Overview of Adsorption Rate Mechanisms

Key Insight

The rate at which proteins and other biopolymers adsorb onto chromatographic stationary phases is determined by specific physical phenomena. Engineering models are developed to represent these phenomena, enabling the interpretation of experimental measurements and the execution of design calculations. The primary focus is on mass transfer processes, as these typically control the overall rate of protein adsorption in most cases.

Protein molecules in the mobile phase encounter a series of resistances that collectively determine the time scale for adsorption and desorption. These resistances include external mass transfer in the fluid surrounding the particle, pore diffusion within the liquid-filled intra-particle pores, kinetic resistance to binding at the liquid-adsorbent interface, and solid or adsorbed phase diffusion. A simplified film model is often used to represent external mass transfer as diffusion across a stagnant film.

While mass transfer in the stationary phase is generally the controlling dispersion factor for protein chromatography columns using porous particles, exceptions exist. For instance, slow binding kinetics can limit the rate in some cases, such as immunoaffinity chromatography where an antibody binds to a surface-bound antigen. Additionally, transport resistance in the mobile phase becomes relevant when using pellicular particles or monoliths, or when mass transfer is enhanced by mechanisms other than pore diffusion.