DELAYED ADSORPTION ON SLOWLY ACCESSIBLE STATIONARY-PHASE SURFACES IN POROUS LIQUID-CHROMATOGRAPHIC COLUMN PACKINGS

Variation in the fractional area of the tailing portion of elution pea k 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 or ders of magnitude longer than those usually used in analytical or prep arative separation. The apparent first-order rate constant was evaluat ed to be of the order of 10(3)-10(4) s(-1) for 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 con stant of the slow adsorption could be explained as being due to slow d iffusion by which solute molecules are transferred to the stationary p hase surface. The stationary phase surface causing the slow adsorption may be considered thermodynamically equivalent with the rest of the s tationary phase surface contained in the column and to which low-molec ular-mass samples have geometrical access requiring considerable time owing to slow pore diffusion.