<P> Chromatography is a laboratory technique for the separation of a mixture . The mixture is dissolved in a fluid called the mobile phase, which carries it through a structure holding another material called the stationary phase . The various constituents of the mixture travel at different speeds, causing them to separate . The separation is based on differential partitioning between the mobile and stationary phases . Subtle differences in a compound's partition coefficient result in differential retention on the stationary phase and thus affect the separation . </P> <P> Chromatography may be preparative or analytical . The purpose of preparative chromatography is to separate the components of a mixture for later use, and is thus a form of purification . Analytical chromatography is done normally with smaller amounts of material and is for establishing the presence or measuring the relative proportions of analytes in a mixture . The two are not mutually exclusive . </P>

Why do the substances separate in a chromatogram
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