How to verify peptide purity before you buy

Purity is the most quoted and least verified number in the peptide trade. Here is a practical way to check it without owning a lab.

Start with a batch-specific COA

Ask for the certificate for the lot you will receive — not a generic product COA. Check that the batch number matches the label and invoice. If the supplier cannot provide this before payment, that is the answer.

Read the chromatogram, not the percentage

A purity figure without a chromatogram is unverifiable. Look for: baseline separation, visible impurity peaks, integration marks, and a retention time consistent with the method. A perfectly clean trace with no minor peaks is unusual for real synthesis and deserves scrutiny.

Confirm identity with mass spec

HPLC tells you the main peak is homogeneous; MS tells you it is the right molecule. The observed mass should match the expected molecular weight within the instrument's tolerance. Ask for the spectrum, not just "MS: conforms".

Check the method details

Column type and dimensions
Mobile phase and gradient
Detection wavelength
Injection volume and sample concentration

Without these, the percentage is not reproducible and cannot be compared across suppliers.

Net peptide content vs HPLC purity

A sample can be 98% pure by HPLC and still contain water, counter-ions or excipients that reduce the actual peptide content. For quantitative work, ask for assay or net peptide content.

A practical checklist

Check	Pass condition
Batch number on COA matches label	Exact match
Chromatogram included	Visible integration and impurity profile
MS spectrum included	Observed mass matches expected
Water content reported	Value stated, method named
Methods described	Column, gradient, wavelength
Release signature and date	Present and consistent

Research use only. This guide is about analytical verification, not human use.

Editor's take

Most purity disputes are documentation disputes in disguise. The buyer and the supplier are often looking at different lots, different methods or different definitions of the same percentage — and the fix is a batch-linked record, not a better argument.

Sources & evidence

Method validation: ICH Q2(R1) — ICH quality guidelines.
Specifications: ICH Q6B — same ICH page.
Water / particulates / endotoxin: USP <731>, <788>, <85> — USP.
Chinese Pharmacopoeia (ChP 2020) Part IV: chapters 0512, 0832, 0903, 1143 — cited by chapter number.

No laboratory testing was performed by the author for this guide; acceptance limits must come from your own specification.

Limitations

This guide describes methods and interpretation, not acceptance limits. Limits must come from your own specification and, where applicable, the relevant pharmacopoeial monograph. No testing was performed by the author for this page.