Conditions: tirzepatide · chromatogram.
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?
Before anything else, check that the lot number on the certificate matches the vial in your hand — that traces back to your material and not to some other lot from some other date.
The test date matters because a certificate six months older than the vial is not evidence about your material, it is evidence about what the material was like half a year ago.
| Test | Answers | Does NOT answer |
|---|---|---|
| RP-HPLC, area % | What fraction of detected material is the target | How much target is present |
| Quantified content | Milligrams of peptide per vial | What the impurities are |
| ESI-MS identity | Whether the molecular weight matches | Purity, or isomeric substitution |
| Peptide mapping | Sequence, localised to a fragment | Quantity |
| Karl Fischer | Water content of the solid | Solvent content |
| LAL endotoxin | Pyrogen load in EU/mg | Sterility |
| Sterility test | Growth in defined media over 14 days | Endotoxin, or bioburden count |
Stated carefully, 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.
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.
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.
Aggregated, published test results and vendor ratings built from submitted batches. Methodology stated, dataset browsable, no listing fees.
Browse resultsA certificate without a method section is giving you a claim rather than a measurement, and claims are not evidence.
The laboratory name and accreditation status can be checked in real time against the testing service's own register, and that two-minute check catches substituted certificates.
It helps to be literal here: comparison across certificates from different laboratories is almost never valid, even when the material is the same, because the methods are almost never the same.
Research-grade certificates carry no regulatory backing and no standing in quality systems, and that limitation is worth being explicit about.
My working rule is that a document without a lot number, a method section and a test date is marketing.
edited 3 Jan 2026 by tobias_maartens — reworded for clarity after a comment
Mechanically, 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.
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.
In practice, 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.
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.
Start from what the certificate claims to measure and what the claimed measurement actually answers, because those are two different questions.
Writing the lot number on your vial in permanent marker is worth doing because it later proves that the certificate traces to your material.
One qualification: a single result from a single vial at a single time is a point estimate with no error bar, and no amount of decimal places changes that.
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.
Ask PeptideStack is a static archive. Posting is closed, but the norms are worth stating: answer the question that was asked, show your working, cite the trial or the certificate, and say plainly where the evidence runs out.