Peptide listings and Certificates of Analysis often include a molecular weight (MW) figure alongside the compound name. It's a small number that does real work — it's the bridge between a peptide's chemical identity and the practical arithmetic of reconstitution and mass spectrometry confirmation.
What molecular weight represents
Molecular weight is the mass of a single molecule of the compound, generally expressed in daltons (Da) or grams per mole (g/mol) — the two are numerically equivalent for this purpose. It's determined by the peptide's exact sequence: the specific amino acids involved and how many of each.
Because sequence determines molecular weight precisely, MW is also one of the values a mass spectrometer measures to confirm identity — a result that doesn't match the expected MW for the stated sequence is a flag that something is off, independent of the purity result.
Why it shows up next to purity
Purity (from HPLC) and identity (partly via molecular weight, from mass spec) answer different questions. A sample can be highly pure by HPLC while still being the wrong compound if identity wasn't independently confirmed — which is exactly why the two are typically reported together rather than either alone.
Where it matters practically
For most day-to-day use, MW is a reference figure rather than something researchers calculate with directly — reconstitution maths generally works from the labelled peptide mass (mg) rather than molecular weight. Where MW does come in handy is comparing sequence variants of a compound, or double-checking a mass-spec result against the expected value for a given sequence.