The following article has been authored by John Heaney.
Does the presence of a resident sampling probe affect the hydrodynamics of a dissolution vessel? Yes, yes it does. The more important questions are how strong is the change in the hydrodynamics and is it strong enough to affect testing?
USP <1092>, an advisory chapter addresses this directly with:
Sampling probes may or may not remain in the vessel throughout the entire run. Sampling probes or fiber-optic probes can disturb the hydrodynamics of the vessel; therefore, adequate validation should be performed to ensure that the probes are not causing a significant change in the dissolution rate.
While USP <711> is a bit less direct in terms of the probes specifically:
If automated equipment is used for sampling or the apparatus is otherwise modified, verification that the modified apparatus will produce results equivalent to those obtained with the standard apparatus described in this general chapter is necessary.
So resident probes, and/or automation are allowed, but only if it has been proven that there is not an effect on the dissolution rate when they are present as compared to manual sampling. This basically puts the responsibility on the lab to prove that the resident cannula or probe is not causing an adverse effect on the dissolution rate.
While time consuming, this isn’t particularly difficult to do. Ideally the lab would run a series of tests alternating vessels between a resident probe or cannula, versus a manual sampling where the cannula is present only during the sampling process. On the first test the odd numbered vessels may be manually sampled, while even numbered vessels are sampled using the resident probes or cannula. Then for a subsequent test the assignments would be reversed. This would help eliminate any bias in the data caused due to the vessel position.
However, depending on the sampling system under evaluation this isn’t always practical. If multiple testers are available, one tester can be assigned to resident probes/cannula while the other is assigned to manual sampling; then for the following test the roles are reversed between the two testers, again to eliminate any bias. Ideally the tests would be run using the same batch of media, tablets and operator.
With the testing complete, it’s then necessary to review the data and perform a comparison to ensure that the resident probes/cannula did not produce significantly different results than the vessels that were manually sampled. Depending on the variation in the test method, what is significantly different may vary from dosage to dosage.
With this data in hand, it’s easy to justify resident probes or cannula along with a fully automated sampling system.