= Secondary drying
\b[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.<Lyophilization of Parenteral (7 93) by the Food and Drug Administration>[Introduction: primary drying by sublimation and secondary drying by desorption.]{ref}
An original mass example shows why this distinction matters quantitatively. Suppose the dry solid contributes $0.250\,\mathrm g$ and a partly dried specimen weighs $0.260\,\mathrm g$. It initially retains $0.010\,\mathrm g$ of water under these assumptions. A later mass of $0.253\,\mathrm g$ would leave $0.003\,\mathrm g$ of water. The removed $0.007\,\mathrm g$ 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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