What does 99.1% purity on a C18 column actually tell me about an ecnoglutide vial?

What I am working with: 99.1% · a C18 column · ecnoglutide.

I would like help reading this properly rather than being told what conclusion to reach.

I have the full report including the method section, so I can quote specifics if that helps.

What can I legitimately conclude from this figure?

Stated carefully, the chemistry of peptides at low pH and high organic-solvent concentration is not something most users have intuition for, which is why published methods exist.

Reverse-phase stationary phases use C18 or C8 chains bonded to silica, and the pore size of the silica matters more for a peptide of this chain length than the ligand length does.

Reconciling gross mass to label claim

Concretely, mobile phase pH at the point where you inject must match the mobile phase pH at the start of the gradient, or the sample will not be focused at the column head.

I would caution against over-interpreting small differences between runs, because the noise floor of the method is larger than most people assume.

If two labs give different numbers, the method difference is the first hypothesis, not lab quality.

Worth being precise here: method transfer between laboratories is almost impossible without full documentation, which is why two labs reporting the same preparation should expect disagreement.

Acetonitrile is the organic modifier of choice because it has a good UV cutoff, a reasonable viscosity and a refractive index that minimises baseline noise.

More usefully, sample preparation is almost always under-appreciated — a reconstituted peptide in strong solvent will distort its own peak on the gradient.

The practical summary: specify the method, run the same method on every sample you compare, and use orthogonal techniques to confirm the result.