The science of airport bomb detection: chromatography

That differential attraction to the stuff surrounding you — that’s what drives chromatography. You walk through the aisles only rarely interacting with the goods on sale, while your shopping partner has much greater affinity for the shelves and stops frequently. By the time you’re at the exit they are still only halfway through the shop — you’ve separated!

That is what happens to molecules. The solvent flows over the matrix (in the shopping list case, the paper) carrying the analytes. The relative affinity of the analyte for the matrix compared with the solvent determines the separation.

If a compound is totally insoluble in the solvent, it stays fixed to the matrix (you may have seen this when spilling water on a shopping list written in pencil). If the analyte is very soluble, it may move as fast as the solvent.

The shopping list example is called planar chromatography. The running ink seems to defy gravity, moving up the paper due to the capillary effect. More common in high-performance chromatography, the matrix is a column with the solvent forced over it, by gravity or pumping.

Using a column makes it easier to change the ratio of solvents by using a pump that can mix multiple materials (usually a mixture of water and a soluble organic solvent such as acetonitrile).

In the case of a gradient separation, the analyte has much higher affinity for the matrix than for the initial solvent mixture. As the solvent mix is changed, the analyte dissolves in the solvent and is carried out of the column separated from materials that are soluble in different solvent ratios.

Sometimes it’s a gas, gas, gas
For gas chromatography, the set-up is a little different. The analytes are gases or volatile liquids (think petrochemicals, plant oils, chemical weapons). Such compounds are usually non-polar and hydrophobic – in other words, they don’t mix well with water.

The compounds are evaporated into an inert carrier gas (analogous to dissolving in a solvent). The carrier gas transports the compound over a hydrophobic matrix contained in a coiled column (often tens of meters long but only micrometers wide).

To improve separation, and allow analysis of materials with a higher boiling point (up to around 300C), the column is placed in an oven. Changing the temperature of the oven affects separation in a similar way to changing the mixture of solvents in liquid chromatography.

Quality control
When separating colored compounds it’s pretty obvious when the process has worked. But how do you know if you’ve separated two colorless compounds, or separated microscopic amounts of analyte?

There are several ways to detect the analytes depending on their chemical and/ or physical properties. Among the more common are:

  • ultraviolet or infrared (non-visible but optical wavelength) absorbance
  • non-visible fluorescence
  • conductivity or pH (how acidic the solution is)
  • collect samples and perform chemical tests
  • mass spectrometry.

Probably the most useful of these is mass spectrometry as it allows the analyst to work out exactly what compound they are seeing without needing prior knowledge of what was in the original analyte mixture.

An ever-developing world
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.

And even more exciting experiments are being done by chromatography instruments on board the Philae probe that detected organic chemicals on the comet 67P/Churyumov–Gerasimenko.

Martin Boland is Senior Lecturer of Medicinal and Pharmaceutical Chemistry at Charles Darwin University. This story is published courtesy of The Conversation (under Creative Commons-Attribution/No derivatives).