Explosives & Handheld Trace Detection

The threat of homemade explosives (HMEs) is not new. From the Oklahoma City bombing in 1995, to the “shoe bomber,” London underground bombings, “underwear bomber,” and attacks in Paris and Brussels in the 2000s, the threat is ever changing. Not only do post-incident crime scenes present danger to responders until secondary devices have been ruled out, but also makeshift laboratories where the bombs are made. Handheld explosives trace detection (ETD) equipment can help responders quickly determine on-scene threats, like Triacetone Triperoxide (TATP) and react appropriately and expediently.

TATP has been used in bombing and suicide attacks, including the 2016 Brussels and 2017 Manchester Arena bombings. It was also used in the explosion that preceded the 2017 terrorist attacks in Barcelona. Terrorists frequently use this chemical because it is relatively easy to make using household supplies. As such, TATP is often produced in makeshift laboratories found inside apartments, homes, or other residential structures. Evidence of TATP manufacture may include glassware such as beakers or flasks, mixers, filtration systems, and distillation equipment. TATP is often kept cold to increase its stability, so ice baths or refrigerators may indicate production. Its instability makes it very dangerous to responders investigating makeshift laboratories. Even trace level quantities can be dangerous if detonated.

TATP Chemical Relevance

Given the instability and volatility of TATP, it is important for responders to know what to look for and how to approach explosion sites or suspected makeshift laboratories where the chemical has been found or handled. TATP appears as a white crystalline powder with a bleach-like odor. Shock, static, sparks, heat, and friction can cause detonations. Large volumes of easily obtained chemicals such as acetone, sulfuric acid, or peroxide-based bleaching formulations can be indicators of TATP production (Figure 1):

• Acetone is found in nail polish remover or paint thinner.
• Sulfuric acid is present in car batteries and acidic drain cleaners.
• Cosmetic or wood bleaching solutions can be a source of concentrated hydrogen peroxide.

These ingredients are not just dangerous on their own, but gaseous byproducts produced by them can be toxic and explosive.

Using Explosive Trace Detection

In a suspected bomb-making environment, both surfaces and containers may be investigated using particle or vapor sampling procedures. Responders should always be aware of the possibility of booby traps or secondary devices, and the dangers of entering homemade laboratories. Numerous technologies can be used for detection, including colorimetric kits (using wet chemistry and color change indicators), Raman spectroscopy (confirmatory tool), ion mobility spectrometry (IMS), and chemiluminescence. The data provided in this article demonstrates TATP detection using a chemiluminescence-based ETD. Detection of trace explosive signatures results from either directly collecting trace quantities of explosive particles, or by sampling vapor that emanates from an explosive source. Particulates can be solid explosive residue or non-explosive particulates contaminated with explosives, with surface contamination occurring in two primary scenarios:

• When explosives are handled or moved, small particulates of explosives can become suspended in the air and settle onto surfaces.
• Surface contamination can also occur through direct contact with explosive-contaminated or contact with another explosive-contaminated surface. Primary transfer occurs when bulk explosive materials (a quantity that can easily be seen) come into direct contact with a surface, such as a person’s hand when handling explosives. Secondary transfer occurs when a surface that was contaminated by primary transfer comes into contact with a second surface. An example of secondary transfer would be a transfer of explosive materials to an identification card, door handle, cellphone, etc.

Vapor – TATP vaporizes very quickly, resulting in a vapor signature that can be readily detected with a handheld ETD. The vapor enters the sensor via direct vapor sampling (Figure 2). The direct vapor method is effective for screening bottled liquids and concealed, high-volatility explosives.

Vapor becomes more dilute as it travels further from the source and mixes with the surrounding air, resulting in lower concentrations of explosives. Vapors can accumulate to higher concentrations in confined spaces, such as a box, bag, or car trunk. When vapor from confined spaces can be directly sampled, the likelihood of detection increases. Advantages to vapor sampling are that it does not require contact with an object to collect a sample and the sample can be cleaner, which may improve sensor performance. Vapor is the suggested method of detection in cases where it is not possible to make contact with the object that needs to be sampled, or when explosives are suspected of being friction sensitive. Environmental factors, such as temperature and wind, affect vapor sampling more than swab sampling. Higher temperatures produce more vapor to be sampled. Concentrated “plumes” of vapor will only be present down current or downwind of the target, due to non-uniform mixing of the explosive molecules in the air. Particulate – Trace particulate residues can be collected from contaminated surfaces using a particulate swipe that is then inserted into the ETD (Figure 3). If a bulk quantity of white powder is suspected as being TATP, it should not be directly sampled. Responders should call the bomb squad and evacuate all personnel to a safe distance.

Particulate screening allows for detection of secondary transfer on personnel and vehicles. Screening is rarely impacted by environmental conditions. Because particulate screening requires contact with the sampled objects, it is not recommended in situations that pose an imminent threat to the screener. Fluorescence and chemiluminescence based ETDs are capable of detecting trace levels of TATP and hydrogen peroxide, materials that are often used in the manufacture of Homemade Explosive Devices. It can be detected in both particle and vapor modes. Detection of TATP is indicated by audible and onscreen alerts (Figure 4). These ETDs use an open sample flow path, which enables a faster clear-down than other methods, so subsequent screenings can be performed quicker.

Summary

TATP is a common threat used by terrorists, because it can be made from easily available household supplies and produced in makeshift laboratories. This unstable chemical is dangerous to first responders in bulk (visible) quantities. Responders should be aware of the unique signs to look for when TATP manufacturing is suspected and understand that trace (invisible) quantities of TATP can be detected by ETDs using particle (swipe) or vapor sampling methods.