Verifying a certificate of analysis.
A certificate of analysis is the document that connects a claim about purity to the material in front of you. This page explains what appears on one, which parts carry weight, and which are worth nothing at all, so that you can assess any supplier's paperwork rather than taking a number on trust.
Purity of what, exactly
A purity figure is a percentage of a total, and the total depends entirely on the method used. HPLC with ultraviolet detection reports the target peak as a share of everything else the detector registered at that wavelength. Anything that does not absorb at that wavelength is not counted, which includes much of the residual solvent and most inorganic salt.
A stated purity of 99% by HPLC therefore describes 99% of what the method measured rather than 99% of the contents of the vial. Material can be 99% pure by peak area and still carry meaningful water and salt weight. This is normal for lyophilised peptide, and it is the reason net peptide content is a separate question from chromatographic purity.
The chromatogram carries the evidence
The summary table on a certificate states a conclusion. The chromatogram is the data that conclusion was drawn from. A clean run shows one dominant peak against a flat baseline, with any impurity peaks clearly separated rather than merged into the main one. A shoulder on the principal peak usually indicates a closely related impurity, often a deletion sequence, sitting underneath the figure being reported.
A certificate that reports a number without attaching the chromatogram is showing you the conclusion while withholding the working.
Mass confirmation answers a different question
Chromatography establishes how much of one component is present. Mass spectrometry establishes whether that component is the compound it is supposed to be, by comparing the observed molecular mass against the theoretical mass for the intended sequence. A report carrying purity without mass confirmation has demonstrated that the vial contains a large quantity of something, without demonstrating what.
Matching the report to your vial
A certificate covers one production batch rather than a product line. It should carry a batch or lot number, and that number should correspond to the label on the vial you received. A generic certificate with no batch number, or one whose batch number does not match what arrived, establishes nothing about the material in your possession.
The test date deserves the same attention. A report dated three years ago describes a batch synthesised three years ago, whatever the date on the file itself.
Who carried out the testing
There is a meaningful difference between a report issued by an independent laboratory and a document produced by the manufacturer about its own material. The second is not worthless, but it amounts to a supplier assessing its own work. An independent report names the laboratory that performed the analysis, and that laboratory can be looked up.
Documentation that establishes nothing
- A purity badge printed on a product page with no document behind it.
- A certificate image with the batch number cropped out or left blank.
- A report naming no laboratory, no method and no test date.
- The phrase "third-party tested" unattached to any specific report.
- A chromatogram reproduced at too low a resolution to read the baseline.
Reference ranges
The following is a general guide to what appears on a report for a lyophilised research peptide.
| Field | What you'd expect |
|---|---|
| Method | RP-HPLC, UV detection at 214 nm |
| Identity | ESI-MS or MALDI-TOF, observed mass vs theoretical |
| Purity | Reported as % peak area, with the chromatogram attached |
| Appearance | White to off-white lyophilised powder |
| Lot | Present, and matching your vial label |
| Date | The date the assay was run, not the date of the PDF |