Peptide content versus purity — the distinction most buyers get wrong

These are two different measurements of two different things and they are routinely conflated, including on certificates.

• Purity (% area, by RP-HPLC) is the fraction of the chromatographically detected material that is the target compound. Water does not absorb at 214 nm. Neither does acetate, and neither does trifluoroacetate to any useful extent. None of them appear in a purity figure at all.
• Peptide content (%w/w, by HPLC assay against a standard, or by amino-acid analysis, or by nitrogen determination) is the fraction of the total mass of the material that is peptide. It counts everything that weighs something.

A lyophilised vial contains four things: peptide, water, counter-ion, and residual solvent plus inorganic residue. They are exhaustive and they must sum to 100 % of the cake mass. The index publishes that mass balance on every record where the measurements exist, closed to within 0.02 pp.

Worked example — a 10 mg semaglutide vial

Measured:

  purity  (RP-HPLC, % area)              98.74 %
  water   (Karl Fischer)                  5.62 %
  TFA     (ion chromatography)            7.41 %
  residual solvent + ash                  0.48 %

Gravimetric mass balance:

  total peptide content = 100 − 5.62 − 7.41 − 0.48   = 86.49 %w/w
  check: 5.62 + 7.41 + 0.48 + 86.49                  = 100.00 %  ✓

Target-compound content (the number that matters):

  target content = 86.49 × 98.74 / 100                = 85.40 %w/w

Recovered mass from a vial weighing 10.00 mg gross:

  recovered = 10.00 × 85.40 / 100                     = 8.540 mg
  % of a 10 mg label claim                            = 85.4 %

A 98.74 %-pure vial delivered 85 % of its label claim.
The purity figure was not wrong. It was answering a different question.

Why suppliers report purity and not content

Purity is cheap, fast, and flattering. Content requires either a certified reference standard of the same compound — which for most research peptides does not exist commercially — or amino-acid analysis after acid hydrolysis, which is a day of work. Water and counter-ion each require their own dedicated instrument. A purity-only certificate costs one injection.

That is not a conspiracy, it is an economic gradient, and it is why the index treats a content figure as the single strongest signal of documentation quality on a certificate. Suppliers who state content are, in this dataset, also the suppliers whose purity figures agree best with independent measurement.

On salt form

Trifluoroacetate is a heavier counter-ion than acetate — 114 Da against 60 — and a peptide with six basic residues shipped as the TFA salt carries roughly 8 % of its mass as TFA against roughly 4 % as acetate. At equal purity and equal water, an acetate-salt vial therefore contains more peptide. One supplier in this index ships acetate by default and the effect is visible in its recovered-mass distribution.