A Certificate of Analysis records what one batch of material was measured to be, by one laboratory, on one date. That is a narrower statement than it is usually taken for, and most of the value in reading a CoA well comes from understanding the edges of what it covers.
Check the header before anything else
Four things sit at the top of any usable certificate:
- the material identity, written as a chemical name rather than a marketing one
- the batch or lot number
- the date of manufacture and the separate date of analysis
- who performed the analysis, named well enough to be contacted
If the batch number on the certificate does not match the batch number on the container in front of you, the certificate describes a different material. Nothing further down the page matters until those two agree.
What a purity figure means
Purity on most peptide certificates is an HPLC figure: the area of the main peak as a proportion of the total peak area the method detected. Two words in that sentence carry the weight — area and detected.
It is a relative measure of what the method could see. An HPLC method tuned for peptide-related impurities reports on peptide-related impurities. Things it is not measuring do not appear as impurity, and so do not reduce the number.
Commonly outside the figure:
- water content, which for a freeze-dried material can be several percent by mass
- residual solvents from synthesis and purification
- counterion mass, typically acetate or trifluoroacetate
- elemental impurities
- endotoxin and microbial content
The practical consequence is worth stating plainly: a material can report high HPLC purity and still be substantially water and salt by weight. Peptide purity and net peptide content are different quantities, and a certificate that gives only the first has not told you the second.
Identity and purity are separate questions
Purity describes how much of the sample is the principal component. It says nothing about whether that component is the compound you ordered. Identity is established separately, usually by mass spectrometry confirming the molecular weight, and sometimes by sequencing.
A sample can be pure and wrong. Look for both answers on the page.
Method matters as much as result
A certificate that reports results without stating the methods used cannot be checked or repeated by anyone. Look for the column type and gradient for HPLC work, the instrument and observed against expected mass for MS, and ideally the chromatogram itself rather than a single extracted number.
A chromatogram shows where impurities sit and whether peaks are resolved. A number on its own shows a number.
What a certificate cannot tell you
It is a measurement of a sample at a moment. It does not establish that a material is suitable for any particular purpose, that the batch has been stored correctly in every hand it has passed through since, or anything at all about a different batch.
It also carries no authority it does not name. A certificate is only as good as the laboratory that signed it, which is why the issuer being identifiable is not a formality.
A short checklist
- Batch number matches the container.
- The issuing laboratory is named and reachable.
- The analysis date falls after the manufacture date.
- Methods are stated, not only results.
- A chromatogram is included, not just a figure.
- Identity is evidenced separately from purity.
If you hold a batch supplied by us, you can check it against our own record on the batch verification page.