Every peptide vendor says their products are tested. The Certificate of Analysis is where that claim becomes checkable. A COA is a lab report tied to a specific lot, and reading it well takes about two minutes once you know what each section is for. This guide walks through a typical certificate top to bottom.
1. The header: who tested it and which lot
Look for three things before reading any result. The name of the laboratory that ran the tests, the lot or batch number, and the date. If the lab named is the vendor itself, the certificate is an internal QC record, not third-party testing. If there is no lot number, the certificate cannot be matched to the vial in your hand. Every Homegrown Peptides vial carries a lot number that maps to its own certificate, which we send on request before you order.
2. Identity by mass spectrometry
This is the line that proves the vial contains the compound on the label. The test measures the molecular weight of what is in the sample and compares it with the theoretical mass of the peptide sequence. The observed and expected values should match within a small tolerance, usually well under one dalton for a peptide of this size. If a COA has a purity number but no mass-spec identity, it has told you the sample is clean without telling you what it is.
3. Purity by HPLC
High-performance liquid chromatography separates the sample into its components and reports what fraction of the total signal comes from the main peak. A result of 99.2% means 99.2% of the detected material eluted as the target peptide, and 0.8% eluted as something else, typically truncated or modified sequences from synthesis. Purity is reported at a specific wavelength, commonly 214 or 220 nanometers for peptides, and the chromatogram itself should be attached or available. Our article on what HPLC purity measures goes deeper on the method.
4. Net peptide content
This is the one most people skip. Lyophilized peptide powder is never 100% peptide by weight. It also contains bound water and counter-ions such as trifluoroacetate left over from purification. Net peptide content tells you what fraction of the powder's mass is actually peptide. A 10 mg vial at 85% net content contains 8.5 mg of peptide. Purity and net content are different questions: purity is about what kind of peptide, net content is about how much of the mass is peptide at all.
5. Appearance and solubility
Usually a single line: white to off-white lyophilized powder, soluble in water. It is a sanity check, not a quality metric, but a discolored powder is a reason to ask questions.
6. Endotoxin and sterility, when present
Bacterial endotoxin testing (often the LAL method) and sterility testing look for biological contamination that HPLC and mass spec are blind to. Not every COA includes them, and their absence is a meaningful gap for any work with cell cultures. We cover the contamination categories a purity number cannot detect in this separate guide.
What a good COA looks like at a glance
- Independent laboratory named, with a lot number and a test date.
- Mass-spec observed mass within tolerance of the theoretical mass.
- HPLC purity of 99% or higher, with the chromatogram available.
- Net peptide content stated, not implied.
- Endotoxin or sterility results for anything destined for cell work.
Red flags
- A single COA reused across many lots or many products.
- A purity figure with no chromatogram and no method stated.
- Rounded results like "99%" with no decimal, on every product, every time.
- A certificate that names no laboratory at all.
Our vendor red-flag checklist covers the rest of the buying decision.
How Homegrown Peptides does it
Every lot of every compound, from retatrutide to BPC-157 to GHK-Cu, is tested by an independent laboratory for HPLC purity and mass-spec identity before it is released, and the lot number on the vial maps to that certificate. Ask for it before you order.
Matching the certificate to the vial
A certificate is only evidence if it describes the material in front of you. Three checks close that loop. The lot number printed on the vial label must appear on the certificate. The product name and nominal mass must match. And the test date should sit before the ship date, not after it. If a vendor cannot produce a lot-matched certificate, the testing claim is unverifiable no matter how good the numbers look.
Storing certificates with your records
Good practice is to file the certificate alongside the lab notebook entry for the experiment that used the lot. Most journals and internal QA reviewers want reagent provenance, and a lot-matched COA is the simplest way to provide it. Record the lot number, purity, net peptide content and reconstitution date together so the whole chain from vial to result can be reconstructed later.
Frequently asked questions
Is 99% purity the same as 99% of the label weight?
No. Purity is the fraction of detected peptide signal that comes from the correct sequence. Net peptide content is the fraction of the powder's weight that is peptide. Both matter and they answer different questions.
Should I be worried if the observed mass differs from expected by a fraction of a dalton?
Small differences within the instrument's stated tolerance are normal. A difference of many daltons suggests a different or modified compound and should be questioned.
Can I get the COA before I buy?
Yes. Email us the product and size and we send the current lot's certificate.
References
- United States Pharmacopeia. General Chapter <1225> Validation of Compendial Procedures.
- Mant CT, et al. 2007. HPLC analysis and purification of peptides. Methods in Molecular Biology.
- Eggen I, et al. 2014. Impurities in peptide drug substances: identification, control and analytical strategies. Journal of Peptide Science.
Research use only. Homegrown Peptides products are for laboratory research and are not for human or animal use. Nothing in this article is medical advice.
