Loss on drying
In short Loss on drying describes how much residual moisture a lyophilisate contains. It is given as the percentage of mass lost on drying.
The figure does not appear on every certificate. Where it is missing, the nominal amount on the vial is the practical working figure.
What is measured is how much mass a sample loses when dried further. What escapes is mostly water left behind in the lyophilisate. Typical values are a few per cent; nothing that has been freeze-dried is completely dry.
The figure has two consequences. First, the water counts towards the nominal amount and so, like the counterion and any bulking agent, reduces the actual peptide content. Second, residual moisture limits how long the material keeps, because water breaks the bonds between the links.
The value is usually determined gravimetrically, by weighing before and after drying, or more precisely by Karl Fischer titration, which measures water specifically.
In practice this means that cold glass should not meet humid room air: it mists up, and the moisture finds its way into the vial. More on this in storing lyophilised peptides in the laboratory.