How does reference-standard error propagate into a cagrilintide content figure?

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

I can do the algebra. I am not confident about the conversion factors.

If there is a standard way to lay this out, I would rather learn that than invent one.

Can someone walk through the arithmetic step by step?

Start from the certificate on the standard itself, because that tells you the uncertainty in every result that uses that standard.

Storing a standard in a freezer is better than storing it at room temperature, but how much better depends on the chemical stability of the standard.

Put another way, using a standard that is older than its certificate validity window is using a material of unknown purity, which defeats the purpose of having a standard.

Ask for the standard's purity when you buy it.

edited 22 Jan 2026 by RP_C18 — removed a claim I could not source

Specifically, the standard is not the sample, and the standard is not a stability indicator — it is a measurement baseline against which all inference depends.

A reference standard that has been opened and used many times is more exposed to contamination and moisture than a fresh standard is.

The relevant detail is that blank injections using only the mobile phase should give a clean baseline, and any peak in the blank is a contaminant that will affect your sample result.

The pharmacopoeial standards specify system suitability requirements for quantitative methods that include replicate injection agreement and that agreement should be tighter than two per cent.

The practical summary: use a standard of declared purity and verified traceability, and check that the certificate is current.

More usefully, a standard of low purity is worse than no standard at all, because low purity introduces a systematic error that is invisible without independent verification.

Comparing your quantitation result to a standard certificate requires that the sample purity be accounted for — if you measure a 95 per cent pure sample against a 99 per cent pure standard, the content result is biased.

If you only pay for one test, pay for quantified content. Purity is the number everyone quotes and content is the number that changes what you do.

Worth being precise here: the cost of a certified standard is cheaper than the cost of one wrong conclusion based on an unverified standard, which is the argument for using them.

A standard of uncertain purity used to measure an unknown sample produces a result whose uncertainty is larger than you will have estimated.

The ICH Q3A and Q3B thresholds for reporting, identification and qualification of impurities are the framework the pharmaceutical industry works to, and they are worth reading even though nothing in the research-grade supply chain is obliged to meet them, because they tell you which numbers a competent analyst would consider worth reporting at all.

In practice: ask for the chromatogram, check the method section, check the lot number against the vial, and set your accept threshold before you see the result rather than after.