Identity and Purity Answer Different Research Questions

A report can contain an identity result, a purity result, or both. Those findings answer different questions. Identity concerns whether the tested sample matches the substance being investigated. Purity concerns composition under a stated measurement approach. Reading one as a substitute for the other can leave a procurement question unanswered.

Read the identity conclusion on its own terms

An identity entry may use words such as consistent, conforms, or matches. Find the method and the comparison underlying that conclusion. Ask what characteristic was examined and what reference was used. A substance name typed into the sample-description box is not automatically an analytical identity finding.

Eurachem describes qualitative analysis as producing a classification, such as the identity of a substance, rather than necessarily a numerical amount. Its qualitative-analysis guide also emphasizes that these conclusions need reliable procedures. A yes-or-no result can therefore require context just as a percentage does.

Read the purity result separately

Next, locate the actual purity result and its basis. Does the report describe a chromatographic area percentage, a mass fraction, or another quantity? Which components or signals enter the calculation? Preserve that wording in your notes rather than replacing every percentage with the broad phrase chemical purity.

The ICH Q2(R2) guidance published by FDA distinguishes identification from assay and impurity testing, and discusses specificity for their intended purposes. It is pharmaceutical guidance, cited here to explain analytical distinctions. Its presence in a source list does not establish regulatory compliance or approval for any research supplier.

Illustrative example: a result in the wrong column

Imagine a fictional report for Material M. The sample-description field says Material M, and a results table says 98.7% area. There is no separate identity conclusion. A reviewer initially writes identity confirmed because the name and a percentage appear on the same page.

A more useful record separates three entries. The submitted sample was described as Material M. The report states 98.7% on an area basis. Analytical confirmation of identity is not documented in the supplied pages. This preserves the available information without turning the sample label into a test result.

Now imagine that a second page contains an identity conclusion with its method reference. Add that evidence to the identity entry, retaining the scope of the stated conclusion. The additional page does not change the area percentage into a measurement of package content, and it does not answer every possible composition question.

Use a two-column review

• Under identity, record the conclusion, method reference, comparison material, and report page.
• Under purity, record the value, units or calculation basis, reported components, and limitations.
• Mark a missing finding as not documented rather than failed.
• Keep a separate entry for content if the procurement specification requires an amount measurement.

Before closing the review, write the question each finding actually answers in one plain sentence. If you cannot describe the basis without adding assumptions, request clarification from the report issuer. A useful document review is specific enough that another reader can see why the identity and purity entries were accepted, questioned, or left unresolved.

Sources

• Eurachem/CITAC: Assessment of Performance and Uncertainty in Qualitative Chemical Analysis
• FDA/ICH: Q2(R2) Validation of Analytical Procedures