The following notes detail the key underlying concepts and applications of the chromatographic techniques detailed in the VCAA Chemistry course:

Chromatography is, first and foremost, a separation technique. It separates mixtures of (usually) organic molecules on the basis of their polarity. The technique can be measured according to the distance the molecules to be separated adsorb onto a stationary phase in comparison to the movement of a mobile phase in the case of paper and thin layer chromatography (the Rf factor) or the time taken for the molecules to pass along a column (for GSC, GLC and HPLC) techniques – the retention time, Rt. In these cases, the separation of the molecules in question will depend on the extent to which they preferentially dissolve in either the polar stationary phase or the nonpolar mobile phase.

The key points to recognise with respect to paper chromatography are that the paper is the stationary phase and, being made from cellulose polymers (which in turn are made up of polar glucose monomers) is highly polar. The mobile phase is the solvent into which the paper is dipped: often water or a water/ethanol mixture. If the molecules to be separated are highly polar themselves they will adsorb more strongly onto the paper and so will move only a relatively short distance. In this scenario, the Rf factor is small. If the molecules are less polar, they will be less likely to adsorb (or ‘stick’) onto the paper and so will be carried a greater distance. In general, the presence of polar groups such as the hydroxyl (-OH) and amine (-NH2) AND/OR a larger molecular weight (and so more dispersion forces) increases the polarity of a molecule.

The same fundamental rules apply to column chromatography in the applications of GSC (Gas Solid Chromatography), GLC (Gas Liquid) and HPLC (High Performance or Pressure Liquid Chromatography). The stationary phase in these instruments is a highly polar solid with a high surface areas such as alumina (GSC) or a similar solid or inert material coated with a high molecular weight ester liquid (GLC and HPLC). The mobile phase will be an inert, nonpolar gas such as nitrogen or argon (GSC and GLC) or a nonpolar liquid (HPLC). The molecules to be separated will either dissolve more readily into the polar coating on the stationary phase (or the solid itself) if thy are polar OR will dissolve more readily into the mobile nonpolar phase (either a gas or a liquid).

In summary, polar molecules dissolve into the polar stationary phase and so have longer retention times. Nonpolar molecules dissolve more readily into the nonpolar mobile phase and so have short retention times, as they are ‘swept’ along the column more readily.