Freeze-drying, also called lyophilization, removes water from a frozen material chiefly by sublimation: solid water passes directly into vapor. Reduced pressure makes that route useful without requiring the whole sample to pass through a liquid-water stage. Primary drying concerns removal of ice by sublimation; secondary drying concerns removal of residual sorbed water by desorption.[ref]
Lyophilization of Parenteral (7/93) by the Food and Drug Administration. Introduction and Glossary, historical July1993 guide: phase-change definition and named stages.
A hypothetical mass balance separates water removal from the measurement convention. Suppose a specimen contains of dry solids and of water. If its final mass is , with no solids lost or chemical change, the residual water is . Its wet-mass mass fraction iswhereas water relative to dry solids is .
Both percentages describe the same imagined sample with different denominators. The mass change establishes water removal under the assumptions, but does not independently prove that sublimation was the mechanism. That claim requires information about the material's phase and the physical process.
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