A freeze-dried vial is described as dry, and it is not. After a lyophilization cycle finishes there is still water in the cake, bound to the molecule and dissolved in the amorphous solid, and the amount of it is one of the better predictors of how long the vial will keep. It is also invisible, unmeasurable by eye, and absent from every certificate in this library.
This post explains what the number is, why it matters, how it is measured when it is measured, and what to do with the fact that you do not have it.
What residual moisture is
Lyophilization removes water in two phases. Primary drying sublimes the ice, which is most of the water by mass. Secondary drying then desorbs the water that was never ice: molecules hydrogen-bonded to the peptide and dissolved in the concentrated amorphous phase between the pores. The lyophilization post walks through both.
Secondary drying is asymptotic. Each additional hour removes less than the hour before, and the cycle is stopped at a point chosen by the manufacturer rather than run to zero. Whatever is left at that point is the residual moisture, reported as a percentage of the solid's weight. Published figures for lyophilized pharmaceutical products commonly sit in the low single digits, and the point is not the number but that it is never nil.
Crucially, this is set after the cake's appearance is already fixed. The structure you can see was determined during freezing and primary drying. Secondary drying changes the water content without changing the look. A visibly perfect cake and a visibly perfect cake holding four percent water are the same photograph.
Why it matters twice
Water is a reactant. Deamidation, aspartate isomerisation, backbone hydrolysis and pyroglutamate formation all consume water. Five of the six degradation routes in the post on which residues degrade first need it. Removing water does not slow those reactions in the way cooling does; it removes a required ingredient. Leaving some in leaves the reaction partly supplied.
Water is a plasticiser. A lyophilized cake is a glass, an amorphous solid below its glass transition temperature, in which molecules are effectively immobilised. Water lowers that transition temperature. Enough of it and the glass transition falls toward or below the storage temperature, at which point the solid is no longer a glass, molecules can move, and both chemical degradation and physical collapse become available. This is why residual moisture and storage temperature are not independent variables: the same cake can be stable in a freezer and unstable on a shelf, and how wet it is decides where that line falls.
The two effects compound. Water supplies the reaction and also unlocks the mobility the reaction needs.
How it is measured
The Karl Fischer method is the reference method and is defined in the pharmacopoeial chapter on water determination. It is a volumetric determination in which iodine is consumed in a reaction that requires water stoichiometrically, so the amount of titrant consumed is a direct measure of water and of nothing else. It is specific, it works at low percentages, and it is destructive: the sample used is gone.
Loss on drying is the cheaper alternative. Weigh the sample, heat it under defined conditions, weigh it again, and report the difference as a percentage. It is simple and it is not a water measurement. Anything volatile leaves during heating, including residual synthesis solvents and, in some cases, part of the counter-ion. A loss-on-drying figure is an upper bound on water, not a determination of it.
The distinction is worth holding onto because the two are often quoted interchangeably. They are different chapters, different methods and different numbers, and only one of them is specific to water.
What this library reports, and what it does not
Every field appearing on any of the 114 certificates in this library was enumerated on 5 October 2026. The complete list is identity, identity method, content, purity, purity method, endotoxin, heavy metals, sterility on some lots, retention time and molecular weight.
There is no moisture line. No loss on drying, no water content, no water activity, on any certificate.
That is not an omission by the laboratory. A certificate of analysis reports the tests within its stated scope, and the scope here is identity, purity, content, endotoxin, heavy metals and conditionally sterility, which the dataset post describes across all 114. Water determination is simply not among them, and the certificate does not claim otherwise. The post on what a peptide certificate does not test for places this alongside the other absences, residual solvents and counter-ion content among them.
What follows from that is a small number of real consequences rather than a general worry.
The content figure is unaffected. This is the part most often misunderstood. Content is a measured mass of peptide determined by assay, not a weight of powder, so water in the cake does not inflate it. A certificate reporting 10.76 mg of content is reporting 10.76 mg of peptide whatever else is in the vial. That is exactly why the content line and not the label is the number to use in any calculation, as content versus purity argues.
