# Nine Checks Before You Buy From a Research Peptide Supplier > Eight of them can be run before you place an order. The early ones eliminate most candidates, and the fourth is the one almost nobody runs. Web page: https://peptidemedixeu.com/en/learn/choosing-a-peptide-vendor-checklist/ · Peptide Medix EU · 4 min read · Updated: 2026-05-27 Choosing a research peptide supplier reduces to nine checks you can verify, eight of which can be completed before any order is placed. This is a fragmented market where documentation quality varies wildly, and the most valuable skill is telling apart claims that can be tested against a document from claims that are merely decorative. What follows is written for anyone comparing two or three suppliers of laboratory reference material. Take the checks in sequence — the first few will rule out most candidates. Everything discussed concerns material. ## 1. A certificate tied to your lot, not a generic PDF Only a document naming your lot number counts. A PDF bearing a product name and a purity figure, with no lot, no analysis date and no batch size, is a marketing asset rather than a certificate. Ask for the COA covering the lot that will be shipped, and when it arrives confirm that the lot on the vial matches the lot on the paper. Every other item on this list depends on that match. The anatomy of the document is explained in how to read a peptide COA. ## 2. HPLC purity accompanied by the chromatogram An unaccompanied number cannot be verified. A genuine reversed-phase HPLC report includes the chromatogram, the gradient conditions, the column, the detection wavelength — usually 214 nm, where [the amide backbone](https://peptidemedixeu.com/en/glossary/peptide-bond/) absorbs — and the integrated peak-area table. The trace reveals whether the main peak is symmetric, whether impurities cluster nearby as deletion sequences do, and whether the integration left out a solvent front that should have been included. What the percentage does and does not mean is [explained under HPLC purity](https://peptidemedixeu.com/en/learn/hplc-purity-explained/) explained. ## 3. Identity confirmed by mass spectrometry Where purity asserts that one thing is present, mass spectrometry asserts that the thing present weighs what it should. A COA quoting ≥99% purity without MS data has demonstrated homogeneity but not identity — the weaker of the two claims. For a small peptide, expect an observed mass within a fraction of a dalton of theory, and [consult mass spectrometry](https://peptidemedixeu.com/en/glossary/mass-spectrometry/) and peptide identity on interpreting the result. ## 4. Net peptide content and salt form — the check almost nobody runs Peptides made synthetically are isolated as salts. What a vial labelled 10 mg holds is 10 mg of powder, and that powder contains counter-ions ([TFA](https://peptidemedixeu.com/en/glossary/tfa-salt/) or acetate), residual water and residual solvent. The portion that is genuinely peptide — net peptide content — usually falls between 70 and 90% and is measured by amino-acid analysis or nitrogen determination. A vendor unwilling to state it is inviting you to guess. ### Worked example: the real cost per milligram 1. The headline figures. Vendor A offers 5 mg for €60, or €12.00 per mg. Vendor B offers 10 mg for €95, or €9.50 per mg — apparently 21% cheaper. 2. Now apply net peptide content. A states 92%: 5 × 0.92 = 4.6 mg of peptide, giving €60 ÷ 4.6 = €13.04 per mg of peptide. B states 78%: 10 × 0.78 = 7.8 mg, giving €95 ÷ 7.8 = €12.18 per mg. 3. The outcome. B remains cheaper, but by 7% instead of 21% — and had B declined to disclose the figure, the whole comparison would have been fiction. 4. Where it really hurts. Any molarity calculation based on label mass instead of net peptide content is off by that same 8–30%. In quantitative work this is a question of data integrity, not of price. ## 5. Third-party testing traceable to a lot The phrase "[independently tested](https://peptidemedixeu.com/en/glossary/third-party-testing/)" carries weight only when the third party is identified, the report is dated and the report cites the lot number. An independent laboratory report covering a lot you cannot purchase establishes that the vendor once had a good batch. Who the laboratories are, what they measure and how to verify a report are set out in third-party testing explained. ## 6. Specifications that match the claim being made Every claim calls for its own tests, and a COA ought to include whichever ones the product's positioning implies. ## 7. Consistency across the range, and sensible vial sizes When a catalogue lists the same molecule at 5, 10 and 20 mg with consistent specifications throughout, it is describing a real supply chain. When it lists hundreds of exotic sequences, all in stock, all at ≥99%, all in one vial size, it is usually describing a drop-ship arrangement where nobody has handled anything. There is a practical dimension to size laddering too: the available sizes should let you reconstitute to a working concentration without ridiculous diluent volumes, as [the reconstitution arithmetic](https://peptidemedixeu.com/en/learn/peptide-reconstitution-calculator-explained/) shows. ## 