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Retentiveness is the ability of a membrane to retain the particle or molecule of interest. It is impractical for filter manufacturers to test the retentiveness of every filter in every possible application. Sometimes, however, the market size for a filter or the criticality of the application makes it economically feasible for the manufacturer to integrate retention testing into routine release testing. Regardless of whether testing is done by the manufacturer or the user, the filter should be validated using methods relevant to the final application.
For example:
  • A filter to be used for analysis of asbestos particles should be tested for removal of all asbestos particles from an air stream. 
  • A sterilizing-grade filter should be tested for quantitative retention of bacteria. 
  • A virus removal filter should be validated for retention of viruses.
In the unique case of UF membranes, retention testing using dextrans or proteins is the only practical way of characterizing performance characteristics. The results of retention testing provide a measure of the NMWL of the membrane.

The need for retention testing is illustrated by the following example.
For example:
Two membranes with identical pore ratings are tested side by side over two hours using a solution containing a known contaminant which should be retained by both membranes. The flow rate of the first membrane starts to decline within minutes of the test starting and continues to decline out to two hours. The flow rate of the second membrane remains constant for the two-hour period. The difference can be explained in two ways:
  1. The first membrane may not have a flow rate as high as the second membrane. 
  2. The first membrane may be retaining the particles more efficiently than the second membrane.
The only way to distinguish between these two possibilities is to analyze the filtrate for particle content and determine the retention efficiency.