# Reading a Peptide Certificate of Analysis: Identity, Purity and the Line Most Buyers Miss > Two questions the document exists to answer, one figure almost everyone misreads, and a worked read-through of a real-shaped certificate. Web page: https://peptidemedixeu.com/en/learn/how-to-read-a-peptide-coa/ · Peptide Medix EU · 4 min read · Updated: 2026-08-07 Two questions justify a peptide certificate of analysis (COA): is what sits in the vial the molecule named on the label — identity — and how much of the peptide present is that molecule rather than related impurities — purity? [Mass spectrometry settles identity](https://peptidemedixeu.com/en/glossary/mass-spectrometry/); HPLC settles purity. The rest of the document — appearance, lot number, counter-ion, a storage note — is supporting metadata. Learning to read one converts it from a PDF that gets filed into the most useful quality signal available to a research buyer. Every catalogue peptide here ships with a COA matched to its lot; what follows walks the document from top to bottom. ## The header: product, lot and date Three things come before any numbers. Start with the product name and catalogue identity, which must correspond exactly to your vial, salt form and any modification included — "acetate", "amide", a fragment numbering such as 176-191. Then the lot number, which has to match what is printed on your vial label; a certificate for some other lot is marketing rather than evidence. Finally the analysis date and, where one is given, a retest or expiry date, which tell you how current the data are. That same act of cross-checking the lot is the heart of verifying [testing by an independent laboratory](https://peptidemedixeu.com/en/glossary/third-party-testing/). ## Identity: the mass spectrometry section Two masses appear in the MS section: a theoretical one calculated from the molecular formula, and an observed one from the instrument, typically by ESI-MS. Reading it takes one step — the observed value should fall within the method's tolerance of the [calculated value](https://peptidemedixeu.com/en/learn/mass-spectrometry-peptide-identity/), usually well inside 1 Da for research-scale QC on small peptides. Take BPC-157 (C62H98N16O22), whose monoisotopic/average mass sits near 1419.5 g/mol: an observed 1419.6 confirms identity, whereas a mass out by +42 (an acetyl that was never removed) or −18 (a dehydration product) names the fault precisely. Multiply-charged species such as [M+2H]²⁺ are ordinary in ESI spectra, and it is the deconvoluted mass that matters. Mass spectrometry and peptide identity goes into the mechanics. ## Purity: the HPLC section What HPLC purity measures is the main chromatographic peak's area as a share of total peak area, normally by reversed-phase HPLC with UV detection near 214–220 nm, where the peptide bond absorbs. When a figure reads ≥99%, related peptide impurities — truncations, oxidation products, deletion sequences — together account for under 1% of the chromatogram area. Two habits are worth forming: record the method conditions (column, gradient, wavelength), since a purity figure is comparable only inside one method, and study the [trace itself](https://peptidemedixeu.com/en/blog/read-peptide-coa-like-chemist/) where it is reproduced, because one clean peak on a flat baseline says more than a bare percentage. What that number captures and what it misses is [set out in HPLC purity](https://peptidemedixeu.com/en/learn/hplc-purity-explained/) explained. ## The line every buyer misreads: purity versus net peptide content Purity by HPLC expresses a percentage of the peptide fraction. Also present in the gross powder are bound water, residual solvent and the counter-ion — [usually TFA](https://peptidemedixeu.com/en/glossary/tfa-salt/) or acetate left from purification. Net peptide content, generally 70–90% by mass and determined by amino acid analysis or nitrogen assay where it is reported, states how much of that gross mass really is peptide. A vial may honestly be ≥99% pure while only about 80% of its milligrams are peptide. Where molarity matters, calculate from net content whenever the COA supplies it; the arithmetic sits in molecular weight, moles and molarity. ## Red flags on a peptide COA - A missing lot number, or one that does not match the vial — the document then certifies nothing at all about your material. - Identity claimed from HPLC retention time alone, with no mass spectrum. Ruling out co-elution of a wrong or truncated sequence is exactly what MS is for. - Purity quoted without method information — no wavelength, column or gradient. Numbers nobody can verify are decoration. - Documents that are implausibly clean: purity of 100.00%, chromatogram images reused across lots, or no analysis date at all. - Vendor paperwork alone where the stakes are high. An in-house COA is the floor; independent confirmation is the upgrade, which is why the vendor checklist ranks third-party verifiability near the top. ## A worked read-through Imagine a certificate for a 10 mg [semaglutide vial](https://peptidemedixeu.com/en/learn/what-is-semaglutide/) (CAS 910463-68-2, C187H291N45O59, MW 4113.58). The header: product and lot correspond to the vial, analysed this year. Appearance: a white lyophilized powder, which matches. MS: theoretical 4113.6 against observed 4113.5, so identity is confirmed. HPLC: 99.3% main-peak area at 220 nm on a stated C18 gradient, meeting the ≥99% specification. Net peptide content: 84%. Read together: the vial holds the right molecule at high purity, and for molar work you would treat it as containing 8.4 mg of actual peptide rather than 10. Skipping that final step is what separates a COA that gets filed from one that gets used. The same reasoning covers every format in the store, from lyophilized vials through to the raw material behind sprays and [oral capsules](https://peptidemedixeu.com/en/collections/oral-capsules/). ## Frequently asked questions ### What belongs on a good peptide COA? The essentials are the product name with its sequence or formula, a lot number matching the vial, the date of analysis, appearance, mass spectrometry showing theoretical against observed mass, and HPLC purity accompanied by method details. Stronger certificates add net peptide content, counter-ion or TFA content and water content. The two omissions that disqualify a document outright are missing lot traceability and missing MS identity. ### Does 99% purity mean the vial is 99% peptide? It does not. HPLC purity concerns the peptide fraction alone: 99% of the peptide molecules present are the target sequence. Water, counter-ion and residual solvent are also in the gross powder, so it is net peptide content — frequently 70–90% by mass — that says how many of the labelled milligrams are peptide at all. A vial can truthfully be 99% pure and truthfully 80% peptide by weight at the same time. ### If there is HPLC, why does a COA need mass spectrometry? HPLC reports how clean the material is, not what it is. An entirely different or truncated peptide can give one beautiful peak at some retention time. Mass spectrometry independently establishes that the molecular mass matches the intended sequence, and characteristic shifts of −18, +16 or +42 even identify particular faults such as dehydration, oxidation or stray acetylation. ### What is the counter-ion line telling me? Peptides purified by reversed-phase HPLC leave the column paired with a counter-ion, usually trifluoroacetate (TFA) or acetate, which stays in the lyophilized salt. Two things follow: the counter-ion adds to gross mass, and residual TFA can disturb some sensitive cell-based assays, which is why acetate salts are sometimes specified for work in cells. ### How do I establish that a COA is genuine? Begin by matching the certificate's lot number to the vial, then look for specifics that can be checked: a named analytical method, results that are plausible and not suspiciously round, an analysis date, and chromatogram or spectrum images that vary from lot to lot. Independent verification is strongest of all — send a sample of your lot to a third-party laboratory and compare. Who tests and at what cost is covered in the third-party testing guide. ### Do capsules and sprays come with COAs too? The raw peptide going into those formats is COA-tested identically — MS for identity, HPLC for purity — and the finished product's specifications give per-unit peptide content. For finished formats the additional questions concern uniformity of content and stability within the formulation, which is why format-specific pages quote both the raw-material purity and the amount per capsule or per spray. --- 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.