8. The language the vendor uses about use This tests compliance and competence together. A research-material supplier writes about research material: no human protocols, no titration schedules, no before-and-after photographs, no claims to treat or prevent anything. A vendor that publishes guidance for human use has told you it is not operating as a research supplier, and that its documentation habits may be equally casual. ## 9. Logistics, support and the response when something fails - Where stock sits and how long transit takes. Domestic stock cuts transit time and removes customs variability. For lyophilized powder, format matters more than speed — see shipping and cold chain. - Packaging. Each vial should arrive individually protected; broken glass reflects a packaging decision rather than misfortune. - A replacement policy for damaged or mismatched lots, published before purchase instead of negotiated afterwards. - A technical contact capable of answering a COA question. Send one before ordering — the quality of the reply is the most informative signal on this list. - Retention of lot samples. Vendors holding retains can re-test a disputed lot; vendors without them cannot. ## Scoring the checklist Treat checks 1 through 4 as pass or fail: a supplier unable to produce a lot-matched COA carrying a chromatogram, an MS result and a net peptide content figure should not be used for quantitative work at any price. Checks 5 to 7 separate good suppliers from merely adequate ones. Checks 8 and 9 predict what the relationship will be like once something goes wrong, as eventually it does. Across our full catalogue we apply the same documentation practice — COAs matched to lots, HPLC and MS on every lot, [declared purity](https://peptidemedixeu.com/en/glossary/purity/) thresholds and format-specific specifications. Related reading: the purity, COA and testing FAQ. ## Frequently asked questions ### Which check matters most? Getting a certificate of analysis that names the exact lot you will be sent. A document carrying a product name and a purity figure but no lot number, no analysis date and no batch reference cannot be connected to your material, and every later verification step rests on that connection holding good. ### What is net peptide content, and why does it alter the price? Because synthetic peptides come out of purification as salts, a 10 mg vial holds 10 mg of powder inclusive of counter-ions, residual water and residual solvent. Net peptide content is the portion that is truly peptide, typically 70–90%. At 78%, a 10 mg vial contains 7.8 mg of peptide, so a €95 vial works out at €12.18 per milligram of peptide rather than €9.50. ### Is a purity number sufficient without the trace? It is not. The percentage is the output of an integration, and without seeing the chromatogram you cannot judge whether the main peak is symmetric, whether impurities sit close beside it as deletion sequences do, or whether the integration dealt correctly with the solvent front. Ask for the trace together with gradient conditions, column and detection wavelength. ### Does high purity guarantee the right peptide? No. Purity shows that a single species predominates; it says nothing about which species that is. The identity claim comes from mass spectrometry, comparing observed mass with theoretical. A certificate quoting 99% purity but offering no MS data has made only the weaker of the two claims. ### How can a third-party testing claim be verified? It only means something when the independent laboratory is named, the report is dated, and the report quotes the lot number you intend to buy. A report covering a batch that is no longer for sale merely proves the vendor once had a good lot. Request the report for your lot, and confirm that its figures agree with the vendor's own certificate. ### What does a vendor's marketing language reveal? A supplier of research material writes about research material — no human protocols, no titration schedules, no treatment claims and no before-and-after photographs. Any vendor publishing guidance for human use has announced that it is not operating as a research supplier, and loose compliance tends to come packaged with loose documentation. ### Is a very large catalogue reassuring? Not by itself. Hundreds of unusual sequences, every one shown in stock, every one at the same purity and every one in a single vial size, usually points to a drop-ship arrangement in which nobody along the chain has touched the material. Consistent specifications across several vial sizes of one molecule is a better indicator than sheer breadth. ### What should I ask before ordering? Put one technical question about a particular COA to them — say, the net peptide content and counter-ion for a named lot. How quickly, how specifically and how accurately they answer forecasts what support will look like the day a shipment arrives damaged or a lot number fails to match. --- For laboratory research use only. This page is provided for scientific and educational information. Materials referenced here are sold strictly for in-vitro laboratory research by qualified professionals — not for human or veterinary use, and nothing on this page is medical advice.