What Does Purity Percentage Mean?

A purity percentage is the most quoted number on a peptide report, and the most often misread. This guide explains what the figure measures, what it leaves out, and what to read beside it.

A share of what the method detects

On a peptide report, purity is usually chromatographic purity: the share of the total detected signal that belongs to the named compound, measured by HPLC with a UV detector. It is reported as area-percent, the main peak's area as a proportion of all integrated peak areas. Understanding HPLC Results explains how those areas are obtained.

A worked example

Suppose, hypothetically, that a chromatogram shows a main peak of 970 area units and three small peaks totalling 30. The total is 1,000, so the area-percent purity is 97.0. If a fourth impurity left the column at the same time as the main peak, its area would be counted as the compound, and the figure would read higher than the sample's true make-up. If an impurity did not absorb at the detection wavelength, it would not be counted at all.

The method is part of the number

Two laboratories can report different purity figures for the same sample if they use different columns, gradients, wavelengths or integration settings. That is why a purity figure means little without the method printed beside it, and why a figure read at one wavelength is not directly comparable with one read at another.

Purity is not content

Purity says what share of the detected material is the named compound. It does not say how much of the compound is in a vial. A lyophilized peptide also contains water and counter-ions, such as acetate or trifluoroacetate remaining from synthesis and purification, which a UV purity method does not detect. Net peptide content, where it is measured, is a separate figure, and the labelled quantity of a vial is a third.

Purity is not identity

A high purity figure says the sample is mostly one component. It does not, on its own, say which component. Identity is a separate test, usually mass spectrometry; see Understanding Mass Spectrometry. A useful report gives both.

Specification and measured figure

A report often prints a specification, the acceptance criterion, beside the measured figure. ICH Q6A describes a specification as a list of tests, references to analytical procedures and appropriate acceptance criteria. The specification is the pass mark; the measured figure is the finding for that sample. A figure that meets its specification says the sample passed that test, by that method, on that date, and nothing more.

Reading a purity figure, step by step

• Find the method: the technique, the column and the detection wavelength.
• Find the batch number and vial size the figure describes.
• Read the specification beside it, if one is printed.
• Look for a separate identity test.
• Do not carry the figure over to another batch or another vial size.

Where this site fits

A purity figure appears on a product page only where a published report states one for the vial size shown. Where a batch report is published, the full document is in the COA Library, with the method beside the figure. The difference between identity and purity is covered further in Purity & Third-Party Testing, Explained, and why a figure belongs to one batch in Why Batch-Level Testing Matters.

Sources and references

• ICH Q2(R2), Validation of Analytical Procedures, International Council for Harmonisation, 2023.
• ICH Q6A, Specifications: Test Procedures and Acceptance Criteria for New Drug Substances and New Drug Products: Chemical Substances. International Council for Harmonisation, 1999. Cited for its definition of a specification; its scope is chemical drug substances.
• USP General Chapter <621>, Chromatography. United States Pharmacopeia.
• USP General Chapter <1086>, Impurities in Drug Substances and Drug Products. United States Pharmacopeia.
• L. R. Snyder, J. J. Kirkland and J. W. Dolan, Introduction to Modern Liquid Chromatography, 3rd edition. Wiley, 2010.

Research use

All materials on this site are supplied for laboratory research use only. They are not for human or veterinary use, and this guide gives no instructions for preparing or using them.