Introduction to Dynamic Vapor Sorption
Dynamic vapor sorption (DVS – water sorption) is a gravimetric technique that measures how quickly and how much of a solvent is absorbed by a sample, such as a dry powder absorbing water. The water uptake describes how hygroscopic the compound is. The hygroscopicity is highly relevant in evaluation of storage conditions and general stability, as “water-containing”/hygroscopic compounds might be quite unstable. Water might cause:
- Formation of hydrates (or even transformation of an hydrate into an anhydrate)
- Transformation of the crystalline form
- Increased reaction rates, i.e. degradation of the pharmaceutical compound.
- “Desalting” of pharmaceutical salts.
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Determination of the water uptake is part of the required physical characterisation of a pharmaceutical compound. In a dynamic vapor sorption measurement the hygroscopicity of the compound is determined at relevant temperatures and humidities. In general, determination of hygroscopicity is quite time consuming: A standard dynamic vapor sorption measurement requires around 24-36 hours, as “time is needed” in order to obtain equilibrium in the system. Not only the properties of the active molecule plays a role for the water uptake: The particle size might also influence the hygroscopicity. The method complies with Ph. Eur. 2.9.39
Instrument and measuring principle, DVS
In a dynamic vapor sorption measurement the water concentration surrounding the sample is varied and the change in mass which this produces is measured. The DVS analyser is based on a vertical nulling microbalance in which the sample and a reference hang-down wire are enclosed in a humidity- and temperature controlled chamber. The humidity is controlled by a dry nitrogen purge steam between a dry- and a wet line (saturated with water). A high precision mass flow controller regulate the propotion of wet and dry gas to obtain the desired humidity.
In a standard set-up the humidity is increased from low to high humidities. As desorption might take place at a different rate, the humidity is following decreased to low humidity while measuring the weight loss. Furthermore, a second run is performed in order to see whether the materials structure or behaviour are changed due to the initial exposure to high humidity. The Dynamic Vapor Sorption Instrument applied by Particle Analytical is a DVS Advantage 1 from Surface Measurement Systems.
|Instrument||DVS Advantage 1 from Surface Measurement Systems|
|Humidity range||0-98 % relative humidity|
|Temperatures||5 to 60 °C|
|Sample amount||10-20 mg|