Short answer
A peptide COA is the laboratory report that proves what is inside a vial. The two most important sections are purity measured by HPLC and identity confirmed by mass spectrometry. Every research-grade peptide should ship with a batch-specific COA.
What is a COA?
A COA is a document issued by a testing laboratory that reports the results of analysis on one specific batch of material. For peptides, the COA answers three questions:
Scientific and regulatory guidance for peptide quality reporting consistently includes these elements: molecule identification, lot number, purity, assay method, and analytical results [1]. A COA is only as good as its traceability: the batch number on the certificate should match the batch number printed on the vial you hold, and the reported data should be reproducible from the named test methods and instruments.
The two tests that matter most
HPLC purity
High-performance liquid chromatography (HPLC) separates the components of a sample. The area under the main peak, compared with the total peak area, gives the chromatographic purity. A research-grade peptide should typically show:
It is worth emphasizing what HPLC purity does and does not tell you. HPLC measures the fraction of the sample that elutes as the main peak; it does not, by itself, prove that the main peak is the peptide you ordered. A well-made but mis-synthesized peptide could in principle show a clean chromatogram. That is why purity is always paired with identity testing.
Mass spectrometry identity
Mass spectrometry (MS) measures the molecular mass of the peptide. The measured mass is compared with the theoretical mass of the expected amino acid sequence. A match within a few mass units (or parts per million at high resolution) confirms identity. Reviews of peptide characterization methods describe HPLC and MS as the backbone of modern peptide quality control [2].
A stronger form of identity testing is tandem mass spectrometry (MS/MS), which fragments the peptide and reconstructs its sequence. Sequence-level confirmation is the gold standard, though not every COA includes it. At minimum, the measured intact mass must match the theoretical mass of the intended sequence, including any modifications.
Understanding HPLC purity in depth
HPLC purity is expressed as a percentage, but reading the number without the chromatogram is a mistake. Two batches can both claim "99%" while telling very different stories:
The chromatogram, not the percentage, is the primary evidence. Look for the shape and symmetry of the main peak, the number and size of impurity peaks, and the consistency of the retention time. The COA should name the column, the mobile-phase gradient, and the detection wavelength, because these determine whether the separation is trustworthy. Two analytical considerations deserve attention: whether the method resolves closely related deletion or truncation impurities, and whether the detection wavelength is appropriate for the peptide's chromophores.
Understanding mass spectrometry identity in depth
The measured mass from mass spectrometry is compared against the theoretical monoisotopic mass of the intended sequence. For a standard peptide, the match should be within a few mass units; at high resolution, the mass error is often reported in parts per million.
Several details matter when reading an MS result:
If the measured mass does not match the theoretical mass, the peptide is either the wrong sequence, incompletely synthesized, or modified differently than claimed. This is the single most reliable indicator of a synthesis problem.
Other sections on a good COA
Net peptide content deserves special mention. A vial labelled "10 mg" often contains the peptide plus counter-ions, water, and buffer salts. The net peptide content states how many milligrams are actually the peptide. For quantitative experiments, this number matters more than the gross fill weight.
Why batch-level testing matters
A COA is only meaningful if it is batch-specific. A certificate that reads the same for every batch a supplier sells is a template, not a test report, and it tells you nothing about the vial in your hand. Genuine batch-level testing means the analytical work was performed on the same lot of material that was shipped to you, with results recorded and archived under a batch number you can look up.
There are several reasons batch-to-batch variability is real and worth guarding against. Synthesis runs can differ in coupling efficiency, yielding different levels of deletion and truncation impurities. Purification runs can differ in how completely they remove those impurities. And storage or shipping conditions can affect water content and stability. Two batches of the same peptide from the same supplier can therefore have measurably different purity and impurity profiles, which matters if your assay is sensitive to those differences.
For research this has a concrete implication: the COA for the exact batch you receive, not a representative certificate, is the document you should archive with your experiment. If results later become hard to interpret, the batch record is the first place to look for an explanation.
Red flags on a COA
Any of these should make you stop and ask questions before ordering. See our supplier red flags guide for the broader purchasing checklist.
A practical COA review checklist
When a COA arrives, a quick structured review takes only a few minutes and catches most problems:
If any item is absent, ask the supplier for it. A supplier that tests every batch and can produce the supporting raw data is the one to keep; a supplier that cannot is a risk you can avoid by choosing elsewhere. This review habit is the practical endpoint of the entire COA guide: not just knowing what a certificate should contain, but checking it, every time, before material enters your workflow.
How to verify a COA before you buy
PEPMAKE publishes batch documentation through its quality verification portal and includes a COA with every order. For a deeper look at the two analytical methods, see HPLC vs mass spectrometry.
FAQ
What is a peptide COA?
A Certificate of Analysis is a laboratory document reporting test results for a specific peptide batch.
What is HPLC purity?
The percentage of the sample that matches the target peptide in an HPLC analysis; 99%+ is typical for research-grade material.
Why is mass spectrometry important?
It confirms the identity of the peptide by comparing measured mass with the expected sequence mass.
Can a COA be faked?
Yes, which is why batch-level verification and independent testing matter.