Chromatography separates the components of a mixture so they can be identified. All forms of chromatography have two phases:• a stationary phase, which does not move, e.g. a solid such as silica on a plate, or a liquid held on a solid support
• a mobile phase, which moves through or over the stationary phase, e.g. a liquid solvent or a gasComponents separate because each one interacts with the two phases to a different extent. A component that is held strongly by the stationary phase moves slowly; a component that spends more time in the mobile phase moves quickly. After a time, the components are at different positions (or leave at different times).OCR identifies two mechanisms:• adsorption: in thin-layer chromatography (TLC), components are adsorbed (held on the surface) by the solid stationary phase to different extents
• relative solubility: in gas chromatography (GC), components have different solubilities in the liquid stationary phaseWorked example: in TLC, compound P moves further up the plate than compound Q. Q is therefore more strongly adsorbed by the stationary phase than P.Chromatography separates the components, but on its own it cannot always identify them: two different compounds may behave in the same way. It is often combined with mass spectrometry (GC-MS) so that each separated component can be identified.
• a mobile phase, which moves through or over the stationary phase, e.g. a liquid solvent or a gasComponents separate because each one interacts with the two phases to a different extent. A component that is held strongly by the stationary phase moves slowly; a component that spends more time in the mobile phase moves quickly. After a time, the components are at different positions (or leave at different times).OCR identifies two mechanisms:• adsorption: in thin-layer chromatography (TLC), components are adsorbed (held on the surface) by the solid stationary phase to different extents
• relative solubility: in gas chromatography (GC), components have different solubilities in the liquid stationary phaseWorked example: in TLC, compound P moves further up the plate than compound Q. Q is therefore more strongly adsorbed by the stationary phase than P.Chromatography separates the components, but on its own it cannot always identify them: two different compounds may behave in the same way. It is often combined with mass spectrometry (GC-MS) so that each separated component can be identified.