Instant Notes: Analytical Chemistry

Peak asymmetry The concentration profile of a migrating solute is fundamentally symmetrical
(Gaussian), only if the solute distribution ratio, D(defined by eqn. (1)), remains
constant over the concentration range of the entire peak, as shown by a linear
sorption isotherm, which is a plot of the solute concentration in the stationary
phase, CS, against that in the mobile phase, CM(Fig. 3(a)). However, curved
isotherms, resulting from changes in the solute distribution ratio at higher
concentration levels, lead to two types of peak asymmetry, or skew, described
as tailingand fronting.
Both tailing and fronting are undesirable, as closely eluting peaks will be less
well separated and retention data less reproducible. Where either occur,
reducing the amount of solute chromatographed will generally improve the
peak shape, but slow de-sorption may still cause some tailing.

126 Section D – Separation techniques

CS

CM

Linear

Gaussian

CS

CM

Convex

Tailing

CS

CM

Concave

Fronting

t tt

(a) (b) (c)

Fig. 3. Sorption isotherms and the resulting peak profiles. (a) Linear isotherm, symmetrical

peak. (b) Curvature towards the CMaxis, tailing peak. (c) Curvature towards the CSaxis,

fronting peak.

Two means of assessing the quality of a chromatographic separation are to

measure the extent of band broadening of individual solute peaks (efficiency)

and the degree of separation of adjacent peaks (resolution).

For column chromatography, a plate number, N(based on the theoretical

plate concept of distillation columns), is used as a measure of efficiency.

Assuming a Gaussian peak profile, Nis defined in terms of the solute retention

time, tR, and the peak width as given by the standard deviation, st(Fig. 4), i.e.

N

2

(8)

In practice, it is much easier to measure either the baseline width of a peak,

Wb, or the width at half height, Wh/2, and two alternative expressions derived

from equation (8) are

N (^16) 
2
(9)
and
tR


Wb
tR


st
Efficiency and
resolution