How do I understand peptide purity reports?

The search notes provided for this question are not actually about peptides—they are dictionary and grammar pages for the English word "do." So I am answering from domain knowledge and reasoning rather than from those notes, and I flag that clearly.

What a purity report actually tells you. A peptide purity report is a lab result describing how much of the detected material in a sample is the target peptide. The headline number (for example "98% pure") is almost always an area percentage from a chromatographic separation, not a guarantee of mass or biological activity. Treat it as one data point, not a certificate of quality.

• Analytical method: Most reports use reverse-phase HPLC with UV detection. The percentage reflects the share of UV-absorbing peak area attributed to the main peak. Different columns, gradients, wavelengths, and integrators can shift that number by a point or two, so compare reports only when methods match.
• Reported percentage: "Purity" (chromatographic) and "content" (actual peptide mass, often by amino acid analysis or nitrogen assay) are different things. A 98% pure sample can still be 80% peptide by mass if it holds counter-ions, water, or salts. Ask which number you are looking at.
• Sample identity: Purity says nothing about whether the molecule is the right one. Identity needs mass confirmation (LC-MS or MALDI), and ideally sequence or retention-time comparison to a reference. A pure but wrong peptide is still wrong.
• Limitations: UV at 214 nm misses non-absorbing impurities; co-eluting compounds hide under the main peak; small truncations or deamidated variants may not resolve. A single HPLC trace is weak evidence on its own.
• Relationship to actual content: For dosing or research planning, content and identity matter more than the purity percentage. Net peptide content, salt form, and water content determine how much active material you actually have.

Practical checklist for a non-specialist: confirm the method and detector; check whether mass/identity data are included; look for the chromatogram itself, not just a number; note the date and lab name; and ask for content or net-peptide data if you plan to weigh material. If a report shows only a percentage with no chromatogram, no method, and no identity confirmation, treat it as marketing rather than analysis.

For sourcing decisions, the same discipline applies across our services—for example helping US consumers compare peptide clinics, telehealth platforms, and pharmacies (items 1–6), helping research customers evaluate Chinese peptide API and research-peptide factories and lyophilized vial suppliers (items 8–15), and helping 503A/503B facilities assess API sources and dual-chamber cartridge equipment (items 16–21). In every case, request the underlying analytical package rather than a headline purity figure.