Which are the main liquid chromatography separation modes? / Liquid-Liquid Chromatography (LLC)

Liquid-liquid chromatography  (LLC) is another separation mode used by chromatographers. The separations in LLC are derived from the partitioning of analytes between two liquids, one of which is held immobile on a stationary solid support. In separations by liquid-liquid (or partition) chromatography the solute molecules are distributed between two immiscible liquid phases, the mobile phase and the stationary phase.

The stationary phase is a liquid film coated on a packing material consisting of 3-10 μm porous silica particles. The properties of a stationary phase are determined by the nature of the organosilane’s alkyl group. If R is a polar functional group, then the stationary phase will be polar. Examples of polar stationary phases include those for which R contains a cyano (-C2H4CN), diol or amino (-C3H6NH2) functional group.

To avoid phase miscibility, the two partitioning liquids must differ greatly in polarity. The stationary phase should be a good solvent for the sample but a poor solvent for the mobile phase. The combination of a polar stationary phase and a nonpolar mobile phase is called normal-phase chromatography.

Reverse phase LLC or HPLC employs a nonpolar stationary phase and a polar mobile phase. Therefore, the compounds which are nonpolar are retained more strongly than the ones which are polar. For example, hydrocarbons would be retained more strongly than alcohols.

Reverse –phase chromatography is the more commonly encountered separation mode used in HPLC. The most common nonpolar stationary phases use an organochlorosilane for which the R  group is an n-octyl (C8) or n-octyldecyl (C18) hydrocarbon chain. Most reverse-phase separations are carried out using buffered aqueous solutions as  a polar mobile phase.

In general, reverse phase LLC or HPLC is especially useful for very nonpolar compounds like high molecular weight hydrocarbons.
Another liquid chromatography separation mode is ion-exchange chromatography. For details please see the post "Ion-Exchange Chromatography (IEC) - Liquid Chromatography Separation Modes".