Airport Explosive Detectors: Technology Behind Secure Skies

Airport explosive detectors are a crucial aspect of airport security, employing various technologies to ensure the safety of passengers and staff. The detection of explosives in airports is a complex process that has evolved over the years, with new technologies being continuously developed to stay ahead of emerging threats. From ion mobility spectrometry (IMS) to advanced imaging technologies and sniffer dogs, the methods used to detect explosives in airports vary, each with its own advantages and limitations. This introduction will explore the different techniques used, their effectiveness, and the challenges faced in ensuring accurate and efficient explosive detection in airports worldwide.

Ion mobility spectrometry

The time it takes for an ion, in IMS, to move a specified distance in an electric field indicates the ion's size-to-charge ratio. Ions with a larger cross-section will collide with more gas at atmospheric pressure and will, therefore, be slower. This method is often used to analyse the swabs from airport pat-downs, where the swab is heated to evaporate the volatile compounds that make up explosives.

IMS was also used in "puffers" at standard checkpoints in several US airports in the mid-2000s. These "puffers" would waft air past stationary passengers and then use IMS to check for explosives. However, these devices were prone to breaking down and were eventually abandoned by 2010.

While IMS is a widely used method for explosive detection, it is not perfect. For example, it requires a vacuum pump and frequent maintenance. Additionally, IMS is just one of several technologies used to detect explosives. Other methods include colourimetric test kits, X-ray detection, and sniffer dogs.

Gas chromatography

In the context of airport security, GC is employed to separate and identify volatile compounds that may be explosive in nature. When a swab is taken from a suspicious bag or person, the swab is heated, causing the volatile compounds to evaporate. These compounds are then separated using GC. The process involves using an inert carrier gas to transport the compound over a hydrophobic matrix contained in a coiled column. The column is often tens of meters long but only micrometers wide.

To improve separation and enable the analysis of materials with higher boiling points, the column is placed in an oven. By adjusting the oven's temperature, the separation of compounds can be enhanced, similar to changing solvent mixtures in liquid chromatography. This process is particularly effective for analytes that are gases or volatile liquids, such as petrochemicals, plant oils, and chemical weapons. These compounds tend to be non-polar and hydrophobic, meaning they do not mix well with water.

The time it takes for a molecule to pass through the GC instrument can provide some initial indication of its identity. However, this information is not always conclusive, as different compounds may have similar retention times. To definitively identify the compounds, additional techniques such as mass spectrometry are employed.

GC plays a crucial role in airport explosive detection by enabling the separation and preliminary identification of volatile compounds. This separation is essential for subsequent analysis and detection of explosive materials, ensuring the safety of air travel.

X-ray detection

X-ray scanners are a common method used to detect explosives at airports. These scanners use electromagnetic waves to penetrate materials and create an image of the contents of a bag. The Transportation Security Administration (TSA) uses X-ray machines to scan carry-on items and checked luggage. The X-ray systems used by the TSA must meet the FDA requirements to limit the exposure of passengers and workers to radiation.

X-ray machines used in airports typically use a dual-energy X-ray system. This system has a single X-ray source that emits X-rays in the range of 140 to 160 kilovolt peaks (KVP). The KVP refers to the amount of penetration an X-ray can achieve. The higher the KVP, the further the X-ray can penetrate. In a dual-energy X-ray system, the X-rays pass through a detector, a filter, and then another detector. The detector picks up the X-rays after they have passed through the item being scanned. The detector then passes the X-rays to a filter that blocks out the lower-energy X-rays. The remaining high-energy X-rays then hit a second detector. A computer circuit compares the readings of the two detectors to better represent low-energy objects, such as organic materials.

Since different materials absorb X-rays at different levels, the image on the monitor allows the operator to see distinct items inside the bag. Items are typically colored on the display monitor based on the range of energy that passes through the object. All X-ray systems use shades of orange to represent organic materials, as most explosives fall under this category. Machine operators are trained to look for suspicious items, including components of an improvised explosive device (IED).

X-ray scanners are also used to scan passengers for weapons or explosives that may be hidden under their clothing. Backscatter passenger scanners use very low-energy X-rays that are reflected back to the machine itself. The amount of radiation received from a backscatter machine is very low, and there is little health risk associated with the scan.

Colorimetric test kits

The use of colorimetric test kits is advantageous because many existing techniques are time-consuming, presumptive, and require technical know-how as well as the use of specific instruments and chemicals. Colorimetric arrays can be fabricated on solid supports, and imaging/processing can be done with commercially available smart devices, allowing for the potential for mass production and deployment. They are also lightweight and portable, meaning they could replace bulky, conventional equipment.

However, one disadvantage of colorimetric tests is that some explosive compounds (such as acetone peroxide) do not contain nitrogen and are therefore harder to detect. In addition, the specificity of colorimetric tests is compromised because multiple analytes react positively. Multiple tests are therefore required to narrow down the possible substance in question.

An example of a colorimetric test kit is the Dry Explosive Test Kit by S2 Threat Detection Technologies, which is a company in Australia that specializes in colorimetric test strips that can detect solid or liquid explosives such as nitrates, chlorates, and peroxides.

Laser detection

Laser-based explosive detection methods are a fairly recent development, given that lasers themselves were only invented in 1958. The field of laser-based explosive detection is still a work in progress, but it has advanced significantly in recent years, and further progress is expected as more exotic laser sources become more common, less expensive, and more readily available.

Lasers offer multiple approaches to explosive detection that are not possible with other techniques. These approaches can be separated into two types: those based on the unique properties of lasers for long-distance propagation of intense energy, and those that are based on molecular and atomic spectroscopy, utilizing the high wavelength specificity that most lasers offer.

Laser-based spectroscopic techniques offer several unique advantages. They can deliver laser pulses at a desired location (using both nanosecond and femtosecond) and collect the spectroscopic signal with excellent detectors now available. Interrogation of suspicious compounds in traces on various backgrounds, either destructively or non-destructively, is made a lot easier by lasers as it is possible to collimate and also focus it at a precise spot from a safe distance.

Laser-based explosive detection methods are expected to dramatically improve the probability of detection and reduce false alarm rates. This suggests that they may evolve into a major new technology area in the next 1–3 years.

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