In a chromatographic separation a mixture of substances is transported by a carrier, called mobile phase, along a not moving surface, called stationary phase. Chromatography is therefore a separation based on two mass separating agents: the mobile phase and the stationary phase. Between the two phases mass transfer processes take place, which lead to a different transport velocity along the surface of the stationary phase for the different components of a mixture. The components reach the end of the stationary phase at different times and can be detected and collected separately.