The following article has been authored by John Heaney.
While automation is prevalent with dissolution testing, manual testing can and still should be done in certain circumstances. This can be when automation is too expensive, when the method is being developed and a certain amount of flexibility is needed, or when sample times are too short to be completed by automated systems. Manual sampling does offer significant flexibility but it also has some factors that need to be considered to ensure consistency.
Sampling height is clearly defined in USP <711> as half-way between the top of the apparatus, be it the top of the paddle blade, or the top of the basket (not the hub), and the media surface. The sampling height can be critical with both APP 1 and APP 2. It’s been shown in a number of studies that the sampling height for paddles can be especially critical as the hydrodynamics within the vessels create zones where there is higher concentration of the drug. With baskets it can be critical as well and for tests that use 500 mL of media and baskets together, ensuring the sample is taken at the correct height can be difficult, especially if there is a short stagger time between vessels.
Using stoppers on sampling cannula can help with those issues. They allow the cannula to be set at a specific height relative to the apparatus and readily reproduced without spending a lot of time aligning everything during the sample. A good stopper will tighten so it can’t move on the cannula and be easy to put into the cover in a very reproducible way.
As long as the stoppers and cannula are adjusted prior to starting the test, the process is simply to slide the cannula through the vessel cover and engage the stopper with the cover. This gives a readily reproducible sampling height every time without the need for expensive automation.
There are many different cannula stopper styles. Which one you should use will depend on your Bath OEM/Model vessel cover and cannula tubing thickness.
