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32 September 2020 Tablets & Capsules as shown in Figure 4. You must be careful with sorption isotherms, however, as some materials might undergo structural changes due to drying or exposure to high humidity, resulting in a change in the sorption isotherm afterwards (see next section). Relating moisture content to water activity Water activity and free moisture content are related by the sorption isotherm, which is measured by dynamic vapor sorption (DVS). In a DVS experiment, a material is forced to different water activity values by exposure to an atmosphere with controlled water vapor pres- sure and temperature. The sample's weight is constantly monitored, and changes are the result of moisture being absorbed into or expelled from the material. A DVS experiment generally starts by drying the sample at 0 percent RH to a water activity of 0.0. After that, the sample is exposed to a series of RH values, while the mass change is monitored. The RH only changes to the next value after the sample mass has reached equilibrium. In this way, the equilibrium mass of free moisture in the system can be plotted as a func- tion of the RH. In the isotherm graphs, the RH equals the water activity times 100 percent, as these points are defined when the system is in equilibrium. Sorption isotherms are unique for each material and are an intrinsic material property. If sorption isotherms are available, the relationship between free moisture con- tent and water activity is defined, and both parameters can be derived from the other by interpolation. Looking at a sorption isotherm, you can evaluate a sample's actual moisture status by either measuring the free moisture content (y-axis, by LOD) or the water activity (x-axis), Figure 4 Relationship between water activity (% RH at equilibrium) and the amount of free water in a sample (m %) m (%) 0.35 0.30 0.25 0.20 0.15 0.10 0.05 0.00 Measuring LOD Measuring A w Relative humidity (%) 0 20 40 60 80 100