QLA Cannula Filters

Cannula Filters - Generally have a large surface area and may affect the hydrodynamics in the vessel. However they have a good flow rate and may be used when high levels of particulates are present to prevent particulates blocking the sample probes. A large porosity range is available. Different manufacturers recommend different filters for use with their sampling cannula and not all are interchangeable because of differences in cannula diameter.

Filter Discs - Some low volume sample probes require an In-Line Filter Disc (or filter frit) which has a lower surface area than a cannula filter. These are designed to keep the filtration process outside the vessel but may suffer from blocking due to the lower surface area.

Filter Tips - Filter Tips are smaller than cannula filters and fit on the end of larger diameter sample probes.

Key Considerations

Most manufacturers use a filter on the end of the sampling cannula. These cannula filters are made of Ultra High Molecular Weight (UHMW) Polyethylene and are completely inert to most compounds.

With filters, porosities are nominal and represent an average pore size rather than an absolute pore size. Thus a 35 µm and 40 µm filter will perform in much the same way.

If in-line filters are used it is important that a small amount of media is pushed back in the reverse direction to wash particles off the filter back into the vessel. Particles are part of the tablet and as such should be kept inside the vessel dissolving with the rest of the tablet.

Failure to do this results in the particles staying on the filter. The effect of this is not only to remove the particles from the media but next time a sample is taken, the media is pulled through the particles stuck to the filter which can give erroneously high readings.

When selecting a filter, it is wise to conduct some tests to ensure that there is no sample absorption onto the filter. If this is a problem then alternative filters and materials are available but external filtration may be required.