A peptide purity percentage is a single figure that usually appears near the top of a certificate of analysis (CoA), the document a vendor uses to describe a batch of a research-use-only compound. It is almost always a High-Performance Liquid Chromatography (HPLC) result: the proportion of the sample's total detector signal that belongs to the target peptide. The number is useful, but it answers a narrower question than most readers assume, and it is only as trustworthy as the raw data behind it. This guide explains what the figure measures, what it silently omits, and why you should read the trace, not just the total.
What does a purity percentage on a CoA actually measure?
When a lab reports a peptide as 99.0% pure, that number almost always comes from HPLC (High-Performance Liquid Chromatography). The instrument pushes the dissolved sample through a column that separates its components by how strongly each one interacts with the column material. A detector records each component as a peak, and the software integrates the area under every peak. Purity is then reported as the target peak's area expressed as a percentage of the total peak area. So 99.0% means the main peak accounts for 99% of the detected signal, and other peaks (impurities, related substances, or breakdown products) make up the remaining 1%. This is a measure of relative composition by peak area, not an absolute weight of pure peptide in the vial, and not proof that the main peak is the molecule the label claims. Keeping that definition in mind is the first step to reading a CoA sceptically.
Why is net peptide content different from HPLC purity?
HPLC purity and net peptide content answer two different questions, and confusing them is one of the most common CoA misreadings. HPLC purity describes how clean the peptide fraction is relative to other peaks. Net peptide content describes how much of the material in the vial is actually peptide by mass. The gap between them exists because a lyophilised (freeze-dried) peptide is rarely just peptide. It also carries water, residual salts from synthesis such as trifluoroacetate (TFA) or acetate, and associated counter-ions. A sample can be 99% pure by HPLC and still be a meaningful fraction water and salt by weight. Net peptide content is measured separately, typically by amino acid analysis (AAA) or nitrogen determination, and it is reported as its own figure. If a CoA shows a high purity percentage but no net-content value, understand that the two numbers are not interchangeable.
| Figure | Question it answers | Typical method |
|---|---|---|
| HPLC purity (% by area) | How clean is the peptide relative to other peaks? | HPLC |
| Net peptide content | How much of the vial's mass is actually peptide? | AAA or nitrogen analysis |
| Identity | Is this the molecule the label claims? | Mass spectrometry (LC-MS) |
Why should you look at the chromatogram, not just the number?
The chromatogram - the plotted HPLC trace itself - is the actual evidence; the purity percentage is only a summary of it. A bare number with no chromatogram attached is a weak signal, because you cannot see how the lab reached it. With the trace in front of you, you can see whether the baseline is stable, whether the main peak is cleanly separated from its neighbours, and whether small peaks were resolved or buried. Integration choices matter: where the software draws the boundaries of each peak changes the reported area. A trace lets a reader judge those choices instead of taking them on faith. This is why PeptideTrust treats a chromatogram, not just a number as a marker of transparency. If you want the full anatomy of the document, our guide on how to read a peptide CoA walks through each element and its red flags. Transparency here is a documentary property, not a marketing claim.
How do purity and identity answer different questions?
A high purity percentage tells you the sample is clean. It does not tell you the clean substance is the peptide the label names. Purity and identity are separate questions answered by separate methods. HPLC measures purity; mass spectrometry (MS), often coupled as LC-MS, confirms identity by measuring the compound's molecular mass and checking it against the expected value. A sample can be 99% pure and still be the wrong molecule, and only an identity method rules that out. A credible CoA therefore pairs an HPLC purity result with an MS identity result, so both questions are covered. If a certificate shows only a purity figure, it has answered half the question. Our explainer on HPLC versus mass spectrometry testing covers why both methods belong on the same document and what each one can and cannot prove.
What transparency signals should you check around the number?
A purity percentage becomes meaningful only when the paperwork around it is verifiable at the source. Rather than trusting the figure alone, treat these as the context that gives it weight:
- A named, independent lab - a third-party laboratory identified by name, not a vague "tested by a US lab."
- A batch or lot number that matches the number printed on the vial, so the result describes your material.
- A date of analysis, because results describe a batch at a point in time.
- The chromatogram itself, attached rather than summarised.
- A verification mechanism - a code or QR that resolves on the lab's own domain, not a PDF hosted only by the vendor.
Accreditation to ISO/IEC 17025, the standard for testing-laboratory competence, is a further strengthening signal. To see how these elements are weighted into a score, read our scoring methodology, and remember that a purity CoA describes composition only, for research use only, and never implies sterility or safety.