the entire cargo hold of an aircraft in a matter of minutes by using its purge valve, thus speeding up passenger and cargo processing times, etc.

At the same time, traditional swabbing has the potential to uncover minute amounts of explosives, which end up on the surfaces of objects due to contamination during the process of packaging the explosives. Newer technologies, which use air sampling to achieve the same results as swabbing, have also attracted a great deal of interest within the security industry.

Before we get too deeply into the technical aspects, let’s have a look at the regulatory side:

In the Aviation Security Manual, 8973, in its various versions, the International Civil Aviation Organization (ICAO) is clear on explosive trace detection (ETD) systems not being the defining factor in a decision regarding clearing baggage, but rather being treated as an integral part of a larger system. It does go further to mention ETD’s significant contribution, and surprisingly goes into a bit of detail on the analysis mechanism, specifically mentioning ion mobility spectrometry (which we will discuss later in the article) and saying that ‘Mass spectrometry has the potential for greater sensitivity and specificity than other techniques.’ Whilst it may be difficult to agree given the emergence of various new methods, this nevertheless provides the aviation security community with fairly clear guidance. Despite the facts mentioned above, the Aviation Security Manual does not offer the same level of detail regarding trace and vapour detection – it simply mentions both as available methods.

Methods of Detection

In order to properly understand the subject, let’s discuss what sampling vapours does, and what obtaining particles via surface wiping allows us to do. For the purpose of an exercise, I will focus on two of the most commonly known explosives – pentaerythritol tetranitrate, better known as PETN, and triacetone triperoxide (TATP, an abbreviation sometimes deciphered as tri-cyclic acetone).

PETN becomes a plastic explosive when mixed with the highly explosive TATP (even trace amounts of TATP can be dangerous if detonated), of the aforementioned Richard Reid’s fame – he attempted (quite unsuccessfully) to mix them in 2001 in order to try and bring down American Airlines flight 63 from Paris to Miami.

Semi-Automated Sample Collection – Vapour

Let’s begin by looking at vapour – TATP vaporises rapidly, and as the result produces a vapour signature, it can be easily and quickly detected with a ‘sniffer’, a handheld ETD device. The vapour is literally sucked into the device, coming into direct contact with the sensor inside. This allows for rapid screening of items, and for the detection of concealed explosives (especially very volatile ones). It is also quite effective in screening liquids in bottles and other containers.

A core chemical property of an explosive is its vapour pressure. Because there is considerable difference in vapour pressure between different explosives, this pressure can be useful in determining between types of explosives.

A drawback to vapour sampling is that it should be sampled from where it can reside in high concentration, meaning closed spaces such as the boot of a car, luggage, boxes, etc. When sampling air from such a closed space, the probability of detection vastly increases. Because vapour becomes more and more diluted the more it travels away from its source, and as it mixes with air and other gases (if present), the explosives concentration decreases, decreasing the probability of detection. This in turn brings in other factors, such as the environment, i.e. temperature, precipitation, movement of air, etc. Incidentally, higher temperatures tend to be more beneficial, as they serve to produce higher amounts of vapour.

There are then clear advantages to using this method to analyse substances in closed spaces, or closed items. One major advantage is the fact that no contact with the object of interest is required, which means decreased sample contamination and subsequently better performance.

Manual Sample Collection – Swabbing

Trace particle detection is often associated with surface wiping. Signatures of trace amounts of explosives can be found by either collecting particles of the explosive (which may be located on the surface of a bag, for example) or by collecting and then sampling vapour generated by an explosive.

A surface can be contaminated due to direct contact with explosives or by an individual having handled explosives and subsequently handled the surface in question. The above scenarios are known as a primary transfer and a secondary transfer, with the contaminated hand being a case of primary transfer and the bag handle being a case of secondary transfer. PETN is a good example of a type of explosive readily discovered by surface wiping (swabbing). However, if there is any sort of white powder, which may be TATP, it should not be touched, and/or sampled directly. If there is no option for using a vapour detection device, it should be left to be handled by bomb disposal professionals.

Swabbing is beneficial for the detection of secondary transfers and is not affected by drawbacks such as the environment.

Its biggest detriment though is a very well-known anecdote in the industry: upwards of 50% of men do not wash their hands after using the restroom, resulting in false positive detection of trace levels of urea nitrate explosives. The same is known to happen with trace amounts of fertilisers and even heart medicine (which contains nitro-glycerine).

