relative solubilities in the stationary and mobile phases. Partition isotherms usually have a longer linear
range than adsorption isotherms, so tailing or fronting of elution peaks is not a particular problem,
except at high concentrations.
Stationary and Mobile Phases
There is a very wide choice of pairs of liquids to act as stationary and mobile phases. It is not necessary
for them to be totally immiscible, but a low mutual solubility is desirable. A hydrophilic liquid may be
used as the stationary phase with a hydrophobic mobile phase or vice versa. The latter situation is
sometimes referred to as a 'reversed phase' system as it was developed later. Water, aqueous buffers and
alcohols are suitable mobile phases for the separation of very polar mixtures, whilst hydrocarbons in
combination with ethers, esters and chlorinated solvents would be chosen for less polar materials.
In principle the wide range of stationary phases used in GLC can be used in liquid–liquid partition
systems, but problems can arise from mutual solubility effects which may result in the stationary phase
being stripped from the column.
Characterization of Solutes
As already described, the rate of movement of a solute is determined by its distribution ratio defined as
The larger the value of D, the slower will be the progress of the solute through the system, and the
components of a mixture will therefore reach the end of a column or the edge of a surface in order of
increasing value of D. In column methods, a solute is characterized by the volume of mobile phase
required to move it from one end of the column to the other. Known as the retention volume, VR, it is
defined as the volume passing through the column between putting the sample on the top of the column
and the emergence of the solute peak at the bottom. It is given by the equation
where VM is the volume of mobile phase in the column (the dead or void volume) and k is the retention
factor which is directly proportional to D but takes account of the volume of each phase. Sometimes k
is used to characterize a solute rather than VR.
If k = 0, then VR = VM and the solute is eluted without being retarded or retained by the stationary phase.
Large values of k, which reflect large values of D, result in very large retention volumes and hence long
retention times. At a constant rate of flow of mobile phase F, VR is related to the retention time, tR, by
the equation