Secondary drying is the freeze-drying stage concerned with removing residual sorbed water after the main ice-removal step. Its central description is desorption, whereas primary drying is described chiefly by sublimation. A sample can therefore have no remaining bulk ice yet still contain water associated with its solid material.[ref]
Lyophilization of Parenteral (7/93) by the Food and Drug Administration. Introduction: primary drying by sublimation and secondary drying by desorption.
An original mass example shows why this distinction matters quantitatively. Suppose the dry solid contributes and a partly dried specimen weighs . It initially retains of water under these assumptions. A later mass of would leave of water. The removed is 70% of the initially residual water, while it is only about 2.69% of the earlier total specimen mass.
A large relative change in residual moisture can therefore accompany a small change in overall mass. That calculation is conditional on unchanged dry-solid mass and no chemical reaction; the mass data alone do not distinguish desorption from every other possible cause of mass loss. The physical interpretation and the choice of denominator must accompany the reported moisture result.
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