The science of airport bomb detection: chromatography

DetectionThe science of airport bomb detection: chromatography

By Martin Boland

Published 12 December 2014

As the holidays draw near, many of us will hop on a plane to visit friends and family — or just get away from it all. Some will be subjected to a swab at the airport to test clothes and baggage for explosives. So how does this process work? The answer is chromatography — a branch of separation chemistry — along with mass spectrometry. Although instrumental chromatography is a mature technology (the first instruments were produced just after WWII), new applications frequently pop up. Some are a matter of scale. Pharmaceutical companies that produce monoclonal antibodies (often used in cancer treatments) make use of capture chromatography to purify their products. On an industrial scale these can be tens of centimeters in diameter and meters in length (typical lab scale systems are a few millimeters diameter and 5-30cm long). Other uses can either be in a specific new application, such as detecting cocaine on bank notes using the gas chromatography systems often seen at airports as bomb and drug detectors.

As the holidays draw near, many of us will hop on a plane to visit friends and family — or just get away from it all. Some will be subjected to a swab at the airport to test clothes and baggage for explosives. So how does this process work?
The answer is chromatography — a branch of separation chemistry — along with mass spectrometry (which I will address in a later article).
The word “chromatography” is roughly translated from Greek as “the science of colors.” The reason for the name becomes obvious when you realize that most people have accidentally performed a simple chromatography experiment.
If you’ve ever spilled water onto a hand-written shopping list, then held it up to let the water run-off, you’ve probably noticed the ink diffuses across the paper, and that the pen’s color is made up from several pigments (if you’ve not, you can do the experiment — try it with a couple of pens of different brands, but the same color). This separation is chromatography.
There are several different types of chromatographic separation. What they all have in common is that a mixture of materials that need to be separated (the analytes) is washed over a solid material (called the matrix), causing the analytes to separate.
That may sound like chromatography is just filtration, or separation by particle size. In some cases, that is almost exactly what happens (size exclusion chromatography is often referred to as gel filtration chromatography).
But most chromatography methods work by some other chemical effect than just the size of the materials being separated, including (but not limited to):

• normal-phase chromatography, such as ink on paper
• reverse-phase chromatography, often used in university lab experiments
• gas chromatography, seen in airport bomb detectors
• “capture” chromatography, used to purify drugs.

Each of these can be performed with one solvent, such as dropping water on your shopping list – known as isocratic (Greek for “equal power”) or with a changing mixture of solvents (known as a gradient).
So how does it work?
Technically speaking, it is the differential affinity of the analyte for the solvent and the solid matrix that drives chromatographic separation. So what does that mean, really?
You’ll need to bear with me here.
Have you ever been shopping with someone who stops to look at things while you’re trying to move though the store as quickly as possible?