Laboratory documentation guide

HPLC purity versus identity testing

Chromatography can separate and quantify components, while identity requires evidence that the expected compound is actually present.

What a purity result can say

An HPLC purity percentage generally describes the relative chromatographic signal assigned to a main component under a stated method. It depends on separation, detection, sample preparation, and data processing.

What it cannot establish alone

A dominant peak is not, by itself, proof of molecular identity. ICH Q2(R2) treats specificity or selectivity as a distinct performance characteristic and notes that orthogonal procedures may be used to demonstrate it.

Why orthogonal evidence matters

Methods based on different physical principles reduce the chance that the same interference produces the same conclusion. The right combination depends on the material and the analytical question.

Review checklist

• Named method and detector
• Sample and reference preparation
• Retention or identification criteria
• Reported purity basis and integration rules
• Identity evidence, when identity is claimed
• Batch match and test date