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Karl Fischer water content on a peptide COA: what % moisture means

Published 6 Nov 2026 · By The QLD Analytical Team

Informational guide for research samples — not medical advice. QLD Analytical arranges independent testing; we do not sell or endorse any product.

Certificates of analysis for research peptides sometimes list a Karl Fischer (KF) water or moisture result as a weight percentage. This guide explains what that figure means, why lyophilised powders still hold water, how moisture interacts with net peptide content, and what to verify before you weigh a sample.

What Karl Fischer measures

Karl Fischer (KF) titration is a specific chemical assay for water. On a research-peptide certificate of analysis it is usually reported as a weight percentage of water in the lyophilised powder. Unlike a simple “loss on drying” figure, KF reacts with water itself, so residual process solvents are not counted as moisture unless the method has been adapted and that is stated.

Two common formats appear on COAs:

• Volumetric KF — suitable when water content is relatively high and sample size is generous.
• Coulometric KF — often preferred for low water levels and small peptide samples, because it generates iodine electrolytically and can resolve smaller absolute amounts of water.

Either way, the result answers one question: how much of the powder’s mass is water under the method used. It does not measure peptide purity, identity or salt form. Those are separate lines on a well-built COA, as outlined in what a COA should show.

Why lyophilised peptides still hold water

Freeze-drying removes bulk water, but it does not produce an anhydrous solid. Peptides are hygroscopic. They can retain bound or surface water after lyophilisation, and they can pick up more from humid air once a vial is opened or poorly sealed. Counter-ions and residual solvents also sit in the same powder matrix, so the material you weigh is a mixture of peptide, salt, residual process chemicals and water.

That is why a vial labelled “5 mg” is a labelled fill of powder, not a guarantee of 5 mg of pure peptide free of moisture. Moisture is one of the reasons net peptide content is usually well below 100% even when HPLC purity looks strong. Related reading: HPLC purity vs net peptide content and our earlier note on residual TFA and counter-ions.

How % water interacts with net peptide content

Net peptide content (NPC) estimates how much of the powder by weight is peptide after accounting for water, residual solvents and counter-ions. A higher Karl Fischer % water lowers NPC for the same HPLC purity and salt form. Two batches of the same sequence can therefore deliver different amounts of peptide per milligram of powder if one is drier than the other.

When you read the certificate:

• Use HPLC purity for the peptide-related peak profile.
• Use mass spectrometry to confirm identity against the peptide mass.
• Use KF water (when reported) as one input to why the powder is not 100% peptide.
• Use net content (when reported) to convert a weighed amount of powder into an estimated amount of peptide.

Do not treat KF % water as a purity score. A sample can be chromatographically clean and still hold several percent water. Conversely, a low water result does not prove high peptide content if the salt load is large.

Reading the water line on a COA

Look for clear method wording, not just a bare percentage:

1. Is the method named (Karl Fischer, coulometric or volumetric)?
2. Are units stated as % w/w (or equivalent) of the sample as tested?
3. Is the result tied to the same batch and sample description as the HPLC and MS lines?
4. If water is “not detected” or below a limit of quantification, is that limit printed?
5. Is net peptide content reported separately, or only water / residual solvent notes?

If moisture is missing entirely, you cannot infer it from HPLC purity. Purity and content remain different measurements.

What to verify before you weigh a sample

Before you use the powder mass in a calculation:

• Match the vial label to the COA batch number and compound description.
• Note the declared salt form and any residual TFA or solvent figures alongside KF water.
• Prefer a reported NPC when you need peptide amount rather than powder amount.
• Minimise open-vial time in humid conditions; water uptake after the COA was issued is not captured on the certificate.
• Record the weighed mass, the COA water / NPC figures you used, and the date the vial was opened.

Those steps will not turn a research COA into a medical document. They simply keep the laboratory arithmetic honest about what was weighed.

Moisture and testing through QLD Analytical

QLD Analytical is a testing intermediary. Samples are analysed offsite by Apex Labs Pty Ltd. Our standard panel covers mass spectrometry identity, HPLC purity and net content. Karl Fischer moisture is not automatically included in every identity-and-purity order. If you need a water figure, check the available tests at order time or ask before you ship the sample.

Enter the compound name, labelled strength and salt form exactly as printed on the vial so the laboratory can interpret mass and content correctly. As with every test we arrange, the result applies only to the sample sent in. Packing and postage details are in our customer guide and FAQ.

Sources

• QLD Analytical — Net content testing (how content differs from purity and why moisture matters).
• QLD Analytical — What a COA should show (identity, purity and content as separate lines).
• Standard analytical references for Karl Fischer titration (volumetric and coulometric) as named on laboratory methods.