How to read a peptide Certificate of Analysis (COA)

A Certificate of Analysis (COA) is the laboratory report that describes one production lot of a peptide. For a wholesale buyer it is the most important document in the transaction: it is the only evidence of what is actually in the vials. A COA has a small number of sections, and each one answers a different question. This guide walks through them in the order a laboratory reviewer would, with a reference table at the end.

What a peptide COA should contain

Lot number: the first thing to check

A COA is only meaningful for the lot it describes. Before reading anything else, confirm that the lot or batch number on the report is identical to the one printed on your vial label or kit box. A COA labelled "sample", "representative" or with no lot number tells you about some other material. WholesalePeptide.us prints the lot code on every vial and lists each lot's report in the COA library.

HPLC purity: read the chromatogram, not only the number

Reversed-phase high-performance liquid chromatography (RP-HPLC) separates the components of a sample as they travel through a column; a UV detector records each one as a peak. Purity is calculated as the area of the main peak divided by the total area of all integrated peaks. That makes the number only as good as the integration behind it.

• Look for the whole trace. Impurity peaks should be visible, not cropped out of the image.
• Look at the peak table. Retention time and area % for each peak should add up to the reported purity.
• Check the wavelength. Peptide bonds absorb strongly near 214–220 nm; methods at other wavelengths can under-count impurities.
• Check the baseline. A noisy or drifting baseline can hide small peaks; integration should start and end on a flat baseline.

Common impurities in synthetic peptides include deletion sequences (a residue missing), truncated chains, incompletely removed protecting groups, and oxidized forms. HPLC sees them as extra peaks but does not say what they are. Our purity floor is 99% by RP-HPLC; any lot below it is not released.

LC-MS identity: observed vs theoretical mass

HPLC cannot tell you which molecule is in the main peak. Mass spectrometry can. In electrospray LC-MS the peptide picks up protons and appears at several mass-to-charge (m/z) values, one for each charge state: [M+H]⁺, [M+2H]²⁺, [M+3H]³⁺ and so on. Software reconstructs the neutral mass from these. The COA should state the observed mass and the theoretical mass calculated from the sequence.

1. Find the theoretical mass for the exact sequence and modifications. Monoisotopic and average mass differ, so compare like with like.
2. Check that the observed mass agrees within the lab's tolerance (often ±1 Da for average mass on a routine instrument, tighter for high-resolution work).
3. If only m/z values are given, divide (m/z × z) − (z × 1.007) to recover the neutral mass for each charge state; they should agree with each other.
4. Be alert to compounds with shared names. "TB-500", for example, is used for both full-length thymosin β4 (about 4.96 kDa) and a short fragment (under 1 kDa). Only the mass tells them apart.

Net peptide content vs vial weight

A vial labelled 10 mg contains 10 mg of lyophilized powder, but that powder is not 100% peptide. Synthetic peptides are purified with trifluoroacetic acid and are usually isolated as TFA or acetate salts; the counter-ions and residual water add weight. Net peptide content, typically measured by amino acid analysis, nitrogen analysis or quantitative HPLC against a reference, reports how much of the powder is actually peptide.

The figures above are illustrative, not a specification. They show why two suppliers quoting the same purity can ship materially different amounts of the target peptide, and why a laboratory method should be calculated from net content rather than from the label weight.

The testing lab and test date

An independent lab has no financial reason to report a better result. A COA should name the lab, give its address or website, carry a report number, and show the date the sample was tested. That date should sit close to the lot's manufacture and release. WholesalePeptide.us lots are tested by an independent third-party laboratory using RP-HPLC (UV 214/220 nm) and LC-MS identity. A lot-specific Certificate of Analysis is published before the lot ships.

How to verify a COA with the lab

1. Note the lab name, report number, lot number and test date from the COA.
2. Find the lab's contact details independently (search the lab, do not rely on contact details printed on the COA alone).
3. Ask the lab to confirm that the report number exists and that the result and lot match. Many labs offer a verification page or QR code for this.
4. Compare the PDF you received with the lab's copy for any differences in purity, mass or dates.
5. Keep the confirmation with your receiving records for that lot.

Common COA problems

• Same chromatogram image reused across different compounds or lots.
• Purity reported to two decimal places with no chromatogram attached.
• Mass result stated as "conforms" with no observed value.
• Lab logo present but lab name, address or report number missing.
• Test date years before the lot could have been made.
• Theoretical mass that does not match the sequence or ignores a stated modification.

Any one of these is reason to ask questions; two or more is reason to choose a different source. For the wider supplier checklist, see how to vet a wholesale peptide supplier. To see how WholesalePeptide.us documents its wholesale peptides, browse the COA library by compound and lot. Laboratories that want lot COAs attached to every invoice can open a wholesale account.