A Certificate of Analysis (CoA) is the single most useful document a research peptide supplier can give you — and the easiest one to misread. It is the supplier's written statement of what a specific batch is, how it was tested and what the results were. Read properly, it tells you whether the material in your hands is what you ordered. Read carelessly, it can give false comfort.
This guide walks through a typical peptide CoA section by section, explains the difference between purity and peptide content, and lists the red flags that should prompt a follow-up question before a batch goes into your workflow.
What a CoA is — and what it is not
A CoA is batch-specific. It describes one manufactured lot, identified by a batch or lot number, and reports the results of tests performed on that lot. A document that describes a compound in general terms, with no batch number, is a specification sheet or a marketing document — not a CoA.
A CoA is also a claim made by the issuer. It is only as reliable as the laboratory and quality system behind it. That is why the most important habits are checking that the CoA matches the physical vials you received, and that the methods and results make analytical sense.
The header: identifying the batch
The top section of a CoA should let you tie the document to the product in front of you. Expect to see:
- Compound name and, ideally, a synonym or code (for example Retatrutide (LY3437943)).
- Batch or lot number — this must match the number printed on your vial labels.
- Manufacturing date and expiry or retest date.
- Quantity or fill per vial (for example 5 mg or 10 mg).
- Issuing organisation and the date the CoA was issued.
If the lot number on the CoA does not match the label on the vial, stop there. Everything below it describes a different batch.
Specification vs result
Most CoAs are laid out as a table with three columns: the test, the specification (the acceptance criterion, such as "≥98.0%") and the result actually obtained (such as "99.1%"). A batch passes when every result falls within its specification.
Read both columns. A result of "Conforms" with no number tells you the batch passed but not by how much. A numeric result is more informative — and a specification that has been quietly loosened (for example "≥95%" on a product advertised at ≥98%) is worth noticing.
HPLC purity: what "≥98%" actually means
Purity on a peptide CoA is almost always measured by reverse-phase HPLC (high-performance liquid chromatography) with UV detection. The sample is separated on a column and each component shows up as a peak on a chromatogram. Purity is reported as the area of the main peak as a percentage of the total peak area.
So "98.6% by HPLC" means that, of everything the detector saw at that wavelength, 98.6% of the signal came from the main compound. The remaining 1.4% is made up of related impurities — typically shorter "deletion" sequences, truncated chains or oxidised forms created during synthesis. Our guide to HPLC purity testing explains how to read the chromatogram itself.
Two practical points:
- Ask for the chromatogram. A purity number with no trace behind it cannot be checked. A good supplier can provide the batch-level HPLC report on request.
- Be wary of implausible precision. Results such as "99.99%" on a long synthetic peptide are unusual and worth questioning.
Identity: is it the right molecule?
HPLC purity tells you how clean a sample is; on its own it does not prove which molecule the main peak is. Identity is normally confirmed by mass spectrometry (MS), which measures the molecular mass and compares it with the theoretical value — for example approximately 4113.6 g/mol for semaglutide or approximately 1419.5 g/mol for BPC-157.
Where identity data is supplied, check that the observed mass is close to the theoretical mass for the compound you ordered. If a CoA reports purity but no identity test, ask how identity was confirmed for that batch.
Purity is not the same as peptide content
This is the most common misunderstanding. A lyophilised peptide powder contains more than the peptide itself: residual water, and counter-ions (often trifluoroacetate or acetate) left over from purification. Net peptide content — the proportion of the powder mass that is actually peptide — is commonly well below 100% even when HPLC purity is very high.
A vial can therefore be "99% pure by HPLC" and still contain somewhat less than its nominal mass of pure peptide. Some CoAs report peptide content, water content (often by Karl Fischer titration) or counter-ion content separately. If precise quantities matter to your work, ask whether peptide content was determined and how the fill weight is defined.
Physical tests, storage and dates
The rest of a typical CoA covers simpler checks:
- Appearance — for most lyophilised peptides, a white to off-white powder or cake.
- Solubility — the solvent the batch dissolves in, for example sterile water.
- Storage conditions — for long-term storage of lyophilised powder, commonly −20 °C, sealed and protected from light and moisture.
- Retest or expiry date — the date until which the stated results are considered valid under the stated storage conditions.
These should be consistent with the product label. Our storage guide covers what those conditions mean in practice.
Red flags worth questioning
None of the following proves a problem on its own, but each is a reason to ask the supplier for clarification:
- No batch or lot number, or a number that does not match the vial labels.
- The same CoA (identical results and dates) supplied for different orders months apart.
- No analytical method named — "tested" with no indication of how.
- Purity reported with no chromatogram available on request.
- Results that sit exactly on a round number every time, or implausibly high precision.
- A document with no issuing organisation, signature or approval reference.
- Dates that do not make sense — for example an expiry date before the manufacturing date.
A quick CoA checklist
- The batch number matches the vial labels.
- The compound name and fill quantity match what you ordered.
- Purity is reported numerically by a named method (usually HPLC), with a chromatogram available.
- Identity has been confirmed, or you know how it was confirmed.
- Every result falls within its stated specification.
- Storage conditions and dates are present and consistent with the label.
- The issuer and approval are identifiable.
How Peptide Direct issues CoAs
Every order from Peptide Direct ships with a batch-specific Certificate of Analysis covering compound specification, batch/lot number, manufacturing and expiry dates, the HPLC purity result (≥98%), the analytical method and storage conditions. Full batch-level HPLC reports, GMP and ISO 9001:2015 certification and supply-chain documentation are available on request, and private label buyers can receive CoAs issued under their own brand. You can read more on our quality assurance page.