Why Peptide Purity Is Reported in Several Different Ways

Is there a single number that means purity?

There is not. ICH Q2(R2) ties the validation of an analytical procedure to its intended use, which is a formal way of saying that a method answers the specific question it was designed for and not a general one. The IUPAC technical report on organic purity lists mass balance, quantitative NMR, thermal methods and other direct assay techniques as the principal routes to a purity value, and treats water, volatile organic compounds and related-structure impurities as categories that require their own determination.

Two documents can therefore report different purity figures for the same material without either being wrong, because they measured different things. This matters most when comparing suppliers. A figure reported without its method and basis cannot be ranked against another figure, and the larger number is not automatically the better material. Before comparing, establish what each number was derived from and which impurity classes it accounted for.

Sources: ICH Q2(R2): Validation of Analytical Procedures (2023); NIST / IUPAC: Methods for the SI value assignment of organic compound purity

What does the mass balance approach actually do?

Mass balance assigns purity by difference rather than by direct measurement of the compound. In the reference-standard characterisation described by McCarthy and colleagues, all detectable impurities, given as peptide-related impurities, counter-ion, water, residual solvents and non-combustible residue, are measured and subtracted from 100 percent to assign purity. The value is only as complete as the list of impurity classes actually determined.

This is why the phrase carries more information than a bare percentage. A mass-balance purity implies that water was determined, that counter-ion was determined, and that residue was determined, each by a method appropriate to it. A percentage taken from a single chromatogram implies none of those things. When a document reports mass balance, the useful follow-up is which classes were included; when it does not, the useful follow-up is what else was measured at all.

Sources: McCarthy et al., Pharmaceutical Research (2023): Reference standards to support quality of synthetic peptides

Why did NIST use four methods on one peptide?

For angiotensin I as a model compound, Josephs and colleagues investigated and optimised four distinct approaches: a full mass balance approach, peptide impurity corrected amino acid analysis, elemental analysis, and quantitative NMR. Four methods for one value is not redundancy. Each route carries different assumptions and fails in different ways, so agreement between them is evidence that no single assumption is carrying the result.

That is the standard of work behind a reference material, and it is deliberately higher than what sits behind a routine batch certificate. A reference standard receives characterisation beyond the tests it will later be used for, precisely because other measurements will be traced to it. Reading these studies is useful for understanding what a purity value can mean, not as a specification any particular catalogue product has been shown to meet.

Sources: NIST: SI traceable purity assignment using angiotensin I; McCarthy et al., Pharmaceutical Research (2023): Reference standards to support quality of synthetic peptides

How should you compare two purity figures?

Record the method and the basis next to every value, and refuse to compare figures whose basis you do not know. Where one document reports a mass fraction determined by quantitative NMR and another reports a percentage from a chromatogram, the two are not on the same scale and ordering them by size is meaningless. Ask which impurity classes each figure accounted for, and treat unanswered classes as unmeasured rather than as absent.

Ask also what the acceptance criterion was, since a result only means something against the limit it was judged by. For Savage Labs records, send support the compound name, vial size, batch identifier if known, and order number if applicable, and ask what is available before ordering. These resources support document literacy for in vitro laboratory research and do not establish suitability for human or veterinary use.