Analyte Adsorption and Distribution

During chromatographic separations, the analytes transfer between the mobile and stationary phases through sorption.

With the continuous addition of the mobile phase solvent or carrier gas, the analytes move along the stationary phase at different rates, depending on the affinities of the components for each phase.

For example, highly polar compounds are adsorbed strongly by polar solids like silica gel or alumina but are not strongly solvated by nonpolar solvents.

So, using a minimally polar solvent in the mobile phase usually gives the best separation conditions on a polar stationary phase.

The migration rate of each analyte depends on the fraction of time spent in the mobile phase, described by the distribution constant—an idealized equilibrium relationship expressed as the ratio of the stationary and mobile phase concentrations.

The distribution constant is larger for components that are strongly retained by the stationary phase and smaller for components that reside primarily in the mobile phase.