Column chromatography follows the same principles as TLC, with the following differences:
• The stationary phase is contained inside a column, rather than applied as a coating on a plate. The column is commonly made of glass, but some are made of metal or other materials.
• The sample to be separated is loaded from the top. The eluting solvent, or mobile phase, is also added from the top.
• The solvent flows down the column by gravity, carrying with it the components of the sample. By the same principles that apply in TLC, the components travel at different rates effecting the separation.
Refer to figures 19.4 (p. 759) and 19.7 (p. 765) for diagrams of typical macroscale and microscale chromatographic columns. The macroscale column can be fitted with a valve to control the flow of solvent. Unfortunately, the microscale column is simply a Pasteur pipette that cannot be fitted with a convenient device to control flow. A small stopper or Teflon cap liner can be used to block the tip to interrupt solvent flow as needed.
It can be seen from fig. 19.4 that as in TLC, the less polar components travel down the column faster than the more polar ones. Once the less polar components are out the column, the polarity of the eluting solvent can be increased to speed up the rate of travel of the more polar components.