MULTIVALENT ION-EXCHANGE MODEL OF BIOPOLYMER CHROMATOGRAPHY FOR MASS OVERLOAD CONDITIONS

The simple model of multivalent ion-exchange biopolymer chromatography is analyzed on the basis of classical quasi-chemical treatments. The rigorous isotherm equation deduced from the stoichiometric displacement model (SDM) was used to stimulate the migration of a solute through the chromatographic column in the isocratic and gradient elution modes. The peak profiles generated for various samples sizes were compared with those obtained on the basis of a Langmuir isotherm. Peak tailing increases with the value of the exponent Z, defined as the ratio of the protein valency to the displacing counter-ion valency. For large Z the asymmetries due to the non-linearity of the isotherm are still present for small sample sizes, but may be reduced by using a displacing counter ion of higher valency. To illustrate the theoretical results, the ion-exchange model was applied to analyse the zonal elution behaviour of bovine serum albumin on a polymeric anion-exchange stationary phase deposited on silica. The effective charge of the protein (m = 8) was determined at infinite dilution with mono- and a divalent counter-ions. This value was introduced into the SDM isotherm to predict the elution behaviours in mass-overload conditions and the maximum loading capacity of the protein was determined. Good agreement between theory and experiment was obtained: for about the same capacity factor at infinite dilution, a larger peak asymmetry due to non-linear effects is found with a monovalent counter ion.