Every reputable supplier provides a Certificate of Analysis, and at a glance they all look much the same: a header, a compound name, a percentage, a signature, occasionally a chromatogram. The uniformity is the problem. It makes a thorough document and a decorative one difficult to tell apart, and the difference matters entirely.
Start with the lot number
Before reading anything else, check that the lot or batch number on the certificate matches the one printed on the vial in front of you.
This sounds trivial and is the most frequent failure. A supplier who publishes one certificate on a product page and ships whatever the current batch happens to be has given you a document about material you do not have. Each synthesis is a separate event with its own impurity profile. A certificate is a statement about one batch and carries no information about any other.
If the certificate carries no lot number at all, it is not a certificate. It is a brochure with a percentage printed on it.
Purity and identity are different questions
This is the distinction that most reading of certificates misses, and it is worth being precise about.
Purity, by HPLC
High-performance liquid chromatography separates the components of a mixture. The sample is pushed through a column, different components emerge at different times, and a detector records what comes off as a series of peaks. Purity is reported as the area of the main peak expressed as a percentage of total peak area.
What that tells you is how much of the sample consists of one substance. What it does not tell you is which substance. HPLC establishes homogeneity, not identity.
Identity, by mass spectrometry
Mass spectrometry ionises the sample and measures mass-to-charge ratio, yielding molecular weight. That figure is compared against the weight calculated from the intended amino-acid sequence. Agreement is good evidence the intended molecule was synthesised.
The consequence
A sample can be 99% pure and still be entirely the wrong compound: one substance, homogeneous, well made, and not what the label says. Purity without identity is an incomplete claim, and a certificate offering only one of the two answers only half the question.
The chromatogram is the evidence
Where a trace is reproduced, look at the whole of it. The baseline should be visible and the time axis should be labelled.
A chromatogram cropped tightly around the main peak has removed precisely the information the assay exists to produce. The interesting part of a purity trace is not the large peak; it is the small ones on either side of it, and whether the tail of the run has been cut off before anything late-eluting could appear.
Fields worth checking
| Field | What to look for |
|---|---|
| Lot / batch | Matches the vial. Non-negotiable. |
| Method | Named explicitly, with column and gradient conditions where possible. |
| Purity | A figure attached to a method, not floating free. |
| Identity | Observed mass against calculated mass. |
| Net peptide content | How much of the vial mass is peptide rather than counterion or water. |
| Date of analysis | Recent, and after the synthesis date. |
| Testing party | Independent laboratory, or the supplier's own bench. |
Net peptide content
A vial labelled 10 mg holds 10 mg of material. It does not necessarily hold 10 mg of peptide.
Synthesised peptides are isolated as salts, commonly acetate or trifluoroacetate, and the counterion carries real mass. Residual water adds more. Net peptide content, usually established by amino-acid analysis, is the proportion that is actually peptide.
The practical consequence is arithmetic: any concentration calculated from the label mass rather than the net figure inherits that discrepancy, and every result downstream inherits it too. Two vials with identical labels and different salt forms do not contain the same quantity of peptide.
Who ran the test
In-house analysis is a supplier grading its own work. That is not automatically dishonest, and a competent in-house laboratory produces perfectly good data, but it is not independent and should not be read as though it were.
Third-party analysis places a laboratory with no commercial interest in the result between the claim and the buyer. Where a certificate is issued by an independent laboratory, its name will be on the document, and it can be looked up.
What a certificate cannot tell you
- How the material was handled after testing. A certificate describes the batch at the moment of analysis. Shipping conditions and storage since then are outside its scope entirely.
- Whether the vial you hold came from that batch. Only the lot number connects the two, which is why it matters more than the purity figure.
- Anything whatsoever about biological effect. A certificate is a statement about chemistry. It says nothing about what the compound does, and any document implying otherwise has stopped being an analytical record.
In short. Match the lot. Confirm both purity and identity were tested and that the methods are named. Look at the full chromatogram. Check net peptide content before doing any arithmetic. Note who ran the test. A certificate that survives those five checks is doing its job.