A COA says 99.2% purity and 6.1 mg content — is that contradictory?

This is my second independent submission on material from the same supplier.

I have the document in front of me and I can read the numbers. What I cannot do is interpret them.

I am reasonably comfortable with statistics and completely uncomfortable with chromatography, or vice versa.

What does this actually establish, and what does it not?

No — 99.2 per cent purity and 6.1 mg content are answers to different questions and cannot contradict each other. Purity is a ratio: 99.2 of every 100 units of detected peak area, with 0.8 per cent belonging to something else. Content is a mass: 6.1 mg of peptide in the vial, measured against a standard. A vial can be 99.2 per cent pure and hold 6.1 mg because the things that make up the difference between gross fill and 6.1 mg — water, counter-ion, a short fill — are not impurity peaks and never appear in a purity figure. Against a nominal 10 mg label, 6.1 mg is 61 per cent of claim, and that is the number that changes your dosing arithmetic. The 99.2 per cent does not.

Put another way, what a useful certificate carries is debatable, but there are certain fields that almost no research-grade certificate includes despite their being standard in pharmaceutical work.

Writing the lot number on your vial in permanent marker is worth doing because it later proves that the certificate traces to your material.

Batch versus lot is a semantic distinction that determines whether one certificate applies to ten vials or to one, and checking this distinction is worth a minute.

Research-grade certificates carry no regulatory backing and no standing in quality systems, and that limitation is worth being explicit about.

Worth noting that none of this is medical advice, and a compound sold for research use carries no regulatory approval in any jurisdiction.

My working rule is that a document without a lot number, a method section and a test date is marketing.

The certificate is only as useful as the lot number, because without traceability it is gossip about someone else's material.

The field you have least reason to trust is the one that looks most impressive, because appearance and accuracy are not correlated.

Mechanically, a purity figure to two decimal places with no supporting chromatogram is giving false precision and is a red flag on the same level as spelling errors.

Industry guidance on this topic is consistent because it comes from first principles rather than convention.

If the certificate does not trace to your vial, it is not evidence about your vial.

What makes a certificate useful is not its appearance but whether you could hand it to another laboratory and they would reproduce the result.

Lot number visibility is the first check, and it is the one most certificates fail — the number is not printed on the vial, or it is on the certificate but the vial is unmarked.

Stated carefully, multiple testing on the same lot reported in a single document suggests internal quality control; identical figures across different lots suggests the same certificate reused.

Be careful not to over-read this — a certificate that is imperfect is not worthless, it is just a starting point for thought rather than an endpoint.

The practical summary: check the lot number, ask for the method, and reserve judgement until you have both.

edited 11 May 2026 by tobias_maartens — tightened the wording; no substantive change

Start from what the certificate claims to measure and what the claimed measurement actually answers, because those are two different questions.

The certificate is a snapshot and nothing else — it tells you about the material on one day from one sample of one lot, and extrapolation beyond that requires logic.

Published inter-laboratory comparisons of the same material routinely find spreads of half a per cent to a full per cent, which sets realistic expectations.

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.

A useful certificate has five elements: the lot number, the analytical method, the reported result with units, the date of analysis, and the primary data or a reference to it.

Test results from a material supplier are different from independent results in ways that matter for interpretation, even if both are accurate.

The caveat is that a certificate is not a guarantee, it is a data point, and treating it as anything else is asking for disappointment.

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.