The lot number on the vial matches the certificate, which at least rules out the easy problem.
I want to know whether this is a real physical effect or an artefact of how it is measured.
What prompted the question is an inconsistency between two sources I otherwise trust.
What is the causal chain, and where does it stop being established?
Start with the method, because Karl Fischer and loss on drying measure different things and disagree in an informative way.
Loss on drying measures the mass lost on heating and includes water, residual solvent and anything else volatile. On a lyophilised peptide the two methods disagree, and loss on drying reads higher by roughly the residual solvent content.
Typical residual moisture after a competent lyophilisation cycle is two to six per cent. Pharmaceutical targets are often one to three; research-grade material commonly runs at the upper end.
Typical residual moisture ranges after lyophilisation are published in formulation literature, with pharmaceutical targets around one to three per cent.
A collapsed cake holds more water. Appearance and moisture are linked.
Analytical standards and reagents with traceable certificates. Every quantitative result you read inherits the accuracy of the standard behind it.
Shop standardsAnswering this needs to know whether you are reconciling gross mass against label claim, since that is the only calculation where the number actually matters.
Sample handling matters: a hygroscopic lyophilised cake picks up atmospheric water in seconds, so a Karl Fischer result depends on how the sample was transferred as much as on the material.
To be exact about it, karl Fischer titration measures water by a specific chemical reaction — iodine consumed stoichiometrically by water — and is therefore unaffected by other volatiles. Coulometric Karl Fischer is the appropriate variant at the low water contents and small sample masses involved here.
Ask for Karl Fischer specifically, not loss on drying, and ask for both if you can.
Ask PeptideStack is a static archive. Posting is closed, but the norms are worth stating: answer the question that was asked, show your working, cite the trial or the certificate, and say plainly where the evidence runs out.