Delayed adsorption on slowly accessible stationary phase surfaces in porous liquid chromatographic column packings

Abstract

Variation in the fractional area of the tailing portion of elution peak in HPLC was analysed over a wide range of mobile phase flow-rates. A slow adsorption-desorption process was found to contribute to solute retention only in the range of column hold-up times by at least two orders of magnitude longer than those usually used in analytical or preparative separation. The apparent first-order rate constant was evaluated to be of the order of 10 3–10 4 s −1 fpr the slow adsorption and the dependence of the rate constant on column temperature and mobile phase composition was analysed. The behaviour of the apparent rate constant of the slow adsorption could be explained as being due to slow diffusion by which solute molecules are transferred to the stationary phase surface. The stationary phase surface causing the slow adsorption may be considered thermodynamically equivalent with the rest of the stationary phase surface contained in the column and to which low-molecular-mass samples have geometrical access requiring considerable time owing to slow pore diffusion.