Peak tailing and mass transfer kinetics in linear chromatography

Abstract

The origin of peak tailing under linear conditions, at very low analyte concentrations, is investigated using the equations of the transport-dispersive model of chromatography. It is shown that a general explanation can be obtained by assuming the existence of two different types of adsorption sites having different equilibrium isotherms and different rates of mass transfer kinetics. Even if the experimental conditions are such that both mechanisms operate linearly, tailing can be observed if the mass transfer kinetics is much slower on one type of sites than on the other. The most pronounced and typical peak tailings occur when the slow type of adsorption sites give a smaller contribution to the retention than the fast type and if the rate constant of mass transfer for the slow sites is between 20 and 2000 times smaller than that of the fast sites. Axial dispersion (caused by molecular and eddy diffusion) dampens the effects. Therefore, peak tailing of kinetic origin will be more important on highly efficient columns than on mediocre ones. An important case in point is that of enantiomeric separations. Chiral phases are designed as two-types-of-sites phases since it is impossible to eliminate the general non-selective interactions between analytes and bonded groups. The chiral selective sites involve strong interactions between the stationary phase and at least one of the enantiomers, conditions which slow down mass transfers.