Separating dissolved substances

What is the connection between finding different colours in ink and identifying people? They both use a similar way to separate substances.

Have you put a spot of ink in the centre of a piece of filter paper then dropped water on the spot? If you have, you will have seen the water slowly spread out from the sides of the spot. As the water moved out across the filter paper it carried away the different substances in the ink. The substances in the ink vary in two ways. They vary in their solubility. This means that some are very soluble in the water and others are less soluble. They also vary in the way they tend to stick to the paper. Some substances readily grip the paper while others have a lower tendency to stick.

When you put water drops on an ink spot the substances dissolve in it, and move with the water across the filter paper. The filter paper has holes called pores in it and we call it a porous material. The water flows from one pore to another and spreads out from the spot in all directions.

As the water moves outwards the substance which is least soluble and has the greatest tendency to stick to paper stops moving. It comes out of the solution, holds onto the paper fibres and forms a coloured ring round the spot. A little later, when the water has moved further out, a substance, which is a little more soluble than the first and with a lower tendency to stick to the paper, comes out of the solution. It settles on the paper fibres and forms a coloured ring. Finally, when the water has moved almost all the way across the filter paper, the last substance in the ink forms a coloured ring. It is the most soluble and has the least tendency to stick so it has travelled furthest from the ink. This method of separating substances is called chromatography. People are often amazed when they test different inks. They find that each ink is a mixture of coloured dyes and the colours of some of the dyes are completely different from the colour of the ink.

Every person is made of cells yet every person is different. The reason for this is a substance in every cell. It is called DNA. This substance is like the instructions to assemble a model or a toy. It tells the cell what to do and also tells the body how to develop. The DNA in one person is different from that in all other people so the instructions in the cells of each person are different and each body develops slightly or greatly different from every other one.

When scientists discovered that everyone had different DNA they wondered if there was simple way to show it. In time they worked out a process which is now widely used today. In this process a few cells are collected from a person and the DNA is removed from them and dissolved. A spot of the dissolved DNA is then put on a porous substance just like ink is put on filter paper. A liquid is then allowed to pass through the spot and different parts of the DNA are carried along and deposited just like the different colours in an ink. The separation of the parts of DNA is a little more complicated than the separation of dyes in ink. The main difference is that a current of electricity is used to move the pieces of DNA along and they form lines in the porous material like a bar code on a cereal packet. When you look at the DNA from different people you can see that each one has its own unique patterns of lines.

Does filter paper need to be in a circle for chromatography to work?

No. You can cut filter paper into strips. Take a strip, put a spot of coloured ink near one end and then carefully dip that end in water. You must make sure not to immerse the spot in the water. The water must be given a chance to seep up through the paper, dissolve the substances in the ink and carry them up the paper. Bands of colour will form above the spot as the different dyes come out of the solution.

Does chromatography involve evaporation?

No, it does not. Some substances are separated by evaporation but in chromatography the water just carries the dissolved substances along. They separate out due to their own properties of solubility and a tendency to stick to the paper.

Can chromatography be used with substances that are not coloured?

Yes, it can. In fact chromatography is used with substances that are not coloured and with other solvents besides water. These solvents are made from oil and are similar to the solvents used in nail varnish. A spot of a colourless substance is placed on chromatography paper, which is similar to filter paper, and the solvent is allowed to pass through it. When the solvent reaches the end of the paper, the paper is sprayed with a liquid called a locating agent. If the colourless substance is a mixture of colourless substances each one will show up as a coloured patch on the paper.

Does chromatography have any uses?

Yes, it has. Chemists use it to test substances to see if they are pure. If a substance is pure only one patch should be found when a locating agent is used. If a substance is impure, two or more patches may be found. Doctors also use chromatography to examine the sugar content of blood. Sugar is carried around in the blood naturally to provide energy to all parts of the body. In a disease called diabetes, large amounts of sugar are carried around in the blood. By using chromatography the doctor can tell if a person may be suffering from diabetes.

Is DNA a coloured substance?

No. It is colourless. When the different parts of the DNA have been separated a locating agent is used to show where they are. The locating agent makes them appear as black bands like those on a bar code.

How do scientists obtain DNA from people?

They can get DNA from blood cells, skin cells or the cells at the root of the hair.

Is identifying people by their DNA useful?
Yes, it is. Each pattern of lines is unique to each person just like the pattern of lines on their fingerprints. Forensic scientists use DNA tests on suspects of a crime. If their DNA matches the DNA of cells found at the scene of the crime this could be evidence to show that they are guilty.