A certificate of analysis records what a laboratory measured on a sample of one lot, on the day it tested it. It is not a description of the product line, not a guarantee, and not a statement about the vial in your hand unless the lot numbers match.
Read in the right order, it takes about two minutes. This is that order.
Start with the lot number, not the results
Find the lot on the certificate and the lot on the vial, and confirm they are the same string. If they differ, nothing else on the page describes your material and the remaining thirteen fields are irrelevant.
This sounds trivial and is the single commonest failure in practice, because suppliers frequently publish one certificate per product rather than per lot. Lot traceability and recordkeeping covers why the lot is the unit that matters.
Then the two dates
A certificate carries a date the sample was received and a date the report was issued. Received must come first, and the gap between them is informative: a sterility result needs incubation time, so a certificate reporting sterility with a two-day turnaround is reporting something other than a completed culture.
A single date, with no receipt date, removes your ability to make that check at all.
Identity, which is the line that matters most and is checked least
Identity answers what the compound is. Purity answers how much of what is present is that compound. A certificate reporting 99.5% purity and no identity test is telling you that something was very pure.
Identity is established by mass spectrometry, and the stronger form fragments the molecule and reads a sequence ladder rather than only weighing it. Mass spectrometry and peptide identity covers the difference, which matters because two different compounds can share a mass.
| Field | What it measures | What it does not tell you |
|---|---|---|
| Lot number | Which batch this document describes | Anything, if it does not match your vial |
| Date received | When the laboratory got the sample | When the material was made |
| Date issued | When testing finished | How long the material has been in storage |
| Identity | What the compound is | How much of it there is |
| Purity | Share of peptide-related material that is the target | What the remainder is, or what the compound is |
| Content | Measured mass of peptide in the vial | Mass of powder, which is higher |
| Endotoxin | Below a stated limit | A measured value, when reported as a limit |
| Heavy metals | Below a stated limit | Which metals were screened, unless listed |
| Sterility | No growth recovered in culture | Anything about endotoxin |
| Appearance | What the laboratory saw | Whether the cake holds water |
| Retention time | Where the peak eluted | Anything lot-specific; it is compound-characteristic |
| Method per result | How each number was produced | Nothing; its absence is the problem |
The fields a complete certificate carries, and the limit of each.
Content is the number to put in a calculation
The label is nominal. Content is measured. A vial labelled 10 mg whose certificate reports 10.4 mg contains 10.4 mg of peptide, and the powder weighs more than either figure because of counter-ion and residual water.
Using the label where the certificate gives content is the most common arithmetic error in this area. Net peptide content covers it, and reconstitution arithmetic works through the consequences.
Three lines that routinely mislead
- Endotoxin reported as "<0.05 EU/mL". That is the assay's reporting limit, the lowest number it will state. It establishes a ceiling, not a measurement, so it cannot rank one lot against another.
- Purity on a blend. A blend certificate usually reports purity and content for one component, so a two-compound 20 mg vial can show roughly 10 mg of content and be entirely correct. Blends and single vials explains the reading.
- Appearance described as a white lyophilised powder. That is what the laboratory saw, and it says nothing about residual moisture, which is fixed after the cake's appearance is already set.
The method line is the completeness test
A result without a named method is a number without a meaning. "Purity 99.6%" is incomplete; "purity 99.6% by RP-HPLC at 214 nm" is a measurement, because the method and the detection wavelength determine what the figure is a ratio of. Scan the document for results that carry no method. That one pass tells you most of what you need to know about how seriously it was produced.
Read the panel for what is absent, not only what is present
A certificate is a positive document: it lists what was measured and says nothing about what was not. The useful habit is to scan for the tests you expected and did not find, because that absence is the real difference between two suppliers quoting the same compound.
Endotoxin is the line most often missing, because it costs money and most research work does not strictly require it. Identity is the line whose absence matters most, because without it purity describes an unknown substance. What a certificate does not test for covers the rest of the usual gaps.
Why retention time is on the page at all
It is the time the compound took to come off the chromatography column, and it is characteristic of the molecule rather than of the batch. A short peptide that barely interacts with the column elutes early; an acylated compound carrying a fatty chain elutes late.
That makes it a consistency check rather than a quality measure. The same compound tested by the same method should land in roughly the same place every time, and a retention time that has moved substantially between lots, with the method string unchanged, is worth a question. Lot-to-lot consistency covers what does and does not legitimately vary.
What to do when something does not reconcile
Ask the supplier, in writing, and keep the answer with the lot record. A discrepancy that is explained is a documented anomaly; one that is not is an open question attached to material you have already used. Comparing suppliers on documentation covers what a good answer looks like.
This sits inside the wider picture of what gets tested and why, which why peptide testing matters sets out across the whole analytical panel.