The following article has been authored by John Heaney.
The USP chapter <711> says that dissolution testers should be free of any noticeable vibration. While the chapter technically addresses the issue, it’s a far cry from the detail of the standards laid out for vessels, paddles, and baskets. The reason for this likely lies with vibration itself.
Vibration has both frequency and amplitude and those can be varied over a near limitless range. Sound is one of the most common forms of vibration that most people deal with on a day-to-day basis and is a good example of how non-audible vibration can vary widely. The sound of a jack hammer at a construction site is greatly different from the notes of a piano. Though in most cases there are certain things that would be considered typical in a lab environment. A small electric motor in the lab may put out a quiet hum, while a medium air compressor would be much louder.
Sources of vibration in the lab can widely vary as well. Pumps, centrifuges, air compressors, and other equipment have the potential to introduce vibration into the environment. There are also architectural things to consider like HVAC units. For example, a dissolution tester located in the middle of a second-floor lab, away from supports, may see more vibration due to lab staff walking around the lab as compared to a dissolution tester located near one of the supports. Conversely, an HVAC unit may send more vibration to benches down the support beams of a structure.
Dissolution testers also have inherent vibration from their components, though most manufacturers will work to limit it. Pumps for circulating water baths, motors, belts and bearings present in the drive system can all contribute to vibration. This is one of the reasons that regular maintenance on dissolution testers is important.
Disintegrating dosage forms (e.g. prednisone tablets) tend to be more susceptible to vibration related issues than non-disintegrating dosage forms (e.g. salicylic acid). This makes sense as within the dissolution vessel a disintegrating dosage form would turn into a finer powder from a tablet or capsule. The smaller particles would be affected more strongly by the vibration than a larger and heavier tablet.
Depending on the apparatus used, and the vibration, how the results are affected can vary. The particles can be driven to disperse further raising dissolution values if the vibration is intense enough. Potentially it can also cause the cone in the bottom of the vessel that forms when using a disintegrating dosage form with Apparatus 2 (paddles) to pack down, or force particles away from the basket for Apparatus 1, which may lower results.
It is because of the variability in how dissolution results can be affected as well as the vibration itself that helps explain why the USP simply states “no noticeable vibration.” There’s not an easy way to specify all the frequencies and amplitudes that may cause a problem.
There are things that can be done within a lab to help deal with vibration.
If the dissolution tester has not been installed, it is a great idea to use a vibration meter to measure bench vibration in possible installation locations. This does not need to be an extended study but should ideally cover things like lab staff walking around the lab and ensuring the HVAC system is active at the time of measurement.
Other equipment on the same bench or nearby that could potentially be an issue should be moved. This would include (but not be limited to) pumps, centrifuges, compressors, tap density testers, and sieves. As a general rule of thumb, if the piece of equipment is very noisy and/or requires hearing protection when used, it should not be near a dissolution tester.
Maintaining the dissolution tester is another important step. Dissolution testers are mechanical. There are typically belts, pulleys, motors, and bearings to worry about and over time these will wear out. For dissolution testers with water baths, the pumps used to circulate the water can potentially develop issues if not routinely checked. Also ensuring the dissolution tester has its weight evenly distributed across all of the feet can help reduce vibration as well. Routine maintenance on the system can prevent issues before they happen and stop unexpected changes in results.
Even though there are not set specifications for vibration, under most circumstances it can be easily managed in a lab environment with a bit of planning.