The purity figure is unaffected. Purity by reversed-phase HPLC is a ratio of peak areas among the species the detector sees at 214 nm. Water is not one of them.
Shelf life is the claim that has no support from these certificates. Without a moisture figure there is no way to read, from the certificate, how far this particular cake is from its glass transition. That is why the expiry date on a vial of this material is a convention rather than a measurement, which is the whole subject of where a lyophilized expiry date comes from.
The closure is doing the work after that. A dry cake is hygroscopic. It will take water from the air through a compromised stopper, and whatever the cycle achieved can be undone in storage. That makes the vial's seal part of the stability story rather than packaging, which is the argument of the stopper and seal post, and it is why repeated entries into a vial are a real consideration.
What a moisture figure would and would not add
It is worth being precise about the value of the number we do not have, because it is easy to overstate.
A Karl Fischer result would tell you how much water is in the cake at the time of test. It would let you reason about the distance to the glass transition and therefore about storage temperature. It would not tell you the peptide's shelf life, which needs a stability study at known conditions rather than a single water measurement, and it would not change identity, purity or content. It would be one more input to a stability argument, not the argument.
So the honest summary is that the absence narrows what the certificate can support, and the things it most obviously supports, what the molecule is, how pure it is and how much of it there is, are untouched.
Frequently asked questions
Does residual moisture mean the vial is wet?
No. It means water remains chemically bound within an otherwise dry solid, typically at a low single-digit percentage by weight. Visible liquid in a vial is a different and much larger problem, usually an incomplete cycle or a breached closure.
Can I see or feel whether a cake holds water?
Not at the levels that matter. The cake's appearance is set during primary drying and the water content during secondary drying, so the two are not linked. A sticky or slumped cake indicates a lot of water, but a normal-looking cake tells you nothing either way.
Does water in the cake mean I am getting less peptide than the label says?
No, and this is the important reassurance. Content is an assayed mass of peptide, not a weight of powder, so moisture does not dilute what the certificate reports. The certificate's content figure is the number to use.
Why does the laboratory not test it?
Because it is outside the stated scope of these certificates, which cover identity, purity, content, endotoxin, heavy metals and sometimes sterility. The certificate reports what it was asked to report, and a water determination was not part of it.
Is loss on drying good enough instead?
It is an upper bound rather than a measurement. Heating drives off everything volatile, including residual solvents, so the figure can exceed the true water content. The Karl Fischer method is specific to water and is the reference method.
What should I actually do about it?
Three things, none dramatic. Use the certificate's content figure rather than the label in any calculation. Keep vials sealed and limit entries, so the closure keeps doing its job. And treat the expiry date as a convention rather than a measured endpoint, because without a moisture figure that is what it is.
References
- Pepstral certificate library. The reported fields across all 114 certificates in src/coa.json were enumerated on 5 October 2026: identity, identity method, content, purity, purity method, endotoxin, heavy metals, sterility, retention time and molecular weight. No water, moisture, loss-on-drying or water-activity field appears on any certificate. pepstral.com/coa.html
- United States Pharmacopeia. General Chapter <921> Water Determination, which defines the Karl Fischer methods, and General Chapter <731> Loss on Drying. www.usp.org
- Tang X, Pikal MJ. Design of freeze-drying processes for pharmaceuticals: practical advice. Pharmaceutical Research 2004;21(2):191-200. Secondary drying as the determinant of residual moisture. doi.org/10.1023/b:pham.0000016234.73023.75
- Carpenter JF, Chang BS, Garzon-Rodriguez W, Randolph TW. Rational design of stable lyophilized protein formulations: theory and practice. Pharmaceutical Biotechnology 2002;13:109-133. Water as plasticiser and the glass transition consequence. pubmed.ncbi.nlm.nih.gov/12596620
Every product mentioned is sold for laboratory research use only and is not for human or animal use. Nothing on this page describes or recommends use of the material sold here in humans or animals.
