Peak tailing and mass transfer kinetics in linear chromatography - Dependence on the column length and the linear velocity of the mobile phase

Although peak tailing in analytical (i.e., linear) chromatography is often considered to be associated merely with slow mass transfer kinetics, we show that it results rather from the use of too short a column, at too high a mobile phase velocity, under such conditions that the mass transfer kinetics is insufficiently fast. The phenomenon is characterized by the dimensionless Stanton number, St=k(f)L/u (with k(f) the rate coefficient of mass transfer, L the column length, and u the linear velocity of the mobile phase) which relates the average residence time of the molecules in the stationary phase (1/k(f)) and the column hold-up time (t(o)=L/u). If there is only one retention mechanism (e.g., in adsorption chromatography, if the surface of the adsorbent phase is homogenous), there is a critical range of Stanton numbers around 1 (depending slightly on the retention factor). If St<1, a split peak effect is observed. For 1<St<10, the peak tails severely and the asymmetry factor, asf(10), is equal to or larger than 1.2. For St>1000, the peak profile is Gaussian and asf(10)=1.0. If there is a mixed mechanism (e.g., with a heterogenous adsorbent surface), the relationship between peak shape and Stanton numbers is more complex as it also depends on the relative contribution of both mechanisms to retention. The peak may tail severely for values of the smaller Stanton number between 10 and 100 and asf(10) may be large. Split peak effect may arise at Stanton numbers below 10. (C) 1999 Elsevier Science B.V. All rights reserved.