What does a Karl Fischer water content test add for liraglutide that a purity figure does not?

Concretely: a Karl Fischer water content test · liraglutide.

These are treated as interchangeable and I do not think they are.

If both are acceptable I would like to know that, so I can stop thinking about it.

Under what conditions does the answer flip?

The part that matters: quantified content is how many milligrams of peptide are actually in the vial, measured against a calibrated reference standard, and it is the only measurement that directly answers that question.

System suitability for a quantitative method is stricter than for purity because a small systematic error in the standard directly translates into an error in the sample result.

If the standard and sample have different absorption coefficients at the detection wavelength, the response factors differ and the inference fails.

Pharmacopoeial guidance on quantitative methods specifies validation steps for linearity, range, accuracy and precision that most research-grade work does not claim to meet.

If a supplier gives you content without the standard's purity, ask them to provide it.

edited 30 Jun 2026 by haze_check — added the method parameters

Mechanically, the short answer is that purity says "what fraction of detected material is the target" while content says "how many milligrams of the target are present," and those are two different things.

The single most common reason for disagreement between a supplier content figure and an independent assay is using different standards.

Comparing content results from different laboratories requires knowing whether they both used certified reference materials or whether one used an in-house standard of unknown provenance.

Quantitation against a standard requires that the standard be traceable to a national metrology institute, and certificates for research-grade standards claim that traceability.

I would not rely on a content figure from a lab that is not willing to state their standard's purity on request.

The practical summary: if you are ordering from a new supplier, budget for content assay on the first lot.