Is loss on drying interchangeable with Karl Fischer?

I have both a purity figure and a content figure, which is why the discrepancy is visible.

I am trying to choose between two options that are usually discussed as though only one exists.

I am not optimising for price, but I am not indifferent to it either.

Which axes does this decision turn on?

The short version: Karl Fischer for water specifically, loss on drying for all volatiles, and the gap between them is your solvent estimate.

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.

Mass shifts and what they usually mean

Stated carefully, a collapsed cake retains more residual moisture than a properly formed one, because the reduced surface area slows secondary drying. Appearance and water content are therefore linked.

The caveat is that a water figure without a method statement is not interpretable, since the two methods give different numbers.

A collapsed cake holds more water. Appearance and moisture are linked.

The part that matters: residual moisture also predicts solid-state stability better than any expiry date, which is a second reason to want the number.

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.

If content and purity are both good but the gross mass does not reconcile, water plus counter-ion is where the missing mass usually is.

A single water figure on one vial says nothing about lot homogeneity.

Water plus counter-ion is where the missing mass usually is.

A vial that reads low on content assay often reads high on water, and the two together explain the discrepancy.

Residual moisture is the dominant variable in solid-state stability. Above roughly five per cent, hydrolysis and deamidation proceed measurably faster than they do at two.

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.

Residual moisture is established as the dominant determinant of solid-state stability in lyophilised peptide and protein formulations.

Residual moisture predicts stability better than any printed date.

edited 3 Nov 2025 by stopper_core — clarified the distinction between purity and content

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.

Nothing here is medical advice, and research-use compounds are not approved for human use.

Two to six per cent is normal. Above that, the cycle was short.

edited 3 Nov 2025 by tare_weight — added the citation requested in comments

Answer first: residual water is part of the mass on the label and not part of the peptide, and for a lyophilised peptide it typically runs between two and six per cent.

Worked reconciliation: a vial with a gross content of 10.0 mg at four per cent water and three per cent trifluoroacetate contains 10.0 × 0.93 = 9.3 mg of peptide. That is a seven per cent shortfall against label with nothing wrong anywhere.

Typical residual moisture ranges after lyophilisation are published in formulation literature, with pharmaceutical targets around one to three per cent.

Ask for Karl Fischer specifically, not loss on drying, and ask for both if you can.