A certificate of analysis is one page that says what a lab measured on one batch. Most buyers read the purity number and stop. Every other line is there for a reason, and a few matter more than the percentage. We walk through them using the retatrutide reports Peptryn publishes, since they are among the most complete we have found.
Start with the product name and identity
The certificate should name the compound and its expected mass. Peptryn's listing gives retatrutide (LY3437943) at about 4,731 Da, CAS number 2381089-83-2. If a certificate gives only a trade name or no mass, you cannot tell what was tested.
Lot number and test date
Peptryn's 10 mg report is tied to lot 2026-06-12 and its 30 mg report to lot 2026-08-31. The lot is what connects the paper to a vial. Compare the number printed on your vial with the one on the certificate. If they differ, the report describes a different batch. The date also tells you how current the test is.
Method
The 10 mg report lists HPLC-UV-MS. HPLC separates what is in the sample and reports how much of it is the main peak. Mass spectrometry checks that the main peak has the right mass. A line that says HPLC alone gives you purity without identity. The HPLC and mass spectrometry guide covers the difference.
Purity
The 10 mg lot reads 99.62% and the 30 mg lot 99.32%. Purity by HPLC is an area percentage, the main peak's share of every peak the detector saw. It does not say how much peptide is in the vial by weight, and it cannot rule out anything the detector does not see. Two lots that differ by a few tenths of a percent look like real runs. A catalog where every product reads exactly 99% deserves suspicion, as the guide to spotting a fake COA explains.
Endotoxin or identity result
The 10 mg report adds a negative endotoxin result, which checks for bacterial contamination. The 30 mg report says identity was confirmed by mass spectrometry instead. The tests differ between the two lots. That is worth noting, though it is not a failure.
The lab, and where the report lives
Both reports name MZ Biolabs, and Peptryn links each certificate to a report hosted by the lab. That link is the useful part, because it lets you check that the lab issued the document instead of taking the vendor's word. We have not read the full lab reports or contacted MZ Biolabs, so we have not confirmed the lab issued them.
What a missing certificate looks like
The Core Power Peptides tirzepatide 50 mg listing has a COA tab. When we it held a review widget and no report, and the page says a certificate is available on request. The site's homepage shows one sample certificate, for a different lot. One sample standing in for a whole catalog is the first red flag in our guide to spotting a fake COA.
A five-minute check before you order
- Find a certificate for the exact product and size you are buying, not a sample.
- Match the lot number to the one on the vial or given at checkout.
- Confirm the method includes mass spectrometry as well as HPLC.
- Open the lab's own report if a link is given, and check the lab name and date.
- Email the vendor for any missing certificate and note how long the answer takes.
We score vendors on three of these checks: an outside lab, a batch match and a stated method. The rules are in the method section. The same checks apply to the muscle-related compounds in our post on the best peptides for muscle growth. For a second vendor's certificates read the same way, see our American Peptides review.
| Row | Outside lab | Batch matched | Method stated | Total |
|---|---|---|---|---|
| Peptryn | Pass, 2 points | Pass, 2 points | Pass, 2 points | 6/6 |
| American Peptides | Pass, 2 points | Partial, 1 point | Pass, 2 points | 5/6 |
| Core Power Peptides | Partial, 1 point | Partial, 1 point | Pass, 2 points | 4/6 |
| Peptides for Sale | Fail, 0 points | Fail, 0 points | Partial, 1 point | 1/6 |
- Pass, 2 points
- Partial, 1 point
- Fail, 0 points
Documentation score out of 6, from each vendor's public pages. Vendor findings , American Peptides . Not a safety or quality score.
Frequently asked questions
What purity should a peptide COA show?
The vendors we reviewed all advertise 99% or higher. The number matters less than whether it is tied to your lot, names its method and comes from an outside lab.
Does a COA prove a peptide is safe?
No. It reports what a lab measured on one batch, such as purity and identity. It says nothing about safety or what a compound does in a body, and nothing on this site is medical advice.