Source of Purity claim inflation

{{Infobox concept | name = Purity claim inflation | subtitle = Quality failure | image = purity-hist.svg | caption = The same material can be described by several defensible numbers, and the largest is usually chosen. | Distinguished from = [[Test report forgery|Forgery]] — the figure here is genuine | Mechanism = Method choice, integration, and framing }} '''Purity claim inflation''' is the presentation of a purity figure in a manner that overstates what it establishes. Unlike [[Test report forgery|forgery]], the figure is generally genuine: what is misleading is the method that produced it, the way it was integrated, or the framing that presents it as answering a question it does not answer.{{r|usp1503}} The most consequential form is the substitution of purity for content. An [[Area percent purity|area percent]] figure is normalised: it describes the proportion of detected material attributable to the main peak, and it is unaffected by how much peptide is present. A vial can be 99% pure and contain half the stated mass. Presenting purity where content is the relevant question is the commonest inflation of all.{{r|usp1503}} Because these practices operate through selection rather than fabrication, they are not detectable by asking whether a report is genuine. They are detectable, if at all, by reading what the report actually measured. == Mechanisms == | Mechanism | Effect on the number | |---|---| | Short gradient | Close-eluting impurities merge into the main peak | | Generous integration boundaries | Shoulders counted as main peak | | Detection wavelength chosen for the analyte | Impurities absorbing elsewhere under-detected | | Reporting purity in place of content | Silence on how much peptide is present | | Quoting the best of several determinations | Selection from a distribution | | Aggregating related substances into one figure | Individual impurities concealed | None of these is necessarily improper. A short gradient may be entirely appropriate for a well-characterised material; aggregate reporting of related substances is standard in some contexts. What makes a presentation inflationary is the gap between what the method establishes and what the figure is used to assert.{{r|usp621}} The detection-wavelength point is the least appreciated. Peptide purity is usually determined at 214 nm, where the amide bond absorbs, so response is roughly proportional to the number of bonds and impurities are detected reasonably. A determination at 280 nm, where aromatic side chains absorb, will systematically under-detect impurities lacking those residues. Both are legitimate methods; they do not answer the same question. == What a purity figure does establish == An area percent purity figure establishes that, of the material detected by that detector under those conditions, the stated proportion eluted as the main peak. That is a real and useful fact and this article should not be read as dismissing it.{{r|usp1503}} What it does not establish: how much peptide is in the vial; that the main peak is the intended molecule; that undetected species are absent; that the figure would be reproduced on a different method. A complete characterisation therefore requires, at minimum, identity by [[Mass spectrometry|mass]], purity by a specified chromatographic method, [[Peptide content|content]], and water. Certificates supplying all four are a minority in submissions to this wiki, though all four are ordinary components of a specification where one exists.{{r|ich_q6b}} A certificate supplying only the second is not thereby dishonest — but a claim about how much active substance a vial contains cannot rest on it.{{r|reports}} == Reading against inflation == The useful questions are about method rather than about number, and are answerable from a well-constructed certificate. Which method produced the figure, at what wavelength, on what column, over what gradient duration? Is the main peak identified, and by what? Is [[Peptide content|content]] reported separately from purity? Are related substances itemised or aggregated? Is water reported, and by [[Karl Fischer titration|Karl Fischer]] or by [[Loss on drying|loss on drying]] — which measure different things?{{r|usp1503}} A document that answers these is checkable, whatever the numbers on it. A document that does not is not checkable, whatever the numbers on it. That criterion is more robust than any threshold on purity, and it does not require the reader to trust the figure at all. None of this is an allegation against any supplier. It is a description of how a genuine number can be made to carry more weight than it can bear, which is a property of the instrument rather than of anyone using it. == References == {{reflist}} <ref name="usp1503">United States Pharmacopeia, General Chapter <1503>, ''Quality Attributes of Synthetic Peptide Drug Substances''.</ref> <ref name="usp621">United States Pharmacopeia, General Chapter <621>, ''Chromatography''.</ref> <ref name="reports">PeptidePedia Wiki community test-report tally, 2024–2026 (self-reported; see [[Project:Sourcing_guidelines]]).</ref> <ref name="ich_q6b">International Council for Harmonisation, ''Q6B: Specifications — Test Procedures and Acceptance Criteria for Biotechnological/Biological Products'' (1999).</ref> == See also == * [[Area percent purity]] * [[Peptide content]] * [[Certificate of analysis]] * [[Test report forgery]] * [[Analytical method validation]] * [[Underfilling]] {{DEFAULTSORT:Purity claim inflation}} [[Category:Quality failures]] [[Category:Certificates of analysis]] [[Category:Method validation]]