| Capture is often the most critical purification stage and has a large influence on the overall success of the process. The target molecule has to be recovered and stabilized as quickly as possible from the crude feedstock. Techniques must be robust, have high loading capacity and handle large feed volumes. Capture can have a major impact on process economy.
In capture the focus is the isolation of the target molecule from crude sample. A unit operation in the capture stage is designed to maximize capacity and/or speed at the expense of some resolution. The chromatographic separation is usually achieved during binding of the product and washing steps. Product can be simply recovered, in concentrated form, by a step change in eluent composition.
Resolution is achieved during capture by selectivity of the adsorbent rather than by shallow gradients or fine beads. Separation of molecules that differ greatly from the product is a minimum requirement. In addition, substances that destroy or destabilize the product must be removed. Finally, water is normally removed to a level where the product is at a useful concentration.
In a typical capture situation, throughput is very important to process large sample volumes in a reasonable time and to keep the scale of equipment as small as possible. The focus is on binding capacity and flow rates. The characteristics of the feed and the anticipated final scale of work, form the basis for the balance between capacity and speed during capture.
If the final scale of work is very large, binding capacity will have high priority, i.e. it will be important to maximize loading capacity in terms of amount of product that can be applied per unit volume of media. The "capacity first" approach has limitations, however, since a certain minimum size of column will be required to handle the volume of feedstock in a reasonable time, irrespective of product binding capacity.
If time is a critical issue, for example because of degradation of the product, flow rate will have higher priority than binding capacity. It might be more important to apply high flow rates, even if this means that a somewhat larger bed volume is used for adsorption of a specific amount of product than is dictated by capacity.
Recovery is important in any preparative situation, especially for high value products. However, a general maxim is that recovery becomes more important further downstream as the value of the product increases. Recovery is influenced by destructive processes in the sample and unfavorable conditions on the column.
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