Mass transfer effects in isocratic non-linear elution chromatography

Abstract

In large-scale liquid chromatography, solute competition for sorbent sites (interference) coupled with mass transfer effects can result in complex solute distribution patterns and concentration dependent migration of solute bands. A rate equation model, which accounts for axial dispersion, film mass transfer, intraparticle diffusion, size exclusion and non-linear isotherms, was used to simulate the effluent histories of multicomponent ion-exchange systems. The method of orthogonal collocation on gradient-directed moving finite elements developed by Yu and Wang in 1989 was used to solve the model equations. The theory was verified by comparing the model predictions with data on amino acid and protein ion exchange and good agreement was obtained. Dimensionless groups were used to identify the local equilibrium regime, in which the effects of axial dispersion, film mass transfer and intraparticle diffusion are negligible. Mass transfer effects were pronounced for systems with small pulse sizes and for solutes with long retention times. For a system with a pulse size about 20 pore volumes C / C i = 10 3, and α (separation factor of a solute against eluent) = 1.6, mass transfer effects were negligible at N p10 3. For low affinity solutes (α < 0.1) of a similar pulse size, local equilibrium was approached at a lower N p. The dimensionless groups were also useful for studying the combined effects of designed and operating parameters. The effects of these parameters on band shape and band spreading were examined for non-linear systems with various solute affinities. The dimensionless groups were also used to establish simple scaling rules for non-linear chromatography. The scaling rules suggest that one can achieve the same degree of separation but significantly higher throughputs by using small particles, short columns and rapid cycles.