Characterization of cross-linked cellulosic ion-exchange adsorbents: 1. Structural properties

Abstract

The structural characteristics of the HyperCel family of cellulosic ion-exchange materials (Pall Corporation) were assessed using methods to gauge the pore dimensions and the effect of ionic strength on intraparticle architecture. Inverse size exclusion chromatography (ISEC) was applied to the S and STAR AX HyperCel derivatives. The theoretical analysis yielded an average pore radius for each material of about 5nm, with a particularly narrow pore-size distribution. Electron microscopy techniques were used to visualize the particle structure and relate it to macroscopic experimental data. Microscopy of Q and STAR AX HyperCel anion exchangers presented some qualitative differences in pore structure that can be attributed to the derivatization using conventional quaternary ammonium and salt-tolerant ligands, respectively. Finally, the effect of ionic strength was studied through the use of salt breakthrough experiments to determine to what extent Donnan exclusion plays a role in restricting the accessible pore volume for small ions. It was determined that Donnan effects were prevalent at total ionic strengths (TIS) less than 150mM, suggesting the presence of a ligand-containing partitioning volume within the pore space.

Keywords: Cellulosic materials; Donnan equilibrium; Electron microscopy; Inverse size exclusion chromatography; Ion-exchange chromatography; Salt-tolerant adsorbents.

Figure 1

Dextran retention volume chromatograms in STAR AX HyperCel (20 mM bis-tris, 100 mM NaCl, pH 7).

Figure 2

PEG retention volume chromatograms in STAR AX HyperCel (20 mM bis-tris, 100 mM NaCl, pH 7).

Figure 3

Dextran calibration curves for S HyperCel (■), STAR AX HyperCel (□), Q Sepharose FF (●), and Q Sepharose XL (○); PEG calibration curve for STAR AX HyperCel (△) also shown; log-normal (---) and Ogston (—) models applied to the dextran calibration curve for STAR AX HyperCel (100 mM NaCl, pH 7).

Figure 4

Dextran calibration curves for (a) STAR AX HyperCel and (b) S HyperCel at different NaCl concentrations (20 mM bis-tris, pH 7).

Figure 5

SEM images of Q (a and b) and STAR AX (c and d) HyperCel. a and c show the resin chemically fixed with OsO₄; b and d show resin without chemical fixation.

Figure 6

SEM images of Q (a and b) and STAR AX (c and d) HyperCel at higher magnification. a and c show the resin chemically fixed with OsO₄; b and d show resin without chemical fixation.

Figure 7

TEM images of Q HyperCel prepared in 10 mM sodium phosphate (a), 50 mM bis-tris (b) and 10 mM sodium cacodylate (c) buffers during fixation.

Figure 8

TEM images of Q HyperCel prepared using standard aldehyde fixation (a and c) and TAGO fixation (b and d); low (a and b) and high (c and d) magnification.

Figure 9

TEM images of STAR AX HyperCel prepared using standard aldehyde fixation (a and c) and TAGO fixation (b and d); low (a and b) and high (c and d) magnification.

Figure 10

Micrographs of HyperCel materials showing void regions. a) TEM image of Q HyperCel without protein adsorbed; b) TEM image of Q HyperCel with 50% of maximum loading of β-lactoglobulin; c) SEM image of STAR AX HyperCel section; d) TEM image of STAR AX HyperCel with 50% of maximum loading of β-lactoglobulin.

Figure 11

Salt exclusion in Q HyperCel (a) and STAR AX HyperCel (b, c). Solution conditions for (a) and (b) were 50 mM bis-tris, pH 7, with TIS adjusted with NaCl. Symbols denote maximum TIS of buffer used for each range of ionic strengths: 200 (●), 100 (■), and 50 (▲) mM TIS. Solution conditions for (c) were 10–100 mM sodium phosphate, pH 7. Symbols denote the maximum concentration of sodium phosphate used for each range of ionic strengths: 100 (■) and 30 (▲) mM sodium phosphate.