Why a Purity Percentage Does Not Prove Identity

The percentage contains no compound name

In an uncorrected peak-area calculation, a peak's area is divided by the sum of the included peak areas. Agilent's technical reference describes this calculation separately from calibrated quantitation. The ratio alone does not identify the compound responsible for the peak. [A2]

Consider two invented samples analysed separately. Each produces a dominant integrated peak with 9,900 arbitrary area units, plus other included peaks totalling 100 units. Assume the same calculation rule and detector conditions for this illustration.

The two percentages are identical because the signal proportions are identical. No compound identity enters the arithmetic. Relabelling the dominant peak would leave the calculation unchanged, although it would change the chemical claim. This is a logical example, not a prediction that two real materials would have identical chromatograms.

It also does not mean that either powder is 99.0% target compound by mass. Detector response and the calculation's denominator must support any conversion to mass. [A2] HPLC analysis explained covers those assumptions; identity, purity and assay separates the three reported attributes.

A familiar retention time still needs discrimination

Retention time is where a signal appears under specified chromatographic conditions. A match to a reference can contribute to identification, but one matching retention time does not exclude every alternative. ICH Q6A explicitly distinguishes a single retention-time match from a sufficiently specific identification procedure. [A3]

Two components may also overlap in one chromatographic feature. If the method does not resolve them, a large integrated peak can contain more than one component. The relevant question is whether the method can distinguish the intended material from plausible interferents, rather than whether the tallest peak looks clean. ICH Q2(R2) addresses this selectivity question. [A1]

An extra peak, shoulder or change in shape can prompt further investigation; it does not name an impurity. See common impurities and what they look like for the distinction between observing a feature and assigning its identity.

A mass match adds evidence with its own limits

For a simple fictional comparison, consider the unmodified linear dipeptides Ala–Gly and Gly–Ala with the same terminal groups. They contain the same residues and have the same elemental composition, but their residue order differs. Their intact molecular masses therefore cannot distinguish that order. This is a composition example, not an experimental spectrum or a claim about a commercial peptide.

The result does not make mass spectrometry unhelpful. It shows why the identity question must be precise. An intact-mass comparison, a fragmentation experiment and a stereochemical analysis do not provide interchangeable information. The mass-spectrometry guide explains what to check in a mass assignment.

When one procedure lacks sufficient discrimination, ICH Q2(R2) recommends combining procedures to address that limitation. [A1] Two test names on a certificate are not enough by themselves: ask what each result establishes and which ambiguity remains. The testing-methods overview provides context for that discussion.

Write a conclusion that matches the evidence

Use the following as a report-reading exercise. Each entry describes an evidence state, not an actual Knockout Labs lot.

Keep the percentage, method name and identity conclusion as separate fields in your notes. If an identity report is missing from the documents supplied to you, write “identity evidence not available for this review”. That records the gap without asserting that no test was performed or that the material is wrong.

Use the quality and batch-documentation hub to locate the applicable evidence route. A high percentage is useful within its defined measurement; it cannot supply an absent identity assignment, sampling connection or quantitative assay.