How to Read a Certificate of Analysis (COA) for Research Peptides
Published October 29, 2026 · 7 min read
A Certificate of Analysis (COA) is the single most useful document a research-material supplier can give you — and one of the easiest to misread. This guide walks through what each part of a typical peptide COA means, what it cannot tell you, and how to spot a certificate that isn't worth the paper it's printed on.
What a COA is (and what it isn't)
A COA is a report from a testing laboratory describing the results for one specific production batch, also called a lot. It is evidence about that batch only. A certificate for lot A says nothing about lot B, even from the same supplier and the same product name.
It also isn't a promise about how a material behaves in your experiment. It documents identity and purity measurements so that you can keep records, repeat work, and compare batches.
Start with the basics: is this certificate about your vial?
Before you look at a single number, check that the paperwork matches the product in your hands. A certificate that can't be tied to your vial is just a marketing image.
- Product name and sequence or molecular formula match what you ordered.
- The lot or batch number on the COA is identical to the number printed on the vial label.
- A test date is shown, and it is recent enough to make sense for the batch.
- The testing laboratory is named, with contact details, and is clearly separate from the seller.
- The methods used are listed (for example HPLC and mass spectrometry).
HPLC purity: what the percentage means
High-performance liquid chromatography (HPLC) separates the components of a sample as it passes through a column. A detector, usually reading ultraviolet light around 214 to 220 nanometres where peptide bonds absorb, draws a chromatogram: a line with peaks. The biggest peak is the main compound; smaller peaks are impurities such as truncated sequences or by-products of synthesis.
The purity figure on a COA is normally the area of the main peak divided by the total area of all peaks. A result of 98.5% means 98.5% of the detected signal came from the main peak. That is a relative measure of what was detected, not the percentage of the powder in the vial that is peptide.
Ask to see the chromatogram, not just the number. A clean, sharp main peak with a flat baseline looks very different from a crowded trace that was integrated to make the headline figure look good.
Mass spectrometry: confirming identity
HPLC tells you how pure a sample is. It does not tell you what the main peak actually is. Mass spectrometry (MS) does that by measuring the mass of the molecules and comparing it with the theoretical value for the expected sequence.
Peptides usually appear with one or more added protons, so the spectrum often shows several peaks for the same compound. For a peptide with a theoretical monoisotopic mass of 1000.5 Da, you would expect a peak near 1001.5 for a single added proton and another near 501.3 for two. Your COA should show an observed mass that agrees with the theoretical mass within the instrument's normal tolerance.
A COA that lists only purity and no identity test leaves a gap: a pure sample of the wrong compound still gives a high percentage.
Purity is not the same as peptide content
Freeze-dried peptide powder contains more than peptide. It also holds counter-ions left from purification (such as trifluoroacetate or acetate) and some residual water. So a material that is 98% pure by HPLC can still contain noticeably less than 98% peptide by weight.
This matters when you calculate concentrations from the weighed mass. Some COAs report net peptide content from amino acid analysis or nitrogen analysis, and some also report water content. If your work depends on exact concentrations, look for those figures or ask the supplier whether they are available.
Other tests you may see
Depending on the material and the lab, a COA may include further results. None of these is required on every certificate, but where a supplier claims a property, the matching test should be there.
- Water content (often by Karl Fischer titration).
- Endotoxin testing, if the material is intended for sensitive biological assays.
- Residual solvents or counter-ion content.
- Appearance, such as white to off-white lyophilized powder.
Red flags that make a COA worthless
Most bad certificates fail in predictable ways. Be sceptical when you see any of the following.
- No lot number, or a lot number that doesn't match the vial.
- The same certificate reused for every lot of a product.
- No chromatogram or spectrum, only a typed summary.
- The testing lab is the seller itself, or can't be identified.
- Purity of exactly 100% or results that are identical across different lots.
- Screenshots or images that look edited, cropped or low resolution.
- A test date that is missing, very old, or later than the shipping date.
How to check a certificate is genuine
If a COA matters to your work, verify it. Contact the named laboratory with the report number and ask whether it issued the document for that lot. Reputable labs will confirm. You can also ask the supplier for the raw chromatogram and spectrum files, which are far harder to fake than a formatted summary.
At Zeplabs, batch-specific Certificates of Analysis are available for every research material, and you can browse them on our certificates page.
Frequently asked questions
- What does a COA for a peptide show?
- It reports test results for one specific batch: identifying details such as the lot number, HPLC purity, mass spectrometry confirmation of identity, the test date, the methods used, and the laboratory that carried out the testing.
- Is 98% HPLC purity good?
- Many suppliers report research-grade materials at 98% or above, but the right threshold depends on your application. Look at the chromatogram as well as the number, and remember purity is a relative measure, not peptide content by weight.
- What is the difference between HPLC and mass spectrometry on a COA?
- HPLC measures how pure a sample is by separating its components. Mass spectrometry confirms identity by measuring the molecular mass and comparing it to the expected value. A good COA has both.
- How can I tell if a COA is fake?
- Check that the lot number matches your vial, that a real testing laboratory is named, and that a chromatogram and spectrum are included. If it matters, contact the laboratory to confirm it issued the report.
Zeplabs research materials are supplied for laboratory research use only and are not intended for human or veterinary use, or for the diagnosis, treatment, cure or prevention of any disease. This guide is general information for laboratory professionals. Always follow the label and documentation for the specific material you are working with.