Except for highly specific affinity methods, batch adsorption techniques are not generally applicable to highresolution separations. Instead, batch adsorption is ideal for so-called capture applications, where entire classes of impurities are to be eliminated in advance of highresolution purification techniques, such as differential elu-tion chromatography. Such bulk adsorptions also serve to reduce large volumes of dilute cell culture fluids or micro-bial lysates to more manageable volumes for further downstream processing.
Whereas solvent extraction is a popular first step in recovery of antibiotics from fermentation broths, adsorption is generally more gentle to proteins, which can be adsorbed without the denaturation often produced by organic extraction (21). Ion exchange adsorbents are preferred for highly polar molecules that, even in neutral form, exhibit low distribution coefficients for extraction.
Selectivity in batch ion exchange adsorption can be enhanced by judicious choice of adsorption buffer conditions (e.g., moderate salt concentration) to exploit differences in properties between the desired product and contaminants. This may be especially true in the case of recombinant proteins, which may be fundamentally different from the host organism's natural proteins. Adsorption equilibria that rely on weak physical interactions such as hydrophobic interaction, hydrogen bonding, and ion exchange are desirable for separations because these low-energy interactions can be readily reversed. Additional selectivity can be introduced during the desorption step so that adsorption processes are not without resolving power. Nonetheless, high selectivity is not the primary requirement for such an early process step.
With the introduction of the more specific nature of affinity interactions, well-thought-out batch adsorptions could accomplish clarification, concentration, and at least partial purification in one integrated step. Creative new configurations such as expanded-bed adsorption to approach this one-step purification ideal are discussed in more detail later ("Expanded Bed Chromatography").
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