The science of airport bomb detection: mass spectrometry

Mass spectrometry will identify those explosive molecules. Marcia Cirillo/Flickr, CC BY-NC-ND

You’ve probably had your hand luggage swabbed after walking through the metal detector at the airport. Whatever molecules were picked up by the swab have been separated using gas chromatography. So how do airport staff know if those molecules are traces of explosives or simply a wayward dribble of tomato sauce?

Mass spectrometry has a wide range of applications in everyday life, and the most recognised is probably airport explosive detectors. In practice, these instruments are calibrated to detect a wide range of prohibited substances other than explosives.

The principle at work is, by swabbing a person’s clothes and possessions, bits of material that the possessions have been in contact with over the past few days will be transferred to the swab.

The swab is then put into the instrument and (usually) heated so the volatile compounds that make up most drugs and explosives will evaporate. These compounds are separated using a form of gas chromatography.

Although it is possible to get an idea of what compounds may be in a sample from the time it takes for them to pass though the chromatography instrument, that information is equivocal:

  • different compounds can have the same retention times
  • optical spectroscopy (the absorbance or fluorescence of molecules at wavelengths close to that of visible light) is not specific enough to determine a compound’s identity
  • other techniques such as nuclear magnetic resonance and chemical analysis require additional analysis.

This is where mass spectrometry comes in.

A common sight in airports. Steve Jurvetson/Flickr, CC BY

What is mass spectrometry?

The short version is that mass spectrometry is the world’s most accurate set of scales, potentially able to measure the mass of single molecules.

The mass of an atom (or molecule) is expressed as the relative atomic (or molecular) mass and is measured in Daltons (Da). One Da = 1.66×10−27 kg.

The principle of mass spectrometry is that it’s possible to measure some property of a molecule, and that property can be related to the molecule’s mass. The mass of a molecule is specific for the atoms that make it up, accurate to several decimal places. Therefore, by accurate measurement of the mass related property, you can infer what atoms are in the molecule.

In practice, it is easier to work with a property of an ion (charged particle) that is formed from the molecule. It doesn’t really matter if this is a positive or negative ion, although positive tends to be more common.

Modern analytical instruments add functions that help to fragment the molecules being weighed. If you first weigh the whole molecule, then weigh the fragments, it is possible to infer the connectivity of the fragments, and thus the structure of the starting molecule.

Talking in a vacuum

One thing to be aware of: all of the following processes are most easily applied in a controlled fashion when the ion is a gas, and that gas is at low pressure (ideally, a vacuum). Once an ion is formed, it can be manipulated by magnetic or electromagnetic fields, and its properties measured by similar methods.

By manipulating the strength of the magnetic fields, it is possible to select for ions that have a particular ratio of charge compared with their mass (a property called M/Z, pronounced “em over zed”).

There are a wide variety of ways to manipulate the ions to measure M/Z. Some of these are more suitable for particular types of ions:

  • some mass spectrometry systems are suited to metal ions (tens of Da)
  • others are better for working with small organic molecules and peptides (tens to a few hundreds of Da)
  • yet others can determine the composition of proteins (from around 1,000 – tens of thousands of Da).

Usually, for tests to give a positive result, the compound collected from the passenger will need to match several known characteristics of a controlled substance (gas chromatography retention time, molecular mass and several fragment masses are common). That multiple redundancy is designed to reduce the incidence of ‘false positive’ tests.

Ball and stick models of TNT (left) and nitroglycerin. Wikimedia Commons

The amount of material needed for detection (called the limit of detection, or LoD) is usually low nanogram (x 10-9) to picogram (x 10-12) quantities.

Beyond air travel, mass spectrometry is used to ‘sniff out’ molecules in space such as on the Philae probe and Martian Curiosity rover. It can even be used as a ‘laboratory interloper detector’.

Some mass spectrometry systems are open to the air and sample the atmosphere of the lab along with a target material – say, a particular perfume or aftershave.

When visitors would come to the lab, the instrument would show a signal at a certain number of Daltons for several hours afterwards – sometimes into the next day, long after the scent of the visitor’s perfume or aftershave was no longer discernible. (By the way, these open-source mass spectrometry systems are good evidence against the chemtrail conspiracy theory – any chemical being dispersed by aeroplanes would be detected just like perfume.)

So next time you get pulled aside to be swabbed for bombs or drugs, take heart that you’re being probed with the same technology that is used to look for traces of life on other planets!

See also: The science of airport bomb detection: chromatography