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Water for Dissolution Testing

 
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Impact of Water

Water is a main component of the dissolution testing procedure. The most important use of water is the preparation of the dissolution media, but it is also used for washing and rinsing the vessels, as well as in the thermostatic bath. In addition, water is usually needed for the analysis of the dissolution samples (to prepare the blanks for UV/Visible spectrometry, to prepare the mobile phases in HPLC, etc.).

The purity of the water used in preparing the dissolution media is important since it is in direct contact with the drug product being tested. Impurities in the dissolution media may interfere with the drug itself or with some of the components of the dosage form, and therefore modify the experimental outcome. In addition, the water used for the final rinse of the vessels and all the components coming in contact with the drug during the test (paddle, basket, etc.) should be of the same purity as the water used to prepare the dissolution media.

The following water contaminants may have an effect on dissolution testing:
  • Organics
    Organic molecules may interact with the drug or with some of the components of the dosage form, and have an effect on the dissolution. They may also interfere with the analytical method used to analyze the samples. 
  • Ions
    Water hardness (calcium and magnesium ions) may form deposits on the vessels. Ions may also have an effect on the pH or on the ionic strength of the dissolution medium. This is especially important in the cases where water is used as the dissolution medium. In addition, ions, in particular metallic ions, may interact with the drug or with some of the components of the dosage form, and alter their physicochemical properties. 
  • Bacteria
    Bacteria may form biofilms and release ions and organic molecules, which may in turn interfere with the test. 
  • Particles
    Particles may participate to the release of the active ingredient from the dosage form, and may interfere with the analysis of the samples. 
  • Dissolved Gas
    The level of dissolved gas, in particular dissolved oxygen, in the dissolution medium has an impact on the dissolution of some drug products. Deaeration of the dissolution media is often recommended.
It is important to note that the sensitivity of the analytical method used to analyze the dissolution method can also influence the choice of water quality used to prepare the dissolution medium. Direct methods of analysis, such as UV/Visible spectroscopy, are usually less sensitive to water quality than separation-based methods such as HPLC or CE. Very sensitive detection methods, providing superior selectivity, are increasingly being used, especially in dissolution testing of complex formulations, such as extended-release formulations. If these very sensitive detection methods, such as mass spectrometry (e.g. in LC-MS), fluorescence or luminescence are used, high quality water should be used to prepare the dissolution media, as these methods are very susceptible to water contaminants.

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