Chromatography Detective Work

We had to do some detective work this week to find out who'd been threatening Miss Mackie's class fish!

ransom note!

A very suspicious-looking ransom note was found in the classroom. Since all the children were at lunch at the time the note was left, we thought that it was probably written by a teacher - but who?
What if the criminal had disguised their handwriting?

The note was written in black pen, so we searched the teachers' desks... they all had black pens!

The pens were different, but when we tried them out the writing all looked very similar. So we used a scientific technique called chromatography to analyse the ink in the note and try to find out which pen had written it.

Can you tell the difference?


Our experiment

Ransom note            Test samples               

1. We cut a strip off the left edge of the ransom note (with the O on it) and drew matching Os from all our four 'suspect' pens on another piece of filter paper, labelled with the suspect's initials.

In the chromatography tank

2. We pegged the piece of ransom note next to the test paper, above a tray of water so that the bottom edges of the papers were just touching the water. We waited as the water moved up the paper and watched what happened.

3. After a few minutes, we dried the papers off and compared the ink patterns. These four 'black' inks were actually very different! Now can you tell which pen wrote the note?


What is chromatography and how does it work?

Chromatography is a method used to separate mixtures.

Very few things consist of a single substance - most things are mixtures of different substances. For example, a perfume may contain dozens of different substances, each one of which can influence the overall smell of it.

Chromatography is used for all sorts of purposes - checking that our food and our medicines don't contain dangerous impurities, testing athletes to ensure they aren't taking any performance-enhancing drugs, and testing our water supply to check that it is not polluted. Police forensic scientists really do use chromatography to identify substances found at crime scenes.

The black ink is actually made up of different colours. These different colours are different substances.
The ink (this is called the sample) is put onto the paper (the stationary phase) and the water (the mobile phase) then moves up the paper.

Some of the substances in the ink stick strongly to the paper, and do not dissolve very well in the water. These colours will not move up the paper very quickly.

Other colours don't stick to the paper very well and are carried along quickly by the water. These move furthest up the strip. You can imagine a competition going on between the paper and the water for the different colours.

Certain colours, like permanent ink, (ink M in our test) will not move at all because they do not dissolve in water. If we want to get them to move we would have to use a liquid that the colour does dissolve in, like alcohol.


Chromatography with felt tip pens

We all had a go at doing some chromatography ourselves. Everyone chose some felt tip pens and put a few small dots of colour about 1cm from the bottom of a strip of filter paper.

We put the filter paper strips into the water and left them for about 5 minutes. Some of the felt tip inks separated into 3 or 4 different colours!

felt pen chromatography

Here are some of the chromatograms we made at Science Club.

felt pen chromatography


> Read more about chromatography


More science to try at home

If you want to do some chromatography at home and don't have filter paper, coffee filters are particularly good.

You can use any felt tip pens, darker colours are best to try as they are often made up of different lighter colours.