Pore Size or Nominal Molecular Weight Limit (NMWL)

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Pore size relates to the filter’s ability to filter out particles of a certain size. For example, a 0.20 micron (µm) membrane will filter out particles with a diameter of 0.2 microns or larger from a filtration stream.

Pore size is determined by one of several techniques:
  • Visual examination using scanning electron microscopy, where a small section of membrane is appropriately treated, put in the microscope, and evaluated, using appropriate imaging software. 
  • Porosimetry is a physical method where liquid is forced into the membrane under pressure and the penetration profile is analyzed mathematically to determine pore size. 
  • Particle challenge uses particles of defined size to determine the minimum size that can be retained by the filter
With ultrafiltration (UF), filter pore size is irrelevant because the pores are so small. These filters are rated according to their nominal molecular weight limit (NMWL) or their molecular weight cutoff (MWCO). For example, a UF membrane rated at 30,000 will exclude a test protein with a molecular weight of 30,000 Daltons. Ninety percent of that test protein will be retained on the upstream side and 10% will pass through into the filtrate, resulting in concentration of the protein.

Although all membranes are rated for a particular pore size, pore size ratings alone are an unreliable measure of filter effectiveness because these ratings vary from manufacturer to manufacturer and from product to product. The function of the filter – is it a prefiltration, clarification, or sterilization filter, for example - and how well the filter performs that function have to be considered.

Bubble Point Integrity Test

Bubble Point is a practical, nondestructive test used for estimating the pore size of microporous filters and confirming the integrity of sterilizing membrane filters and filter systems. It is the most widely used non-destructive integrity test.

Bubble point is based on the fact that liquid is held in the pores of the filter by surface tension and capillary forces. The minimum pressure required to force liquid out of the pores is a measure of the pore diameter. The pressure required to force liquid out of a liquid-filled capillary must be sufficient to overcome surface tension and is a direct measure of effective tube diameter.
Diagram showing derivation of Bubble Point Test formula
Diagram showing derivation of Bubble Point Test formula
Formula for Bubble Point Test:
Formula for Bubble Point Test
Where:
P = bubble point pressure
d = pore diameter
k = shape correction factor
cos θ = liquid-solid contact angle
ó = surface tension

The bubble point test is a sensitive visual technique and is performed routinely as part of the Merck Quality Assurance Program. The bubble point test detects minor filter defects and out-of-size pores and correlates with the bacteria passage.

Bubble Point Procedure for Filters

  1. Wet the filter with the appropriate fluid, typically water for hydrophilic membranes or an alcohol for hydrophobic membranes. 
  2. Place the filter into the holder and cover the membrane with the wetting fluid. 
  3. Pressurize the system to about 80% of the expected bubble point pressure which is stated in the manufacturer’s literature. 
  4. Slowly increase the pressure until a single, continuous stream of bubbles comes through the membrane. 
  5. Read the bubble point pressure from the pressure gauge.

Bubble Point Procedure for Devices

  1. Wet the filter with the appropriate fluid, typically water for hydrophilic membranes or an alcohol/water mixture for hydrophobic membranes. 
  2. Pressurize the system to about 80% of the expected bubble point pressure which is stated in the manufacturer’s literature.
  3. Slowly increase the pressure until rapid continuous bubbling is observed at the outlet. 
  4. A bubble point value lower than the specification is an indication of one of the following:
    • Fluid with different surface tension than the recommended test fluid 
    • Integral filter, but wrong pore size
    • High temperature 
    • Incompletely wetted membrane
    • Non-integral membrane or seal

    Merck:/Freestyle/BI-Bioscience/Chromatography-Sample-Prep/membrane-images/wettability-bubble-graph.jpg
    Bubble point test procedure for devices