A peptide certificate of analysis (COA) exists to answer two questions: is the material in the vial the molecule on the label (identity), and what fraction of the peptide content is that molecule rather than related impurities (purity)? Identity is established by mass spectrometry; purity by HPLC. Everything else on the document — appearance, lot number, counter-ion, storage note — is supporting metadata. Knowing how to read a peptide COA turns it from a PDF you file away into the single most useful quality signal a research buyer has. Every catalog peptide here ships with a lot-matched COA; this guide walks the document top to bottom.
The header: product, lot and date
Check three things before reading any numbers. First, the product name and catalog identity should match your vial exactly, including salt form and any modification — "acetate", "amide", a fragment numbering like 176-191. Second, the lot number on the certificate must match the lot printed on your vial label; a COA for a different lot is marketing, not evidence. Third, the analysis date and, where given, a retest/expiry date tell you how current the data are. Cross-checking the lot is also the core of third-party testing verification.
Identity: the mass spectrometry section
The MS section states a theoretical (calculated) mass from the molecular formula and an observed mass from the instrument, usually by ESI-MS. Reading it is a one-step check: the observed mass should match the theoretical mass within the method's tolerance, typically well under 1 Da for research-scale QC of small peptides. For example, BPC-157 (C62H98N16O22) has a monoisotopic/average mass around 1419.5 g/mol; an observed 1419.6 confirms identity, while a mass off by +42 (an unremoved acetyl) or −18 (a dehydration product) tells you exactly what is wrong. Multiply-charged species — [M+2H]²⁺ and so on — are normal in ESI spectra and the deconvoluted mass is what counts. The deeper mechanics are in mass spectrometry and peptide identity.
Purity: the HPLC section
HPLC purity is the area of the main chromatographic peak as a percentage of total peak area, usually by reversed-phase HPLC with UV detection around 214–220 nm, where the peptide bond absorbs. A ≥99% figure means related peptide impurities — deletion sequences, oxidation products, truncations — sum to under 1% of chromatogram area. Two reading habits matter: note the method conditions (column, gradient, wavelength) because a purity number is only comparable within a method, and look at the chromatogram itself if reproduced — a clean single peak with flat baseline is worth more than a bare percentage. What the number does and does not capture is unpacked in HPLC purity explained.
The line every buyer misreads: purity vs net peptide content
HPLC purity is a percentage of the peptide fraction. The gross powder in a vial also contains bound water, residual solvent and the counter-ion — commonly TFA or acetate from purification. Net peptide content (typically 70–90% by mass, determined by amino acid analysis or nitrogen assay when reported) states how much of the gross mass is actually peptide. A vial can honestly be ≥99% pure while only ~80% of its milligrams are peptide. For molarity-critical work, calculate from net content when the COA provides it; the arithmetic lives in molecular weight, moles and molarity.
Line-by-line reference
| COA field | What it establishes | What to check |
|---|---|---|
| Product name / sequence | What was ordered | Matches label, salt form, modifications; sequence notation per peptide sequences explained |
| Lot number | Traceability | Identical to vial label |
| Appearance | Gross physical state | "White lyophilized powder" (blue for copper peptides like GHK-Cu) |
| MS observed vs theoretical | Identity | Agreement within stated tolerance |
| HPLC purity | Peptide-fraction purity | Meets spec (≥98% or ≥99%); method stated |
| Net peptide content | True peptide mass | Use for molar calculations when present |
| Counter-ion / TFA content | Salt form | Relevant for sensitive cell assays |
| Water content | Residual moisture | Part of gross-mass accounting |
| Storage / retest date | Stability frame | Follow with storage practice |
Red flags on a peptide COA
- No lot number, or a lot that does not match the vial. The document then certifies nothing about your material.
- Identity by HPLC retention time alone, with no mass spectrum. Co-elution of a wrong or truncated sequence is exactly what MS exists to rule out.
- A purity claim with no method information — no column, wavelength or gradient. Unverifiable numbers are decoration.
- Implausibly clean documents: 100.00% purity, reused chromatogram images across lots, or a certificate with no analysis date.
- Vendor-only paperwork for high-stakes work. An in-house COA is the baseline; independent confirmation is the upgrade, which is why the vendor checklist puts third-party verifiability near the top.
Worked read-through
Take a hypothetical certificate for a 10 mg vial of semaglutide (CAS 910463-68-2, C187H291N45O59, MW 4113.58). Header: product and lot match the vial, analyzed this year. Appearance: white lyophilized powder — matches. MS: theoretical 4113.6, observed 4113.5 — identity confirmed. HPLC: 99.3% main-peak area at 220 nm on a stated C18 gradient — meets the ≥99% specification. Net peptide content: 84%. Interpretation: the vial contains the right molecule at high purity, and for molar work you would treat the vial as 8.4 mg of actual peptide, not 10. That last step is the one most readers skip, and it is the difference between a COA that is filed and a COA that is used. The same logic applies to every format in the store, from lyophilized vials to the raw material behind sprays and capsules.