The short version
When a research peptide is listed at ≥99% HPLC purity, the supplier is saying that under the stated chromatographic method, the main peak accounts for at least 99% of the integrated signal attributed to analytes in that run. It is a quality metric for laboratory materials — not a clinical grade and not a guarantee of performance in every assay.
Nextday Peptides uses that threshold across much of the catalog because it is a common expectation for in-vitro and analytical work. Always confirm the lot COA rather than relying only on the product-page claim.
What HPLC is actually measuring
High-performance liquid chromatography separates sample components as they move through a column. A detector records peaks over time. Purity software then assigns area percentages. The headline number is only meaningful relative to that method: column, gradient, wavelength, sample solvent, and integration rules.
Compounds that do not absorb at the monitored wavelength, or that co-elute with the main peak, can sit outside what the purity percentage “sees.” That is why identity methods (often MS) appear alongside HPLC on stronger COAs.
Purity is not identity
A 99% peak still needs a name. Identity testing asks whether that peak is the intended sequence or small molecule. Mass spectrometry, amino-acid analysis, or comparison to a reference standard answers a different question than peak-area purity.
When you file paperwork for a study, record both: the HPLC result and the identity method listed for that lot. Our COA reading guide walks through matching those fields to the vial label.
Practical checks for buyers
Compare the product page claim to the COA for your batch. Note units and rounding. Prefer lot-specific PDFs from the COA Library over screenshots of an older lot. If a blend SKU is on the order, understand whether the COA covers the blend or each component — and request clarification when the file is ambiguous.
Counterfeit or mislabeled research chemicals are an industry-wide problem. HPLC on a COA helps, but receiving controls at your institution still matter: visual check, lot match, and quarantine until documentation is filed.
RUO framing
Purity language on this site describes research materials. It is not a potency claim for human use, not a bioavailability statement, and not an invitation to self-experiment. See Research Use Only peptides explained and the Educational Articles Disclaimer for the boundaries we enforce on copy.