What a purity claim really tells you - and what it doesn't

How is peptide purity measured?

The purity number is not a fixed property of the peptide. It is the result of an HPLC measurement (high-performance liquid chromatography). An HPLC device separates the sample into its components in a column. Purity is calculated from the ratio of the main peak (your peptide) to the sum of all measured peaks in the chromatogram.

This value depends heavily on the measurement conditions. The choice of column and mobile phase (liquid) determines which impurities become visible. A study with 30 combinations of five columns and six mobile phases demonstrates this "complementary selectivity": each condition shows a different picture of purity.

The detection wavelength also influences the result. For eptifibatide acetate, for example, maximum absorption is at 219 nm and 275 nm. Since different wavelengths detect components with varying intensity, a different setting would also yield a different purity number.

What is the difference between purity, identity, and content?

Purity, identity, and content (mass) answer three different quality questions for peptides. While purity indicates the proportion of the target peptide relative to impurities, identity (e.g., through reference comparison) checks whether it is really the right substance. Content (assay) determines the actual mass of the peptide in the sample.

An example illustrates this: a vial can have a purity of 98%, but only be half full (low content). Or the main substance is a completely different compound (wrong identity). Professional guidelines such as those from the ICH or EDQM therefore strictly separate these categories into their own test procedures.

What safety statements are missing from the purity number?

A purity number is not a safety guarantee for a peptide. It does not tell you whether the remaining percentage of impurities is toxic or harmless. To ensure the safety and efficacy of peptide drugs, comprehensive specifications are needed that go far beyond a single percentage value.

Moreover, the number says nothing about identity, since the main peak could be a wrong peptide. A certificate of analysis (CoA) also always applies only to a specific batch. Without this direct batch reference, a purity statement is analytically worthless for you.

Other critical quality aspects such as sterility, endotoxins (bacterial toxins), or the correct folding of the peptide are not captured by the purity number. These characteristics must be confirmed through separate test methods.

Why are complete details in the certificate of analysis (CoA) important?

A valid CoA (Certificate of Analysis) must clearly list the test methods, results, and acceptance criteria. The WHO uses standardized templates for this purpose to make the trade in starting materials safe. If details on method, wavelength, or date are missing, the result cannot be seriously verified.

Without chromatograms as representative data, you cannot assess the specificity of the measurement. If the documented conditions are missing, the analysis is not reproducible. Only metrological traceability - an unbroken chain of calibrations - makes the purity statement credible.

How counterfeiters exploit the purity statement for peptides

In unregulated markets, a single purity number is easy to fake. Without method details, you cannot reproduce the analysis, which makes the number unfalsifiable. If the date or report number is missing, counterfeiters often use the same document for completely different batches.

It is easier to invent a plausible number than to create a consistent analysis protocol with validated parameters. How complex the testing is shows the Belgian customs: for seized peptides, several orthogonal methods such as LC-MS/MS were often needed to clarify the true identity.

How is peptide purity measured?

What is the difference between purity, identity, and content?

What safety statements are missing from the purity number?

Why are complete details in the certificate of analysis (CoA) important?

How counterfeiters exploit the purity statement for peptides

Sources