Peak Asymmetry (Peak Skew)
Few chromatographic peaks are perfectly Gaussian (symmetrical), most exhibiting a degree of
asymmetry due to tailing or fronting (p. 82). It is common practice to quantify the degree of asymmetry,
or skewness, using one of a number of formulae that incorporate the front and rear half-widths of the
peak as defined in Figure 4.13. A perpendicular is drawn from the peak maximum to the baseline and
the horizontal distances from this perpendicular to the front and rear edges of the peak (A and B) are
measured at a specified height above the baseline, usually 5 or 10% of the peak height. The asymmetry
factor, As, is defined as B/A and the tailing factor, Af, as (A + B)/2A. For a Gaussian peak, A=B and both
As and Tf are exactly 1. Values of 0.9 (fronting) to 1.2 (tailing) are considered acceptable, whereas
outside this range an improvement in the chromatography should be sought. Comparisons should
always be based on the same formula, although differences between As and Tf for the same peak are
small when the values are close to 1. Some laboratories use As whilst others prefer Tf.
Figure 4.13
Determining peak-asymmetry and
peak-tailing factors. Peak asymmetry = B/A,
and peak tailing factor = (A + B)/2A.Kinetic Effects
The ultimate width of a peak is determined by the total amount of diffusion occurring during movement
of the solute through the system, and on the rate of mass transfer between the two phases. These effects
are shown diagrammatically in Figure 4.14. Both diffusion and mass transfer effects are inter-dependent
and complex, being made up of a number of contributions from different sources. Because they are
kinetic effects, their influence on efficiency is determined by the rate at which the mobile phase