Stagnant mobile phase mass transfer in chromatographic media: Intraparticle diffusion and exchange kinetics

Pulsed field gradient nuclear magnetic resonance has been successfully appl ied to a direct and detailed experimental study of topological and dynamic aspects involved in the exchange of small, nonsorbed fluid molecules betwee n the intraparticle pore network and the interparticle void space in chroma tographic columns packed with spherical-shaped, porous particles. The appro ach provides quantitative data about the effective, intraparticle diffusion coefficients (and tortuosity factors) and about the associated, diffusion- limited mass transfer kinetics, including stagnant boundary layer contribut ions. In view of the recorded exchange kinetics, an analytical description for solute diffusion into/out of spherical particles is offered and address es the influence of the particle size distribution and particle shape on th e observed mass transfer rates and calculated diffusivities. The combined a nalyses of the steady-state intraparticle pore diffusion data and the assoc iated exchange kinetics with Peclet numbers up to 500 reveals the existence of external stagnant fluid where all the interparticle fluid-side resistan ce to diffusion is localized. It is represented by a thin stagnant boundary layer around the particles and can be accounted for by the introduction of a hydrodynamically effective particle diameter which is found to depend on the Peclet number. The approach appears to be promising for a selective, d etailed study of the boundary layer dynamics. Concerning the investigation of different chromatographic media and intraparticle morphologies, we demon strate that: the actual correlation (or randomness) of interconnection betw een intraparticle pores of different size has a profound effect on the obse rved tortuosity factors and the diffusion-limited stagnant mobile phase mas s transfer kinetics. Compared to intraparticle pore networks with a random assignment of different pore sizes, hierarchically structured bidisperse po rous particles offer a superior network topology, which can form the basis for an increased chromatographic performance.