Chromatography is the name given to a series of separation techniques that use two states of matter, one stationary solid and the other a mobile liquid of gas, to separate the components of a liquid or gaseous mixture. Chromatography is very good at separating organic compounds such as dyes, mixtures of drugs and poisons. Separation occurs according to the difference in affinity of the components in the mixture for the stationary and mobile states.

There are three main types of chromatography:

• Paper chromatography
  
• Liquid - column chromatography
  
• Gas chromatography

Paper chromatography
The simplest chromatography to do is paper chromatography. Paper chromatography uses absorbent paper, such as blotting paper, filter paper or even paper towel, as the solid stationary state and water, or another liquid solvent, as the mobile state.

Experiment: Separation of ink dyes from a black marker pen.

Many water soluble marker pens use two or three different dyes to produce their black color. Make sure the pen you use is of the water soluble type and try the following:

1. Cut the paper into a strip some 10 or so cm long and 3 - 4 cm wide.
   
2. Mark the paper with a spot from the black pen at about 3 cm from the bottom of the paper. (You may darken the spot by marking the same spot a number of times with your pen.)
   
3. Hang the paper over the edge of a jar or tall container with the spot about 3 cm from the bottom of the jar.
   
4. Remove the paper and place about 2 cm of water in the jar. (The liquid level must be less than the level of the spot.)
   
5. Carefully lower the paper into the jar, making sure the end of the paper is in the water, but the spot is not!
   
6. Allow to stand quietly and watch as the water soaks up the paper carrying the dye colors with it.
   
7. Remove the paper and allow to dry. See how the different colors are now separated on the paper.

You can repeat this experiment with different colors and pens. Some will be single colors and others multiple colors. You can also try methylated spirits or mineral turpentine as solvents.
Hint: If using solvents other than water, place a lid over the jar and keep it in a well ventilated area, like a fume hood or open window. These solvents are toxic and flammable and must be handled with care.

Liquid-column chromatography and gas chromatography

Paper chromatography is suitable for simple separations of colored dyes, but is not much good for separating complex mixtures such as urine drug test samples. For these more complex separations liquid-column chromatography and gas chromatography are preferred.

With liquid-column chromatography a glass column is lightly packed with a stationary solid, such as aluminium oxide powder, and the mixture is poured into the top of the column with the mobile carrier solvent. Each component then passes through the chromatography column at a different rate and collected in a flask.

Gas chromatography is used to detect minute quantities of substances in a mixture. It operates in much the same way as the liquid-column chromatography, just that a gas is used as the carrier instead of a liquid and the mixture to be separated is first vaporised before injecting into the gas chromatography column.