2026-09-07

Purity Is Not Identity

Identity asks, “What is this?” Purity asks, “What else is present, and how much did this method detect?” A sample can produce a signal consistent with the target and still have an unresolved composition. A high purity number can also be calculated under a method that does not establish the target’s full identity.

Keep the two claims separate. This is the fastest way to stop a COA from doing work it never performed.

Identity is a matching problem

An identity test compares an observation with expected evidence: a mass-to-charge signal, retention time, spectrum, peptide map, reference standard response, or another characteristic. Confidence improves when more than one independent feature agrees and the method can distinguish plausible look-alikes.

Mass spectrometry can support a molecular-mass match. It may have limited power against isomers or epimers with identical masses. HPLC retention time can add information, but retention time alone is not a unique identity certificate. The method has to show what was compared and how specificity was assessed.

The PubMed review on synthetic peptide therapeutics describes why sequence variants, stereoisomers, degradation products, and process-related impurities need deliberate characterization.

Purity is a measurement convention

“99% pure” is incomplete without the denominator and method. It might mean chromatographic area percentage, an assay against a reference standard, mass fraction, or another convention. These results are not interchangeable.

Area percentage tells you about detector response among the signals included in that chromatogram. It can understate or omit compounds that co-elute, do not absorb well, fall outside the method’s scope, or sit below the reporting threshold. A quantitative assay answers a different question and depends on calibration, accuracy, precision, and matrix behavior.

ICH Q2(R2) uses “fitness for intended purpose” as the central validation idea. The intended purpose decides which performance characteristics matter. A method built for a broad purity screen is not automatically a specific identity method.

Three statements, three evidence burdens

Consider these claims:

• “A signal is consistent with semaglutide.” This needs an identity method, reference information, and a sample-to-lot link.
• “The sample is 98% pure.” This needs the definition of purity, method, units, calibration or calculation basis, and acceptance criterion.
• “This is equivalent to an approved product.” This needs evidence about formulation, manufacturing, controls, labeling, authorization, and clinical context. A COA does not supply that package.

The third claim is where category confusion becomes dangerous. Chemical similarity does not establish pharmaceutical equivalence or therapeutic equivalence.

A better report-reading habit

Underline every noun in the claim: target, form, lot, sample, product, person. Then underline every verb: detected, identified, quantified, passed, approved, works. Ask whether the report supports that verb.

On the sample review, record the exact analyte name, form, method, units, reference standard, lot, date, and missing information. The Explore library can help with surrounding evidence; the semaglutide overview is a place to keep product and trial context separate from analytical claims.

A clean conclusion might read: “The report shows a mass signal compatible with the named analyte and a method-specific chromatographic result. It does not establish all impurities, formulation equivalence, sterility, stability, or clinical performance.” That sentence is less exciting than “verified,” and far more useful.

Limitations

Analytical conclusions are method- and sample-specific. This article cannot assess a particular certificate, laboratory, product, or jurisdiction. It is educational only and does not provide medical, dosing, injection, reconstitution, administration, vendor, or purchase guidance.

Sources

• PubMed: Characterization of Synthetic Peptide Therapeutics Using LC-MS
• PubMed: Separation of peptide isomers and conformers by UPLC
• ICH Q2(R2): Validation of Analytical Procedures
• DailyMed: Wegovy label