COA EDUCATION: HOW TO READ A CERTIFICATE OF ANALYSIS
A Certificate of Analysis (CoA) is the most important document in research peptide purchasing. Here is what each element means, what good looks like, and what red flags to watch for.
1.What a CoA tells you — and what it cannot
A CoA is a document produced by a testing laboratory confirming the identity and purity of a specific batch of a compound. A good CoA tells you: the compound identity was confirmed (not just assumed from the vendor's label), the purity percentage of that compound in the tested sample, what testing methodology was used, which laboratory produced the result, and the batch number the CoA applies to.
What a CoA cannot tell you: whether the synthesis process was clean, whether endotoxins or residual solvents are present at low levels that HPLC does not detect, whether the vial you actually received matches the tested batch, or whether the compound is sterile. A CoA is necessary but not sufficient evidence of quality.
The distinction between compound identity and purity: a CoA showing 99% purity on HPLC tells you that 99% of what was in the tested sample was a compound that eluted at the same retention time as the reference standard. It does not tell you that the remaining 1% is safe — it just tells you 1% is not the target peptide. High purity does not equal complete absence of other concerning substances.
CoAs should be batch-specific. A vendor who posts one CoA for a compound without batch numbers, or who posts the same CoA for multiple restocking events, is either not testing per batch or posting documentation that does not match what you received. Batch-linked CoAs are the standard for any serious vendor.
When you receive your order, compare the batch number on the vial label to the batch number on the CoA. If they match, the documentation is potentially valid for your specific product. If there is no batch number on the vial, or the numbers do not match the available CoA, that is a serious quality control concern.
2.HPLC testing: what it measures
High-Performance Liquid Chromatography (HPLC) is the industry standard for peptide purity analysis. The method works by dissolving the sample in a solvent, then pushing it through a column packed with material that interacts differently with different molecules based on their polarity and size. Each compound travels through the column at a characteristic speed and elutes at a specific retention time.
The output is a chromatogram — a graph showing detector signal vs. time. Each peak represents a compound in the sample. The area under each peak corresponds to the quantity of that compound. Purity is calculated as: [area of target peak] / [total area of all peaks] × 100%.
HPLC purity numbers for research peptides should be above 98% for a quality vendor. 95-97% is acceptable but not optimal. Below 95% indicates significant impurities that raise questions about synthesis quality and what specifically those impurities are.
HPLC can miss compounds that co-elute with the target peptide — meaning a different molecule that travels through the column at the same speed will overlap with the target peak and be counted as part of the target purity. This is why mass spectrometry is valuable as a complementary test.
The HPLC method used matters: reversed-phase HPLC with a C18 column is the standard for peptide purity. If a CoA does not specify the method, it is less informative than one that does. Gradient conditions, wavelength of detection, and column specifications all affect whether impurities are detected. A professional CoA includes all of these details.
3.Mass spectrometry: verifying compound identity
Mass spectrometry (MS) identifies compounds based on their molecular mass and fragmentation pattern. When a compound is ionized and broken into fragments, the pattern of fragment masses is essentially a molecular fingerprint. MS confirmation verifies that the compound you received matches the molecular weight and fragmentation profile of the target peptide.
HPLC alone can tell you that something elutes at the expected retention time with high purity. MS confirms that what is eluting is actually the target molecule — not an analog, a partially synthesized fragment, or a different compound that happens to have similar chromatographic behavior.
LC-MS/MS (liquid chromatography combined with tandem mass spectrometry) is the gold standard combination. It provides both purity quantification (from the HPLC component) and identity confirmation (from the MS component) in a single analysis. The best vendors provide LC-MS or LC-MS/MS CoAs.
What to look for on an MS CoA: the observed molecular weight should match the theoretical molecular weight of the target peptide within 1-2 Da (dalton). For most research peptides, the molecular weight is published in the scientific literature and can be verified. If the observed mass does not match, the compound is not what it claims to be.
Not all vendors include MS data — it adds cost and requires more specialized equipment. An HPLC-only CoA from a reputable, community-vetted third-party lab is still meaningful. MS adds a layer of certainty, particularly valuable for expensive compounds or compounds where the specific peptide sequence matters (e.g., distinguishing CJC-1295 no DAC from CJC-1295 DAC, which have different biological properties).
4.Red flags and community verification
Red flags on a CoA: (1) No date — old CoAs from years prior may not apply to current stock. (2) No batch number — the document cannot be linked to what you received. (3) In-house testing only, no named external lab — vendor-self-reported testing has obvious conflict of interest. (4) Purity given as a range ('95-100%') rather than a specific value — indicates imprecise measurement. (5) Missing lab accreditation or no verifiable lab information.
Red flag: vendor uses the same CoA across multiple different compound listings (changing only the compound name in the header). This occurs and is a sign of fabricated documentation. Each compound and each batch should have a unique CoA document.
Community verification adds significant value. Independent testing by community members — where a user orders a product, sends a sample to an independent lab at their own expense, and posts the results publicly — provides data that is free from vendor self-interest. r/Peptides maintains threads aggregating community test results for major vendors. One independent test is meaningful; 10 consistent independent tests across multiple users is strong evidence.
Endotoxin testing on a CoA is a quality-differentiating feature. Bacterial endotoxins (lipopolysaccharides, or LPS) can cause fever, chills, and potentially serious reactions when injected. Pharmaceutical manufacturing has strict endotoxin limits. Research vendors are not required to test for endotoxins, but the best ones do. A CoA that includes LAL (limulus amebocyte lysate) endotoxin testing, showing results within safe limits for parenteral use, is a significant quality indicator.
The practical checklist: named third-party lab with verifiable contact information or accreditation, a date within the last 12 months, a batch number that matches your vial, HPLC purity above 98%, and ideally MS confirmation. These criteria, met together, represent genuinely good documentation. Any one of them missing is a caution flag; multiple missing is a reason to look elsewhere.
Sources & Studies
HPLC purity testing standards for peptide pharmaceuticals
USP Expert Committee, United States Pharmacopeia, 2023
Rosenfeld L. et al., J Pharm Biomed Anal, 2014