Source of Underfilling

{{Infobox concept | name = Underfilling | subtitle = Quality failure | image = vial.svg | caption = Nominal fill is a label claim; delivered mass is a measurement. | Failure = Delivered mass below nominal | Not detected by = Purity determination | Detected by = Gravimetric fill check, or [[Peptide content|content]] with fill mass }} '''Underfilling''' is the delivery of less material in a container than the quantity stated on its label. It is a distinct failure from impurity and is invisible to the determination most often supplied: [[Area percent purity|area percent purity]] is normalised and does not change with the amount of material present.{{r|usp1503}} The quantity a purchaser cares about is the mass of peptide in the vial. That is the product of three things — the fill mass, the [[Peptide content|peptide content]] of the solid, and the purity — and a certificate reporting only the last of these constrains it hardly at all.{{r|usp1503}} Underfilling has been documented in independently examined research-chemical material. It is also the failure most easily checked without a laboratory, since it requires only a balance of adequate resolution and an empty vial of the same type for tare.{{r|reports}} == Why purity does not detect it == Chromatographic purity is computed as the area of the main peak divided by the total integrated area, expressed as a percentage. Both terms scale with the amount injected, so the ratio is invariant to concentration over the linear range of the detector.{{r|usp621}} A consequence is that a half-filled vial and a correctly filled vial of the same material give the same purity figure. The determination is answering a question about composition, and fill is a question about quantity. The same reasoning applies to identity determinations. A [[Mass spectrometry|mass spectrum]] confirms what a molecule is, not how much of it there is, unless the method is explicitly quantitative against a standard. == What does detect it == | Determination | What it gives | |---|---| | Gravimetric fill check | Mass of solid in the vial | | [[Peptide content]] | Fraction of that solid that is peptide | | Water by [[Karl Fischer titration]] | Fraction that is water | | Counterion content | Fraction that is salt | | Quantitative assay against a standard | Peptide mass directly | The arithmetic that matters is straightforward. A vial containing 5.0 mg of solid at 80% peptide content contains 4.0 mg of peptide, whatever its purity figure says; the same vial at 60% content contains 3.0 mg. Content in the 70–90% range is ordinary for a [[Trifluoroacetate counterion|trifluoroacetate-salted]] lyophilisate, and figures below that are not rare.{{r|usp1503}} Gravimetry is accessible outside a laboratory. Weighing a sealed vial against an empty vial of the same type gives an approximate fill mass; the approximation is limited by variation in the glass and closure, which for small fills can be comparable with the fill itself. Weighing several units of the same type reduces that error and also reveals vial-to-vial spread, which is often the more informative observation.{{r|reports}} == Overage and its interpretation == Some fills exceed nominal deliberately. A fill target set above the label claim compensates for losses in reconstitution and transfer, and an overage of a few per cent is ordinary in licensed manufacture; it is not a defect.{{r|ich_q7}} This cuts against a naive reading of gravimetric results in both directions. A vial weighing more than nominal is not evidence of generosity or of contamination, and a vial weighing slightly less is within ordinary process variation. What is informative is a systematic shortfall across units, or a spread far wider than a fill process should produce. Reports submitted to this wiki that record fill mass generally record it across several units for this reason. A single vial is uninformative about a fill process; ten are considerably more so.{{r|reports}} == 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_q7">International Council for Harmonisation, ''Q7: Good Manufacturing Practice Guide for Active Pharmaceutical Ingredients'' (2000).</ref> == See also == * [[Peptide content]] * [[Area percent purity]] * [[Vial filling and stoppering]] * [[Certificate of analysis]] * [[Trifluoroacetate counterion]] * [[Purity claim inflation]] {{DEFAULTSORT:Underfilling}} [[Category:Quality failures]] [[Category:Fill and finish]] [[Category:Quality and testing]]