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Jul
A certificate of analysis (COA) is the document that turns a supplier’s purity claim into something you can check. Most research buyers never read past the headline percentage — which is a shame, because the headline number is the least informative part of the document. This guide walks through what each section means, how to verify a COA belongs to the material in your hand, and which patterns should give you pause.
Research use only. This article concerns analytical documentation for laboratory research compounds. It is not medical guidance.
A COA is a batch-specific analytical record. The critical word is batch-specific: it describes one production lot, tested on one date, not the compound in general. A COA that cannot be tied to the lot number on your vial tells you nothing about your material.
A COA issued by an independent laboratory rather than the manufacturer is materially stronger evidence, because the party making the purity claim is not the party verifying it.
The product name, lot or batch number, manufacture date and test date. Cross-check the lot number against the label on your vial. This is the single most important check and the one most often skipped.
Usually recorded as “white to off-white lyophilized powder”. Deviations — yellowing, discolouration, visible particulate — are worth querying, though a barely visible cake at low fill masses is normal and not a defect.
High-performance liquid chromatography separates the sample into its components. Detection is typically by UV absorbance at 214 nm (the peptide bond) or 280 nm (aromatic residues). Purity is reported as the area of the main peak as a percentage of total peak area.
Two things follow from that definition, and both matter:
This is why a 99% peptide can still be, say, 85% peptide by total vial mass once water and counterion are accounted for. Neither figure is dishonest; they measure different things.
Mass spectrometry confirms the compound is what it claims to be. The observed molecular weight should match the theoretical molecular weight for the sequence, typically within a fraction of a dalton for high-resolution instruments.
HPLC without MS tells you the sample is pure. It does not tell you it is the right compound. A COA with purity data but no mass confirmation leaves the identity question open — this is the most common substantive gap in COAs circulating in this market.
Usually determined by Karl Fischer titration and reported as a percentage. Lyophilized peptides are hygroscopic and typically retain a few percent residual water. High water content reduces net peptide per vial and can shorten shelf life.
Synthetic peptides are commonly isolated as salts — most often trifluoroacetate (TFA) from purification, sometimes acetate or hydrochloride. Counterion mass is real mass in the vial that is not peptide.
TFA is worth knowing about specifically: it is cytotoxic at sufficient concentration and can interfere with cell-based assays. If your work is cell-based, the counterion identity on the COA is not a footnote.
Where reported, this is the fraction of total vial mass that is actually peptide, after water and counterion. It is the number most directly relevant to concentration calculations, and it is the number least often published.
If the COA includes the chromatogram rather than only the summary table, look at the trace itself:
Check that the reported retention time and the integration window are shown. A purity figure quoted without the underlying trace cannot be independently assessed.
Routine synthesis and purification for research-grade material typically lands in the 95–99% range by HPLC peak area. Higher figures are achievable for short, well-behaved sequences; unusually long or hydrophobic sequences are often lower.
Not necessarily. HPLC purity is a ratio among UV-detected species. Net peptide per vial depends additionally on water content and counterion mass. Two vials both certified at 99% can differ in actual peptide content.
It contributes mass that is not peptide, and it can be biologically active in its own right. Trifluoroacetate in particular can affect cell viability assays at concentrations reached in normal use.
It is better than none, but the party producing the material has an interest in the result. Independent third-party testing removes that conflict, which is why we publish third-party reports rather than in-house figures.
Our published COA library is organised by compound and lot. Match the lot number printed on your vial. If the document you need is not listed, contact us with the lot number.
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