axis. In such cases, the concentration profiles will be asymmetrical or skewed, an appearance described
as tailing (Figure 4.11(b)) or fronting (Figure 4.11(c)). Both these effects are undesirable as they lead to
poor separations and unreliable quantitative data. Fronting, which produces peaks with sloping front
and sharp rear boundaries is more likely to occur in systems where partition forms the basis of the
separation process, and where the solute has a small distribution ratio. Tailing produces peaks with
sharp leading edges and long sloping rear boundaries. It is particularly likely to occur where adsorption
is involved in the separation process. The diagrams show that both effects become more pronounced at
high concentrations and are therefore symptomatic of overloading the column or surface with sample.
Restricting the sample size so as to operate over the linear region of the sorption isotherm is a
recognized means of preventing fronting and tailing.
Figure 4.11
Sorption isotherms and concentration profiles.
(a) Linear isotherm; Gaussian profile. (b) Curved isotherm; 'tailing'.
(c) Curved isotherm; 'fronting'.Adsorption Systems
Separations in which surface adsorption is the predominant sorption process depend upon polarity
differences between solute molecules. Those which are highly symmetrical or consist of atoms with
similar electro-negativities are relatively non-polar, e.g. C 5 H 12 , C 6 H 6 , CCl 4. The presence of functional
groups leads to an increase in polarity, thus, C 5 H 11 OH is more polar than C 5 H 12 , C 6 H 5 NO 2 is more polar
than C 6 H 6 , and CHCl 3 is more polar than CCl 4. The more polar the molecule, the more tenaciously it
will be adsorbed by a polar surface. The approximate order of increasing strength of adsorption is:
alkanes < alkenes < ethers < esters < ketones and