The Identity Problem

Part 1 — Laboratory Notebook

Why a 0.036-dalton difference is the hardest number in peptide identity

Parts per million sound impressive until they are converted into daltons at the molecular weight of the thing being measured.

Part 2 — Explainers

Twelve things a real certificate has, and what their absence means

The header identifies the batch, the table records the tests, and the footer says who is answerable. Two of the three are usually incomplete.

Part 3 — The Supply Chain

The batch number is the only thing connecting the page to the vial

What happens to traceability when bulk material is subdivided, repackaged and relabelled two or three times before sale.

Part 4 — Laboratory Notebook

What a mass spectrometer does to a peptide before it measures anything

A brief and unromantic tour of the interface between a liquid sample and a vacuum.

Part 5 — The Supply Chain

Who signed this, and what did they undertake by signing it

What the Journal asks when a document arrives unsigned, and what the answers have been.

Part 6 — Analytics

Date of analysis, date of manufacture, and the gap between them

The interval between manufacture and analysis is the most under-read figure on the page, and the one most likely to matter by the time a vial is opened.

Part 7 — Explainers

What a purity certificate is silent about

A purity figure is silent on peptide content, on water, on counter-ion, on sterility, on endotoxin and on stability. Each of those silences has a price attached.

Part 8 — Laboratory Notebook

Why VendorInvestigate can tell you the mass is right and not that the sequence is

The Journal’s standing position: a mass that matches is necessary evidence of identity and nowhere near sufficient.