You’re waiting in line at airport security, looking forward to a family vacation. Just as you load your carry-on items into a tray for the baggage scanner, things slow down. An airport security agent approaches the person in front of you and swabs their hands, shoes and bags. Then they insert the swab into a mysterious machine.
What just happened, and why? Well, this procedure checks if a passenger has come into contact with a bomb. The equipment at airport security uses ion mobility spectrometry to look for small amounts of explosive chemicals that may be left behind on their clothes and belongings. This method uses the size and shape of molecules to determine if they come from an explosive chemical.
There are many ways to detect explosives. But ion mobility spectrometry works particularly well at airports. It doesn’t require a laboratory and it’s fast--you get results in seconds! The readings are even automated, so staff don’t need to be experts in chemistry to understand the results.
Screening luggage and passengers for explosives involves three main steps:
Step one: Heating the sample
An airport security agent rubs a special swab over a passenger’s luggage, hands and clothing. The swab picks up particles that could contain small amounts of explosives.
The agent then inserts the swab into a machine called an ion mobility spectrometer, which heats the sample. Many explosive chemicals aren’t gases at room temperature, but heat brings them into the gas phase.
Step two: Forming ions
Before the ion mobility spectrometer can detect any explosive molecules, they need to be ionized. That means they need to be converted to a form with a charge.
To have a charge, a molecules must have either more or fewer electrons than protons. This happens in the machine’s ionization area. A small radioactive source, a high electrical voltage or ultraviolet light does the ionizing.
Explosive molecules tend to have a section that is very electronegative. In other words, they strongly attract electrons. So these molecules have more electrons than protons, forming negatively charged ions when they are ionized.
Did you know? Molecules used as explosives often contain multiple nitro groups (-NO2).
Step three: Separating the Ions
Once ions are formed, an electronic shutter directs them into a tube called the drift region. The time that it takes an ion to pass through the drift region is its drift time. Different substances have different drift times.
In the drift region, an electric field draws the ions toward a detector, and a flow of gas molecules (nitrogen or air) opposes the ions' path. When the ions hit the detector, it sends an electrical signal to a computer programmed with the drift times for explosive chemicals. If the computer detects a suspicious drift time, it alerts the security agent.
Diagram of the drift region of an ion mobility spectrometer.
Size and shape
An ion’s drift time depends on ion mobility. Ions that reach the detector faster have more ion mobility. Ions that gained more than one extra electron during ionization have higher charges and therefore more mobility. Ion mobility is lower for larger ions because they bump into gas molecules more often in the drift tube. These collisions slow down the ions.
Imagine you were trying to roll a ball through some pylons on a soccer field. You would probably hit more pylons if you were using an enormous beach ball than a golf ball, right? That’s kind of how ion mobility works.
The shape of an ion is also very important. Some shapes are more likely to collide with gas molecules than others. For example, look at the diagram below. The cube and the rectangular prism have the same volume, but different shapes. See how the blue slice slice of the rectangular prism is larger than the blue slice of the cube? This would make the rectangle more likely to hit things.
Now look at the explosive molecules in the image below. Do you see how their different sizes and shapes would help an ion mobility spectrometer identify them, based on their drift times?
Space-filling models of from left to right: TNT, HMX, and ethylene glycol dinitrate. HMX (middle) is the biggest and so would have the most collisions and longest drift time. Ethylene glycol dinitrate (right) is the smallest and would have the fewest collisions and shortest drift time. TNT (left) is in between. Grey atoms are carbon, white atoms are hydrogen, red atoms are oxygen, and blue atoms are nitrogen.
Prepared by the author using Chem 3D Pro 14.0.
Meanwhile, back at the airport...
Let’s get back to the airport. If the system detects explosive molecules on the passenger in front of you, it doesn’t necessarily mean they’re a terrorist. The system can sometimes return a false positive result. In other words, it can identify explosives by mistake. But just to be sure, security staff will have a much closer look at the passenger and their belongings.
Of course, if the system doesn’t detect any explosives, the passenger can continue on to their flight. Then it’s your turn!
Did you know? The famous explosive 2,4,6-trinitrotoluene (TNT) was first produced by the German chemist Joseph Wilbrand while he was trying to make dyes!