Understanding Peptide Purity & COAs
A certificate of analysis is the only real window into what's actually in a vial — if you know how to read one. Here's how, including the number most people overlook.
This guide explains how to read and verify peptide certificates of analysis. It is educational only and not medical advice. Sources are listed at the end.
Why COAs Matter
With an approved pharmaceutical, regulation guarantees a quality floor — inspected facilities, validated processes, and accountability if something goes wrong. Research compounds have no such floor. As we cover in our guide to research vs pharmaceutical peptides, the supplier's own standards are the standard.
That makes the certificate of analysis the single most important document in this field. It's the difference between "trust us" and "here's the evidence." And a buyer who can read one is far harder to mislead.
What a COA Is
A certificate of analysis is a lab report on a specific sample. An independent laboratory receives a vial, runs analytical tests on it, and documents what it found: what the compound is, how pure it is, and how much of it is present.
The key phrase is "a specific sample." A COA is not a general statement about a product line — it's a snapshot of one batch, tested on one date. That's why batch numbers and dates matter as much as the results.
Anatomy of a COA
Most reputable third-party reports contain the same core elements. Here's what each one is for:
| Field | What it tells you |
|---|---|
| Testing laboratory | Who ran the analysis. An independent lab carries far more weight than in-house testing. |
| Task / report number | The unique identifier for that specific test. Used for verification. |
| Sample name | The product submitted, including its label claim (e.g. "Retatrutide 20 mg"). |
| Batch number | Which production run was tested. Without it, you can't tie the report to your vial. |
| Dates | When the sample was received and analysed. Recency matters. |
| Content / quantity | How much active compound was actually found — compare against the label claim. |
| Purity | What percentage of the material is the target peptide rather than impurities. |
| Verification key | A code allowing you to confirm the report directly with the lab. |
Reading the Purity Number
Purity is usually measured by HPLC (high-performance liquid chromatography), which separates the sample into its components and measures how much of each is present. A purity of 99% means 99% of the peptide material is the target compound; the remaining 1% is related impurities from synthesis.
| Purity | Interpretation |
|---|---|
| 99%+ | Excellent — the standard good suppliers aim for |
| 98–99% | Good; the common benchmark for research-grade material |
| 95–98% | Acceptable for some research uses, but below the usual expectation |
| Below 95% | Poor — ask why |
| No number given | The biggest red flag of all |
Don't obsess over decimal points
The practical difference between 99.05% and 99.35% is essentially nothing — both are excellent. Chasing hundredths of a percent is a distraction. What actually matters is that a number exists, comes from an independent lab, is tied to a real batch, and sits comfortably above the 98% mark.
The Number People Overlook
Here's the most useful thing on this page. Almost everyone fixates on purity — and purity alone can't tell you whether you're getting what you paid for.
Purity is a percentage, not a quantity
Consider a vial labelled 20 mg that tests at 99% purity but contains only 10 mg of peptide. The purity is superb — whatever is in there is almost entirely the right compound. But there's half as much of it as the label claims. Someone measuring doses from that vial would be using half of what they intended, and the purity figure would never reveal it. Always check the content figure against the label claim, not just the purity percentage. Underfilled vials are one of the most common quality problems in this market, and they hide perfectly behind a good purity number.
A well-filled vial typically tests at or slightly above its label claim — a small overfill is normal and is what you want to see, because it means the full labelled amount is genuinely there after manufacturing losses.
A Worked Example
Here's one of our own third-party reports, and how to read it:
Reproduced from our published test reports. See the full document on our lab tests & COA page.
How to read it: The purity of 99.051% is excellent. More importantly, the content came back at 20.80 mg against a 20 mg label claim — a slight overfill, which is exactly what a properly filled vial looks like. The batch number ties the report to a specific production run, the analysis date is recent, and the verification key means you don't have to take our word for any of it. That combination — independent lab, identified batch, both numbers reported, independently verifiable — is what a good COA looks like.
What a COA Doesn't Tell You
Being straight about the limits matters as much as explaining the strengths. A purity and content report tells you a great deal — but not everything.
Sterility
A standard purity test says nothing about whether the vial is sterile. Sterility testing is a separate analysis, routine for pharmaceuticals and not standard for research material.
Endotoxins
Bacterial cell-wall fragments can cause fever and worse. Endotoxin testing is mandatory for injectable pharmaceuticals and is a separate test from purity.
Your Specific Vial
A COA covers a sample from a batch, not the individual vial in your hand. It's strong evidence about the batch — not a guarantee about every unit in it.
What Happened Afterward
Testing happens at one moment. Heat exposure in shipping or poor storage since then can degrade a peptide that tested perfectly. See our storage guide.
Red Flags
Things that should make you ask questions:
- No COA at all, or one that only appears after you push for it repeatedly.
- No batch number — a report that can't be tied to a production run can't be tied to your vial.
