An impurity can be limited without being named individually in a specification, and it can be tracked without its chemical structure being established. These are different situations. Reading unspecified and unidentified as synonyms can make a report appear to answer more, or less, than it actually does.
Keep specification status separate from identity
ICH Q3A distinguishes an unspecified impurity, covered by a general criterion, from an unidentified impurity, whose structure has not been established. It also recognises individually specified impurities that remain unidentified.ICH Q3A(R2) — Impurities in New Drug Substances (opens in a new tab)
Q3A explicitly excludes peptide drug substances from its scope. Its terminology helps explain these words; its numerical thresholds must not be copied onto research peptides as universal limits.ICH Q3A(R2) — Impurities in New Drug Substances (opens in a new tab)
| Question | Possible answer |
|---|---|
| Is it individually listed and limited? | Specified or covered by a general impurity criterion |
| Has its structure been established? | Identified or unidentified |
| How is it tracked analytically? | A defined peak label or other analytical description |
| How was its amount obtained? | A stated quantitative method or response assumption |
A report should make its own usage clear. If the terminology differs, ask what the laboratory means rather than assuming the label has a single universal implementation.
An unnamed peak can still be tracked
In an original example, a table lists impurity U-1 at relative retention 0.82. That label can connect a result to a feature under the defined method. It does not, by itself, identify the molecular structure of that feature.
Relative retention is method-dependent. A peak at a similar relative position in another method is not necessarily the same compound, and several compounds can be insufficiently resolved within one apparent peak.
Tracking becomes more useful when the method, peak assignment and comparison records are consistent. A label that is reassigned to whichever peak is largest makes comparisons across reports difficult.
Read the percentage on its actual basis
If U-1 is reported as 0.20% area, the value describes a proportion of integrated detector response under the method. It is not automatically 0.20% of the powder mass or a measured quantity per vial.
An impurity can respond differently from the target peptide. Converting relative signal into an amount may require a suitable standard, response factor or justified assumption. The report should identify which approach was used.
A total impurity percentage can also conceal different distributions. In an original signal-area illustration, one feature at 0.60% and three features at 0.20% each have the same summed area, while presenting different individual-feature questions.
The largest individual impurity and the total are therefore different summaries. Neither tells the reader the structures of unnamed peaks, and neither describes compounds that the method does not adequately detect.
Keep unresolved identity visible in the conclusion
Ask whether the report is intended to quantify a known impurity, monitor an unidentified feature, or survey a broader impurity profile. Those objectives require different levels of assignment and analytical coverage.
An unnamed peak does not automatically establish a dangerous contaminant. Equally, a small percentage or a passing general criterion does not establish that an unknown substance is harmless. A safety conclusion requires evidence beyond a chromatographic label.
A precise summary might state that the method reported a feature at a defined relative retention and area percentage, with its structure unresolved. That preserves the useful observation without assigning a chemical name, origin or biological effect that has not been demonstrated.
Sources and further detail
- ICH Q3A(R2) — Impurities in New Drug Substances (opens in a new tab)
Preamble and glossary read. Used only for the distinction between specified, unspecified and unidentified; peptide substances are explicitly outside its scope. No reporting, identification or qualification threshold is imported.
Sources checked 19 September 2026. Worked examples are illustrative unless a supplied report is explicitly identified. This article has not undergone independent scientific peer review.