mobile phase

Meaning: The mobile phase is the fluid that moves over or through the stationary phase, carrying the sample mixture with it. In thin-layer and column chromatography it is a liquid solvent (the eluent). In gas chromatography it is an inert carrier gas. Each compound partitions between the two phases. Compounds that spend more time in the mobile phase move faster, and compounds that stick to the stationary phase lag behind. These differences in speed separate the mixture.

Equation: Rf = dcompound / dsolvent front, where dcompound is the distance the spot traveled and dsolvent front is the distance the solvent traveled, both measured in meters from the origin line. Rf is dimensionless and falls between 0 and 1.

On the MCAT: A typical setup uses a polar stationary phase such as silica or alumina (normal phase). Nonpolar compounds usually travel farther and elute first, while polar compounds are retained longer. Making the mobile phase more polar raises the Rf of the compounds, especially polar ones. In reversed-phase chromatography the stationary phase is nonpolar, so the order flips. You may be asked to read a TLC plate or to predict which compound elutes first.

Common confusion: Students sometimes treat the mobile phase as the sample. It is the moving solvent or gas, and the sample is the analyte. Retention depends on each compound's relative affinity for the two phases, so you must compare the polarity of the compound, the stationary phase and the mobile phase together.