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Standard USP 1&2 dissolution apparatus use 1000ml vessels or larger. These are generally suitable for test volumes of 500ml upwards. Whilst there are a few methods that use this equipment for volumes as low as 300ml the media flows become less reproducible and special lower volume vessels with mini paddles or baskets are used instead. 200ml and 100ml vessels are common.
The lowest volume that can be used in a 100ml vessel is about 40ml. For volumes lower than this other methods have to be used such as USP4 or USP7.
Small volume dissolution is a useful technique when tablets only contain very small amounts of active ingredient, or for extended release tablets where small amounts are released and may be difficult to detect using HPLC or UV-Vis. Sometimes the API may also have a low response to this type of detection which may make the problem worse.
Using multiple tablets in one vessel is never allowed as the tablets may stick to each other and affect the dissolution rate and of course individual tablets that have very high or low amounts of API are hidden in an average result. Using small volumes increases the concentration of the sample and allows for proper detection.
Small volume dissolution is not discussed in the USP and although a 250ml vessel is detailed in the Chinese Pharmacopiea this is not widely used. However a particular configuration may be referred to in some of the individual monographs for particular dosage forms.
There are no given dimensions for the paddles or vessels, and there are variations between different suppliers.
Measuring the standard USP dimension of height/centering/wobble are difficult as often the calibration tools will not work inside the smaller vessels, and there is debate over whether the standard USP settings should be used or adjusted to take account of the smaller vessel dimension. As an example the height between the bottom of the paddle/basket is often set to 5, 10 or 25mm.
Any method developed using small volume apparatus should be robust and able to discriminate variations in different batches.
It is important that “sink conditions” are maintained – that is that the total volume of media is enough to dissolve at least 5-10 tablets.
The small volume kit will only work with some dissolution testers. It will not work on testers that have directly heated vessels or those tha have thermistors in the shafts. Please contact us to discuss what is required for your bath.
There are several parts that need to be changed to covert a normal tester into one for small volume dissolution. The parts required are:
The stirring element (paddle or basket) is always supplied as a spin on/off accessory. If you do not have the spin shafts on your bath then you will need to order both the paddle/basket hub and the upper shaft.
There are different lengths of upper shaft, and these are 9.8mm diameter (fit Hanson/Agilent/Varian/Distek). They do not fit Erweka/Pharmatest or Sotax baths directly, please contact us for these.
The baskets are sold separately to the basket hub
There are two different 200ml vessel sizes – the Agilent vessel is a little longer than the Distek vessel though they are of similar diameter. The 100ml vessel is shorter but the same diameter.
The vessel adaptor is required to position the vessel in the centre of the vessel position. The vessel adapters are designed to fit into the original 1000ml vessel holes. It is recommended to buy the adapter with the vessel cover.
