A certificate of analysis (COA) is the laboratory's record of what was found in one specific batch. A good COA lets you check, line by line, that the material in your vial is the compound on the label, at the purity claimed. A weak COA can look impressive and still prove almost nothing.
This guide walks through a typical peptide COA from top to bottom.
The header: who, what and which batch
The top of the report must answer three questions:
- Who tested it? The laboratory's name and address. An independent third-party lab is stronger evidence than the seller's own test. Accreditation to ISO/IEC 17025 shows the lab's methods are audited.
- What was tested? The compound name, the declared amount (for example 10 mg) and the sample description.
- Which batch? A batch or lot number, and the test date. The batch number on the COA must match the number printed on your vial. A COA without a batch number cannot be linked to any vial.
Identity: is it the right molecule?
Identity is usually confirmed by mass spectrometry (MS), often coupled to liquid chromatography (LC-MS). The report lists the theoretical molecular weight and the observed mass. For a large peptide (around 4.7 kDa for retatrutide), electrospray MS shows several multiply charged ions; the lab calculates the mass from them. Observed and theoretical values should agree closely.
Purity without identity is not enough: a sample can be very pure and still be the wrong peptide.
Purity: how much of the peak is the target?
Purity is measured by HPLC (high-performance liquid chromatography) and reported as a percentage of the total peak area. A result such as "99.2% (HPLC, 214 nm)" means that, at that detection wavelength, 99.2% of the detected material eluted as the main peak. Our HPLC explainer covers what this number does and does not tell you.
Peptide content: purity is not quantity
A lyophilised peptide powder is never 100% peptide by weight. It also holds water and counter-ions (such as acetate or trifluoroacetate) left from purification. Net peptide content tells you how much of the powder's weight is the peptide itself. Some COAs report an assay instead: the measured amount of peptide in the vial, compared with the declared amount.
Water and counter-ions
Water content is often measured by Karl Fischer titration. Counter-ion content may be listed too. Both explain the gap between the powder's weight and its peptide content.
Endotoxin
Some reports include a bacterial endotoxin result from a LAL (Limulus amebocyte lysate) test, given in EU per mg. It is a measure of contamination from bacterial cell-wall material during manufacture.
Red flags
- No batch number, or a batch number that does not match the vial.
- The same COA reused across many batches or for months.
- No laboratory name, or a lab you cannot find or contact.
- Purity reported without the method (HPLC) or without identity (MS).
- A scanned image that cannot be checked with the lab.
R3TA publishes the COA for every batch on the lab reports page, and prints the batch number on every vial so the two can be matched.