Sample dropping is not something which often leads to errors in the dissolution rate but it is an area which is frequently misunderstood by users. With baskets the sample is introduced in a dry basket. If all the samples are to be started at the same time this is easy as the drive head is simply lowered and the baskets enter the media together. This should be used with an automatic sampler.
The USP allows for a 2% variation in the sampling time. This is considered to be the elapsed time rather than the total test time. If the first sample is required after 5 minutes, then the user has 6 seconds to withdraw a sample from 6 positions — this is only possible with an automatic sampler. Otherwise the sample introduction needs to be staggered to allow enough time for the manual samples to be taken.
If a staggered drop is required the baskets must be introduced one at a time which is more difficult or impossible on most systems.
With paddles the USP requires that the dosage form is dropped on a stationary paddle. This prevents the risk of a tablet getting chipped or broken if it lands on a spinning paddle. If the paddle is spinning it may also prevent the tablet from sitting in the wrong position in the vessel. This can have a small but significant effect on the dissolution rate.