The paper gives a summary of successful methods for measuring protein mass transfer kinetics in ion exchange matrices along with models needed to interpret experimental results. Both macroscopic methods (isocratic elution, gradient elution, batch adsorption, frontal analysis) and microscopic methods are considered. In all cases the main focus is the determination of the effective intraparticle diffusivity in order to permit a comparison of different stationary phases and provide a basis for predicting chromatographic process performance. Experimental results for representative systems are evaluated alongside the experimental and modeling aspects. Practical criteria for the selection of experimental conditions and the application of different models are discussed along with the advantages and disadvantages of the various approaches.

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We describe a method to measure protein mass transfer kinetics in ion exchange adsorbents for preparative chromatography based on the use of radioactively labeled protein. The method was developed and evaluated using lysozyme as a test protein with the three commercial strong-acid cation exchangers SP-Sepharose-FF, SP-Sepharose-XL, and S-HyperD. Iodination with 125I was used to label the protein, which was added in trace amounts (approximately 0.1%) to an unlabeled protein solution. The solution was recirculated through a shallow bed of the adsorbent particles and the radioactivity accumulated in the bed measured with a gamma-counter as a function of time. Radiotracer-based kinetics measurements were found to be in good agreement with results obtained with a conventional shallow-bed technique, provided that freshly labeled protein solutions were used. The method has advantages in terms of simplicity, ability to deal with adsorption from complex mixtures, and the potential for measurements under tracer diffusion conditions. Kinetics results obtained for the three different stationary phases were generally consistent with previous studies. Protein mass transfer can be described by a pore diffusion model with a nearly salt-independent pore diffusivity for SP-Sepharose-FF and by a homogeneous diffusion model with a saltindependent adsorbed phase diffusivity for S-HyperD. However, it appears that a more complex model, accounting for parallel pore and surface diffusion, is needed to describe protein mass transfer in SP-Sepharose-XL. The modeling results were found to be correlated with the apparent pore sizes determined by inverse SEC.

Mathematical models for pore-diffusion-controlled batch adsorption that take an ar

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