We have become familiar with the routine at airports. Your carry-on bags are passed through an X-ray machine, after which an officer will often swab your bag with a piece of fabric that is then placed inside a box-like instrument. Within a few seconds you get the all-clear signal and are free to head towards your gate. 

But what do these instruments actually do? When luggage is bombarded with X-rays, some of the rays pass through and some do not, depending on what they encounter. The more dense a material, the less transparent it is to X-rays. Different substances will have unique densities and the densities of various explosives have been determined. By comparing the densities detected to the known densities of a host of suspect substances, the machine can provide an early warning of potential danger.

The instrument that analyzes the swabs is an “ion mobility spectrometer.” When the swab is inserted, a gust of a carrier gas dislodges some of the molecules that have been collected from the luggage. These molecules are then subjected to bombardment by electrons, commonly from a Nickel-63 isotope source. The bombardment creates ions that are swept through a tube where they are subjected to an electric field resulting in a separation by mass, size and shape of the molecules. These ions are detected and compared with those produced by known samples.

Though ion mobility scanners can detect a wide range of molecules and elements, they are commonly tuned to focus on nitrogen, an element found in almost all explosives except triacetone triperoxide (TATP). Christened “Mother of Satan,” this notorious explosive has been used by terrorists around the world because it is powerful and relatively easy to make from readily available acetone, hydrogen peroxide and hydrochloric acid. No great expertise is required to cook up a batch but care is needed because the molecule is unstable and detonates easily. No doubt some would-be terrorists are no longer with us because their preparation exploded prematurely.

The infamous shoe bomber, Richard Reid, tried to ignite a TATP fuse hidden in his shoes with a match to trigger a larger explosion but luckily was subdued by other passengers. The 2005 London bombings were carried out by radical Islamic terrorists using 4.5 kilograms of homemade TATP explosives. They killed 52, injured around 700, and terrorized a nation. The reason that we are not allowed to carry liquids aboard airplanes is that in 2006 terrorists planned to synthesize TATP from liquid precursors aboard flights originating from London’s Heathrow airport. 

The secret to triacetone triperoxide’s instability lies in its molecular structure. The molecule has highly unstable connections between oxygen atoms and these bonds readily break. The decomposition of a single solid phase molecule of triacetone triperoxide yields four gas phase molecules, one of ozone and three of acetone. The sudden production of these gas phase molecules creates an enormous pressure resulting in the very rapid movement of the surrounding air. The impact of the quickly moving air molecules is what causes the blast damage. 

Detection of TATP is difficult but some technologies are emerging. Although TATP is a solid, it does exert vapour pressure, meaning that some molecules are given off as a gas. When these gaseous TATP molecules enter a sensor, they encounter a catalyst that converts TATP back into its constituent parts, acetone and hydrogen peroxide. The sensor also contains a dye that changes colour when oxidized by hydrogen peroxide. By detecting these colour changes, the highly sensitive portable scanner can detect fewer than two parts per billion of TATP. 

Of course an explosive in luggage can only be detected if the luggage is inspected. In the case of the Belgian airport bombing this past March, the explosive was set off in the pre-screening area. That’s why hand held detectors about the size of a thick pen have been developed for use by officers who patrol all areas. Much of the research on such detectors is classified to keep terrorists from knowing the conditions under which the presence of TATP can be spotted. Making explosives requires chemical knowledge and so does foiling terrorists.  

Joe Schwarcz is the director of McGill­ University­’s Office for Science and Society. Read his blog at www.mcgill.ca/oss.