How do you read a certificate of analysis?

What does an HPLC purity number actually measure, and what doesn't it tell you?

High-performance liquid chromatography separates the components of a sample and measures the relative area of each resulting peak, which is what produces the familiar purity percentage on a certificate. USP General Chapter 621 governs the system suitability and method requirements for this kind of chromatographic testing across pharmaceutical substances [1].

What this number measures is the proportion of detected material that elutes as the expected main peak relative to other peaks, under the specific conditions of that run. What it does not measure is absolute identity. A peak in the expected position is consistent with the target molecule, but position alone does not prove the molecule present is what it is assumed to be; a different compound eluting at a similar retention time under the same conditions could produce a similar result.

What does mass spectrometry confirm that HPLC purity does not?

Mass spectrometry measures the mass-to-charge ratio of ionised molecular fragments, which is what allows it to confirm molecular identity rather than only relative abundance. This is the complementary test to HPLC: where HPLC purity answers how much of the detected material is the main component, mass spectrometry answers whether the main component is actually the molecule it is claimed to be.

A certificate reporting an HPLC purity percentage alone, without a mass-spectrometry identity confirmation, has established relative abundance but has not independently confirmed what that abundant substance actually is.

What does neither HPLC nor mass spectrometry tell you?

Sterility and endotoxin contamination are separate concerns entirely, tested by different methods, and a certificate reporting only chromatographic and spectrometric results says nothing about either. USP General Chapter 85, the Bacterial Endotoxins Test, is harmonised across USP, the European Pharmacopoeia and the Japanese Pharmacopoeia specifically for detecting bacterial endotoxin contamination [6]. Neither HPLC purity testing nor mass-spectrometry identity testing screens for this.

A product could show a clean purity peak and a correctly confirmed molecular identity by mass spec, and still be contaminated with bacterial endotoxin, because neither of those two tests looks for it. This is not a hypothetical gap: a 2024 peer-reviewed study testing online-sourced semaglutide products, in the same unregulated supply-chain category as many research-use-only peptides, found endotoxin present in every sample tested, alongside purity results as low as 7.7 percent against a 99 percent claim [7]. The purity and endotoxin findings in that study came from separate tests, illustrating exactly why a certificate needs to report on both to mean anything about overall product safety.

How is a certificate's underlying test method itself supposed to be validated?

A validated method is what makes a certificate's numbers trustworthy in the first place, and this is a formal, defined process, not an informal quality claim. FDA's analytical procedures guidance requires validation data to be generated under approved protocols following current good manufacturing practices, with predetermined acceptance criteria using qualified instrumentation [2]. USP General Chapter 1010 separately governs how the resulting data should be statistically treated, including methods for assessing whether different analytical procedures produce equivalent results [4].

The same FDA guidance requires that reference standards used in this testing, the known samples a new batch is compared against, carry their own qualification reports and certificates [3]. A certificate that reports a number without disclosing the method used, without confirming that method was validated, and without identifying the reference standard against which the sample was compared, is providing far less assurance than the document's formal appearance suggests.

What does a European equivalent look like?

The European Pharmacopoeia structures its monographs for pharmaceutical substances around the same underlying distinctions. A Ph. Eur. monograph separates an identification section, using methods like infrared spectrophotometry and thin-layer chromatography, a tests section covering impurities, and a separate assay section measuring active substance content, typically by chromatography or titration [5].

This mirrors the US framework's separation of identity, purity, and impurity testing into distinct measurements rather than one combined quality score, reinforcing that any single-number purity claim, in either regulatory system, reports on only one axis of quality.

What would change this answer

A specific USP general chapter or FDA guidance addressing minimum certificate disclosure standards for research-use-only chemicals, rather than regulated pharmaceutical ingredients, would close this answer's largest gap. A citation-ready USP chapter for mass-spectrometry-based peptide identity confirmation, distinguishing it explicitly from chromatographic purity testing, would strengthen that section specifically.