Purity, identity and content: three different questions

Start with the question the method answers

An analytical result is useful when the method is appropriate for its purpose. Identifying a compound, measuring its amount and assessing impurities are related tasks, but they are not interchangeable. ICH Q2(R2) distinguishes these analytical purposes. This guideline provides a framework for method validation; it is not evidence that a particular product has been tested or meets a specification.

What a chromatographic percentage means

A reported peak-area percentage depends on the separation, detection and integration used. It describes the measured chromatogram under those conditions. It should not be converted automatically into a claim about the mass of active material in a vial, every possible contaminant, or microbiological quality.

In HPLC, components travel through a column and interact differently with its stationary phase, allowing some components to separate into distinct peaks. Substances that overlap, respond weakly to the detector or fall outside the method's scope may not be adequately represented by the resulting percentage.

There is no single purity percentage that makes every research material suitable for every experiment. Suitability depends on the intended work and the specifications established for it.

What identity testing adds

Mass-spectrometric information can support identification when interpreted with the method and any other characterisation required. It is not a blanket demonstration that every sequence variant or impurity is absent. A matching molecular mass alone may not distinguish compounds with the same mass but different structures.

Content testing answers a further question: how much of the specified material is present? A chromatographic area percentage and a measured amount in milligrams are different results. Neither establishes sterility or an endotoxin level; those require separate, appropriate tests.

Read the report at batch level

Check the sample identifier, lot, date, units, methods and reported results. Distinguish a test that was performed from one that is not listed. A result from one batch or presentation should not be assigned to all products from the same brand.

Use the report library to find the relevant document, then consult the comprehensive-testing article for a fuller explanation of methods. This short guide is about interpreting a result; it does not establish a universal purity guarantee.