Purity and identity describe different attributes. A sample may be dominated by one compound while the evidence does not establish that the compound is the intended peptide. When report conclusions seem to conflict, first ask whether the tests were answering the same question.
Translate each result into the question it answers
A chromatographic area percentage describes the distribution of integrated signal under a particular method. Identity testing asks whether the analysed material matches the intended substance using evidence that can distinguish relevant alternatives.
ICH Q6A treats identification, assay and impurities as distinct test categories and cautions that a single chromatographic retention time is not sufficiently specific for identification. Its drug-specification context supports the distinction; it does not approve a particular research material.ICH Q6A — Specifications: Test Procedures and Acceptance Criteria for New Drug Substances and New Drug Products (opens in a new tab)
| Statement | Question still open |
|---|---|
| The main peak is 99% of integrated area | Is the main peak correctly assigned? |
| An intact mass matches an expectation | Does the available evidence distinguish relevant alternatives? |
| An identity test is inconclusive | What prevented a reliable assignment? |
| An identity criterion fails | Is the failure analytical, documentary or material-related? |
These findings are not votes to be averaged. They contribute different evidence to the description of the sample.
Read a high-area result with an unresolved assignment
In an original fictional case, a report gives 99.3% main-peak area and states that the submitted sample does not meet the specified identity criterion. The high percentage does not make the identity statement disappear.
The first question is which peak supplied the percentage and how it was assigned. If the dominant peak belongs to another compound, a high area percentage may simply describe a relatively uniform wrong material. That is one hypothesis, not the conclusion to assume.
Other possibilities include an incorrect reference, an unsuitable identity method, a sample mix-up or a reporting error. Each requires its own evidence. A reader cannot identify the cause from the two summary lines alone.
Investigate the link between the two measurements
Establish whether the purity and identity results came from the same original sample, the same prepared portion and the same report version. A combined summary can hide different sample origins or stages.
Then read the identity target precisely. Sequence, terminal modifications, disulfide arrangement, salt form and analytical mass convention can affect what matching means. The relevant distinction depends on the compound and the claimed test scope.
If an intact mass is involved, ensure that the comparison is between compatible quantities. An ion mass-to-charge value is not an uncharged molecular mass, and a reported adduct needs interpretation within the method.
These checks identify possible mismatches in interpretation. They are not permission to relabel an unexplained spectrum or invent a structural assignment that the laboratory did not make.
Choose evidence that addresses the remaining question
The next useful measurement depends on the unresolved alternatives. A method that only repeats the same selectivity limitation may reproduce the original ambiguity. Complementary evidence is useful when it can discriminate between the remaining possibilities.
The investigation should state which assignment is supported, which criterion was applied and whether the original result was corrected for a documented reason. If the evidence remains inconclusive, retain that description.
For the report reader, the conclusion can stay narrow: the available area result does not resolve the identity discrepancy. This avoids both an unsupported reassurance and an unsupported accusation about the supplied material.
Sources and further detail
- ICH Q6A — Specifications: Test Procedures and Acceptance Criteria for New Drug Substances and New Drug Products (opens in a new tab)
Section 3.2.1 identification, assay and impurity distinctions read. No claim that any single technique fully establishes every structural feature. The 99.3% case is original.
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.