mobile phase
/ MOH-bul FAYZ /
Think of a river carrying leaves downstream. The water keeps moving and sweeps everything along, but a leaf that keeps snagging on the muddy bank drifts more slowly than one floating free in the current. In chromatography the moving water is the mobile phase — the flowing fluid that ferries the sample past the stationary phase.
Formally, the mobile phase is the liquid or gas that flows continuously through a chromatographic system, carrying sample components along. A component spends part of its time dissolved in or swept by the mobile phase (moving) and part stuck to the stationary phase (paused); only while in the mobile phase does it advance.
It matters because changing the mobile phase changes how eagerly each component lets go of the stationary phase, and so reshapes the entire separation — chemists tune its strength, and sometimes vary it during a run, to coax overlapping peaks apart. The honest caveat is that a mobile phase too strong washes everything through together with no separation, while one too weak leaves slow components stuck for an impractically long time.
In liquid chromatography a chemist might use a blend of water and methanol as the mobile phase, gradually raising the methanol fraction during the run so that even the stickiest compounds are eventually flushed off the column.
The flowing fluid; a component moves only while riding it.
In gas chromatography the mobile phase is an inert carrier gas such as helium or nitrogen and is chemically passive; in liquid chromatography the solvent's chemistry actively shapes the separation.