Laboratory reference · Updated August 2026
How to read a Certificate of Analysis
Many suppliers state that material is lab tested. Fewer publish the document, and fewer still publish the one matched to the batch in the box. This is a plain guide to what each field on a certificate measures, and which parts of the page carry actual evidence.
01 What the document is
A Certificate of Analysis is a report issued by an analytical laboratory describing what a specific lot of material was found to contain. It is tied to one batch rather than to a product line, and it carries a date. Two vials sharing a label but not a lot number have two different certificates.
That distinction carries most of the weight. A certificate that is not matched to the lot in hand describes some other material, tested at some other time.
02 The four questions a certificate answers
Layouts differ between laboratories, but the substantive content generally reduces to four questions.
| Field | What it answers | Typical method |
|---|---|---|
| Identity | Is the molecule present the one named on the label? | LC-MS / MALDI-TOF |
| Purity | How much of the sample is the target molecule rather than related impurities? | RP-HPLC, UV |
| Endotoxin | Is bacterial endotoxin present, and at what level? | LAL assay |
| Traceability | Which lot was tested, on what date, and by which laboratory? | Lot no. + dates |
03 Identity is a measurement, not an assertion
Mass spectrometry separates the sample and measures the mass-to-charge ratio of what comes off the column. A complete report states an observed mass alongside the expected mass, and the two should agree closely. A certificate that declares identity without showing that comparison is asserting rather than demonstrating.
The spectrum itself, or at minimum the observed and theoretical values side by side, is what makes the claim checkable. A line reading "conforms" with nothing behind it is not a measurement.
04 What a purity percentage actually measures
Purity on most certificates comes from reverse-phase HPLC and is reported as area percent: the area of the target peak divided by the total area of every peak the detector registered, at a single wavelength. It is a useful figure, and it is not the same quantity as mass purity.
Anything the detector cannot see at that wavelength never enters the total, which is why water content and residual counterion appear as separate lines on thorough certificates. A sample can report 99% by HPLC area and still carry meaningful water and salt content by mass.
05 Endotoxin is reported separately for a reason
Bacterial endotoxin is not detected by the methods above, so it requires its own assay. The standard approach is the LAL test, and results are normally expressed in endotoxin units per milligram. A certificate that covers identity and purity while remaining silent on endotoxin has simply not answered that question.
06 Dates, lot numbers, and the signing laboratory
A complete certificate names the laboratory that performed the work, carries a lot or batch number, and gives both an analysis date and a retest or expiry date. Independent testing means the laboratory is a different entity from the supplier. That distinction is easy to blur in marketing copy and straightforward to check on the document itself.
07 Six things that invalidate a certificate
- No laboratory named anywhere on the document.
- No lot number, or a lot number that does not match the vial it arrived with.
- No analysis date, or a date that never changes from one batch to the next.
- A chromatogram reproduced as an image with no axis labels, no scale, and no way to read it.
- A purity figure with no method and no wavelength attached.
- "Lab tested" published as a claim on a website, with no document available on request.
None of these establish that a material is defective. They do establish that the certificate is not evidence, and a certificate that is not evidence is a design element.