Difference between related substances and chromatographic purity

In analytical chemistry, especially in pharmaceutical testing, related substances and chromatographic purity are terms used to assess the quality and purity of a substance or product. Here’s how they differ:

Related Substances:

• Refers to impurities that are structurally related to the main compound or active pharmaceutical ingredient (API).
• Related substances are often impurities formed during the synthesis, degradation, or storage of the drug substance or drug product.
• In regulatory terms, related substances are specified by individual limits (typically as a percentage of the main compound) and are reported individually and collectively.
• Testing for related substances ensures that no impurity exceeds a safe limit, which is often required by regulatory guidelines (like ICH guidelines).

Chromatographic Purity:

• Refers to the overall purity of the main compound as determined by chromatography, usually high-performance liquid chromatography (HPLC).
• Chromatographic purity is calculated by comparing the area of the main peak to the total area of all peaks (including impurities) in a chromatogram.
• This measure gives a general percentage of how pure the main compound is but doesn’t provide detailed information about each impurity.
• While chromatographic purity indicates the purity of the main peak, it is a less specific measure than related substances testing because it does not account for each impurity individually.

Key Differences:

• Focus: Related substances testing focuses on individual impurities, while chromatographic purity focuses on the overall purity of the compound.
• Information Provided: Related substances testing provides detailed information on each impurity, while chromatographic purity provides an overall purity percentage.
• Regulatory Importance: Both are essential, but related substances testing is particularly critical for regulatory compliance to ensure safety and efficacy.

At a glance