Underfilling (revision 16)

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 is normalised and does not change with the amount of material present.^[1]

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 of the solid, and the purity — and a certificate reporting only the last of these constrains it hardly at all.^[1]

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.^[2]

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.^[3]

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 spectrum confirms what a molecule is, not how much of it there is, unless the method is explicitly quantitative against a standard.

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-salted lyophilisate, and figures below that are not rare.^[1]

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.^[2]

• Peptide content
• Area percent purity
• Vial filling and stoppering
• Certificate of analysis
• Trifluoroacetate counterion