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With the focus on physical testing of the dissolution tester, it is easy to forget some of the basic things that can make it run better.
The tester should be well maintained, clean and in general good operational condition. A dirty tester can lead to dirt in the mechanisms causing longer term problems quite apart from giving an overall impression about the importance of dissolution testing in the laboratory.
The waterbath should be full to the recommended line. A partly full waterbath will not control vessel temperature as efficiently or evenly.
Examine drive belts periodically. The individual shafts are usually controlled by a tensioned, toothed belt running between them. Over time these can degrade, cause dust and vibration and eventually poor rotation speed.
Check for worn or faulty gears and wheels. They can damage the belt and cause speed problems.
Check also for possible motor surging. Caused by unstable electricity or faulty and worn motor, this variability in speed, even short term, will cause a problem. This can particularly be a problem on older dissolution testers.
The USP Dissolution Toolkit states:sembly
"All vessels and individual parts of the stirring elements (shafts, baskets, paddles or paddle blades) should be uniquely identified, documented, and kept in the same position in the same test assembly for all dissolution runs. For ease of identification and record keeping, apparatus positions on the vessel support plate of the dissolution test assembly should be identified systematically."
This is good advice because if things should fail at any stage, then it is a lot easier to identify the reason if only one variable can be changed at a time. Keeping everything uniform from test to test aids with that considerably.
While hardware variables are the easiest to measure and may have the largest direct effect of dissolution results, there are a number of environmental parameters which can have a significant effect as well.
Vibration is a problem which may not be immediately apparent but that can have a significant effect on dissolution results.
Essentially, vibration = energy. Extra energy into the system is likely to speed up the dissolution rate and the USP calibrators are especially sensitive to vibration.
Neither the USP nor the FDA place an actual figure on acceptable vibration, both stating 'no significant' as the acceptable level. (In reality this is around 0.1-0.2mil displacement at the vessel).
Significant vibration basically equated to whether or not you can feel vibration through your fingers at the rim of the vessel. If you can then there is a potential issue.
The most common source of vibration is from the water-pump heating the tester waterbath. In some cases these pumps ate actually physically attached to the tester and in some cases are a separate unit. An easy and quick way of isolating the pump is to place it on some high density foam to dampen the transmission of vibration to the bench.
Care also needs to be taken with the sighting of the tester as well. While it may not be possible to feel significant vibration at the time of qualification, intermittent vibration from other lab equipment such as centrifuges fume cupboards refrigerators, dishwashers or air conditioning units can all contribute. Equipment such as this should never be placed on the same bench as the dissolution tester.
For the reasons of vibration above, laboratory benches should be as solid as possible. All-wooden benches might look nice, but can transmit vibration easily and so care needs to be taken of that.
Benches should also be level. The dissolution tester will have its own levelling capability but it is a lot easier when the basic environment is right. Level is particularly easy to measure with the correct tool.
