Identity, Purity and Assay: What Each Result Establishes

Three questions to keep separate

The word purity needs a qualifier. HPLC area purity is one measurement; it does not describe every substance that might be present. Similarly, “assay” can mean a content measurement or a biological activity measurement. Read what was measured before comparing two results with the same label. ICH Q6B treats biological potency and quantity as distinct attributes, although a justified relationship can sometimes be established. [A3]

A worked example: three results, three conclusions

The following is an invented teaching example, not a Knockout Labs certificate or a product specification. Assume the documents concern the same sample and the methods are suitable for their stated purposes.

There is no contradiction simply because one result is a percentage and another is a mass. They have different denominators. The purity calculation compares detector signals; the content result concerns the amount measured in the sample. For the detailed distinction between gross material and compound content, see net peptide content.

If a label states a different mass from the assay, first establish whether both values refer to the same chemical form and reporting basis. Do not explain away a discrepancy by assuming it is water, a counter-ion or solvent. Those explanations require their own evidence; assay, water content and residual solvents covers that question.

Why identity needs its own evidence

A clean-looking chromatogram could belong to a different compound. Identification needs enough discrimination to separate the intended material from relevant alternatives. ICH Q6A specifically cautions against relying on a single chromatographic retention time for identification and recognises combinations of methods. [A1]

Use why purity does not prove identity for that distinction. Where a report uses a mass match, mass spectrometry and compound identity explains the remaining questions about charge, sequence order and stereochemistry.

A percentage needs a method behind it

Ask which signal was measured, which peaks entered the calculation and whether the method separated relevant components. A number copied without those details loses much of its meaning. HPLC analysis explained shows the calculation and its limits.

For a quantitative result, method performance also matters. Selectivity concerns interference from other substances; accuracy concerns agreement with an accepted reference value; precision concerns agreement between repeated measurements. Repeatedly obtaining a similar number does not, on its own, establish that the number is accurate. These are separate validation characteristics in ICH Q2(R2). [A2]

Read the batch record in this order

1. Match the sample. Check product identity, chemical form, lot code and laboratory report identifier. A report for another lot does not fill a gap in this one.
2. Read each result with its method and unit. Separate identity findings, chromatographic percentages and quantitative content. Record an unreported result as unreported.
3. Check the comparison basis. Look for the reference, specification, reporting basis and applicable limits. A “pass” without its criterion leaves a question unanswered.
4. Keep the conclusion within the evidence. Do not turn one measured attribute into a claim about all quality attributes or suitability for an experiment.

The analytical testing methods guide helps identify the kind of evidence to request. The quality documentation hub explains the wider record, and batch reports provides the route for asking what is available for the exact lot. For packaging wording, read how the 99%+ purity claim must be substantiated.