[Discussion] can we stop arguing about purity until somebody posts a number

Slightly embarrassed to be asking this, but: can we stop arguing about purity until somebody posts a number.

My first certificate had a compound name, a percentage and a logo. I thought that was normal. It is not, and now I know what to ask for.

How to ask for a batch record without it turning into a negotiation.

One email, three requests: the batch-specific certificate for lot X, the method line behind it, and the storage condition the stability statement was written against. No preamble, no explanation of why you want it.

What comes back is the signal. A full document with the column and gradient printed on it, inside a day, tells you there is a real quality system behind the operation. That is the test I run before I order from anyone new, and it costs nothing.

Why the same vial can honestly produce three different numbers.

Purity by area percent is relative to what the detector saw on one gradient at one wavelength. Net peptide content subtracts counterion, water and residual solvent. Mass of peptide per vial is net content times fill weight, and fill weight has its own tolerance.

So a document reading 99% purity, 6% acetate and 4% water is describing a vial that is about 90% peptide by mass, and every one of those numbers is honest. The mistake is treating the headline figure as the answer to "how much peptide did I get".

Corrections welcome, especially the pedantic ones. Pedantry is how this board earns its reputation.