Purity is the number everyone advertises. Fill weight is the number that quietly changes every concentration you calculate.

Two instruments, two different questions. A report with only one of them answers half of what you need to know.

Most peptide degradation happens before the first measurement. Handling, solvent choice, and the moment you write on the vial.

Three numbers decide whether a lab report means anything. Here is what each one is actually measuring, and what it cannot tell you.