How to Read a Peptide Certificate of Analysis

A COA is a record that a stated set of analytical tests was performed on a stated lot of material, by a stated party, on a stated date, producing stated results. It is a factual report, not a warranty and not a certification in the accreditation sense of that word. Its authority comes entirely from specificity: who measured what, how, and on which material.

That framing tells you how to read one. Every field either narrows the document to a particular physical batch or describes a particular measurement. Fields that do neither are decoration.

The top of a COA identifies the intended molecule, usually with some combination of product name, one-letter or three-letter amino acid sequence, molecular formula, average molecular weight, and a CAS registry number.

Check these against each other rather than accepting them individually. The sequence, the formula and the molecular weight are not three independent facts — the sequence determines the other two. If a document lists a sequence whose composition does not reconcile with the formula printed beside it, something has been copied rather than measured. This is a genuinely common defect on low-quality paperwork and it takes about a minute to catch.

Watch for modification notation, because it changes the molecule. A leading Ac- indicates an acetylated N-terminus; a trailing -NH2 indicates a C-terminal amide rather than a free acid; D- before a residue denotes the D stereoisomer. Each of those carries a mass consequence, and a COA that omits them while the product is in fact modified is describing a different compound from the one in the vial.

This is the field that makes the rest of the document mean anything. A lot or batch number ties the analysis to one specific run of material, and it should appear both on the certificate and on the physical container. If those two do not match, the certificate describes something you do not have.

Expect at least a date of manufacture and a date of analysis; many certificates also carry a retest date. A retest date is not an expiry in the pharmaceutical sense — it is the date by which the issuer suggests the material be re-examined if it is to be relied on. A COA with no lot number and no dates is a template.

Usually a single line: white to off-white lyophilised powder, or similar. It reads as filler but it is a real specification, and it is the one entry you can verify yourself the moment you open the box. A lyophilised solid may present as a compact cake or as a loose powder depending on vial geometry and the freeze-drying cycle used, and neither indicates a problem. Colour that departs markedly from the stated description, or a solid that has visibly collapsed or gone sticky, indicates moisture ingress and is worth raising before anything else happens to the vial.

The headline figure, and the most widely misread. Purity by HPLC is an area-percent: the area under the main chromatographic peak as a percentage of the total integrated peak area in the run.

Three qualifiers should accompany it, and their absence is meaningful:

• Column and mobile phase. Almost always reversed-phase C18 with a water/acetonitrile gradient and an acidic modifier, typically 0.1% trifluoroacetic acid. Different stationary phases resolve different impurities.
• Gradient and run time. A steep, short gradient pushes everything through together and hides impurities under the main peak. A shallow gradient over a longer run separates them. The same material can report meaningfully different numbers under the two, which is why the method belongs on the certificate.
• Detection wavelength. Usually 214 nm or 220 nm, where the peptide backbone's amide bonds absorb. Note the consequence: anything that does not absorb there is invisible to this measurement. Salts, counter-ions and water do not appear in an HPLC purity figure at all.

A chromatogram printed alongside the number is worth more than the number. On a real trace you can see the baseline, the peak shape, the retention time, and whether small satellite peaks were integrated or quietly excluded. Peaks that shoulder into the main peak are the ones to look at. HPLC vs. mass spectrometry explains what this measurement can and cannot establish.

HPLC answers "how much of this sample is one thing?" Mass spectrometry answers "is that thing the molecule it is supposed to be?" They are separate questions and a COA that reports only the first has left identity unestablished.

The mass spectrometry section should show an observed mass alongside the theoretical mass calculated from the sequence, and the two should agree within the instrument's stated tolerance. For electrospray ionisation, which is standard for peptides, expect a series of multiply charged ions rather than a single peak — a 3,357.9 g/mol peptide may appear as [M+2H]2+ near m/z 1,679.9 and [M+3H]3+ near m/z 1,120.3. A deconvoluted mass is the number to compare against theory.

A mass that is 18 lower than expected suggests a dehydration; 17 lower can indicate an amide where a free acid was expected, or the reverse; a mass short by a whole residue's worth points to a deletion sequence that co-eluted with the target. These are exactly the impurities that chromatography alone is worst at catching.

The field most often missing, and the one that determines how much peptide you actually received.

Lyophilised peptide is not pure peptide by mass. It carries counter-ions from the final purification step — commonly trifluoroacetate or acetate — along with residual water and any salts remaining from synthesis. Net peptide content is the percentage of the vial's total mass that is peptide, and for a basic sequence carrying several positive charges it can sit well below the chromatographic purity figure. Content is determined by a different technique entirely, typically amino acid analysis or quantitative nitrogen determination, not by HPLC.

The distinction matters whenever you weigh out or reconstitute to a target concentration. Peptide purity vs. net peptide content works through the arithmetic and the salt-mass correction.

A COA should say who performed the analysis and carry an authorising signature or an analyst identifier. A certificate issued by the synthesising laboratory is normal and useful; a certificate issued by a laboratory with no commercial interest in the result is stronger evidence, because the party reporting the number is not the party selling the material.

"Third-party tested" as a phrase, unaccompanied by an identifiable testing party, is not evidence of anything. If independent analysis is claimed, the certificate itself should carry the testing organisation's name and contact details.

To make the checklist concrete, here is the same framework applied to our own material. Quill Peptides supplies research peptides at ≥99% purity, verified by reversed-phase HPLC and confirmed by mass spectrometry at an independent third-party laboratory, with a certificate of analysis issued against each batch rather than against the product line.

Read that sentence the way this guide has taught you to read any supplier's. It names a method (RP-HPLC and MS, not "lab tested"). It separates purity from identity. It specifies that the analysis is per batch, which is the claim that generic paperwork cannot make. And it should still prompt you to check the lot number on your vial against the lot number on the document you receive — because that check is yours to make, not ours to assert. A supplier's description of its own process is a starting point for verification, never a substitute for it.

• Purity stated with no method, no column, no wavelength.
• A number but no chromatogram, or a chromatogram cropped so the baseline and axes are gone.
• No mass spectrometry section — identity never established.
• No net peptide content and no salt form, so the quantity in the vial is undefined.
• A blank, generic, or "typical" lot field.
• No analysis date, or a date that is identical across every product in the catalogue.
• No issuing party and no signature.
• The same certificate image reused across different strengths or different compounds.

None of these makes material bad. All of them make a document unable to tell you whether it is good, which for a piece of paper whose only purpose is to tell you that is the same problem.

• HPLC vs. Mass Spectrometry
• Peptide Purity vs. Net Peptide Content
• How to Choose a Peptide Supplier
• Peptide Glossary