Introduction to Sampling
Almost every dissolution test relies on samples being taken to produce a result and anything which affects the accuracy of a sample should therefore be avoided. Sampling errors are very common and are often the cause of erroneous results. The sampling position is critical but so to is the type of sampling probe used.
The sampling position is important particularly for tests with baskets as there is often a concentration profile between the bottom of the vessel and the stirring element. It is less noticeable with paddles as the extra agitation provides for a more uniform flow profile. The USP defines the sampling point as "A zone half way between the top of the media and top of the paddle or basket not less than 1cm from the vessel wall". Therefore the sampling position will be different for 500ml and 900ml tests. This can create problems for equipment that uses ‘through the shaft’ sampling.