What Is Peptide Purity, and How Is It Measured?
Purity is one of the most important numbers on a research peptide’s Certificate of Analysis, and one of the most misunderstood. Here is what purity actually measures, how laboratories determine it, and how to read it with confidence.
What purity means
Purity is the proportion of a sample that is the intended peptide, as opposed to impurities such as truncated sequences, deletion products, or leftover synthesis byproducts. It is expressed as a percentage. A purity of 99 percent means that, by the method used, 99 percent of the detected material is the target peptide. That qualifier is doing real work: purity is method-dependent, and it is the validated method that determines which impurities the analysis can see at all.[2]
How purity is measured
The standard method is high-performance liquid chromatography, or HPLC. In HPLC, the sample is separated into its components as it passes through a column, and each component appears as a peak on a chromatogram. Purity is calculated from the area of the target peptide’s peak relative to the total area of all peaks, which is why you often see purity described as “percent by HPLC area.” The compendial chapter behind that number also sets system suitability requirements a run has to meet before the result counts, including a general peak symmetry window of 0.8 to 1.8.[1] A clean chromatogram with one dominant peak and only small side peaks indicates high purity.
Purity is not the same as identity
A high purity figure tells you the sample is mostly one compound, but not which compound. That is where mass spectrometry comes in. Mass spec measures the molecular weight of the peptide and confirms it matches the expected sequence. The ability of a method to tell the target apart from everything else in the sample has a formal name, specificity, and it is one of the characteristics a method must demonstrate during validation.[2] A trustworthy COA shows both: HPLC for purity and mass spec for identity. Our guide on how to read a COA breaks down each section.
Purity versus net peptide content
These two are often confused. Purity, measured by HPLC, describes the quality of the peptide portion of the sample. Net peptide content describes how much of the vial’s total mass is actually peptide, as opposed to water, salts, or counterions left over from synthesis. It is a separate determination, usually made by amino acid analysis after hydrolysis rather than by chromatography, which is why the two figures can differ so widely on the same vial.[3] A vial can be high purity and still contain a meaningful fraction of non-peptide mass, so it is useful to understand both figures.
Why purity matters for research
Impurities are variables. In research, an unknown impurity can confound results or reduce reproducibility. Knowing the purity, and confirming the identity, lets you attribute your findings to the compound you intended to study rather than to a contaminant you did not know was there.
How to read purity on a COA
On a Certificate of Analysis, look for the purity percentage, the method used (HPLC), and ideally the chromatogram itself, along with a mass spec result confirming identity. Confirm that the COA’s lot number matches the lot on your vial, since purity is measured per lot, not per product.
References
Sources are limited to compendial standards and regulatory guidance describing how purity is measured and validated. Each was checked on 11 August 2026.
- United States Pharmacopeia, General Chapter <621> Chromatography. Harmonised through the Pharmacopeial Discussion Group at Stage 4, official with USP-NF 2022 Issue 3 on 1 December 2022; adds a general peak symmetry requirement of 0.8 to 1.8. PDG harmonisation record.
- International Council for Harmonisation, Q2(R2) Validation of Analytical Procedures, FDA Guidance for Industry, final, March 2024. FDA guidance page.
- United States Pharmacopeia, General Chapter <1052> Biotechnology-Derived Articles — Amino Acid Analysis. PDG Stage 6, official May 2018. PDG harmonisation record.
Every lot at USA Made Peps is independently third-party tested. You can look up any lot’s COA or browse the catalog.
Related reading
- Research Peptide Glossary: Key Terms Explained
- What Made in the USA Means for Research Peptides
- All research guides
- Frequently Asked Questions
- BPC-157 and TB-500: What the Blend Actually Contains
Related reading: what >98% purity actually means, with every lot COA, and how to read a Tesamorelin COA.
All products are sold strictly for research use only (RUO) and are not for human or animal consumption.