A Certificate of Analysis is the document a supplier provides to evidence what is in a particular batch. It is the difference between a claim on a website and a measurement someone actually made. But COAs vary enormously in quality, and a well-formatted PDF is not the same thing as a rigorous one.
This is a guide to reading them properly.
The header: batch identity
Before anything else, check that the certificate refers to the batch you actually received. A COA should carry a lot or batch number that matches the number on your vial. If it does not, or if the vial carries no batch number at all, the document is a specimen rather than evidence.
Also look for the manufacture date and, where given, a retest or expiry date. A certificate issued three years ago for a compound you received last week is describing a different physical sample.
Who performed the analysis
This is the single most informative line on the document and the one most often skipped. There are three broad situations:
- The manufacturer tested its own output. Common, and not inherently dishonest, but it is a self-report.
- An independent laboratory tested it. Materially stronger. The lab should be named, not described as “an accredited third party”.
- No lab is named at all. Treat the numbers as unverified.
If a supplier advertises third-party testing, the lab's name should appear on the certificate. “Independently tested” with no named laboratory is a claim about a claim.
Identity: mass spectrometry
Purity tells you a sample is clean. Identity tells you it is the right compound, and the two are independent. A sample can be 99% pure and be 99% pure something else.
Identity is normally established by mass spectrometry, usually LC-MS or MALDI-TOF. The certificate should show:
- The theoretical (calculated) molecular weight for the stated sequence
- The observed molecular weight from the instrument
These should agree closely. A discrepancy of a few daltons is not a rounding error — it can indicate a missing or substituted residue, or an unexpected modification. If a COA reports purity but no identity confirmation, it is answering half the question.
Purity: reading the chromatogram, not just the number
Most certificates state an HPLC purity percentage. The better ones include the actual chromatogram, and it is worth looking at.
What to look for:
- A single dominant peak with a clean baseline either side.
- Peak shape. A sharp, symmetrical peak is what you want. Heavy tailing or a shoulder on the main peak can indicate a co-eluting impurity that the integration has quietly folded into the main peak area.
- The gradient and column conditions. Purity figures are method-dependent. A shallow gradient separates closely related impurities that a steep one will merge into the main peak, producing a flattering number.
- Where the baseline was drawn. Integration is a judgement call, and a generous baseline inflates purity.
A certificate with a stated percentage and no chromatogram is asking you to trust the integration without seeing it. As covered in our piece on HPLC purity versus net peptide content, that percentage also does not tell you how much of the powder is peptide.
Net peptide content and water content
Stronger certificates report net peptide content (typically by amino acid analysis) and water content (typically by Karl Fischer titration). These are the figures that let you work out how much peptide is actually in the vial, which is what your concentration calculation depends on.
Most COAs in the research peptide market omit both. Their absence is not disqualifying, but their presence is a strong signal that the supplier's analytical work is thorough.
Counter-ion and residual solvents
The counter-ion — usually trifluoroacetate, sometimes acetate or hydrochloride — should be stated, because it affects the mass of solid you are weighing out. Residual solvent analysis, where present, quantifies leftovers from the purification step such as acetonitrile.
Appearance, solubility and storage
Straightforward but worth checking against what you actually received. If the certificate says “white to off-white lyophilised powder” and your vial contains something visibly different — discoloured, oily, or partially collapsed — that is a discrepancy worth raising before you use it.
Red flags
In rough order of seriousness:
- No batch number, or a batch number that does not match your vial.
- No named laboratory.
- No mass spectrometry — purity without identity.
- A purity percentage with no chromatogram.
- Identical certificates across different batches. Real analytical results vary slightly between runs. Byte-identical figures across lots suggest a template, not a measurement.
- Suspiciously round numbers. Genuine HPLC integration produces figures like 98.7% or 99.2%, not a clean 99.0% on every batch.
- A theoretical mass that does not match the sequence given. Worth checking by hand if you have the sequence — it catches copy-paste errors and worse.
The honest summary
A COA is evidence, not proof. It tells you what a particular laboratory measured on a particular sample on a particular day, using a method they chose. That is genuinely useful, and much better than nothing — but it is worth reading as a document with authorship and method, rather than as a certificate of truth.
The practical test is simple: ask a supplier for the batch-specific COA for the lot you are being sold, before you order. How they respond tells you a great deal.
Our full range, with CAS numbers, molecular formulae and storage conditions on every product page, is at All Peptides. If you are purchasing on behalf of an organisation and need documentation for a purchasing process, see Trade & Laboratory Accounts.
All compounds supplied by ASPeptides are for in vitro laboratory research use only. They are not medicinal products and are not approved for human or veterinary use.