HPLC Purity vs. Net Peptide Content: What "99% Pure" Means on a COA

Byline: Momentum Editorial

Summary

A purity percentage on a peptide certificate of analysis is meaningful only when the report prints both the figure and enough context to know what was measured. HPLC purity (when a chromatographic method is named) usually describes the relative area of the main peak versus other peaks in that chromatogram. Net peptide content (when stated) describes how much of the weighed solid is peptide versus counter-ions, residual moisture, or other non-peptide mass. A label that only says “99% pure” without a method or content definition is incomplete documentation — record exactly what the COA prints.

Why two different numbers appear on peptide paperwork

Laboratories and buyers sometimes use “purity” loosely. Analytical literature and pharmacopeial chromatography chapters treat chromatographic purity as a peak-area (or related) assessment of the analyte versus other detectable species under the stated method. Separately, peptide manufacturers may report net peptide content after accounting for salts and water. These answers different questions: “How clean is the main peak relative to other peaks?” versus “What fraction of the solid is peptide?”

If a COA does not name a method, do not invent “HPLC” in your lab notebook beyond what is printed (see Academy 04).

What “99% pure” usually claims — and what it does not

When a report prints a purity percentage and names a chromatographic method, the figure typically reflects that method’s purity assessment for the tested sample. It does not, by itself, prove:

• identity (often a separate mass-spectrometric or other identity test when named);
• that every strength of the same catalog name shares the same result;
• net peptide content, unless the report also states content.

Quantity-only reports (mass found in the vial without a purity %) are not purity claims — omit purity in inventory logs for those lots.

Net peptide content in plain language

Net peptide content answers how much of a weighed aliquot is peptide after non-peptide mass is considered. A high chromatographic purity and a lower net content can both be true: the peptide peak can be clean while the solid still includes acetate, trifluoroacetate, or moisture. File both numbers when both are printed; do not convert one into the other.

How to read the fields on a Momentum Labs COA

1. Match lot on the vial to the library row.
2. Confirm product name and strength.
3. Copy purity % only if printed.
4. Copy method only if named (example on file: Liquilabs “Peptide Screening” on an SS-31 report). Janoshik reports in the prior VERIFY inventory often leave method unnamed — do not add HPLC by assumption.
5. Note lab and test date exactly.

Walkthrough: How to Read a Peptide Certificate of Analysis.