Free researcher resource
The Peptide Buyer's Starter Kit
A vendor-neutral guide to reading a certificate of analysis, judging purity honestly, and vetting a research-peptide supplier before you buy. The full kit is on this page, free to read.
Start with the Certificate of Analysis
A certificate of analysis (COA) is a batch-specific document issued for one manufactured lot. It records what the material is, how pure it is, and the methods used to measure both. Because the results are lot-specific, a COA for one batch does not describe another.
Before anything else, confirm the certificate exists, is readable, and matches the vial in front of you.
What to check on a COA
- Compound name and full sequence
- Lot or batch number that matches the vial label
- Analytical methods used (HPLC for purity, MS for identity)
- Date of analysis and the issuing laboratory
- CAS number, molecular formula, and molecular weight
- Physical form and storage guidance
Read purity honestly: HPLC vs. net peptide content
Purity and quantity answer two different questions. HPLC purity is a relative figure: of the material the detector sees, what share is the target peak. Net peptide content is an absolute figure: of the total mass in the vial, how much is actually peptide rather than bound water, salts, and counter-ions.
Same number, different vial
Two vials can both report 99% pure by HPLC and still hold different amounts of peptide. HPLC purity describes the detected material. Net peptide content describes the vial. Ask a supplier for both, and compare net peptide content when the amount of peptide matters to your work.
Confirm identity with mass spectrometry
HPLC tells you how pure the material is. Mass spectrometry tells you what it is. MS confirms that the molecule's measured mass matches the value calculated from its sequence. Purity without identity does not prove you have the right compound, so a complete COA reports both.
On the MS section, look for
- Observed mass compared with theoretical mass
- The method noted, for example ESI-MS or MALDI-TOF
- The spectrum or chromatogram shown, not just a single number
Third-party vs. in-house testing
In-house QC is the manufacturer testing its own batch. Third-party testing is an independent laboratory with no stake in the outcome. Both have value, but independent verification carries more weight for identity and purity.
Fast and routine, but the manufacturer is grading its own work. Useful when methods and limits are disclosed.
An independent lab verifies the lot. Stronger evidence for identity and purity claims when the lab is named.
Either way, confirm
- The issuing laboratory is named, not anonymous
- Testing is available for the specific lot you are buying
- The report can be matched to the certificate (report ID, QR, or lookup)
Supplier red flags to avoid
A polished storefront is not evidence. These are the patterns that should make you pause, regardless of how the site looks.
- "View COA" buttons that open nothing, a blank page, or a generic sample
- Certificates with no lot number, or a lot that does not match the vial
- Any human-use, dosing, or administration guidance
- Comparisons to brand-name drugs, or therapeutic outcome claims
- No research-use-only labeling on the product or across the site
Regulatory basics
You do not need to be a regulatory expert, but a few terms help you read a supplier correctly.
- Research use only (RUO): sold for laboratory research, not for human or veterinary use
- 503A and 503B describe compounding pharmacy categories under US law; a research supplier is not a compounding pharmacy and should not present itself as one
- A COA is not FDA approval. It is an analytical record of identity and purity for a lot, and says nothing about safety or suitability for use in humans
Frequently asked questions
What is a certificate of analysis (COA)?
A COA is a batch-specific document issued for one manufactured lot. It records the material's identity, its measured purity, and the analytical methods used to determine both. Because the results are lot-specific, a COA for one batch does not describe another, so the lot number on the certificate should match the vial in hand.
What is the difference between HPLC purity and net peptide content?
HPLC purity is a relative figure: of the material the detector sees, what share is the target peak. Net peptide content is an absolute figure: of the total mass in the vial, how much is actually peptide rather than bound water, salts, and counter-ions. Two vials can report the same purity and still hold different amounts of peptide, so it helps to compare both.
Why does a COA include mass spectrometry?
HPLC tells you how pure the material is; mass spectrometry tells you what it is. MS confirms that the measured mass matches the value calculated from the sequence. Purity without identity does not confirm you have the right compound, so a complete COA reports both.
Is third-party testing better than in-house QC?
In-house QC is the manufacturer testing its own batch; third-party testing is an independent laboratory with no stake in the outcome. Both have value, but independent verification carries more weight for identity and purity. Either way, the issuing laboratory should be named on the certificate.
What does "research use only" (RUO) mean?
RUO means the material is sold for laboratory research only, and is not for human or veterinary use, not for diagnostic use, and not for human consumption. A COA is an analytical record, not an FDA approval, and says nothing about safety or suitability for use in humans.
What are the biggest supplier red flags?
Certificates that open nothing or show a generic sample, a missing lot number or a lot that does not match the vial, any human-use or dosing guidance, comparisons to brand-name drugs, and no research-use-only labeling on the product or across the site.