Automated Sample Collection

I’ve recently discovered a very unique ‘add-on’ technology – an interesting Israeli company, TraceTech, has developed a unique, patented technology that can be used with virtually any commercially available ‘sniffer’ style ETD. The process involves manipulating the air around and inside screened items in order to extract particles and/or vapours, which may have accumulated in and on the item. This decreases the time spent screening each item to around 15 seconds (as opposed to the usual trace detection process, which leads to increased screening times) and decreases per passenger workloads for frontline personnel.

Trace vs. Vapour

Some schools of thought suggest that surface wiping is more effective than vapour sampling for certain types of explosives, and vice versa, however all concur on the fact that there is no ‘best’ sampling technique – everything depends on the operational requirements, environmental conditions, etc.

The difference in the aforementioned schools of thought led to the proliferation of dual technology devices, so a lot of it is really interchangeable today. This is largely because, firstly, many devices offer both technologies for dual use (in ‘either/or’ and ‘both’ configurations), and secondly, several devices employ vapour collection methods, however the ultimate analysis is that of a trace detector.

Both trace and vapour may have stopped Richard Reid, the ‘Shoe Bomber’, and Umar Farouk Abdulmutallab, the ‘Underpants Bomber’, however both PETN and TATP tend to have higher chances of being discovered by swabbing and vapour detection respectively, making dual technology devices the real winners in both cases.

Regardless of the sample acquisition method, the analysis of the sample can be done by different types of technology, as discussed below, as no methodology analysis can be completed without viewing the underlying mechanisms.

The Technology Behind the Scenes

The market is full of different machines for the detections of trace signatures of explosives. The best-known technology is ion mobility spectrometry (IMS). IMS is closely related to mass spectrometry (which is actually directly mentioned by ICAO in the Aviation Security Manual), where molecules are ionised and then transported through an electric field, which is placed in a vacuum. The key difference for the IMS is that it operates at atmospheric pressure. The time it takes an ion to cover a particular distance through an electric field whilst inside an IMS shows that ion’s ‘size-to-charge’ ration since larger ions hit more gas particles (at atmospheric pressure) and subsequently move at a slower speed.

There are also multiple technologies that are direct derivatives of IMS, such as ion trap mobility spectrometry (ITMS), as well as differential mobility spectrometry (DMS).

At the other end of the spectrum, there are the amplifying fluorescent polymers (AFP), which use molecular recognition to serve as a sort of light switch for the fluorescence of a polymer, which becomes blue under ultraviolet light but remains colourless when there is a chemical reaction with nitrogen-based components. There are also methodologies, which compare different light spectrum measurements on the surface of an object of interest.

This brings us to the once popular chemiluminescence – measuring light coming from excited particles – which is seen in products coming from such industry veterans as Scintrex.

Another analysis method which should be mentioned is atmospheric pressure electron ionisation in which molecules are charged selectively, and then analysed via ion mobility spectrometry. Essentially, this is a method that has the potential to improve the sampling abilities for IMS analysis.

Many devices also utilise a so-called gas chromatography method, in addition to ion mobility spectrometry or mass spectrometry, to allow the separation of molecules before they go through the detection process. This allows for a much higher performance, as well as adding additional metrics. For example, the time a molecule takes to pass through a gas chromatographer can be used to identify its type. One of the key issues with gas chromatography is that it is necessary to use bottled gas, which requires changing (quite often, I have to add), introducing additional maintenance costs and decreasing the system’s ease of use. When sitting idle, the gas evaporates (bleeding, to use a more professional term) – there are additional issues such as oxidation, chemical degradation, etc. Additionally, the gas component needs to work very quickly, so as not to slow down the speed of the analysis, because these devices are usually used in high traffic areas (screening points, etc.).

“…there is no ‘best’ sampling technique …”

Conclusion

We live in an exciting time, with technology advancing in leaps and bounds, and with malignant actors trying to stay ahead of the game, leaving those on the front lines scrambling to implement new methods. Because of this, and many other contributing factors, I’m keen to remind readers that no amount of technology will help if there is no proper process and procedure in place. Stay vigilant and stay safe.


Eugene Gerstein
Eugene Gerstein

Eugene Gerstein is the business development director for Westminster Aviation Security Services Ltd. With over 20 years of experience in 42 countries across the globe, he has worked on large international infrastructure projects, primarily in the airport and defence industries, as well as having spent years in AVSEC and ground operations. Prior to his commercial activities, Eugene was a military officer and served in law enforcement, involved in counter-narcotics and counter-terrorism task forces. He can be contacted at e.gerstein@wass-ltd.com.