Separation is achieved by elution of the mobile phase through the stationary phase. What does this mean?...

Transport...

The mobile phase transports a sample through the stationary phase. Mobile phases come in a variety of forms and are what usually define the different types of chromatography. For example, gas chromatography uses an inert gas to carry a sample through a column that contains the stationary phase. The mobile phase is chosen so as not to react with either the sample nor the stationary phase.

Interaction...

The process of continually adding mobile phase to force the sample through the stationary phase is called elution. The stationary phase is solid or crystalline in form and it is specifically chosen for the way in which it interacts with both the mobile phase and the sample. The sample exists in an equilibrium with the mobile and stationary phases, continuously passing back and forth between the two. Each component of the sample mixture has a unique interaction with the stationary phase. This mass-transfer action can be accounted for by a number of different interactions, from basic phsyical size differences between the particles in the sample components and the stationary phase to ionic affinity between the stationary phase and the sample components. Overall, there is a characteristic affinity of each sample component for the stationary phase, and those components that have a stronger affinity for the stationary phase will remain with it longer and will not be found in the mobile phase as often. Because elution only can occur when the substances are travelling in the mobile phase, mixture components will be eluted at varying times. Thus the mixture is separated by the unique interaction of its components with the stationary phase.

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The above Quicktime Movie gives a more abstract, yet visual depiction of the equilibrium that we are describing. The white and black balls represent the stationary and mobile phases, while the green balls represent the analyte that moves between the two phases.

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