Ion-exchange and electrodialysis 159 As the adsorption is a surface effect, the available surface area is a key parameter. For industrial processing the maximum surface area to volume should be used to minimise plant size and product dilution. It is possible for a l ml bed of ion-exchanger to have a total surface area >100 m2. The ion-exchange material is normally deployed in packed beds, and involves a compromise between large particles(to minimise pressure drop)and small particles to maximise mass transfer rates. Porous particles are employed to increase surface area/volume. However, the surface must also be accessible to the solute molecules, and hence materials with an enormous surface area due to the presence of minute pores may be of very limited use, because much of this surface is inaccessible even to small solute molecules. Manufacturers of ion-exchange materials generally quote the exclusion limit of products with respect to molecular size. Particularly in the case of biopolymers, the shape of the pores and the three-dimensional structure of the solute may be a further consideration Capacity The capacity of an ion-exchanger is defined as the number of equivalents of exchange ble ions per kilogram of exchanger but is frequently expressed in meq/g(usually in the ry form), and can be determined by titration of the charged groups with strong acid or ase.This property depends on the nature of the fixed ions as well as the available surface area. Most commercially available materials have capacities in the range 1-10 uivalents/kg of dry material Blinding and fouling The operational life of an ion-exchanger, or at least the time between major clean-up campaigns, is limited by blinding or fouling. This is non-specific adsorption onto the matrix surface, or within the pores, which effectively reduces the capacity, and certainly affects the choice of ion-exchanger for a particular separation. The susceptibility of an on-exchanger to blinding or fouling with a particular feedstock may exclude its use fo that function despite having otherwise excellent binding capacity and specificity for the molecules in question. For example, the presence of significant lipid levels in a feedstock may exclude the use of some exchangers for protein separations Elution The choice of method of elution depends on the specific separation required. In some cases the process is used to remove impurities from a feedstock, while the required compound(s) remains unadsorbed. No specific elution method is required in such cases, although it is necessary to regenerate the ion-exchanger with strong acid or alkali. In other cases the material of interest is adsorbed by the ion-exchanger while impurities are washed out of the bed. This is followed by elution and recovery of the desired solute(s) In the latter case the method of elution is much more critical- for example, care must be taken to avoid denaturation of adsorbed protei Elution of the adsorbed solute is effected by changing the ph or the ionic strength of the buffer, followed by washing away the desorbed solute with a flow of buffer Increasing the ionic strength of the buffer increases the competition for the charged sites on the ion-exchanger. Small buffer ions with a high charge density will displaceIon-exchange and electrodialysis 159 As the adsorption is a surface effect, the available surface area is a key parameter. For industrial processing the maximum surface area to volume should be used to minimise plant size and product dilution. It is possible for a 1 ml bed of ion-exchanger to have a total surface area >IO0 m2. The ion-exchange material is normally deployed in packed beds, and involves a compromise between large particles (to minimise pressure drop) and small particles to maximise mass transfer rates. Porous particles are employed to increase surface area/volume. However, the surface must also be accessible to the solute molecules, and hence materials with an enormous surface area due to the presence of minute pores may be of very limited use, because much of this surface is inaccessible even to small solute molecules. Manufacturers of ion-exchange materials generally quote the exclusion limit of products with respect to molecular size. Particularly in the case of biopolymers, the shape of the pores and the three-dimensional structure of the solute may be a further consideration. Capacity The capacity of an ion-exchanger is defined as the number of equivalents of exchange￾able ions per kilogram of exchanger but is frequently expressed in meq/g (usually in the dry form), and can be determined by titration of the charged groups with strong acid or base. This property depends on the nature of the fixed ions as well as the available surface area. Most commercially available materials have capacities in the range 1-10 equivalents/kg of dry material. Blinding and fouling The operational life of an ion-exchanger, or at least the time between major clean-up campaigns, is limited by blinding or fouling. This is non-specific adsorption onto the matrix surface, or within the pores, which effectively reduces the capacity, and certainly affects the choice of ion-exchanger for a particular separation. The susceptibility of an ion-exchanger to blinding or fouling with a particular feedstock may exclude its use for that function despite having otherwise excellent binding capacity and specificity for the molecules in question. For example, the presence of significant lipid levels in a feedstock may exclude the use of some exchangers for protein separations. Elution The choice of method of elution depends on the specific separation required. In some cases the process is used to remove impurities from a feedstock, while the required compound(s) remains unadsorbed. No specific elution method is required in such cases, although it is necessary to regenerate the ion-exchanger with strong acid or alkali. In other cases the material of interest is adsorbed by the ion-exchanger while impurities are washed out of the bed. This is followed by elution and recovery of the desired solute(s). In the latter case the method of elution is much more critical - for example, care must be taken to avoid denaturation of adsorbed proteins. Elution of the adsorbed solute is effected by changing the pH or the ionic strength of the buffer, followed by washing away the desorbed solute with a flow of buffer. Increasing the ionic strength of the buffer increases the competition for the charged sites on the ion-exchanger. Small buffer ions with a high charge density will displace