- Purity without content — a percentage with no quantity leaves the underfill question wide open.
- In-house testing only — a supplier grading its own homework isn't independent verification.
- No verification key, or one that doesn't check out with the issuing lab.
- Identical figures across different products — two unrelated compounds returning the same numbers to three decimal places indicates a document error at best.
- Very old reports presented as current, with no recent testing behind them.
- Content well below label claim that the seller hasn't acknowledged or explained.
How to Verify a COA
The best part of modern third-party testing is that you don't have to trust the seller's PDF at all. Reputable labs let you check reports directly.
1. Find the Key
Look for the verification key or report number, usually at the foot of the document.
2. Go to the Lab
Visit the testing laboratory's own verification page directly — type the address yourself rather than following a link from the seller.
3. Check It Matches
Confirm the compound, batch, content, and purity on the lab's record match the document you were given.
4. Ask if Unsure
A supplier confident in their testing will happily walk you through a report. Reluctance is itself informative.
You can see our published third-party reports on our lab tests & COA results page, and read more about quality and sourcing in our guides to peptide safety and research vs pharmaceutical peptides.
Common Questions
What is a peptide COA?
A certificate of analysis is a laboratory report on a specific sample. An independent lab tests a vial and documents what it found: the identity of the compound, its purity as a percentage, and how much active material is actually present. Crucially, a COA covers one batch tested on one date — it's not a general claim about a product line, which is why batch numbers and dates matter as much as the results themselves.
What purity should a peptide be?
98% or higher is the standard benchmark for research-grade peptides, and good suppliers typically achieve 99%+. Between 95% and 98% is acceptable for some purposes but below normal expectation; under 95% warrants an explanation. Don't fuss over decimal points — 99.05% and 99.35% are practically identical. What matters is that a number exists, comes from an independent lab, is tied to a real batch, and sits comfortably above 98%.
Why does the content figure matter more than purity?
Because purity is a percentage, not a quantity. A vial labelled 20 mg could test at 99% purity while containing only 10 mg — the material inside is almost entirely the right compound, but there's half as much as claimed, and the purity figure would never reveal it. Underfilled vials are a common quality problem that hides perfectly behind an impressive purity number. Always compare the content found against the label claim.
Should the content be exactly the label amount?
Slightly above is ideal. A vial labelled 20 mg that tests at 20.8 mg shows a small overfill — normal, and a good sign, because it means the full labelled amount is genuinely present after manufacturing losses. Content meaningfully below the label claim is the concern, since it means every dose measured from that vial delivers less than intended.
What doesn't a COA tell me?
Several important things. A standard purity report says nothing about sterility or endotoxins (bacterial fragments that can cause fever) — those are separate tests, mandatory for injectable pharmaceuticals but not standard for research material. It also covers a sample from a batch rather than your individual vial, and it reflects one moment in time: heat exposure during shipping or poor storage afterward can degrade a peptide that tested perfectly.
How can I verify a COA is genuine?
Reputable testing labs provide a verification key or report number, usually at the foot of the document, that lets you confirm the report on the lab's own website. Go to the laboratory's verification page directly — type the address yourself rather than clicking a seller's link — and check that the compound, batch, content, and purity match the document you were given. A supplier confident in their testing will happily help you do this.
What are the biggest COA red flags?
No COA, or one produced only after repeated requests. No batch number, meaning the report can't be tied to your vial. Purity given without a content figure. In-house testing rather than independent. Missing or invalid verification keys. Identical results appearing across different products — two unrelated compounds returning the same figures to three decimal places signals a document error at best. And old reports presented as current.
Want to see our testing?
We're glad to share the certificate of analysis for any product, walk you through how to read it, or point you to the verification key so you can check it yourself. We answer every message personally, with no pressure and no upsell.
Contact Us on WhatsAppImportant disclaimer: The information in this guide is general educational content only. It is not medical advice, laboratory advice, or a personalized recommendation. Certificates of analysis reflect testing of a specific sample from a specific batch on a specific date; they are evidence about that batch rather than a guarantee about any individual vial, and standard purity and content testing does not assess sterility or endotoxin levels. Nothing on this page should be read as an indication that research compounds are suitable for human use. Products sold by Peptides Costa Rica are intended for laboratory and research purposes only. Always consult a qualified healthcare professional before beginning any peptide protocol.
- United States Pharmacopeia: Analytical procedures for peptide identity, purity, and content determination.
- FDA: Current Good Manufacturing Practice (cGMP) requirements — identity, purity, sterility, and endotoxin testing for injectables.
- Journal of Chromatography A: HPLC methods for peptide purity determination and impurity profiling.
- Analytical Chemistry: Mass spectrometry for peptide identity confirmation and molecular weight verification.
- USP General Chapter <85>: Bacterial endotoxins test.
- Nature Reviews Drug Discovery: "Therapeutic peptides: current applications and future directions."
