How to read a peptide Certificate of Analysis, field by field

Before walking the fields, hold onto the distinction that organizes all of them, because it's the one most certificates blur. A COA is making two independent claims. The first is identity: the molecule in the vial is the peptide named on the label. The second is purity and content: how much of the material is that peptide, and how much is something else. These are answered by different instruments, and a certificate that reports one is not reporting the other. A purity of 99% tells you nothing about whether the 99% is the peptide you ordered — that's an identity question, and it needs its own measurement.

With that in mind, here is what each field means and how to read it critically.

Identity is established by mass spectrometry, typically electrospray LC-MS, which confirms the intact mass of the molecule and, through fragmentation (MS/MS), verifies its amino-acid sequence.¹ A rigorous COA reports this as an explicit identity confirmation, not merely an implied one. High-resolution MS (LC-HRMS) is the sensitive form: it can confirm composition and profile impurities even when they co-elute chromatographically, working at mass tolerances around 5 ppm.² One honest limit worth knowing: accurate mass alone cannot separate isobaric species — leucine from isoleucine, or iso-aspartate from aspartate — without retention-time context and MS/MS, so identity is a convergence of evidence, not a single number.²

What good looks like: an explicit MS-based identity confirmation. A red flag: a certificate that reports a purity percentage but never confirms the molecule's identity at all.

Purity is almost always reported as a reversed-phase HPLC area-percent, conventionally measured at 214 or 220 nm where the peptide bond absorbs.¹ This is the conventional metric, and it's a useful one — but it is a relative measurement of the UV-absorbing, chromatographically-resolved material, not a mass fraction. It systematically overstates how much peptide is actually in the vial, because anything that doesn't absorb UV is invisible to it, and anything that co-elutes with the main peak is counted as if it were product.

Two things separate a rigorous purity field from a decorative one. First, the method should be disclosed — column chemistry (typically C18), detection wavelength, gradient — because "99%" means little without knowing how it was measured. Second, the actual chromatogram should be shown, so the peak shape and the impurity cluster around it can be seen rather than summarized. A single percentage with no method and no trace is a number asking to be trusted rather than verified.

Here is the field that separates a serious certificate from a cosmetic one, and the one most often missing: net peptide content. A lyophilized peptide is not pure peptide by mass. It ships with counter-ions, bound water, and residual solvent, none of which register on a UV trace. Net peptide content is the mass-balance quantity that accounts for all of it — it identifies and subtracts every non-peptide component to report how many milligrams of actual peptide are present. The pharmacopeial formulation states it directly: purity is computed as the chromatographic-area purity multiplied by the gravimetric fraction, subtracting counter-ion, water, residual solvents, and non-combustible residue.¹

The magnitude of what area-percent misses is not small. For a peptide with several basic residues, the trifluoroacetate (TFA) counter-ion alone can account for nearly a quarter of the vial's mass — for angiotensin II it is close to 25%, quantified separately by validated ¹⁹F-qNMR because TFA never appears in a UV chromatogram.³ A certificate reporting a net peptide content below the label weight isn't showing a defect; it's showing honesty about what area-percent leaves out. A certificate that reports only an HPLC percentage and no net content is quietly letting you assume the powder is the peptide.

The fields that account for that missing mass each deserve a glance. The counter-ion — TFA or acetate — is disclosed because it is real, non-chromophoric mass and because acetate is often preferred over TFA for handling reasons; either way, its identity and amount belong on a rigorous certificate. Water content is measured by Karl Fischer titration, which remains the gold-standard method for water specifically, capturing mass that no chromatographic method sees.⁴ Residual solvents from synthesis and purification are similarly quantified. Orthogonal methods like quantitative NMR are valuable here precisely because their near-universal detection catches analytes — water, solvents — that escape HPLC-UV entirely.⁴ None of this shows up in a purity percentage, and all of it is mass you paid for.

Beyond the peptide itself, a thorough panel screens the material for contamination it could carry. A heavy-metals screen checks for elemental impurities — arsenic, cadmium, lead, mercury — that no peptide-specific method would detect. A rigorous panel may also include endotoxin and microbial screens, which characterize the cleanliness of the material — the level of bacterial endotoxin present and whether microbial growth is detected. Read these strictly for what they are: analytical characterization of the material's cleanliness. They describe the material; they do not qualify it for any particular use, and nothing about their presence on a certificate implies otherwise.

A certificate is only as credible as your ability to confirm it belongs to the vial in your hand. Two fields carry that weight. A lot or batch number ties the certificate to a specific production run rather than to a generic sample. And an independent verification mechanism — a code or link that lets you confirm the certificate is authentic and matches its lot — is what turns a PDF from a claim into a checkable record. A per-lot certificate you can independently verify is a fundamentally stronger document than an undated, unnumbered "sample COA" that could describe anything.

Pulling the threads together, a small set of red flags reliably distinguishes a rigorous certificate from a decorative one. Be skeptical of a certificate that reports a purity percentage with no identity confirmation — purity without an MS identity check answers only half the question. Watch for a bare percentage with no disclosed method and no chromatogram, which asks for trust it hasn't earned. Note the absence of net peptide content, which lets you assume the powder mass is peptide mass when counter-ion and water can be a quarter of it. Be wary of no counter-ion or water disclosure, no lot number, and no way to verify the document. Individually these are gaps; together they describe a certificate doing marketing rather than analysis. A strong COA, by contrast, answers identity and purity separately, discloses its methods, reports net content, accounts for the non-peptide mass, and can be verified against its lot.

Reading a COA well means reading past the headline percentage to the structure underneath it: identity confirmed by mass spectrometry, purity reported as a disclosed-method HPLC area-percent with its chromatogram, net peptide content assigned by mass balance, the non-peptide mass accounted for, contamination screened, and the whole thing tied to a verifiable lot. The number at the top is necessary. Everything below it is what makes the number mean something.