 Paper chromatography is a method for analyzing complex liquid mixtures (like ink) by separating them into the chemicals from which they are made. Chromatography is often used by police crime laboratories to identify substances that might be in liquids like blood and urine. Here's how paper chromatography works. A sample of the liquid (in our example it's ink) is placed near the end of a strip of porous paper similar to the paper that coffee filters are made from. The paper strip is then suspended over water so that only the tip of the paper below the sample is immersed in the water. 
 
The water will begin to soak into the paper, moving upwards as it does. As it passes the sample spot, it will carry chemicals from the sample along with it.Here's the special part: the different chemicals in the sample are made from molecules that move at different speeds as they make their way through the paper. This may be because some are attracted to the paper molecules more strongly than others, or perhaps some are larger and find it more difficult to move through the paper. Because some molecules are slowed by the paper, while others aren't (as much), the different chemicals will spread out as they climb higher and higher. This means we can use this method to separate different chemicals from each other. If they are coloured differently (as in ink), you will be able to see all the different chemicals in the ink as different colours on the paper. The separated molecules can be further tested to help identify them. In our case, students were able to identify the pen being used by comparing the 'rainbow' of chemicals to other rainbows made by pens they had collected. 
 In our sample of black ink (it was black before the water was added), note that the colour black is really a mixture of several colours, including blue and violet. In order for paper chromatography to work, the ink or other sample must be soluble in water. If it isn't, you might try rubbing alcohol or some other solvent. Chromatography doesn't need paper to work. Chemicals can also be separated by using a thin layer of silica on glass ('thin film chromatography'), by using a gas ('gas chromatography'), or even by using another liquid ('liquid chromatography') |