Why does residual moisture matter if the material looks dry?

Because the water that matters is not visible. A cake can look perfectly dry and still hold several percent moisture by mass, bound within the solid. That water participates in hydrolysis and deamidation regardless of whether the material appears wet, which is why the figure is measured rather than judged by eye.

Is loss on drying an acceptable substitute?

It is an approximation. Loss on drying counts every volatile component as water and can miss tightly bound water entirely, so the number can be wrong in either direction with no way to tell which from the result alone. Karl Fischer is specific to water; loss on drying is specific to nothing.

Does opening a vial change its water content?

Yes. Freeze-dried peptide draws moisture from room air, and in humid conditions the uptake begins immediately. This is why vials are sealed under vacuum or inert gas, and why a moisture figure describes the material as analyzed rather than as it stands now.

Analysis & documentation

Water content, and why Karl Fischer is the method

Updated August 13, 2026

Residual moisture is mass that is not peptide, and it is the main thing that decides how long material keeps. One method measures it specifically; the common alternative only approximates it.

Two reasons the figure matters

Water in a freeze-dried vial counts twice against you, in ways that have nothing to do with each other.

It is mass that is not peptide, so it reduces net peptide content directly. And it is the reagent in the main routes by which peptides break down, so it governs how long the material stays as analyzed. A vial with a clean chromatogram and high residual moisture is clean now and will not stay that way as long as a drier one.

That second point is why the figure belongs beside the purity result rather than in a technical annexe. Purity describes the present. Moisture is the best single predictor of the future on the whole certificate.

What Karl Fischer actually measures

Karl Fischer titration is specific to water. The chemistry consumes water in a reaction involving iodine and sulfur dioxide, and the amount consumed is measured directly. Nothing else in a typical peptide sample participates, which is the whole point.

Two variants appear on certificates. Volumetric titration adds iodine from a burette and suits samples with substantial water content. Coulometric titration generates iodine electrically and measures the current required, which makes it far more sensitive — the right choice for freeze-dried material, where the quantity being measured is small.

A certificate naming the variant is telling you something about the sensitivity of the result. One that reports a moisture figure with no method attached has reported a number without its error bars.

Why not simply weigh it dry

The obvious alternative is loss on drying: weigh the sample, heat it, weigh it again, call the difference water.

It is cheaper, and it is not specific. Anything volatile leaves during heating — residual solvent, trace acetic acid, ammonia — and all of it is counted as water. Meanwhile water bound tightly to the material may not leave at all, and goes uncounted.

So loss on drying can read high and low simultaneously, for different reasons, and there is no way to tell from the number which is happening. It approximates moisture. Karl Fischer measures it.

The sampling problem

Freeze-dried peptide is hygroscopic, which makes measuring its water content awkward in a way most analyses are not: the act of opening the container changes the thing being measured.

Material exposed to room air begins taking on moisture immediately, and in a humid room the change is quick enough to matter. Careful laboratories handle Karl Fischer sampling in a dry environment for exactly this reason, and a figure produced without that care is likely to overstate the moisture rather than understate it.

It also means the number describes the material at the moment of analysis. A vial opened and left standing has a different water content afterwards, and no certificate can speak to that.

Reading the figure

Context for what you are looking at:

Freeze-dried peptides commonly report residual moisture in the low single digits by mass. Well-dried material sits at the lower end of that.
A figure in the high single digits or above suggests incomplete secondary drying, and often accompanies a collapsed cake.
The number should be paired with a method. Coulometric on freeze-dried material; volumetric suggests either a wetter sample or a less sensitive setup.
Compare it against the date of analysis. Moisture measured a year ago describes a vial that has been sealed since — or one that has not.

Common questions

Why does residual moisture matter if the material looks dry?: Because the water that matters is not visible. A cake can look perfectly dry and still hold several percent moisture by mass, bound within the solid. That water participates in hydrolysis and deamidation regardless of whether the material appears wet, which is why the figure is measured rather than judged by eye.
Is loss on drying an acceptable substitute?: It is an approximation. Loss on drying counts every volatile component as water and can miss tightly bound water entirely, so the number can be wrong in either direction with no way to tell which from the result alone. Karl Fischer is specific to water; loss on drying is specific to nothing.
Does opening a vial change its water content?: Yes. Freeze-dried peptide draws moisture from room air, and in humid conditions the uptake begins immediately. This is why vials are sealed under vacuum or inert gas, and why a moisture figure describes the material as analyzed rather than as it stands now.

Check a certificate

Certificates are published per lot. The number printed on a vial retrieves the analysis of that specific batch.

Look up a lot number

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