Types of Sampling Cannulae

There are several different types of sampling probes. The most common are cannula probes. These are thin tubes that are attached to syringes and are used to withdraw samples manually. Sometimes it is necessary to use filters on the end of the sampling probe or cannula. These can often influence the dissolution result as they have significant volume. If possible, inline filters should be used although sometimes this is not possible. For manual sampling, different length probes exist for different test sizes and probes can be used together with stoppers to ensure that the same depth is reached each time.

Stationary probes sit in the vessel for the entire duration of the test. The amount of turbulence that this adds to the media is dependent on the volume of the probe. If a cannula filter is attached to the end of the probe then the effect can be significantly higher (inline filters are better if there is a back-flush capability). As the probes are stationary the sampling points are very reliable and repeatable — although different probes should be used for 500 and 900ml tests.

Moving sampling probes are probes that are attached to a moving manifold. The probes move in to the media when the sample is taken and then are withdrawn until the next sample is required. The benefits of these probes are that the do not interfere with the media at all when they are not sampling, however the movement in and out of the media will add a small ‘plunger effect’. The sampling point can easily be adjusted by changing the sweep of the manifold.

The hollow shaft sampling method is popular with some baths — the method has yet to be adapted by all manufacturers. The sample is withdrawn from a small gap in the stirring shaft. This method is a little controversial and has not yet been accepted by the USP although many people use this method successfully. There are two main points of contention. The first is that it is impossible to back-flush the line with stirring media — or if no back-flushing is used there will be a chance of cross contamination. The second is that the sampling height is constant — it is the same height for both 500 and 900ml tests — therefore one of these positions will not meet the USP guidelines.