Almost every research peptide vendor claims 99% purity, and almost none of that claim is independently checkable from the homepage. The nine checks below are ordered by how much signal they carry per minute spent, and every one of them is about documentation rather than marketing.
1. Lot-specific COAs, not a generic PDF
The first question is whether the certificate of analysis changes when the lot changes. A single PDF served for every unit of a product is a specification sheet, not a certificate. A real one names a lot number that matches the vial, carries a date of analysis, and reports results from a run on that batch. If a vendor cannot produce the COA for the specific lot you received, batch traceability does not exist and neither does reproducibility. The reading method is in how to read a peptide COA.
2. Mass spectrometry, not chromatography alone
HPLC answers "how much of this sample is one thing". It does not answer "is that thing the peptide on the label". Only mass spectrometry — or an orthogonal identity method — closes that gap. A COA with a beautiful chromatogram and no mass data has verified homogeneity, not identity. This distinction catches the single most consequential failure mode in the category: correctly pure, wrong molecule. See mass spectrometry and peptide identity.
3. Stated method conditions on the chromatogram
Column chemistry, gradient slope, run time and detection wavelength determine what a purity number means. A steep gradient hides closely eluting impurities under the main peak; detection at 280 nm barely sees a sequence without tryptophan or tyrosine. A vendor that publishes conditions is inviting scrutiny; one that publishes only a percentage is avoiding it. Background in HPLC purity explained.
4. Independent third-party verification
In-house testing is necessary and not sufficient. The meaningful signal is whether an external laboratory has run the same lot and whether that report is published rather than merely referenced. "Third-party tested" as a badge with no laboratory named and no report attached is a claim about a claim. What good verification looks like, and how to commission your own, is set out in third-party testing explained and defined at third-party testing.
5. Net peptide content disclosure
A vial labelled 5 mg contains 5 mg of solid, not 5 mg of peptide. The difference is counter-ion, residual water and any excipient, and for basic sequences purified as trifluoroacetate salts it is not a rounding error. A supplier that states net peptide content is one whose molarity figures you can rely on; one that never mentions it is leaving a systematic overestimate in every calculation downstream.
6. Complete product data as a default
Sequence, CAS number, molecular weight, molecular formula where relevant, purity method and available sizes should be on the product page, not available on request. This is a surprisingly good proxy for everything else, because a vendor that maintains structured specification data has a data model behind the catalogue rather than a spreadsheet of prices. Cross-check the specification against the COA and against the mass spectrum — three numbers that must agree.
7. Size range and per-milligram transparency
Multiple sizes per molecule signal that a supplier serves laboratories with different sample requirements rather than a single consumer format. It also lets you compute cost per milligram and compare honestly across vendors, which headline prices obscure. Small sizes matter for method development; large sizes matter once a protocol is fixed. Browse the range across the full catalogue.
8. Storage and shipping practice that is actually described
Ask what temperature the material ships at, what the transit protection is in summer, and what condition the cake should arrive in. Lyophilised peptides are far more heat-tolerant than most buyers assume, but a collapsed or melted cake is a real signal and a vendor should tell you what to do about it rather than treating it as a complaint. The physics is covered in shipping and cold chain, and the condition-by-condition guidance in how to store peptides.
9. Labeling discipline as a legitimacy signal
This one is counter-intuitive but reliable. A supplier operating properly in the research reagent category does not publish human-use instructions, outcome imagery, testimonials or protocol schedules, because doing so would recategorise its products regardless of any disclaimer. A site that reads like a pharmacy is telling you it has not thought carefully about the category it operates in — which is also a statement about how carefully it thinks about everything else. The regulatory framing is in what research use only means.
Scorecard
| Check | What good looks like | Red flag |
|---|---|---|
| COA | Lot-specific, dated, matches vial | One PDF for every unit sold |
| Identity | MS with observed mass stated | Chromatogram only |
| Method | Column, gradient, wavelength published | A bare percentage |
| Independence | Named external lab, report attached | Unsupported "third-party tested" badge |
| Content | Net peptide content stated | Gross vial weight only |
| Specifications | Sequence, CAS, MW on the page | "Available on request" |
| Sizes | Three or more per molecule | Single consumer-style format |
| Logistics | Documented shipping and storage conditions | Silence, or blame on the carrier |
| Labeling | Specification-led product pages | Protocols, testimonials, outcome imagery |
How to run the check in ten minutes
Pick one product you know well — BPC-157 is a good test case because its molecular weight of 1,419.55 Da and 15-residue sequence are easy to verify. Open the product page and note the stated sequence, CAS and molecular weight. Request the COA for a current lot. Check that the observed mass on the spectrum corresponds to the stated molecular weight, that the chromatogram carries axes and conditions, and that the lot number appears in all three places. A vendor that survives that exercise on one product will usually survive it on the rest. The longer institutional checklist is in how to choose a research peptide vendor, and ordering and logistics questions are collected in the shipping and orders FAQ.