There are three types of standard solution that need to be prepared when analysing dissolution samples:

(I) Primary standard (PS)
(II) Secondary standard (SS)
(III) Working standard (WS)

(I) Preparation of PS:

Accurately weigh the amount of target drug substance into a volumetric flask and dissolve with organic solvent. If the drug substance is readily soluble and stable, methanol can be used. If not, DMSO or a proportion of DMSO can be used to increase drug solubility.

(II) Preparation of SS:

SS is required to improve the accuracy of drug concentration and has an intermediate drug concentration that is used for preparing WS.
In most cases, if the drug substance is neutral, typically a mixture of methanol and water (e.g. 75:25 %v/v)  is suitable for our biorelevant media. For pH sensitive drugs, an appropriate buffer can be used instead of water to increase the solubility of drug in the SS. Prepare SS by diluting PS with a potentially suitable diluent to the appropriate drug concentration.

(III) Preparation of WS

Use the biorelevant medium to dilute SS to prepare WS.

Note: if diluent is required, use the same diluent that was used for the preparation of SS. Typically, a diluent is needed unless dilution is being carried out with 1:2 dilution ratio.

Download our Biorelevant Dissolution Guide here.