Protein adsorption on cation exchangers: Comparison of macroporous and gel-composite media

The protein uptake equilibrium and kinetics and the breakthrough behavior of recently developed commercial chromatography media are evaluated using lysozyme as a model solute. One of the adsorbents, known by the trade name POROS 50, is a macroporous matrix based on a styrene-divinylbenzene copolymer. The other adsorbent, known by the trade name HyperD, is a composite obtained by filling the pores of high-porosity polystyrene-coated silica particles with a polyacrylamide-based hydrogel. Both materials are designed for preparative and process ion-exchange chromatography of proteins at high speed and have a strong cation exchange functionality. The equilibrium and transport properties of lysozyme in each adsorbent are studied in batch experiments. The maximum equilibrium uptake capacity in a 10 mM sodium phosphate buffer at pH 6.5 is 160 +/- 5 mg/cm(3) of particle volume for the macroporous adsorbent and 260 +/- 10 mg/cm(3) for the gel-composite material. With the macroporous adsorbent mass transfer appears to be dominated by macropore diffusion with an effective pore diffusivity of 1.1 x 10(-7) cm(2)/s, except during the initial saturation of the outermost layer of the particle, when external film mass transfer is controlling. An idealized two-step model of this process is found to be consistent with the experimental data. With the gel-composite adsorbent, however, mass transfer appears to be dominated by homogeneous gel diffusion;with an;effective pseudo-homogeneous diffusivity of 7.5 x 10(-9) cm(2)/s at high protein concentrations and by the external film resistance at low protein concentrations. For both adsorbents, breakthrough profiles obtained experimentally at elevated flow rates in packed columns are in good agreement with predictions based on the batch measurements and external film coefficients predicted from literature correlations. In spite of the fact that it possesses a larger particle size, the gel-composite adsorbent appears to be superior to the macroporous medium, exhibiting a dynamic capacity at 10% breakthrough more than a factor of 2 larger at mobile phase velocities in the range 0-5000 cm/h.